Gas turbine emissions

Last, the relationship between combustor exit temperature distribution and turbine section durability will be discussed.. Engine system-level requirements and supporting combustor charac

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Tai ngay!!! Ban co the xoa dong chu nay!!!

Gas Turbine emissions

The development of clean, sustainable energy systems is one of the grand challenges of our time Most projections indicate that combustion-based energy conversion systems will remain the predominant approach for the majority of our energy usage Moreover, gas turbines will remain a very significant technology for many decades to come, whether for aircraft propulsion, power generation, or mechanical drive applications This book compiles the key scientific and technological knowledge associated with gas turbine emis-sions into a single authoritative source The book has three parts: the first part reviews major issues with gas turbine combustion, including design approaches and constraints, within the context of emissions The second part addresses fundamental issues associated with pollutant formation, modeling, and prediction The third part features case studies from manufacturers and technology developers, emphasizing the system-level and prac-tical issues that must be addressed in developing different types of gas turbines that emit pollutants at acceptable levels

Timothy C Lieuwen is professor of aerospace engineering and executive director of the

Strategic Energy Institute at the Georgia Institute of Technology Lieuwen has authored one textbook, edited two books, written seven book chapters and more than 200 papers, and received three patents He chaired the Combustion and Fuels Committee of the International Gas Turbine Institute of the American Society of Mechanical Engineers (ASME) He is also on the Propellants and Combustion Technical Committee of the American Institute of Aeronautics and Astronautics (AIAA), and he previously served

on the AIAA Air Breathing Propulsion Technical Committee He has served on a variety

of major panels and committees through the National Research Council, Department

of Energy, NASA, General Accounting Office, and Department of Defense Lieuwen

is the editor in chief of the AIAA Progress in Astronautics and Aeronautics series and

is serving or has served as an associate editor of the Journal of Propulsion and Power, Combustion Science and Technology , and the Proceedings of the Combustion Institute

Lieuwen is a Fellow of the ASME and received the AIAA Lawrence Sperry Award and the ASME Westinghouse Silver Medal Other recognitions include ASME best paper awards, the Sigma Xi Young Faculty Award, and the NSF CAREER award

Vigor Yang is the William R T Oakes Professor and chair of the School of Aerospace

Engineering at the Georgia Institute of Technology Prior to joining the faculty at Georgia Tech, he was the John L and Genevieve H McCain Chair in Engineering at the Pennsylvania State University His research interests include combustion instabilities in propulsion systems, chemically reacting flows in air-breathing and rocket engines, com-bustion of energetic materials, and high-pressure thermodynamics and transport Yang has supervised more than forty PhD and fifteen MS theses He is the author or coauthor

of more than 300 technical papers in the areas of propulsion and combustion and has lished ten comprehensive volumes on rocket and air-breathing propulsion He received the Penn State Engineering Society Premier Research Award and several publication and technical awards from AIAA, including the Air-Breathing Propulsion Award (2005), the Pendray Aerospace Literature Award (2008), and the Propellants and Combustion

pub-Award (2009) Yang was the editor in chief of the AIAA Journal of Propulsion and Power (2001–9) and is currently the editor in chief of the JANNAF Journal of Propulsion and Energetics (since 2009) and coeditor of the Cambridge Aerospace Series He is a Fellow of the American Institute of Aeronautics and Astronautics, American Society of Mechanical Engineers, and Royal Aeronautical Society

Cambridge Aerospace Series

Editors:

Wei Shyy

and

Vigor Yang

1 J M Rolfe and K J Staples (eds.): Flight Simulation

2 P Berlin: The Geostationary Applications Satellite

3 M J T Smith: Aircraft Noise

4 N X Vinh: Flight Mechanics of High-Performance Aircraft

5 W A Mair and D L Birdsall: Aircraft Performance

6 M J Abzug and E E Larrabee: Airplane Stability and Control

7 M J Sidi: Spacecraft Dynamics and Control

8 J D Anderson: A History of Aerodynamics

9 A M Cruise, J A Bowles, C V Goodall, and T J Patrick: Principles of Space Instrument

Design

10 G A Khoury (ed.): Airship Technology, Second Edition

11 J P Fielding: Introduction to Aircraft Design

12 J G Leishman: Principles of Helicopter Aerodynamics, Second Edition

13 J Katz and A Plotkin: Low-Speed Aerodynamics, Second Edition

14 M J Abzug and E E Larrabee: Airplane Stability and Control: A History of the Technologies

that Made Aviation Possible, Second Edition

15 D H Hodges and G A Pierce: Introduction to Structural Dynamics and Aeroelasticity,

Second Edition

16 W Fehse: Automatic Rendezvous and Docking of Spacecraft

17 R D Flack: Fundamentals of Jet Propulsion with Applications

18 E A Baskharone: Principles of Turbomachinery in Air-Breathing Engines

19 D D Knight: Numerical Methods for High-Speed Flows

20 C A Wagner, T Hüttl, and P Sagaut (eds.): Large-Eddy Simulation for Acoustics

21 D D Joseph, T Funada, and J Wang: Potential Flows of Viscous and Viscoelastic Fluids

22 W Shyy, Y Lian, H Liu, J Tang, and D Viieru: Aerodynamics of Low Reynolds Number

Flyers

23 J H Saleh: Analyses for Durability and System Design Lifetime

24 B K Donaldson: Analysis of Aircraft Structures, Second Edition

25 C Segal: The Scramjet Engine: Processes and Characteristics

26 J F Doyle: Guided Explorations of the Mechanics of Solids and Structures

27 A K Kundu: Aircraft Design

28 M I Friswell, J E T Penny, S D Garvey, and A W Lees: Dynamics of Rotating Machines

29 B A Conway (ed.): Spacecraft Trajectory Optimization

30 R J Adrian and J Westerweel: Particle Image Velocimetry

31 G A Flandro, H M McMahon, and R L Roach: Basic Aerodynamics

32 H Babinsky and J K Harvey: Shock Wave–Boundary-Layer Interactions

33 C K W Tam: Computational Aeroacoustics: A Wave Number Approach

34 A Filippone: Advanced Aircraft Flight Performance

35 I Chopra and J Sirohi: Smart Structures Theory

36 W Johnson: Rotorcraft Aeromechanics

37 W Shyy, H Aono, C K Kang, and H Liu: An Introduction to Flapping Wing

Aerodynamics

38 T C Lieuwen and V Yang (eds.): Gas Turbine Emissions

Gas Turbine Emissions

cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Mexico City

Cambridge University Press

32 Avenue of the Americas, New York, NY 10013-2473, USA

www.cambridge.org

Information on this title: www.cambridge.org/9780521764056

© Timothy C Lieuwen and Vigor Yang 2013 This publication is in copyright Subject to statutory exception and to the provisions of relevant collective licensing agreements,

no reproduction of any part may take place without the written permission of Cambridge University Press.

First published 2013 Printed in the United States of America

A catalog record for this publication is available from the British Library.

Library of Congress Cataloging in Publication data

Lieuwen, Timothy C.

