Energy and the environment m j moran (auth )

When one of the two systems is a SUitablyidealized system called an exergy reference environment or simply, an enVironment, and the other is some system of interest, exergyis the maximum

VOLUME 15

The titles published in this series are listed at the end of this volume.

ISBN 978-94-010-5943-5 ISBN 978-94-011-4593-0 (eBook)

DOI 10.1007/978-94-011-4593-0

Cover design: River basin dendrites generated by the constructal principle

ofthermodynamic optimization subject to constraints (see pp 21-22 in this book; also M.R Errera

& A Bejan, Fractals, VoI 6, No 3, pp 245-261, 1998)

Printed an acid-free paper

AII Rights Reserved

© 1999 Springer Science+Business Media Dordrecht

Originally published by Kluwer Academic Publishers in 1999

Softcover reprint of the hardcover 1st edition 1999

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical,

includ ing photocopying, record ing or by any information storage and

retrieval system, without written permission from the copyright owner

fuf~ VllM.J MORAN

Exergy analysis, costing, and assessment of environmental impact 1ABEJAN

AE BERGLES

AD KRAUS

RK SHAH, B THONON, D.M BENFORADO

Opportunities for heat exchanger applications in environmental systems 49

F FRAN<;A, C LAN, J.R HOWELL

The production of improved plastic materials by chaotic mixing of

G.DJ SMITH, G.C SNEDDEN, R.D STIEGER

Advances in the measurement of convective heat transfer coefficient

in gas turbine applications

J.H KIM and K.-W YOU

Thermally affected flows in power plants

A.C VOSLOO

Advances in the technology of liquid synfuel production from coal

D.A NIELD

Some geophysical problems involving convection in porous media

with application to energy and the environment

Evaluation of energy efficient and environmentally acceptable pure

and zeotropic refrigerants in air-conditioning and refrigeration

G.P GREYVENSTEIN and P.G ROUSSEAU

Application of heat pumps in the South African commercial seCtor

Index

175185

This book brings together the work of some of the world leaders on energy research andthe environmental impact of energy technologies "Energy" and "environment" arekeywords for two of the most important directions in contemporary thermal sciences.Many new advances and reassessments of old results have been made in the 1980s and1990s This book provides a bird's-eye view of the current state of the art, and in whatdirections the field is expanding.

Most representative of these new developments is the field of engineeringthermodynamics and energy engineering This field has experienced a real revolution inthe two decades since the energy crisis The renewed emphasis on higher efficiencies,energy-resources conservation and environmental impact of energy systems hastransformed the discipline and practice of thermodynamics The methods of exergyanalysis, entropy generation minimization, and thermoeconomics are the most visibleresults of this revolution

In exergy or availability analysis the engineer uses the second law of

thermodynamics (in addition to the first law) to establish theoretical limits to the design

of proposed energy systems and measures of the departure of real systems ,from theirtheoretical limits Losses-their size and distribution through a complex system-aredetermined on a rigorous scientific basis The minimization of these losses is theobjective of thermodynamic optimization (entropy generation minimization) Lossesare measured in terms of exergy destruction or entropy generation, and are expressed asfunctions of the physical parameters (geometry, size, materials) of the device Theirminimization is carried out subject to the constraints that account for the size of thedevice and its time of operation In thermoeconomics the thermodynamic losses arecombined with other costs into a comprehensive cost function that is subjected toconstrained minimization The three methods are now the standard in modern thermalengineering education and practice, and are ideally suited for computer-aided analysis,design and optimization

This book is timely for two reasons First, these new methodologies have beendeveloped through individual papers at annual conferences, mainly in the 1980s and1990s This book puts these methodologies in perspective, through a team of highlyqualified authors and a comprehensive list of contributed chapters

Another direction that defines what is modern in thermal sciences is the area of heattransfer augmentation (enhancement, intensification) This is an extremely importantscience and art Its objective is to improve thermal contact between heat-exchangingentities, for example, between a fluid and a solid wall The augmentation methods thathave been devised over the years are extremely diverse: special wall structures(roughness, fins, dimples), wall or fluid motion (vibrations, pressure-wave forcing),fluid additives, wall coatings, and mechanical accessories such as wall scrapers andvortex generators

vii

Although the heat transfer augmentation field is young, it is developed enough sothat it can be reviewed systematically This book is an ideal vehicle for providing thisview, especially since a large segment of the world power generation industry is based

on burning coal in power plants that require highly efficient and reliable heat-exchangeequipment Augmentation techniques are generally applicable in heat exchanger design,

in fact, they form the backbone-the limiting technology-in the development ofcompact heat exchangers Another extremely important application of heat transferaugmentation is in the efficient cooling of electronic packages, where, again, theminiaturization evolution is ruled by the progress made on heat transfer augmentationmethods

There are at least two major environmental areas that are covered in this book,because they go hand-in-hand with the energy issues described until now The first isthe environmental impact of energy systems To bring this issue and make it anintegral part of the optimization of the enregy system is one of the contributions ofthermoeconomics The local degradation of the environment is one of the importantcosts that must be included in thermoeconomic optimization The environment-itsproperties, and how they vary in time-plays also a central role in exergy analysis Thevery concept of exergy requires an unambiguous understanding of the state of theenvironment, and whether this state will be altered by the operation of the power plantthat is being designed

The second environmental area covered in this book is convection in porous media.This field experienced an astonishing growth during the past decade, and now is one ofthe most active in thermal sciences Its developmment is comparable with that ofclassical convection (transport by the flow of a pure flluid): governing principles are inplace, experimental data continue to stimulate improvements in the governingprinciples, and there is an abundance of practical applications From an environmentalstandpoint, the fundamental aspects that are covered in this book are relevant tounderstanding the spreading of contaminated fluids through the ground, the leakage ofheat through the walls of buildings, and the flow of geothermal fluids through theearth's porous crust

The research material selected in this book has been used in a week-long workshopformat, in front of an audience of researchers and practicing engineers The venue wasthe USA-RSA Energy and Environment Workshop, held on June 8-12, 1998, at theUniversity of Durban-Westville, South Africa The authors and the participants in thisworkshop greatfully acknowledge the support received from the National Foundation(USA), the Foundation for Research Development (RSA), the University of Durban-Westville and Duke University They also acknowledge the substantial assistanceprovided by Dr G Govender in the organization of the workshop

This book was prepared by Deborah Alford at Duke University

The editors acknowledge with gratitude the support received from the following

colleagues in organizing the Energy and the Environment workshop:

Professor Mapule F Ramashala, Vice-Chancellor and Principal, University ofDurban-Westville, South Africa

Ms Hannekie Botha, Science Liaison Centre, Foundation for Research

Development, South Africa

Ms Jill Sawers, Manager: Competitive Industry, Foundation for ResearchDevelopment, South Africa

Dr Ganasagren Govender, Workshop Secretary, University of

Durban-Westville, South Africa

Ms Patricia J Tsuchitani, Division of International Programs, National ScienceFoundation, USA

Professor Ashley F Emery, Thermal Transport and Thermal Processing

Program, National Science Foundation, USA

Professor F Hadley Cocks,' Chairman, Department of Mechanical Engineeringand Materials Science, Duke University, USA

Professor Devendra P Garg, Department of Mechanical Engineering andMaterials Science, Duke University, USA

A.B., P.V & D.G.K

OF ENVIRONMENTAL IMPACT

M.J MORAN

The Ohio State University

Department of Mechanical Engineering

Columbus, OH43210, USA

1 Introduction

The method ofexergy analysis enables the location, cause, and true magnitude of

energy resource waste and loss to be determined Such information canbeused inthe design of new energy-efficient systems and for improving the performance ofexisting systems Exergy analysis also provides insights that elude a purelyfirst-law approach For example, on the basis of first-law reasoning alone, thecondenser of a power plant may be mistakenly identified as the componentprimarily responsible for the plant's seemingly low overall performance Anexergy analysis correctly reveals not only that the condenser loss is relativelyunimportant, but also that the steam generator is the principal site of

irreversibilities within it

When exergy concepts are combined with principles of engineering economy,the resultis known as thermoeconomics Thermoeconomics allows the real cost

sources at the component level to be identified: capital investment costs, operatingand maintenance costs, and the costs associated with the destruction and loss ofexergy Optimization of thermal systems can be achieved by a carefulconsideration of such cost sources From this perspective thermoeconomics is

exergy-aided cost minimization.

Discussions of exergy analysis and thermoeconomics are provided in (1-4)

2 Defining Exergy

Anopportunity for doing work exists whenever two systems at different states areplaced in communication because, in principle, work canbedeveloped as the twoare allowed to come into equilibrium When one of the two systems is a SUitablyidealized system called an exergy reference environment or simply, an

enVironment, and the other is some system of interest, exergyis the maximumtheoretical useful work (shaft work or electrical work) obtainable as the systemsinteract to equilibrium, heat transfer occurring with the environment only.(Alternatively, exergy is the minimum theoretical useful work reqUired to form aquantity of matter from substances present in the environment and to bring thematter to a specified state.) Exergy is a measure of the departure of the state of the

system from that of the environment, and is therefore an attribute of the systemand environment together Once the environment is specified, however, a valuecan be assigned to exergyinterms of property values for the system only, so exergycan be regarded as an extensive property of the system

Exergy can be destroyed and generally is not conserved A limiting case iswhen exergy would be completely destroyed, as would occur if a system were tocome into equilibrium with the environment spontaneously with no provision to

obtain work The capability to develop work that existed initially would be

I

A Bejan et al (eds.), Energy and the Environment,1-10.

© 1999 Kluwer Academic Publishers.

completely wasted in the spontaneous process Moreover, since no work needs to

be done to effect such a spontaneous change, the value of exergy can never be

negative

Although the term environment is selected to suggest kinship with the natural

environment, the exergy reference environment used in any particularapplication is a thermodynamic model Models with various levels of specificityare used to evaluate exergy The exergy reference environment is typicallyregarded as composed of common substances existing in abundance within theEarth's atmosphere, oceans, and crust The substances are in their stable forms asthey exist naturally, and there is no possibility of developing work frominteractions - physical or chemical - between parts of the environment Althoughthe intensive properties of the environment are assumed to be unchanging, theextensive properties can change as a result of interactions with other systems.Kinetic and potential energies are evaluated relative to coordinates in theenvironment, all parts of which are considered to be at rest with respect to oneanother

For computational ease, the temperature To and pressure Po of theenvironment are often taken as standard-state values, such as 1 atm and

25°C (nop).However, these properties may be specified differently depending onthe application To andPo might be taken as the average ambient temperature andpressure, respectively, for the location at which the system under considerationoperates Or, if the system uses atmospheric air, To might be specified as theaverageairtemperature Ifboth air and water from the natural surroundings areused, Towould bespecified as the lower of the average temperatures for air andwater Further discussion of the exergy reference environment is provided in Sec.3

When a system is in equilibrium with the environment, the state of the system

is called thedead state At the dead state, the conditions of mechanical, thermal,

and chemical equilibrium between the system and the environment are satisfied:the pressure, temperature, and chemical potentials of the system equal those ofthe environment, respectively In addition, the system has no motion or elevationrelative to coordinates in the environment Under these conditions, there is nopossibility of a spontaneous change within the system or the environment, norcan there be an interaction between them At the dead state, the value of exergy iszero Another type of equilibrium between the system and environment can be

identified This is a restricted form of equilibrium where only the conditions ofmechanical and thermal equilibrium must be satisfied This state of the system iscalled the restricted deadstate At the restricted dead state, the fixed quantity of

matter under consideration is imagined to be sealed in an envelope impervious tomass flow, at zero velocity and elevation relative to coordinates in theenvironment, and at the temperature To and pressurePo'

3 Control Volume Exergy Rate Balance

Exergy balances canbewritten in various forms, depending on whether a closedsystem or control volume is under conSideration and whether steady-state ortransient operation is of interest Owing to its importance for a wide range ofapplications, an exergy rate balance for control volumes at steady state ispresented here

Exergy can betransferred across the boundary of a control volume by threemeans: exergy transfer associated with work, exergy transfer associated with heat

transfer, and exergy transfer associated with the matter entering and exiting thecontrol volume All such exergy transfers are evaluated relative to theenvironment usedto defme exergy Exergy is also destroyed by irreversibilitieswithin the control volume At steady state

0=:LEqj - Wcv +:LEi - :LEe -ED

(1)

or

rates of exergy transfer

rate of exergy destruction

(1b)

(2)

where Wct)is the work rate excluding theflow work. Qj isthe time rate of heattransfer at the location on the boundary of the control volume where theinstantaneous temperature is Tj The associated rate of exergy transfer is

(3)

The specific exergy transfer terms ei and ee are expressible in terms of four

components: physical exergy e PH • kinetic exergy eRN , potential exergy e PT , andchemical exergy e CH :

where h and s denote, respectively, the specific enthalpy and specific entropy,and v and z denote velocity and elevation relative to coordinates in theenvironment, respectively The subscript 0 denotes the restricted dead state.