Gas turbine emissions / Timothy C Lieuwen, Vigor Yang.

pages cm – (Cambridge aerospace series; 38) Includes bibliographical references and index.

ISBN 978-0-521-76405-6 (hardback)

1 Gas-turbines – Environmental aspects 2 Gas-turbines – Combustion

3 Combustion gases – Environmental aspects I Yang, Vigor II Title

TJ778.L524 2013 621.43′3–dc23 2012051616 ISBN 978-0-521-76405-6 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party Internet Web sites referred to in this publication and does not guarantee that any content on such Web sites is, or will remain, accurate or appropriate.

PART 1 OVERVIEW AND KEY ISSUES

1 Aero Gas Turbine Combustion: Metrics, Constraints, and

System Interactions 3

Randal G McKinney and James B Hoke

2 Ground-Based Gas Turbine Combustion: Metrics, Constraints,

and System Interactions 24

Vincent McDonell and Manfred Klein

3 Overview of Worldwide Aircraft Regulatory Framework 81

Meredith B Colket III

6 Gaseous Aerosol Precursors 154

Richard C Miake-Lye

7 NO x and CO Formation and Control 175

Ponnuthurai Gokulakrishnan and Michael S Klassen

8 Emissions from Oxyfueled or High-Exhaust Gas

Recirculation Turbines 209

Alberto Amato, Jerry M Seitzman, and Timothy C Lieuwen

Contents

Contents

viii

PART 3 CASE STUDIES AND SPECIFIC TECHNOLOGIES:

POLLUTANT TRENDS AND KEY DRIVERS

9 Partially Premixed and Premixed Aero Engine Combustors 237

Christoph Hassa

10 Industrial Combustors: Conventional, Non-premixed, and

Dry Low Emissions (DLN) 290

Thomas Sattelmayer, Adnan Eroglu, Michael Koenig, Werner

Krebs, and Geoff Myers

Alberto Amato, Georgia Institute of Technology, Atlanta, Georgia, U.S.A

Meredith B Colket III, United Technologies Research Center, East Hartford, Connecticut, U.S.A

Willard Dodds, General Electric Aviation Company, Cincinnati, Ohio, U.S.A

Alan H Epstein, Pratt & Whitney Company, East Hartford, Connecticut, U.S.A

Adnan Eroglu, Alstom Power, Inc., Baden, Switzerland

Ponnuthurai Gokulakrishnan, Combustion Science & Engineering, Inc., Columbia, Maryland, U.S.A

Christoph Hassa, German Aerospace Center, DLR, Linder Hoehe, Cologne, Germany

James B Hoke, Pratt & Whitney Company, East Hartford, Connecticut, U.S.A

Michael S Klassen, Combustion Science & Engineering, Inc., Columbia, Maryland, U.S.A

Manfred Klein, National Research Council, Ottawa, Ontario, Canada

Michael Koenig, Siemens Energy Inc., Orlando, Florida, U.S.A

Werner Krebs, Siemens AG, Fossil Power Generation Division, Muelheim an der Ruhr, Germany

Timothy C Lieuwen, Georgia Institute of Technology, Atlanta, Georgia, U.S.A

Vincent McDonell, University of California, Irvine, California, U.S.A

Randal G McKinney, Pratt & Whitney Company, East Hartford, Connecticut, U.S.A

Richard C Miake-Lye, Aerodyne Research, Inc., Billerica, Massachusetts, U.S.A

Geoff Myers, GE Energy Company, Greenville, South Carolina, U.S.A

Thomas Sattelmayer, Technische Universität München, Garching, München, Germany

Jerry M Seitzman, Georgia Institute of Technology, Atlanta, Georgia, U.S.A

Contributors

When I first became interested in jet engines, smoke trails from the then ultramodern Boeing 707s were an arresting feature of that modern world Ten years later, smoke was regulated and the U.S Federal Aviation Administration had canceled the Boeing 2707 supersonic airliner program in the midst of growing environmental concerns Back in the early 1960s, ground-based gas turbines were a very small business and concern for the environment was only minor Over the five decades since the 707, the role of gas turbines in our society has greatly expanded, and con-

cern regarding their emissions has grown even faster Now, the electric power

gen-eration gas turbine business has outgrown that of aircraft engines and emissions have become a market discriminator Indeed, large fortunes have been won and lost on the basis of the emissions performance of land-based gas turbine engines

On the aero engine side, emissions performance is now featured in engine

market-ing campaigns

Combustion emissions might be thought an arcane topic It is certainly complex

It is also of great importance to our society given the dominance of gas turbines for aircraft propulsion and power generation There are three, basically indepen-

dent, complicated problems associated with gas turbine emissions – the design of low-emissions combustors, the prediction of the effects of emissions on human health and the global environment, and the formulation of balanced and effec-

tive policy and regulation These challenges are important to three very different groups – technical folk, businesspeople, and policy makers and regulators This book will be of interest to them all

For the technical community, the science of how emissions are generated in a gas turbine combustor and their interactions with the atmosphere has always been a fascinating but challenging subject The relatively recent concern for climate change has increased the complexity of the atmospheric science problem, especially for air-

craft engines, from one mainly concerned with local air quality at low altitude to more complex interactions at the tropopause and in the stratosphere During the last fifty years, design engineers have risen to the environmental challenge by realizing combustors with much lower emissions while at the same time significantly increas-

ing reliability and life One important aspect of combustor engineering, however, has

Foreword

Alan H Epstein

Foreword

xii

not changed over this time – we still do not have the technology needed to predict gas turbine emissions from first principles The lack of first principles capabilities drives up product development costs and business risk

Policy makers and regulators, who are not necessarily technical experts in the fields they regulate, face interesting challenges as well These can be grouped into three general categories – technical, political, and diplomatic Technical questions include, for example, consideration of currently unregulated emissions such as very small particulates and CO2, as well as the role uncertainty plays in resolving con-flicting requirements such as NOx and CO2 Political challenges abound and include issues such as how to best balance environmental protection with economic growth and how to balance local air quality with global climate change Gas turbine emis-sions have also become a major diplomatic challenge Aviation is the most interna-tional of endeavors, both in manufacture and operation Most engines have parts and major subsections designed and manufactured in several countries Aircraft take off and land in different countries thousands of times a day and so fall under the pur-view of more than one regulator It is critical to the efficient operation of the world’s air transportation system that regulations be harmonized across the globe This is the job of the International Civil Aviation Organization (ICAO), a branch of the United Nations with 189 member states Getting 189 countries to agree on anything has never been easily or quickly achieved The rise of climate change as a major world-wide issue with its attendant political and economic implications has only increased the complications of international rule making

From the point of view of technical and policy folks, gas turbine combustor emissions bring fascinating challenges For the business community, the fascination turns to dread Why the dichotomy? The confluence of regulation and technical chal-lenge generates business uncertainty and risk, with financial penalties large enough

to destroy a business Manufacturers of ground-based engines are often tually responsible for the price of the electric power not produced if an engine is deficient An engine that does not meet local air quality standards cannot be oper-ated, and may incur liabilities that dwarf the price of the engine Manufacturers

contrac-of aircraft engines face similar challenges; that is, until an engine meets emissions requirements, it will not be certified by regulatory authorities Such engines cannot

be legally shipped, and so the airplanes, which cost ten times more than the engine, cannot be delivered Gas turbine development can cost up to two billion U.S dollars,

so long production runs are needed to amortize the cost The business risk ciated with emissions regulations is further amplified by the long-lived nature of the products Engines typically have service lives of thirty years or more Over this time span, emissions regulations usually change Increased stringency can reduce the residual value of an engine, hinder sales, and even prohibit operation of engines

asso-in the field Additional uncertaasso-inty is asso-introduced by the degree to which regulations are not harmonized across political boundaries since niche markets cannot support high development costs Thus, business planning for gas turbine emissions is a chal-lenge – and a concern

Foreword xiii

These are hard problems These are interesting problems These are important problems at the confluence of engineering, regulation, and business This book is the first to cover both the technical and regulatory aspects of gas turbine emissions With chapters authored by some of the world’s experts in their respective fields, it has the breadth and depth to be of interest to all the stakeholders It is valuable for experts

in the field and informative for those just getting involved

The development of clean, sustainable energy systems is one of the grand challenges

of our time Environmental and energy security concerns, coupled with growing energy demand, require us to increase, diversify, and optimize the use of energy sources while reducing the adverse environmental impacts of energy production, transmission, and use In particular, we are confronted with four interacting issues: climate change, local air and water quality, energy supply, and energy security Global warming has led to significant discussions about reductions of carbon dioxide emissions Meanwhile, concerns about energy security and supplies for a growing uti-

lization base are driving us to consider broader and more reliable energy resources Finally, local air quality concerns are driving interest in other pollutants that lead to, for example, acid rain or photochemical smog, and that have additional implications for the management of power plant operations and emissions

Gas turbines will continue to be an important combustion-based energy version device for many decades to come, for aircraft propulsion, ground-based power generation, and mechanical-drive applications At present, gas turbines are

con-a principcon-al source of new power-genercon-ating ccon-apcon-acity throughout the world, con-and the dominant source for air-breathing flight vehicles as well Over the last decade, power generation from alternative sources, such as solar and wind, has significantly increased Nevertheless, most projections indicate that the relative fraction of energy supplied by these sources will remain small, even several decades from now These projections also indicate that gas-turbine-based combined cycle plants will continue to represent the majority of new power generation capacity Moreover, as the supply of intermittent renewables grows, gas turbines will play an increasingly important role in stabilizing the electrical grid, where the supply and demand of electric power must match at every instant in time The topic of gas turbine emis-

sions, both traditional pollutants (NOx, CO, UHC, particulates) and CO2, is clearly

of significant interest

In the aviation sector, emissions regulations continue to tighten Climate change may lead the worldwide community to begin taxing carbon emissions for aircraft, and cloud formation associated with water vapor emissions continues to be an area

of research Particulate and NOx emissions can significantly influence local air quality

Preface

Preface

xvi

and can be controlled by appropriate combustor designs Changes to engine cycles and pressure ratio to increase fuel efficiency, however, generally promote the pro-duction of emissions such as NOx, and, thus, maintaining safe, reliable, low-emission aircraft engines is an increasingly important issue

The present volume compiles the key scientific and technological knowledge associated with gas turbine emissions into a single authoritative source The book consists of three parts The first part provides an overview of major issues relat-ing to gas turbine combustion, including design approaches and constraints, at both the component and system levels, within the context of emissions It also addresses approaches to meeting regulatory requirements Important considerations for design optimization are discussed across all metrics of significance for gas turbine operation, including cost, safety, and reliability The second part addresses funda-mental issues associated with pollutant formation, characterization, modeling, and prediction This part treats aerosol soot precursors, soot, NOx, and CO In addition, it includes a chapter on emissions from gas turbines with significant levels of exhaust gas recirculation, or whose exhaust will be used for enhanced oil recovery or seques-tered in geologic formations; in these cases, the emissions-related concerns are quite different The third part of this book presents case studies from manufacturers and technology developers, emphasizing the system-level and practical issues that must

be addressed in developing different types of gas turbines that emit pollutants at acceptable levels It is our hope that this book will provide a valuable resource to workers in this field, as a foundation both for scientists researching various aspects

of gas turbine emissions and for technology developers who translate this mental knowledge into products

funda-This book would not have been possible without assistance from many uals Peter Gordon encouraged this project and supported us throughout Our assis-tant Glenda Duncan was a tremendous help a great help in the numerous tasks associated with preparing the text We owe a great debt of gratitude to Jong-Chan Kim for his enormous effort in editing figures and ensuring that the illustrations are

individ-of the highest quality Dilip Sundaram deserves special appreciation for indexing the book

Part 1

Overview and Key issues

tors has greatly improved, they remain an important design challenge.

This chapter will describe how the combustor interacts with the rest of the engine and flight vehicle by describing the relationship between attributes of the engine and the resulting requirements for the combustor Emissions, a major engine performance characteristic that relies heavily on combustor design, will be introduced here with more detail found in following chapters The wide range of operating conditions a combustor must meet as engine thrust varies, which is a major challenge for combustor design, will also be described Last, the relationship between combustor exit temperature distribution and turbine section durability will be discussed

1.2 Overview of selected aircraft and engine requirements and their

relation to Combustor requirements

Aircraft gas turbine engines have been used in many different sizes of aircraft since their introduction in the 1940s Small aircraft such as single-engine turboprops use engines of low shaft horsepower, which are of small physical size Business jets and smaller passenger aircraft may use turbojets or turbofans with thrust in the range of several thousand pounds, usually with two engines per aircraft The other extreme includes four-engine aircraft with turbofan engine thrusts as high as seventy thousand pounds and very large twin-engine aircraft with thrust per engine in the one hundred thousand pound class These thrust designs are also physically very large, with fan diameters over 100 inches In all of these applications, the engine system imposes a common set of requirements upon the combustor, as summarized in Table 1.1

1 Aero Gas Turbine Combustion: Metrics, Constraints, and System Interactions

Randal G McKinney and James B Hoke

Aero GT Combustion

4

As shown in Figure 1.1, these requirements are interdependent Years of design and development within the industry have produced successive designs that improve upon all of the requirements concurrently Although emissions are the focus of this text, each of these other requirements interacts with the emissions constraints and will be introduced briefly

1.3 Combustor effects on engine Fuel Consumption

Gas turbine engines are Brayton cycle devices An ideal version of such a cycle prises isentropic compression, addition of heat at constant pressure, and isentropic expansion through the turbine Figure 1.2 is a simplified schematic of the effect of such a cycle on the pressures and temperatures in the engine In real engines, all

com-of the processes incur some loss com-of performance versus the ideal, manifested as a stagnation pressure loss in the combustor Combustion systems incur pressure losses because of flow diffusion and turning, jet mixing, and Rayleigh losses during heat addition (Lefebvre and Ballal, 2010) However, at most power conditions, the effi-ciency with which the fuel chemical energy is converted into thermal energy is very high, typically greater than 99.9 percent “Low” levels of 98 to 99.5 percent can be seen at low-power levels In general, though, the combustion system is a small para-sitic effect on overall fuel consumption

Table 1.1 Engine system-level requirements and supporting combustor characteristics

Engine requirement Combustor characteristic Optimize fuel consumption High combustion efficiency and low combustor pressure loss Meet emissions requirements Minimize emissions and smoke

Wide range of thrust Good combustion stability over entire operating range Ground and altitude starting Easy to ignite and propagate flame

Turbine durability Good combustor exit temperature distribution Overhaul and repair cost Meet required combustor life by managing metal temperatures

and stresses

Emissions

Altitude relight

Figure 1.1 Combustor performance requirements are interrelated.