To evaluate the chemical exergy, the exergy component associated with thedeparture of the chemical composition of a system from that of the environment,the substances comprising the system are referred to the properties of a sUitablyselected set of environmental substances For discussion of alternative sets ofsubstances tailored to particular applications, see [2, 4) Exergy analysis isfacilitated, however, byemploying astandard environment and a corresponding

table of standard chemical exergies Standard chemical exergies are based on

standard values of the environmental temperature To and pressure Po - forexample, 298.15K(25°C) and Iatm, respectively A standard environment alsoconsists of a set of reference substances with standard concentrations reflecting asclosely as possible the chemical makeup of the natural environment Thereference substances generally fall into three groups: gaseous components of theatmosphere, solid substances from the lithosphere, and ionic and noninonicsubstances from the oceans The chemical exergy data of Table 1 correspond to twoalternative standard exergy reference environments, called here model I andmodel II, that have gained acceptance for engineering evaluations

Although the use of standard chemical exergies greatly facilitates theapplication of exergy principles, the term standardissomewhat misleading sincethere is no one specification of the environment that suffices for all applications.Still, chemical exergies calculated relative to alternative specifications of theenvironment are generally in good agreement For a broad range of engineeringapplications the simplicity and ease of use of standard chemical exergiesgenerally outweigh any slight lack of accuracy that might result Inparticular, theeffect of slight variations in the values of To and Po about the values used todetermine the standard chemical exergies reported in Table 1 can be neglected

TABLE 1 Standard molar chemical exergy, e CH(kJ / krool),

of various substances at 298 K and Po'

Substance Formula Modell' ModelII"

4.Exergetic Efficiency

The exergetic efficiency (second law efficiency, effectiveness, or rationalefficiency) provides a true measure of the performance of a system from the

thermodynamic viewpoint To define the exergetic effiCiency both a product and a

jitel for the system being analyzed are identified The product represents thedesired result of the system (power, steam, some combination of power and steam,etc.) Accordingly the definition of the product must be consistent with thepurpose of purchasing and using the system The fuel represents the resourcesexpended to generate the product and is not necessarily restricted to being anactual fuel such as a natural gas, oil, or coal Both the product and the fuel areexpressed in terms of exergy

For a control volume at steady state whose exergy rate balance reads

where the rates at which the fuel is supplied and the product is generated are EF

and Ep respectively ED'and EL denote the rates of exergy destruction and exergyloss respectively Exergy is destroyed by irreversibilities within the controlvolume, and exergy is lost from the control volume via stray heat transfer,material streams vented to the surroundings and so on

The exergetic effiCiency shows the percentage of the fuel exergy provided to acontrol volume that is found in the product exergy Moreover the differencebetween 100% and the value of the exergetic efficiency, expressed as a.percent, isthe percentage of the fuel exergy wasted in this control volume as exergydestruction and exergy loss To apply Eq (7). decisions are reqUired concerningwhat are considered as the fuel and the product Table 2 provides illustrations forfour common components Similar considerations are used to write exergeticefficiencies for systems consisting of several components, as for example a powerplant

Exergetic efficiencies can be used to assess the thermodynamic performance of

a component plant or industry relative to the performance of similarcomponents, plants orindw~tries.By this means the performance of a gas turbine,for instance, can be gauged relative to the typical present-day performance level ofgas turbines A comparison of exergetic efficiencies for dissimilar devices - gasturbines and heat exchangers, for example -isgenerally not significant however.The exergetic efficiency is generally more meaningful objective and useful thanother efficiencies based on the first or second law of thermodynamics includingthe thermal efficiency of a power plant the isentropic efficiency of a compressor

or turbine and the effectiveness of a heat exchanger The thermal effiCiency of acogeneration system is misleading because it treats both work and heat transfer ashaving equal thermodynamic value The isentropic turbine efficiency does notconsider that the working fluid at the outlet of the turbine has a highertemperature (and consequently a higher exergy that may be used in the nextcomponent) in the actual process than in the isentropic process The heat

exchanger effectiveness fails to identify the exergy destruction associated with thepressure drops of the heat exchanger working fluids.

TABLE 2 The Exergetic Efficiency for Seleded Components at Steady State"

Component

Thrblne or Expander

2

Extraction Thrbine Pump,Compressor,or Fan

2

Heat Exchanger'

Hot stream

Cold slream

E:

• For discussion, see Bejan et aI (19%).

• This definition assumes that the purpose of tbe beat exchanger is to beat the cold stream {T, ;:: To>.

5 ExergyCosting

Since exergy measures the true thermodynamic values of the work, heat, and otherinteractions between the system and its surroundings as well as the effect ofirreversibilities within the system, exergy is a rational basis for assigning costs.This aspect of thermoeconomics is calledexergy costing.

Referring toFig 1 showing a steam turbine-electric generator at steady state,the total cost to produce the electricity and exiting steam equals the cost of theentering steam plus the cost of owning and operating the device This is expressed

by the costrate balance for the turbine-generator:

(8)

where (;e isthe cost rate associated with the electricity, (;1 and (;2 are the costrates associated with the entering steam and exiting steam, respectively, and Z

accounts for the cost rate associated with owning and operating the system, each

annualized in$ per year, for example

E2 • c2

Figure 1 Steam turbine/electric generator used to discuss exergy costing.

With exergy costing, the cost ratesC1, ~ ,and Ce are evaluated in tenns of theassociated rate of exergy trdIlsfer and a unit cost.Equation (8) then appears as

6 Improving Thermodynamic Effectiveness

Various methods canbeused to improve thennodynamic effectiveness All suchmethods should achieve their objectives cost-effectively, however

To improve thennodynamic effectiveness it is necessary to deal directly withinefficiencies related to exergy destruction and exergy loss The primary

contributors to exergy destruction are chemical reaction, heat transfer, mixing,and friction, including unrestrained expansions of gases and liquids To deal withthem effectively, the principal sources of inefficiency not only should beunderstood qualitatively, but also determined quantitatively, at leastapproximately Design changes to improve effectiveness must be done judiciously,however, for the cost associated with different sources of inefficiency can bedifferent For example, the unit cost of the electrical or mechanical powerrequired to provide for the exergy destroyed owing to a pressure drop is generallyhigher than the unit cost of the fuel required for the exergy destruction caused bycombustion or heat transfer.

Since chemical reaction is a significant source of thermodynamicinefficiency, it is generally good practice to minimize the use of combustion Inmany applications the use of combustion equipment such as boilers isunavoidable, however In these cases a significant reduction in the combustionirreversibility by conventional means simply cannot be expected, for the majorpart of the exergy destruction introduced by combustion is an inevitableconsequence of incorporating such equipment Still, the exergy destruction inpractical combustion systems can be reduced by minimizing the use of excess airand by preheating the reactants In most cases only a small part of the exergydestruction in a combustion chamber can be avoided by these means.Consequently, after considering such options for reducing the exergy destructionrelated to combustion, efforts to improve thermodynamic performance shouldfocus on components of the overall system that are more amenable to betterment

by cost-effective conventional measures In other words, some exergy destructions

and exergy losses can be avoided, others cannot Efforts should be centered on those thatcan beavoided.

Nonidealities associated with heat transfer also typically contribute heavily

to inefficiency Accordingly, unnecessary or cost-ineffective heat transfer must beavoided Additional gUidelines follow:

• The higher the temperature T at which a heat transfer occurs in cases where

T > To' the more valuable the heat transfer and, consequently, the greater

the need to avoid heat transfer to the ambient, to cooling water, or to arefrigerated stream

• The lower the temperature T at which a heat transfer occurs in cases where

T < To ' the more valuable the heat transfer and, consequently, the greater

the need to avoid direct heat transfer with the ambient or a heated stream

• Exergy destruction associated with heat transfer between streams variesinversely with the temperature level Accordingly, the lower the temperaturelevel, the greater the need to minimize the stream-to-stream temperaturedifference

Although irreversibilities related to friction, unrestrained expansion, andmixing are often secondary in importance to those of combustion and heattransfer, they should not be overlooked, and the following guidelines apply:

• Minimize the use of throttling; check whether power recovery expanders are

a cost-effective alternative for pressure reduction

• Avoid processes using excessively large thermodynamic driving forces(differences in temperature, pressure, and chemical composition) Inparticular, minimize the mixing of streams differing significantly intemperature, pressure, or chemical composition

• The greater the mass rate of flow, the greater the need to use the exergy of thestream effectively.