1.4 Fundamentals of Emissions Formation 5

1.4 Fundamentals of emissions Formation

The pollutants emitted by engines that are of most interest are carbon monoxide (CO), unburned hydrocarbons (UHC), nitric oxides (NOx), and particulate matter

(PM or smoke) At low-power conditions, the inlet combustor pressure and

temper-ature are relatively low, and reaction rates for kerosene-type fuels are low Liquid fuel must be atomized, evaporated, and combusted, with sufficient residence time

at high enough temperatures to convert the fuel into CO2 If the flow field permits

fuel vapor to exit the combustor without any reaction, or, if partially reacted to

spe-cies of lower molecular weights, UHC will be present If a portion of the flow field

subjects a reacting mixture to a premature decrease in temperature via mixing with

cold airstreams, these incomplete or quenched reactions lead to the production of

CO, as detailed in Chapter 7

At high power conditions, high air pressures and temperatures lead to fast tions, with the result that CO and UHC are nearly zero At these elevated tempera-

reac-tures, emissions of NOx and PM become more prevalent NOx can be formed through

several processes, but the dominant pathway is thermal NOx, as described by the

extended Zeldovich mechanism, also detailed in Chapter 7

O = 2O

N +O= NO+N

N O = NO ON+OH = NO+ H

2 2 2

The formation rate is exponentially related to the temperature in the flame, peaking near stoichiometric conditions Thermal NOx emissions can be reduced by limiting the time the flow spends at the high temperature and/or by reducing the maximum temperatures seen in the flame via stoichiometry control Other NOx formation

Fan flow Core flow

Fan Core

Thrust

Power to operate fan + some thrust

Compressor Core flow

tem-is high enough to be vtem-isible, as was often the case in early gas turbines, it tem-is referred

to as smoke or soot Recently, the more general term particulate matter (PM) has

been used to describe this emission Modern engine smoke levels are invisible but still possess large quantities of very small soot particles and aerosol soot precursors (see Chapter 5) at the exhaust Emerging research on the effect of PM on health and climate focuses more attention on measuring, modeling, and understanding the processes governing PM production

These relationships between engine power conditions and emissions production lead to the behavior shown in Figure 1.3 As shown in the figure, levels of UHC and

CO are highest at low power and drop quickly with increasing thrust Conversely,

NOx and PM increase with engine power and are typically maximized at maximum power Chapters 5 and 7 discuss these emissions formation processes in more detail

1.5 effect of range of Thrust and starting Conditions

on the Combustor

Flight gas turbine engines must provide a range of thrust and thrust response to power the aircraft mission Missions vary depending on the aircraft application Commercial aircraft and military transports have similar missions Military fighters and other specialized aircraft can have very different missions because their use is not exclusively for the transport of payload between two points Design require-ments are also very different for commercial and military applications Military fighter engines are often designed for maximized thrust developed per unit weight

so that the maneuverability of the aircraft is maximized Military fighter engines also fly at a wide range of thrust throughout the flight envelope and must undergo fre-quent rapid thrust transients Typically, commercial engines are designed for maxi-mum fuel efficiency per unit thrust They fly at high altitude to achieve the best fuel efficiency and often do not have to endure the aggressive and numerous thrust transients of military fighter engines Engine combustors must operate stably and efficiently over the full range of operating conditions, and must reliably relight if an engine shutdown or flameout should occur in flight

1.5.1 Engine Mission Characteristics

A typical commercial engine mission consists of ground starting, taxi, takeoff, climb

to altitude, cruise, deceleration to flight idle and descent, approach, touchdown,

1.5 Effect of Range of Thrust and Starting Conditions on the Combustor 7

thrust reverse, and taxi in The extremes in combustor operating conditions drive the overall design approach The combustor must meet performance, operability, and emissions metrics over the full range of operation In order to do so, it must operate

at the following extremes:

1 Minimum fuel-air ratio – This occurs during decelerations from high power to low power Flight decelerations normally occur when descending from high alti-tude cruise and during approach throttle movements They can also occur in emergencies Minimum fuel-air ratio typically depends on the thrust decay rate,

as the time response of the engine turbomachinery that governs the airflow is much longer than that of the fuel flow Risk of weak extinction (flameout) is highest during decelerations

2 Minimum operating temperatures and pressures – These occur at flight and ground idle conditions Low pressure and temperature challenges combustion efficiency due to slower fuel vaporization and chemical kinetics

3 High operating temperatures and pressures – These occur at takeoff, climb, thrust reverse, and cruise conditions These conditions result in the bulk of NOxformation and the most severe liner metal temperatures

4 Ignition conditions – Ignition normally occurs on the ground but also

occasion-ally in flight Ignition is required at near surrounding ambient pressure and temperature High altitude and extremely cold conditions are typically the most challenging to achieve ignition, flame propagation, and flame stabilization These conditions lead to low temperature (−40ºF) and pressure (4 psia at 35,000 ft.) combustor inlet conditions

Thus, the combustor design must meet the performance, emissions, and durability requirements at low- and high-power operations without compromising stability

0 10 20 30 40 50 60

Figure 1.3 Emissions versus power level for the PW4084.

1.5.2 Fixed-Geometry Rich-Quench-Lean (RQL) CombustorsFixed-geometry combustors have been used in the gas turbine industry since its inception Early designs used multiple cans in a circumferential array The cans transitioned through an annular duct to the turbine (Figure 1.4a) Later designs used an annular duct geometry that allowed for reduced overall length and weight (Figure 1.4b) Annular combustors also have reduced liner surface area relative to can-annular combustors and therefore use less cooling All designs use multiple fuel injectors to provide spray atomization and fuel-air mixing Achieving good atomiza-tion and fuel-air mixing is critical for efficient combustion, low emissions, and good temperature uniformity into the turbine Normally, the fuel is injected in the front end

of the combustor and flow recirculation is created to provide a stabilization region for the combustion process This is typically accomplished with air swirlers, which leads to vortex breakdown and flow recirculation The stabilization zone promotes recirculation of hot product gases forward to the incoming fuel spray, thereby pro-viding a continuous ignition source and faster fuel droplet evaporation Accelerated droplet evaporation is critical to high-efficiency combustion at low-power conditions, when low air inlet temperatures are insufficient to provide fast enough evaporation

(a)

(b)

Figure 1.4 (a) Can-annular combustor (Pratt & Whitney JT8D-200); (b) RQL annular bustor (IAE V2500).

com-1.5 Effect of Range of Thrust and Starting Conditions on the Combustor 9

If continuous ignition is not provided at low power, the vaporization and reaction times can exceed the combustor residence time and flameout occurs

The airflow distribution in a fixed-geometry combustor must be selected to achieve both low- and high-power performance requirements Conditions at the combustor inlet vary significantly between low-power idle and high-power takeoff conditions At idle, inlet temperature, pressure, and global fuel-air ratio are relatively low At takeoff, the opposite is true (Figure 1.5) The operating temperatures and

pressures are largely a function of the engine thermodynamic cycle; therefore the most significant parameter for the combustor designer to consider is the fuel-air ratio Because air is introduced in stages along the length, the designer can tailor the airflow distribution to achieve key performance metrics This creates a distri-

bution in fuel-air ratio along the length of the combustor, leading to variations in local temperature as power level is adjusted The difference in fuel-air ratio between high-power takeoff and low-power deceleration and idle conditions is critical because

it determines the range of local fuel-air ratio in the front end of the combustor For most modern gas turbines, the difference is large enough that the front end operates fuel rich (f/a > 0.068 for jet fuel) at takeoff conditions Consequently, fixed-geometry combustors are referred to as rich-burning or rich-quench-lean (RQL) designs This refers to the rich front-end fuel-air ratio that is diluted (quenched) by additional airflow in the downstream section of the combustor to reach the fuel-lean conditions

at the combustor exit The RQL-type design has several advantages and challenges, which are discussed later in this chapter

As previously described, the challenges at low power are combustion efficiency and stability The local fuel-air ratio in the RQL combustor front end at idle is designed to generate high recirculating gas temperatures (Figure 1.6) Therefore, the local fuel-air ratio should be near the stoichiometric (f/a ~.068 for jet fuel) fuel-air ratio to achieve high combustion efficiency High combustion efficiency minimizes unburned hydrocarbon and carbon monoxide emissions that predominate at idle Some increase in NOx emissions is generated by the hot front end, but emissions at idle are not significant when compared to high power By designing for near stoichio-

metric conditions at idle, stability can be ensured at deceleration conditions, where minimum fuel-air ratio occurs If the minimum fuel-air ratio during deceleration is

Pressure

Idle

Temperature

Steady state fuel-air ratio

Transient decel fuel-air ratio Figure 1.5 Combustor operating conditions.

tempera-in the high-temperature, oxygen-rich quench region Thus, the front-end airflow level must be set with understanding of the formation and oxidation processes The NOx

emissions are formed in both the front end and quench regions at high power NOxformation is exponentially a function of gas temperature, but also depends on the residence time at the local temperature The highest rate of formation occurs in the quench region because it is the region where peak temperatures occur However, time at peak temperature in the quench region is relatively short due to high mixing rates In contrast, the formation of NOx in the front end is not negligible because it has relatively longer residence time due to the flow recirculation The presence of cooling flow in the front end also leads to NOx formation when it interacts with the fuel-rich gas mixture

CO HC

Threshold temperature

Gas residence time in combustor

Compressor exit

Turbine inlet

1.5 Effect of Range of Thrust and Starting Conditions on the Combustor 11

Recent advances have shown that substantial reductions in residence time and

NOx can be achieved without compromising combustor stability and low-power

performance Use of fuel injectors that produce small droplets uniformly dispersed within the airflow and rapid air jet mixing has enabled the residence time reduction These advanced RQL combustor designs (Figure 1.8) have demonstrated NOx reduc-

tion of over 50 percent when compared to early annular combustors They are also shorter and have lower volumes to reduce residence times Reduced-length combus-

tors are lighter and also have reduced surface area requiring film cooling Advanced cooling schemes have been deployed to minimize NOx emissions and temperature

streaks into the turbines

Overall, the RQL combustor has demonstrated excellent service history Because

it does not require complex controls to modulate fuel between injectors, it has

dem-onstrated very good reliability It also has inherently favorable stoichiometry for stability because the front-end airflow is minimized for NOx control purposes The

front-end airflow is established as the minimum amount required for smoke control

If the fuel-air ratio range between high power and low power is large, the airflow required to control smoke can be larger than desirable for flame stability during decelerations In these instances, the selected minimum transient fuel-air ratio must

be raised to protect flight safety and reliability In turn, raising the minimum fuel-air ratio limit increases the time required to decelerate the engine and can result in

a safety risk during emergencies If the deceleration time cannot be met with the revised minimum fuel-air ratio, then stability must be addressed by other means, such

as by clustering fuel injectors provided with either more fuel or reduced airflow This

NOxSmoke

Threshold temperature

Gas residence time in combustor

Compressor exit

Turbine inlet

Rapid NOxformation

Rich combustion

Aero GT Combustion

12

zone remains above the weak extinction level locally and protects against flameout

at worst-case deceleration conditions

The critical challenges for the RQL design approach are smoke and liner bility As previously discussed, uniform mixing of fuel and airflow in the injectors can result in reduced smoke levels When the fuel injector stoichiometry is fuel rich overall, the uniformity of the fuel-air distribution within the injector becomes crit-ical A poorly mixed injector with a wide distribution will have regions that range from fuel lean to very fuel rich The latter can produce the bulk of the smoke in the combustor This occurs because the highest smoke generation often takes place

dura-in the most fuel-rich regions where there is sufficient residence time Because the front end is designed with gas recirculation to achieve stability, these zones can produce smoke Thus, the mixing and recirculation patterns are critical to smoke control

The presence of fuel-rich and stoichiometric gases also introduces a liner bility challenge Because modern gas turbines operate at high temperatures and pressures, peak gas temperatures can exceed 4200ºF Metallic liners have a practical temperature limit of <2000ºF for designs that meet typical durability life require-ments Therefore, the liner must be cooled to prevent failure Virtually all aero engine combustors feature hot side film cooling Film cooling provides a protective layer of airflow on the liner surface that prevents convective heat transfer from high temper-ature gas However, when fuel-rich gases in the front end interact with cooling, the film air provides oxidant for high-temperature combustion Therefore, the presence

dura-of cooling air increases NOx formation in the forward portion of the combustor In the aft section of the combustor, cooling does not readily mix radially and there-fore decreases gas temperatures near the walls The result is higher temperatures in Figure 1.8 Advanced RQL combustor (Pratt & Whitney PW1500 TALON X).