• The lower the temperature level, the greater the need to minimize friction

The effectiveness of using energy resources (oil, natural gas, and coal) hasimproved markedly over the last two decades Still, usage varies widely evenwithin nations, and there is room for improvement Compared to some of itsprincipal international trading partners, for example, U.S industry as a wholehas a higher energy resource consumption on a per unit of output basis and

generates considerably more waste

For industries where energy resources are a major contributor to operatingcosts, an opportunity exi~ts for cost savings by improving thermodynamiceffectiveness using means such as discussed in Sec 6 This is a well-known andlargely accepted approach today A related but less publicized aspect concernseffluent streams The waste from a plant is often not an unavoidable result ofplant operation but a measure of its inefficiency: The less efficient a plant, themore unusable by-products it produces, and conversely Effluents not producedowing to greater efficiency require no costly cleanup and do not impose a burden

on the environment

Cleanup efforts have customarily featured an end-oj-the-pipe approach thataddresses the pollutants emitted from stacks, ash from incinerators, thermalpollution, and so on Increasing attention is being given today to what goes into the pipe,however This is embodied in the concept of designjor the environment

(DFE), in which the environmentally preferred aspects of a system are treated asdesign objectives rather than as constraints The aim in DFE is to anticipatenegative environmental impacts throughout the life cycle and engineer them out

In particular, efforts are directed to reducing the creation of waste and tomanaging materials better, uSing methods such as changing the processtechnology and/ or plant operation, replacing input materials known to be sources

of toxic waste with more benign materials, and doing more in-plant recycling.Thermoeconomics, with its rational approach to costing and well-definedmeasures of true efficiency, is especially suited for use in DFE Still, this is anaspect of thermoeconomics that has lagged somewhat and deserves moredevelopment by the thermodynamics community

A related area of application involving exergy that merits further development

is in assessing environmental impact Anunderlying idea is that the exergy of aneffluent stream may serve as index of the stream's influence on nature That is,the exergy of such a stream might be an indicator of its potential for drivingdamaging processes in the natural environment, Alternatively, the exergy of aneffluent stream might becorrelated to observed types of environmental damage.Though occasionally mentioned in the literature over the past two decades,exergy-aided assessment of environmental impact largely remains in its infancy.Recent efforts have suggested intriguing possibilities [7-9], but much remains to bedone

8.Closure

Thermoeconomic analyses combine thermodynamic and economic principles forengineering decision making An abundant and rapidly growing technicalliterature may be consulted for the myriad aspects of this field For an

introduction, [I] is recommended A few salient aspects are noted to close the

9 References

1 Bejan, A., Tsatsaronis, G., and Moran, M.: Thennal Design and Optimization, John Wiley & Sons, New York, 1996.

2 Moran, MJ.:Availability Analysis - A Guide to Efficient Energy Use,ASME Press, New York, 1989.

3 Moran, M.1 and Shapiro, H.N.:Fundamentals of Engineering Thennodynamics, 3rd ed John Wiley

& Sons, New York, 1996.

4 Kotas,1.1.: The Exergy Method of Thennal Plant Analysis,Krieger, Melbourne, FL, 1995.

5 Ahrendts, J.: Reference states,Energy Int J.,5(1980),667-677.

6 Szargut, 1, Morris, D.R., and Steward, F.R.: Exergy Analysis of Thennal, Chemical and Metallurgical Processes, Hemisphere, New York, 1988.

7 Rosen, M.A and Dincer, 1.: On Exergy and Environmental Impact,Int J Energy Research, 21(1997),

643-654.

8 Creyts, lC and Carey, V.P.: Use of extended exergy analysis as a tool for assessment of the environmental impact of industrial processes, inProceedings of the ASME Advanced Energy Systems Division,AES-Vol 37 (1997), ASME, New York, 129-137.

9 Makarytchev, S.Y.: Environmental Impact Analysis of ACFB-Based Gas and Power Cogeneration,

Energy Int 1.,(1998), to appear.

Duke University

Department ofMechanical Engineering

and Materials Science Durham, NC 27708-0300, USA

1 Introduction

The objective of this chapter is to review a modern transformation in the teaching,research and practice of energy engineering: the increasingly important roles played bythermodynamics (especially the second law) in problem formulation, modeling anddesign optimization This methodology is known as thermodynamic optimization, orentropy generation minimization (EGM) and was first recognized in a 1982 book [1].The most recent review [2] shows that the use of this method is expanding at anaccelerated pace, and that it has recently acquired alternate names such as finite time orendoreversible thermodynamics In this chapter we illustrate the application of themethod through examples selected from refrigeration

The method relies on the simultaneous application of principles of heat and masstransfer, fluid mechanics, and engineering thermodynamics, in the pursuit of realisticmodels of processes, devices, and installations By realistic models we mean modelsthat account for the inherent irreversibility of engineering systems The objective of theEGM method is to identify designs (i.e., systems, configurations) in which the entropygeneration is minimum According to the Gouy-Stodola theorem [1-3],

(1)

a minimum entropy generation design characterizes a system with minimum destruction

of available work (exergy)

The method consists of dividing the system into subsystems that are in local (orinternal) thermodynamic equilibrium The subsystem properties are governed by thelaws of engineering (classical) thermodynamics Entropy is generated at the boundariesbetween subsystems, as heat and mass currents through the boundaries These flows areaccounted for through the rate equations taken from the disciplines of heat and masstransfer and fluid mechanics

In this way, the total rate of entropy generation is expressed as a function of theoverall physical characteristics (e.g., finite size) of the greater system Only then canthe total entropy generation be monitored and minimized by properly varying thephysical characteristics of the system The subsystems can be macroscopic (two ormore in a system), or infinitesimally small (an infinite number) In other words,

II

A Bejan et al (eds.), Energy and the Environment, I 1-22.

© 1999Kluwer Academic Publishers.

system modeling and entropy generation analysis can be pursued at the macroscopiclevel or at the differential level: examples of both abound in [1-3].

It is instructive to begin this review with a brief look at why in EGM we need to rely

on heat transfer and fluid mechanics, not just thermodynamics Consider the mostgeneral system-environment configuration, namely a system that operates in theunsteady state Its instantaneous inventories of mass, energy, and entropy are M, E,and S The system experiences the net work transfer rate W, heat transfer rates(Qo,Q\ ,···Qn) with n+ 1temperature reservoirs (To, Tt .,Tn), and mass flow rates(min' mout ) through any number of inlet and outlet ports Noteworthy in this array ofinteractions is the heat transfer r~te between the system and the environmental(atmospheric) temperature reservoir, Qo, on which we focus shortly

The thermodynamics of the system consists of accounting for the first law and thesecond law[3],

We select the environmental heat transfer Qo as the interaction that is allowed tofloat as Sgen varies Historically, this choice was inspired (and justified) byapplications to power plants and refrigeration plants, because the rejection of heat to theatmosphere was of little consequence in the overall cost analysis of the design.Eliminating Qo between Eqs (2) and (3) we obtain

W= - ~(E- ToS) + i(l- TO)Qi +I,m(h - Tos) - I,m(h - Tos) - TOSgen (4)

The work transfer rate in the limit of reversible operation (Sgen = 0)is clearly

Wrev = - ~(E - ToS)+ i(l- TO)Qi +I,m(h - Tos) - I,m(h - Tos) (5)

In engineering thermodynamics, each of the terms on the right-hand side of equation (5)

is recognized as an exergy of one type or another [3-5], and the calculation of Wrev isknown as exergy analysis In exergy analysis we need only the laws ofthermodynamics Subtracting Eq (4) from Eq (5) we arrive at the Gouy-Stodolatheorem Eq.(1):

In Eq (6) Wrev is fixed because the heat and mass flow interactions (other than Qo)arefixed

Pure thermodynamics ends, and EGM begins, with Eq (6) The lost power

(Wrev - W) is always positive, regardless of whether the system is a power producer(e.g., power plant) or a power user (e.g., refrigeration plant)

The critically new aspect of the EGM method-the aspect that makes the use ofthermodynamics insufficient, and distinguishes it from exergy analysis-is theminimization of the calculated entropy generation rate To minimize the irreversibility

of a proposed design, the analyst must use the relations between temperature differencesand heat transfer rates, and between pressure differences and mass flow rates He or shemust relate the degree of thermodynamic nonideality of the design to the physicalcharacteristics of the system, namely to finite dimensions, shapes, materials, finitespeeds, and finite-time intervals of operation For this the analyst must rely on heattransfer and fluid mechanics principles, in addition to thermodynamics Only by varyingone or more of the physical characteristics of the system, can the analyst bring thedesign closer to the operation characterized by minimum entropy generation subject tosize and time constraints

3 Optimal Cooling of Thermal Insulation Structures

To appreciate the engineering origins of thermodynamic optimization, it is useful tonote that the field of low temperature refrigeration was the first where EGM became anestablished method of optimization and design As an application of Eq.(6), it is easy

to prove that the power required to keep a cold space cold is equal to the total rate ofentropy generation times the ambient temperature, with the observation that the entropygeneration rate includes the contribution made by the leakage of heat from TO into thecold space [1-3] The structure of a cryogenic system is in fact dominated bycomponents that leak heat toward lower temperatures, e.g., mechanical supports,radiation shields, electric cables, and counterflow heat exchangers The minimization ofentropy generation along a heat leak path consists of optimizing the path in harmonywith the rest of the refrigerator of liquifier

Figure Ishows a mechanical support of lengthLthat connects the cold end of themachine (TL) to room temperature (TH)' The rate of entropy generation inside thesupport shown as a vertical column is

dQ / T - (Q.+dQ) / (T+dT) = QdT / T2, because dT «T The local heat leakdecrement dQ is removed by the rest of the installation, which is modeled as reversible.The heat leak is also related to the local temperature gradient and conduction cross-section A,

~= fTH ldT

According to variational calculus, the heat leak function that minimizes the Sgen

integral(7)subject to the firite-size constraint (9) is obtained by finding the extremum

of the aggregate integral iHL FdT whose integrand F is a linear combination of the.integrands of Eqs (7)and (8), F= Q / T2 +Ak/ Q, and 'A is a Lagrange multiplier TheEuler equation reduces in this case to aF/aQ= 0,w~ich yields Qopt =('Ak)II2T. TheLagrange multiplier is determined by substituting Qopt into the constraint (9) Theresults are

(10)

(11) A (fT kl/2 )2

Sgen,mm = -L TH dT

L T

Equation (7) was provided by thermodynamics and Eq (8) by heat transfer: togetherthey prescribe the optimal design (l0, 1.1), which is characterized by a certaindistribution of intermediate cooling effect (dQ / dT)opt Any other design, Q(T), willgenerate more entropy and will require more power in order to maintain the cold end ofthe support at TL Quantitative and older examples are given in [1-3]

4 Optimal Distribution of Heat Transfer Area

Classical heat transfer provides information on the relationship between heat transferrates, temperature differences and types of surfaces Interesting and very basicconclusions follow if we think beyond fundamental heat transfer, and ask questionsinspired by thermodynamics and potential fields of application Heat exchangers arealways present in the design of power plants and refrigeration plants In the model ofFig 2, the operation of an actual (irreversible) refrigeration plant can be approximated

by the two heat exchangers and the inner (reversible) refrigeration cycle executed by theworking fluid [1, 2]

The ambient temperature (TH)'cold space temperature (TL),refrigeration load Q,

and the capacity flow rates of the two air streams (CH ,CL ) are assumed known Theevaporator temperature is uniform (Tmin)and lower than the temperature of the cold airstream (CL). Similarly, the condenser temperature is uniform (Tmax)and higher than thetemperature of the ambient air stream (CH). In other words, contrary to the impressiongiven by Fig 2, Tmaxand Tmin embrace from above and from below the temperaturerange occupied by the two unmixed streams The refrigeration cycle executed in thespace shown between Tmax and Tmin is modeled as reversible, in other words,

QL= tLCL(TL - Tmin ) tL = 1 - exp [- ~~] (15,16)

in which UoandAnare the overall heat transfer coefficient and area of the condenser, and

U and A are the corresponding parameters of the evaporator Consider now the totalthermal conductance constraint,

Ifwe introduce the thermal conductance allocation fraction y= UoAo / (UA)tot or

I - Y=UA / (UA)tot, the numerical minimization of F with respect to y yields theYopt values reported in Fig 3 We find that, in general, the total thermal conductancemust be divided unequally between the two heat exchangers It is easy to verify that(UA)opt must be divided equally (Yopt= 1/2) only when the two air capacity rates areequal (CH=CL)

A special limit of the model of Fig 2 is represented by (CH' CL)~ 00, or(No,NL) ~ 0, which corresponds to air streams temperatures (TH,Td that do notchange significantly while in contact with the condenser and evaporator surfaces In thislimit the model of Fig 2 becomes identical to the oldest model of a heat transfer-

irreversible refrigerator [1], and the optimal design subjected to the (UA)lalconstraintrequires Yapt= 1/2,or

Figure5 The optimal allocation of heat transfer area: comparison between the results based on the Alol

and (UA)'n' constraints [2).