1.5 Effect of Range of Thrust and Starting Conditions on the Combustor 13

the midstream The midstream peaked temperature profile also increases the

hot-test streak temperature exiting the combustor Therefore, aft cooling airflow affects the temperature profile and uniformity entering the turbines Consequently, it is desirable to reduce cooling throughout the combustor Improved liner designs have enhanced heat transfer efficiency, enabled emissions reductions, and strengthened turbine durability The evolution of liner cooling designs will be discussed in a later section

1.5.3 Fuel-Staged Combustors

Having discussed RQL approaches, we next consider fuel-staged combustors, which have seen limited use in commercial aircraft service First-generation designs were introduced in the 1990s, and updated designs are scheduled for release in future engines The overall approach in a fuel-staged combustor is to control the combus-

tion stoichiometry through use of fuel injection in multiple locations Where the fixed-geometry RQL combustor injected fuel and air as uniformly as possible in the front end of the combustor, the staged combustor deliberately provides for mul-

tiple airflow and fuel flow zones The objective is to achieve fuel-lean combustion conditions for NOx reduction at high power The fuel-lean conditions keep gas tem-

peratures low and virtually eliminate the highest temperatures associated with

stoi-chiometric conditions that exist in the RQL design

The lack of fuel-rich and stoichiometric combustion creates two immediate efits when compared to an RQL design The first is that the fuel-lean flame produces very low levels of soot emissions This means that carbon particulate emissions have the potential to be lower from fuel-staged combustors Significant future efforts are required to characterize the full range of particulates emitted from both types of combustors (see Chapter 5) The second benefit is that the staged lean combustor

ben-requires less film cooling air for the liner Because the lean reaction produces less soot,

it is less luminous, resulting in reduced radiation heat load on the liner Additionally, because the peak gas temperatures are lower, the convective heat loading is reduced These factors allow for reduced liner cooling flux This air can in turn be used for emissions control or to improve combustor exit temperature uniformity

In a fuel-staged combustor, a large amount of airflow is mixed with the fuel at the injection point, so that fuel-lean conditions are achieved at high power with all fuel injectors flowing The large amount of airflow and fuel-lean conditions pose a stability challenge at low power due to the fuel-air ratio lapse that occurs between high power and low power To mitigate the stability risk, some of the fuel injectors are turned off at low power This allows for the control of the combustion stoichi-

ometry at idle to ensure high combustion efficiency The zone that operates at low

power is referred to as the pilot zone, and the high-power fuel injectors are referred

to as the main zone A difficult challenge for staged combustor designs is the

tran-sition between operating with only the pilot at low power and all fuel injectors at high-power conditions The transition often occurs at mid-power conditions such as approach thrust where fuel-air ratio, pressure, and temperature are not as high as

Aero GT Combustion

14

cruise, climb, and takeoff Therefore, the local fuel-air ratio in the main stage may

be unfavorable for efficient combustion at the lower temperatures and pressures Consequently, more complex staging systems may be required where the main stage fuel injectors are turned on at different overall fuel-air ratios so that high efficiency

is maintained These fuel-air ratios are referred to as staging points Initial designs

were applied to engines with relatively low fuel-air lapse levels These designs were operated with two fuel stages and a single staging point More recent designs applied

to engines with higher fuel-air ratio lapse may require more than a single fuel staging point to maintain staging efficiency

Staging can also affect engine acceleration time from idle to higher power ditions This is because of two factors The first is the aforementioned combustion efficiency near the staging point Lower efficiency results in reduced heat release and slower acceleration The second is potential delay time to deliver fuel to the main fuel injectors If some of the fuel flow is needed to fill fuel manifolds and fuel injectors, a delay occurs in the time to achieve combustion heat release and engine acceleration Therefore, it is desirable to keep the main stage fuel system as filled as possible to achieve prompt acceleration when the throttle is moved However, a full

con-main stage fuel system is vulnerable to fuel coking Fuel coking refers to the hard

carbonaceous compounds formed in the internal passages of the fuel system when the fuel undergoes pyrolysis reactions when it is heated in the absence of air Such compounds can block or reduce the flow of fuel through the main stage hardware Coking is most common inside the fuel injectors because they are exposed to the high temperatures inside the diffuser casing In the extreme, coking can limit thrust

by limiting fuel flow Most modern engines have idle air temperatures near or above the level at which significant coking occurs (400ºF) This air is in contact with the main stage fuel injectors containing the stagnant fuel To prevent fuel coking, cool-ing and insulation features must be incorporated to prevent fuel from contacting passage walls over the critical temperature for coking Some designs use the pilot fuel flow to cool the stagnant main fuel injectors Other possibilities include using air pressure to purge the fuel from the most vulnerable areas

A final challenge to the fuel-staged combustor designer is combustion instability

Combustion instability refers to temporal fluctuations in the heat release Such tuations can be attributed to several mechanisms, typically involving excitation of natural fluid mechanic instabilities in the flow or fuel-air ratio oscillations In the extreme, instabilities can damage hardware and result in engine damage and failure All combustors have risk of instability, but staged lean combustors have been more prone to them It is unclear if this tendency is related to differences in acoustic driving resulting from heat release distribution differences or to changes in acoustic damping

fluc-as the combustor is modified for lean-staged operation (Lieuwen and Yang, 2005).1.5.4 Ignition and Engine Starting

Gas turbine combustors are required to ignite on the ground and in flight Ignition

in flight is rare because it occurs after unplanned engine shutdown The combustor

1.5 Effect of Range of Thrust and Starting Conditions on the Combustor 15

should ignite promptly after the fuel is turned on and provide efficient

combus-tion to accelerate the engine to idle power Delayed ignicombus-tion can cause excess fuel accumulation in the combustor and increased pressure pulses at light off Increased pressure pulses can result in compressor stall that prevents engine acceleration to idle On the ground and at low-speed flight conditions, the engine rotors are turned with a starter to provide airflow to the combustor for ignition and combustion At higher-speed flight conditions, the ram airflow turns the rotor in a process referred

to as windmilling Ignition energy is typically delivered with a spark igniter At least

two igniters are placed in the typical annular combustion chamber to provide

redun-dancy in the event of a failure The spark produces plasma sufficient to initiate the combustion reaction The ignited reactants must then be transported to an area where the reaction can stabilize and propagate to the other fuel injectors in the combustor The same features that provide flame stability at idle and deceleration conditions are relied upon at sub-idle starting operations The pressure at light off is usually near the outside ambient pressure because the rotors are not producing sig-

nificant work However, at higher flight speeds, the total pressure is typically slightly higher than ambient due to the stagnation effect Temperatures at ignition are highly dependent on the thermal state of the engine For the first start of the day on the ground, temperatures are usually only slightly higher than ambient Altitude relight temperatures are highly dependent on the amount of time the engine has been shut down For quick relight attempts less than a minute after shutdown, temperature

at the combustor inlet can be greater than 200ºF If the engine is shut down and windmilling for thirty minutes or longer, the air temperature is closer to the outside ambient

Most commercial aircraft must meet requirements for both ground and altitude starting The ground starting requirements include a range of ambient temperatures and airport altitudes Typical ground starting ambient temperature requirements vary between −40 and 120ºF Airport altitude requirements typically range between sea level and eight thousand feet Altitude relight requirements are typically expressed

on a flight envelope (Figure 1.9) There is a high-speed windmilling envelope and a

lower-speed starter assisted envelope The maximum altitude required for air

start-ing depends on the aircraft Commercial airliners normally require altitude relight capability of at least twenty-five thousand to thirty thousand feet Business jets often require capability at thirty-five thousand feet because of their higher cruising altitude