By minimizing numerically F with respect to x= AoIA tot ,we find that xopt emerges as afunction of CH/CL ,UofU, and

(23)

Figure 4 shows that the optimal Ao is generally not equal to A The optimal way ofdividing Atot among the heat exchangers depends on the total surface available (NtoJ, andthe relative magnitude of the external parameters of the heat exchangers, namely theratios CdCHand UlUo

The results of the optimization subject to the Atot constraint (Fig 4) attain asimple form in the limit in which the two heat exchanger surfaces are bathed byisothermal (well mixed) fluids, e.g., ambient air at To and TL This limit is the same assetting (No, Nd ~0 in the model of Fig 2, and the minimization of F with respect

to x=AolAtot yields

The optimal allocation of a finite heat exchanger inventory in a refrigeration plantwith three heat exchangers can be pursued by using the same method [2]

5 Optimal Refrigeration for Electronic Packages

Consider the cooling of a space filled with heat-generating components such aselectronic packages or superconducting windings [2] In the arrangement shown inFig 6 the space operates at the temperature Te below the ambient To, and generatesJoule heat at the rate Q The coolant is a stream of single phase fluid of flow rate ril

and specific heat cpo The contact area A between the heat-releasing surfaces and thecoolant is fixed The inlet temperature is controlled by a reversible steady-staterefrigerator that receives the stream from room temperature, and rejects heat to theambient, Qo

We assume that the coolant is well mixed at the temperature Tout inside the cooledpackage is matched by the enthalpy gained by the coolant

Q= mcp (Toul - Tin)

(26)(27)

The spent stream of temperature TOUI is discharged into the ambient We also assumethat the refrigerator operates reversibly, and the pressure drop experienced by mbetween inlet (To) and outlet (Toul) is zero Accordingly, the first law and second lawstatements for the refrigerator are

electronic packages

It is now convenient to nondimensionalize Eqs (32) and (33) as

'tin = 'te - M-l

where

(34)(35)

It can also be shown that the optimal inlet temperature('tin,opt)decreases monotonically

as'tedecreases [2]

6 Constructal Theory: Optimization in Natural Systems

In this chapter we reviewed the fundamentals of thermodynamic optimization, andillustrated the method by solving three generic problems in refrigeration: the optimalcooling of the insulation structure, the optimal spatial distribution of heat transfersurface, and the optimal coolant flow rate for a low-temperature electric system Weshowed how the method leads to analytical or numerical results that describe the relationbetwe¢n optimized thermodynamic performance and imposed physical constraints (e.g.,size, heat transfer coefficient) Further improvements can be sought by relaxing theconstraints: the method tells the designer up front how sensitive thermodynamicperformance will be to changes in the constraints

On the fundamental and pedagogical side of energy engineering, this chapter showedthat it is possible to construct simple but instructive models of processes and devices.This is done by taking into account their inherent irreversibility and combiningthermodynamics with heat transfer and fluid mechanics at the earliest stages ofmodeling Each individual component or assembly can then be optimized, byminimizing the respective entropy generation The optima revealed by the entropygeneration minimization method mean that similar trade-offs exist (and are worthsearching for) in the much more complex systems models that are optimized by designengineers Reviews of the current breadth of the field and its history are provided in[2, 7-9]

A new direction in constrained thermodynamic optimization is the prediction ofspatial and temporal organization innatural flow systems, animate and inanimate [3]. It

has been found that the tree network of so many natural systems can be deduced inpurely deterministic fashion from a single principle: the geometric minimization ofresistance in volume-to-point flow The same principle applied to point-to-point flow

generates the round sections of natural ducts, and the watermelon-slice sections of rivers of all sizes.

cross-The result is the constructal theory proposed in [3, 10-15]: the shapes and

structures of natural nonequilibrium systems are results of a process of optimizationsubject to global constraints Additional examples of temporal organization anticipatedbased on constrained thermodynamic optimization are the natural frequencies ofbreathing, heart beating and turbulent flows This is an ideal time to extendthermodynamic optimizations over biophysics, and to bridge the gap between fields thatuntil today were thought to be separate

Acknowledgement This work was supported by the National Science Foundation

7 References

I. Bejan, A.: Entropy Generation through Heat and Fluid Flow, Wiley, New York, 1982.

2. Bejan, A.: Entropy Generation Minimization, CRC Press, Boca Raton, FL, 1996.

3. Bejan, A.: Advanced Engineering Thermodynamics, second edition, Wiley, New York, 1997.

4. Moran, MJ.: Availability Analysis: A Guide to EJjicient Energy Use, ASME Press, New York, 1989.

5. Bejan, A., Tsatsaronis, G and Moran, M.: Thermal Design and Optimization, Wiley, New York, 1996.

6. Klein, S.A.: Design considerations for refrigeration cycles, Int i Refrigeration 15 (1992),929-933.

7 Bejan, A.: Entropy generation minimization: the new thermodynamics of finite-size devices and

finite-time processes, J Appl Phys 79 (1996), 1191-1218.

8. Bejan, A.: Engineering advances on finite-time thermodynamics, Am.J.Phys.62 (January 1994), 12.

11-9 Bejan, A.: Notes on the history of the method of entropy generation mi~imization (finite time

thermodynamics), J Non-Equilib Thermodyn 21 (1996), 239-242.

10 Bejan, A.: Constructal-theory network of conducting paths for cooling a heat generating volume, Int.

i Heat MassTran.~t'er 40 (1997), 799-816.

II. Bejan, A.: How nature takes shape, Mech Eng 119 (October 1997),90-92.

12 Bejan, A and Errera, M R.: Deterministic tree networks for fluid flow: geometry for minimal flow

resistance between a volume and one point, Fractals 5 (1997), 685-695.

13 Nelson, Jr., R A and Bejan, A.: Constructal optimization of internal flow geometry in convection, J.

Heat Transfer120 (1998), 357-364.

14 Bejan, A., Ikegami, Y and Ledezma, G A.: Constructal theory of natural crack pattern formation for

fastest cooling, Int J Heat MassTran.~t'er 41 (1998), 1945-1954.

IS Bejan, A and Ledezma, G A.: Streets tree networks and urban growth: optimal geometry for

quickest access between a finite-size volume and one point, Physica A 255 (1998), 211-217.

Rensselaer Polytechnic Institute

Department ofMechanical Engineering

Aeronautical Engineering and Mechanics Troy, NY 12180-3590, USA

1 Introduction

Energy and materials savingcon~iderations,as well as economic incentives, have led toefforts to produce more efficientheat exchange equipment Common thermal-hydraulicgoals are to reduce the size of a heat exchanger required for a specified heat duty, toupgrade the capacity of an existing heat exchanger, to reduce the approach temperaturedifference for the process streams, or to reduce the pumping power The first twoobjectives translate to an increase in the average heat flux of the heat exchanger, or theencouragement of high heat fluxes In the case of systems with a specified heatdissipation, the goal is to cool the device, or accommodate a high heat flux, at moderatetemperature difference Implicit in these objectives, energy reduction (improvement offirst law efficiency) and temperature difference reduction (improvement of second lawefficiency) are important to global environmental protection

The study of improved heat transfer performance is referred to as heat transfer

enhancement, augmentation, or intensification In general, this means anincrease in heat transfer coefficient Attempts to increase "normal" heat transfercoefficients have been recorded for more than a century, and there is a large store ofinformation A recent survey by Bergles et al [I] cites 5676 technical publications,excluding patents and manufacturers' literature The rather recent growth of activity inthis area is clearly evident from the yearly distribution of publications presented inFig 1

Enhancement techniques can be classified either as passive methods, which require

no direct application of external power (typical surfaces and devices shown in Fig 1), or

as active methods, which require external power The effectiveness of both types of

techniques is strongly dependent on the mode of heat transfer, which may range fromsingle-phase free convection to dispersed-flow film boiling A classification ofenhancement techniques is shown in Table 1

Two or more of the above techniques may be utilized simultaneously to produce anenhancement that is larger than either of the techniques operating separately This is

termed compound enhancement.