At the highest altitudes and in extreme cold, combustor ignition conditions can

be very challenging Pressures of less than five psia and temperatures below 0ºF are typical for an engine that windmills until cold These conditions inhibit the atomiza-

tion of fuel and vaporization of droplets Low temperature and pressure also slow the reaction kinetics that promotes stabilization and propagation of flame Therefore, design of the combustor should provide for three key features that enable ignition: a good fuel spray, a favorable airflow velocity, and the proper spark igniter location

Small fuel droplets are critical to the formation of vapor necessary for

igni-tion Two types of fuel injectors are typically used: pressure atomizing and airblast

Aero GT Combustion

16

atomizing (Figure 1.10) The former uses high pressure to push fuel through a small orifice to generate the spray The fuel can also be swirled prior to passing through the orifice to provide angular momentum that produces a spray cone Airblast atom-ization uses the energy of the airflow to produce the spray The fuel is typically deliv-ered to a cylindrical surface between two swirling airstreams The cylindrical surface develops a thin film of fuel as a result of the action of the swirling inner airstream

As the thin film reaches the tip of the cylinder, the shear between the two airstreams atomizes the film into a spray Airblast atomizer performance degrades as the air pressure drop decreases and should not be used if insufficient airflow is available

to atomize the fuel This occurs when windmill ignition is attempted at very low airspeed and when insufficient starter torque limits rotor speed in assisted starts Often, airblast fuel injection systems will be supplemented with pressure atomizers

in the locations where the igniters are placed Such injectors that incorporate both

pressure atomizing and airblast features are referred to as hybrid or duplex injectors

Increased fuel flow at the igniter locations can help achieve ignition This additional nonuniform fuel flow is provided by upstream valves and is usually only present at low-power settings Manifolds that deliver the fuel must be designed in such a way

as to achieve the desired distribution of fuel

Successful ignition also requires a favorable velocity in the region near the spark plug and the stabilization zone Because most combustors are swirl stabilized, the recirculation of flow can be used to transport the ignited spark kernel to the sta-bilization zone However, even a properly designed stabilization zone can result

in poor ignition characteristics This problem can stem from two factors The first

is local velocities that are too high to sustain the reaction surrounding the spark This results in the convective heat loss from the reaction kernel exceeding the heat

Mach number

0 10 20 30

40 Successful ignition

No ignition x10 3 ft

Figure 1.9 Altitude relight envelope (B777 with PW4084 engine).

1.6 Turbine and Combustor Durability Considerations 17

released by the reaction, quenching the reaction This situation is created when there

is insufficient volume and cross-sectional area in the stabilization zone for the

quan-tity of airflow present Therefore, care must be taken to ensure that local velocity does not exceed the flame propagation speed at light-off conditions The other cause

is improper igniter placement If the igniter is placed in an area with flow

direc-tion away from the recirculadirec-tion zone, the reacdirec-tion kernel can be carried out of the back end of the combustor Igniters also must be placed in an area where the fuel spray provides sufficient local fuel-air ratio to achieve ignition Conditions at igni-

tion are often relatively high in overall fuel-air ratio because of low airflow levels, but wide variations exist in local fuel-air ratio As a result, spark igniters are often placed at the downstream edge of the flow recirculation so that it receives robust fuel-air mixture from the conical fuel spray, but also provides reverse flow direction for stabilization

1.6 Turbine and Combustor durability Considerations

The combustor has a significant impact on turbine durability and consequently impacts engine performance The temperature distribution at the combustor exit affects the cooling airflow required to protect the airfoils and platforms in the tur-

bines This cooling airflow, in turn, reduces the engine performance by diverting flow from the mainstream so that it is not used to produce work The cooling also causes mixing losses if it is introduced as low momentum film on the airfoils Combustor film cooling is required for aero engines because the metallic liners are exposed

to the high-temperature combustion process Combustor cooling itself does not affect engine performance because it is added upstream of the turbines However,

Air Air Fuel

Primary fuel

Secondary fuel

Airblast fuel injector

Duplex fuel injector

in air swirler

Figure 1.10 Fuel injector types.

Aero GT Combustion

18

combustor cooling does reduce the amount of airflow available to control emissions and mix out temperature streaks Therefore, it is desirable to minimize the amount

of cooling flow used

As previously mentioned, the combustor exit temperature distribution has a large impact on the amount of turbine cooling required and, thus, the engine per-formance Combustor exit temperature quality is normally described in terms of the radial average temperature profile and the hottest streak intensity These are

referred to as radial profile factor and pattern factor, respectively They are typically

described as nondimensional parameters:

radial profile factor = (Tra−T )/(Te e−T )i

where Tra is the average temperature at a given radial position, Ti is inlet temperature, and Te is the mass averaged exit temperature The pattern factor is given by:

pattern factor = (Tstr−T )/(Te e−T )i

where Tstr is the maximum temperature anywhere in the combustor exit annulus,

commonly called the streak temperature The radial profile factor is determined as a

function of radial position at the entrance to the turbine The maximum pattern tor occurs at only one spatial position in the combustor exhaust (Figure 1.11) Often this location is the result of random hardware variation and cannot be assumed repeatable from engine to engine However, the radial distribution of pattern factor

fac-is also of interest to the turbine designer Turbine static hardware (vanes and outer air seals) are impacted by the local gas temperatures while rotating blades are impacted predominantly by the radial average temperature profile because they rotate too fast for metal temperatures to respond to local effects Thus, the static hardware cooling level is often set to protect against the hottest pattern factor streak, even though it occurs in only one place It is useful to know the radial distribution of pattern factor

so that static hardware cooling can be distributed more in the core region, where the hottest streak is likely to occur, and less near the walls, where the average tempera-tures are lower because of the effects of combustor cooling

To achieve the target radial temperature profile and low pattern factor, the designer must control the mixing of fuel and air in the combustor To achieve the low-est possible pattern factor, the designer would premix all of the airflow with the fuel

at the combustor front end This would produce a flat, uniform temperature profile at the combustor exit However, considering liner cooling requirements, operability, and radial profile requirements, this approach is not practical for aero engines In practical aero engine designs, cooling air is injected into the combustor in a way such that it provides a protective film near the combustor walls As such, it generally does not mix readily with the other airstreams and the fuel in the combustor Liner cooling is there-fore not effective at controlling pattern factor, but can be effective at providing cooler radial average profile near the inner and outer walls The airflow not used for liner cooling is used to control radial profile shape and pattern factor In RQL designs, the bulk of the non-cooling airflow enters through air jets downstream of the front end

in the liner walls In fuel-staged designs, most of the airflow is incorporated into the

1.6 Turbine and Combustor Durability Considerations 19

fuel injection swirlers at the front end of the combustor Therefore, the mixing

pro-cesses to achieve uniform exhaust temperatures are quite different In swirling flow mixers, multiple airstreams are often used to create shear layers that promote mix-

ing Fuel is injected into the airstreams so that it is dispersed and mixed with the air Fuel injection is usually accomplished with jets, thin films, or pressure sprays Swirling airstreams may be co-swirling or counter-swirling Designers have successfully used both approaches Counter-swirling airstreams produce the highest mixing rates, but result in low net swirl if not designed with unequal flow quantities These principles are applied to both RQL and lean-staged designs because good fuel injector and swirler mixing is required for both design approaches