It should be emphasized that one of the motivations for studying enhanced heattransfer is to assess the effect of an inherent condition on heat transfer Some practicalexamples are: roughness produced by standard manufacturing, degassing of liquids with

23

A Bejan et al (eds.) £nergy and the Environment, 23-35.

©1999Kluwer Academic Publishers.

TABLE I Classification of enhancement techniques.

Passive Techniques

Treated surfaces

Rough surfaces

Extended surfaces

Displaced enhancement devices

Swirl flow devices

Coiled tubes

Surface tension devices

Additives for fluids

Active Techniques

Mechanical aids Surface vibration fluid vibration Electrostatic fields Suction or injection Jet impingement

Compound Enhancement

Rough surface with a twisted tape swirl flow device, for example

high gas content, surface vibration resulting from rotating machinery or flowoscillations, fluid vibration resulting from pumping pulsation, and electrical fieldspresent in electrical equipment

The surfaces in Fig 2 have been used for both single-phase and two-phase heattransfer enhancement The emphasis is on effective and cost-competitive (proved orpotential) techniques that have made the transition from the laboratory to commercialheat exchangers Broad reviews of developments in enhanced heat transfer are available[2-5]

2000

1980 1960 1940 1920 1900

Fi~ure 2 Tubes for enhancement of single-phase heat transfer: (a) corrugated or spirally indented tube

with internal and external protuberances, (b) integral external fins, (c) integral internal fins, (d) deep spirally fined tube with internal and external protuberances, (e) static-mixer inserts,

and (f) wire-wound insert.

2 Single-phase Free Convection

With the exception of the familiar technique of providing extended surfaces, the passivetechniques have little to offer in the way of enhanced heat transfer for free convection.This is because the velocities are usually too low to cause flow separation or secondaryflow However, the restarting of thermal boundary layers in interrupted extendedsurfaces increases heat transfer so as to more than compensate for the lost area

Mechanically aided heat transfer is a standard technique in the chemical and foodindustries when viscous liquids are involved The predominant geometry for surfacevibration has been the horizontal cylinder, vibrated either horizontally or vertically.Heat transfer coefficients can be increased 1O-fold for both low-frequency/high-amplitudeand high-frequency/low-amplitude situations It is, of course, equally effective and morepractical to provide steady forced flow Furthermore, the mechanical designer isconcerned that such intense vibrations could result in equipment failures

Since it is usually difficult to apply surface vibrations to practical equipment, analternative technique is utilized whereby vibrations are applied to the fluid and focusedtoward the heated surface With proper transducer design, it is also possible to improveheat transfer to simple heaters immersed in gases or liquids by several hundred percent.Electric fields are particularly effective in increasing heat transfer coefficients in freeconvection Dielectrophoretic or electrophoretic (especially with ionization of gases)forces cause greater bulk mixing in the vicinity of the heat transfer surface Heattransfer coefficients may be improved by as much as a factor of 40 with electrostaticfields up to 100,000 V Again, the equivalent effect could be produced at lower capitalcost, and without the voltage hazard, by simply providing forced convection with ablower or fan

3 Single-phase Forced Convection

3.1 GENERAL CONSIDERAnONS

The present discussion emphasizes enhancement of heat transfer inside ducts that are

primarily of circular cross section Typical data for turbulence promoters inserted insidetubes are shown in Fig 3 As shown in Fig 3(a), the promoters produce a sizableelevation in the Nusselt number, or heat transfer coefficient, at constant Reynoldsnumber, or velocity However, as shown in Fig 3(b), there is an accompanying largeincrease in the friction factor By using performance evaluation criteria, both heattransfer and friction can be considered [7]

(2)

In general, the maximum enhancement of laminar flow with many of thetechniques is the same order of magnitude, and seems to be independent of the wallboundary condition The enhancement with some rough tubes, corrugated tubes,inner-fin tubes, various static mixers, and twisted-tape inserts is about 200 percent.The improvements in heat transfer coefficient with turbulent flow in rough tubes(based on nominal surface area) are as much as 250% Analogy solutions for sand-grain-type roughness and for square-repeated-rib roughness have been proposed Astatistical correlation is also available for heat transfer coefficient and frictionfactor, as described below

The following correlations are recommended for tubes with transverse repeatedribs or helical repeated ribs (Fig 2a), with turbulent flow (Ravigururajan andBergles, [8]):

of the transverse-repeated-rib type mitigate the deterioration in heat transferdownstream of stagnation

100Gr - - - - - - - ,

100

10 A WALL PROTUBEREJ",;£S

B AXIALl.Y SUPPORTED DISCS

C TWISTED TAPE WITH AXIAL

CORE

D TWISTED lJIPE

101""'<"' -, -.,:: , -,

B 01

Figure3 Typical data for turbulence promoters inserted inside tubes:

(a) heat transfer data, (b) friction data [6].

(4)

Extended surfaces can be considered "old technology" as far as most applications areconcerned The real interest now is in increasing heat transfer coefficients on theextended surface Compact heat exchangers of the plate-fin or tube-and-center variety useseveral enhancement techniques: offset strip fins, louvered fins, perforated fins, orcorrugated fins Coefficients are several hundred percent above the smooth-tube values;however, the pressure drop is also substantially increased, and there may be vibrationand noise problems

Internally finned circular tubes are available in aluminum and copper (or copperalloys), Fig 2(b) Correlations (for heat transfer coefficient and friction factor) areavailable for laminar flow, for both straight and spiral continuous fins Turbulent flow

in tubes with straight or helical fins was correlated by Carnavos [9]:

so that a wider range of geometries and fluids can be handled without resort to extensiveexperimental programs

Many proprietary surface configurations have been produced by deforming the basictube The "convoluted," corrugated," "spiral," or "spirally fluted" tubes (Fig 2(a» havemultiple-start spiral corrugations, which add area, along the tube length A systematicsurvey of the single-tube performance of condenser tubes indicates up to 400% increase

in the nominal inside heat transfer coefficient (based on diameter of a smooth tube of thesame maximum inside diameter); however, pressure drops on the water side are about 20times higher

Displaced enhancement devices are typicaIIy in the form of inserts, with theelements arranged to promote transverse mixing (static mixers, Fig 2(c» They areused primarily for viscous liquids, to promote either heat transfer or mass transfer.Displaced promoters are also used to enhance the radiant heat transfer in high-temperature applications Inthe flue-tube of a hot-gas-fired hot water heater, there is atrade-off between radiation and convection Another type of displaced insert generatesvortices, which enhance the downstream flow Delta-wing and rectangular-wingpromoters, creating vortices that co-rotate or counter-rotate, have been studied Wire-loop inserts (Fig 2(t) have also been used for enhancement of laminar and turbulentflow