Jet mixing is dependent on the arrangement, size, and upstream conditions

influ-encing the jets The penetration depth of a jet, Y, is proportional to the jet diameter,

dj, and the square root of the momentum ratio, J:

Y d J~ jwhere the momentum ratio J is given by:

J = ( U )/( U )ρj j2 ρg g

Uj and Ug denote the jet and cross flow velocities, respectively Specific correlations depend on the geometry of the duct and the arrangement of the jets (Lefebvre and Ballal, 2010)

The penetration of the jet can be controlled by the sizing, pressure loss, and upstream flow quantity Efficient mixing of upstream gases also requires jet spacing dense enough to mix within the combustor length allocated Therefore, the arrange-

ment and size of the jets are critical to the spatial delivery of airflow to the critical regions where it is needed to mix temperature streaks and provide target radial aver-

Aero GT Combustion

20

conditions For example, a row of smaller holes in the aft end of the combustor can effectively cool the inner portion of the radial distribution because of their limited penetration At low-power conditions, staged lean combustors often have worse uni-formity because of the reduced number of fuel injectors operating RQL combustors are more dependent on the jet mixing to deliver both the radial profile and pattern factor targets Researchers have conducted numerous experimental studies to deter-mine the optimum jet arrangement for mixing flow in a duct Holdeman found that the optimum arrangement was given by:

H = ( / )p h J where p is the hole pitch, h is the duct height, and H is the characteristic parameter

(Holdeman, 1993) For unopposed holes in a duct, the optimum value for H is 5

(Figure 1.12) This finding was for a uniform axial cross flow, but is often a good arrangement from which to begin optimization For real combustors where the upstream flow is swirling, computational fluid dynamics analysis is useful to refine the distribution Rig testing is required to determine the maximum pattern factor because CFD calculations typically cover a single fuel injector sector and thus do not provide random effects

Combustion imposes two different types of heat loads on the liner The first is radiation from the flame to the surfaces The second is the convective effect of hot gases contacting the liner cooling films The convective load can result in film tem-perature above metal temperature in some areas of the combustor The radiation flux is given by:

q = 0.5(1+ )( Tr w g g4 T )

g w 4

where εw is the wall emissivity, εg is the gaseous emissivity, αg is the gas ity at the wall temperature Tw, and Tg is the radiating gas temperature The gaseous emissivity is dependent on the flame luminosity and gas temperature, which in turn depends on the combustion stoichiometry Rich combustion tends to produce highly luminous soot Therefore, the forward section of a RQL combustor tends to have more radiant heat load than a staged lean combustor, which produces very little soot The midsection of a RQL combustor produces peak gas temperatures as the stoichiometry transitions from fuel rich to fuel lean This region of a RQL also has higher gaseous emissivity than a lean-staged combustor does

absorptiv-P

h

Figure 1.12 Jet mixing in a duct.

1.6 Turbine and Combustor Durability Considerations 21

Convective heat load on the liner is dependent on the local gas temperature and velocity and its interaction with the cooling film The effectiveness of the film is crit-

ical to maintenance of acceptable metal temperatures because the film temperature

is a key driver in the heat flux:

q = h(Tc film−Tmetal)where qc is the convective flux, h is the convective heat transfer coefficient, Tfilm is the film temperature, and Tmetal is the liner surface metal temperature Film tempera-

ture is dependent on the local gas temperature and film effectiveness Film

effec-tiveness depends on the nature of the film (slot flow, discrete holes, etc.) and the ratio of the cooling momentum to the mainstream flow momentum The momentum

ratio is referred to as the blowing parameter In zones where the mainstream flow

momentum is low and the blowing parameter is high, the cooling film effectiveness

is reduced This occurs most commonly in the front end of the combustor Note that for equivalent film effectiveness, the RQL design will have higher film temperature because it reaches higher peak gas temperature than the staged lean combustor

Cooling strategies for the outside of the liner wall vary widely with design (Figure 1.13) Outside cooling is important because it balances the hot side heat flux With high backside cooling effectiveness, higher hot side heat flux can be tolerated Initial liners made use of simple louvers that created slot films on the liner hot side and had minimal heat transfer on the outside of the liner wall The louver length was determined by the distance that film effectiveness could be maintained Louvered designs later evolved to incorporate more effective backside cooling strategies for the louver lips All continuous ring louver liners fail because of thermal fatigue cracks The cracks result from high thermal stresses on the full ring hoop As oper-

ating temperatures increased, more effective liner designs were required because

Combustor liner history

Sheet metal liner

Double pass liner

Aero GT Combustion

22

cracking was an even more severe problem This led to tiled liner (floatwall) tors that have a cold structure that carries mechanically attached panel tiles The cold carrying structure virtually eliminates the hoop stresses that caused fatigue cracks

combus-in prior designs The tiles have multiple fcombus-in structures to augment convective heat transfer incorporated on the backside Film cooling air is first passed through these fins to provide high effective heat transfer levels More recent designs have relied on film cooling holes to increase liner cold side heat transfer rates

RQL design typically requires a higher cooling flux than a staged lean-type design However, this difference is only significant if it prevents the achievement of other objectives Cooling air generally stays near the liner walls of the combustor It causes the exit temperature profile to be cooler near the walls and more peaked at the mid-span of the exit plane This is generally desirable for turbine durability because

it reduces heat loading on the turbine platforms and seals Using modern liner ing technologies, the cooling flow allocation has not limited the achievement of pro-file and pattern factor targets in either type of combustor design However, cooling can have a significant impact on emissions and combustion stability

cool-Cooling affects emissions most significantly at low power when the inlet air temperature is lowest At low temperatures and fuel-air ratio, liner film cooling can quench the near-wall combustion process, resulting in the generation of unburned hydrocarbons and carbon monoxide This occurs predominantly in the front end

of the combustor, where swirling and recirculating flow contacts liner film cooling These effects occur in both RQL and lean-staged designs Such quenching can result

in disruption of flame stability in the extreme if the heat released is not sufficient

to sustain continuous ignition At high power, cooling can result in formation of

NOx emissions in the front end of RQL combustors The NOx forms when fuel-rich front-end gases contact film cooling The region where the contact occurs produces stoichiometric combustion temperature and the highest NOx formation rates In lean-staged combustors, the cooling air does not increase NOx because the fuel-air mixture is already leaner than stoichiometric and cooling causes a reduction in com-bustion temperature

Future aircraft engine cycles will require improved thermal and propulsive ciency to meet aggressive fuel burn goals and address CO2 emission concerns Current cycles have sufficient differential between the coolant and target metal tempera-tures to allow effective cooling within allowable flow budget As cycle temperatures increase, improved liner cooling technology or increased temperature materials will

effi-be required to maintain low enough cooling fluxes to meet all combustor metrics

1.7 summary

Gas turbine combustors remain an interesting and complex design challenge Balancing the many requirements in an environment demanding low cost, low weight, low emissions, and excellent safety and reliability is often difficult Many of the subjects introduced in this chapter will be discussed in more depth in subsequent chapters of this book to provide a better understanding of these issues