Twisted-tape inserts have been widely used to improve heat transfer in both laminarand turbulent flow Correlations are available for laminar flow, for both uniform heatflux and uniform wall temperature conditions Turbulent flow in tubes with twisted-tape inserts has also been correlated Several studies have considered the heat transferenhancement of a decaying swirl flow, generated, say, by a short twisted-tape insert

3.3 ACTIVE AND COMPOUND TECHNIQUES

Under active techniques, mechanically aided heat transfer in the form of surface-scrapingcan increase forced convection hat transfer Surface vibration has been demonstrated toimprove heat transfer to both laminar and turbulent duct flow of liquids Fluid vibrationhas been extensively studied for both air (loudspeakers and sirens) and liquids (flowinterrupters, pulsators, and ultrasonic transducers) Pulsations are relatively simple toapply to low-velocity liquid flows, and improvements of several hundred percent can berealized

A novel, active enhancement concept for a variable roughness, heat exchanger tube

is reported by Champagne and Bergles [10] Shape-memory-alloy wire coils ride along asupport structure in close proximity to the inner-tube (or outer-tube) wall of a single-phase heat exchanger At low wall temperture (efficient heat transfer), the coils are closetogether An excessive tube-wall temperature (low heat transfer coefficient) results inextension of the coils It has been demonstrated that coil extension does produce asubstantial heat transfer enhancement Since the extended coils, at the present time,must be reset by manual or mechanical means (owing to limitations in materialprocessing and training techniques of shape-memory alloys), this is classified as anactive enhancement technique

Some very impressive enhancements have been recorded with electrical fields,particularly in the laminar flow region Improvements of at least 100% were obtainedwhen voltages in the lO-kV range were applied to transformer oil Itis found that evenwith intense electrostatic fields, the heat transfer enhancement disappears as turbulentflow is approached in a circular tube with a concentric inner electrode

Compound techniques are slowly emerging area of enhancement that holds promisefor practical applications, since heat transfer coefficients can usually be increased aboveeach of the several techniques acting alone Some examples that have been studied are

as follows: rough tube wall with twisted-tape inserts, rough cylinder with acousticvibrations, internally finned tube with twisted-tape insert, finned tubes in fluidized beds,externally finned tubes subjected to vibrations, rib-roughened passage being rotated, gas-solid suspension with an electrical field, fluidized bed with pulsations of air, and a rib-roughened channel with longitudinal vortex generation

4 Pool Boiling

Selected passive and active enhancement techniques have been shown to be effective forpool boiling and flow boiling/evaporation Most techniques apply to nucleate boiling;however, some techniques are applicable to transition and film boiling

Itshould be noted that phase-change heat transfer coefficients are relatively high.The main thermal resistance in a two-fluid heat exchanger often lies on the non-phase-change side (Fouling of either side can, of course, represent the dominant thermalresistance.) For this reason, the emphasis is often on enhancement of single-phaseflow On the other hand, the overall thermal resistance may then be reduced to the pointwhere significant improvement in the overall performance can be achieved by enhancingthe two-phase flow Two-phase enhancement would also be important in double-phase-change (boiling/condensing) heat exchangers

As discussed elsewhere, surface material and finish have a strong effect on nucleateand transition pool boiling However, reliable control of nucleation on plain surfaces isnot easily accomplished Accordingly, since the earliest days of boiling research, therehave been attempts to relocate the boiling curve through use of relatively grossmodification of the surface For many years, this was accomplished simply by areaincrease in the form of low helical fins The subsequent tendency was to structuresurfaces to improve the nucleate boiling characteristics by a fundamental change in theboiling process Many of these advanced surfaces are being used in commercial shell-and-tube boilers.

Several manufacturing processes have been employed: machining, forming,layering, and coating In Fig 4(a), standard low-fin tubing is shown Figure 4(c)depicts a tunnel-and-pore arrangement produced by rolling, upsetting, and brushing Analternative modification of the low fins is shown in Fig 4(d), where the rolled fins havebeen split and rolled to a T shape Further modification of the internal, Fig 4(e), orexternal, Fig 4(f), surface is possible Knurling and rolling are involved in producingthe surface shown in Fig 4(g) The earliest example of a commercial structured surface,shown in Fig 4(b) is the porous metallic matrix produced by sintering or brazing smallparticles Wall superheat reductions of up to a factor of ten are common with thesesurfaces The advantage is not only high nucleate boiling heat transfer coefficients, butthe fact that boiling can take place at very low temperature differences

The structured boiling surfaces developed for refrigeration and process applicationshave been used as "heat sinks" for immersion-cooled microelectronic chips

The behavior of tube bundles is often different with structured-surface tubes Theenhanced nucleate boiling dominates, and the convective boiling enhancement, found inplain tube bundles, does not occur

Active enhancement techniques include heated surface rotation, surface wiping,surface vibration, fluid vibration, electrostatic fields, and suction at the heated surface.Although active techniques are effective in reducing the wall superheat and/or increasingthe critical heat flux, the practical applications are very limited, largely because of thedifficulty of reliably providing the mechanical or electrical effect.

Compound enhancement, which involves two or more techniques appliedsimultaneously, has also been studied Electrohydrodynamic enhancement was applied

to a finned tube bundle, resulting in nearly a 200 percent increase in the average boilingheat transfer coefficient of the bundle, with a small power consumption for the field

5 Convective Boiling/Evaporation

The structured surfaces described in the previous section are generally not used for intubevaporization, due to the difficulty of manufacture One notable exception is the HighFlux surface in a vertical thermosiphon reboiler The considerable increase in the lowquality, nucleate boiling coefficient is desirable, but it is also important that more vapor

is generated to promote circulation

Helical repeated ribs and helically coiled wire inserts have been used to increasevaporization coefficients and the dryout heat flux in once-through boilers

Numerous tubes with internal fins, either integral or attached, are available forrefrigerant evaporators Original configurations were tightly packed, copper, offset stripfin inserts soldered to the copper tube or aluminum, star-shaped inserts secured bydrawing the tube over the insert Examples are shown in Fig 6 Average heat transfercoefficients (based on surface area of smooth tube of the same diameter) for typicalevaporator conditions are increased by as much as 200 percent

A cross-sectional view of a typical micro-fin tube, now widely used, is included

in Fig 5 The average evaporation boiling coefficient is increased 30-80 percent Thepressure drop penalties are usually less; that is, lower percentage increases in pressuredrop are frequently observed Twisted-tape inserts are generally used to increase theburnout heat flux for subcooled boiling at high imposed heat fluxes (l08 - 108W/m2),

as might be encountered in the cooling of fusion reactor components Increases inburnout heat flux of up to 200 percent have been reported at near-atmospheric pressure,but the enhancement drops off rapidly as the pressure is increased above 2 Mpa

Since pressure drop is important to the stability of parallel cooling channels, recentwork has emphasized both burnout and pressure drop [12]

Figure5 Inner-fin tubes for refrigerant evaporators: (a) Strip-fin insert; (b) Star-shaped inserts;

(c) Micro-fin.