Design and Optimization of Thermal Systems Episode 1 Part 3 pptx

It is important to recognize that thermal systems arise in many diverse fields of engineering, such as aerospace engineering, manufacturing, power generation, and air conditioning.. Vari

It is important to recognize that thermal systems arise in many diverse fields of engineering, such as aerospace engineering, manufacturing, power generation, and air conditioning Consequently, a study of thermal systems usually brings in many additional mechanisms and considerations, making the problem much more com-plicated than what might be expected from a study of thermal sciences alone.

1.3.2 A NALYSIS

The analysis of thermal systems is often complicated because of the complex nature of fluid flow and of heat and mass transfer mechanisms that govern these systems As a result, typical thermal systems have to be approximated, simpli-fied, and idealized in order to make it possible to analyze them and thus obtain the inputs needed for design Following are some of the characteristics that are commonly encountered in thermal systems and processes:

1 Time-dependent

2 Multidimensional

3 Nonlinear mechanisms

4 Complex geometries

5 Complicated boundary conditions

6 Coupled transport phenomena

7 Turbulent flow

8 Change in phase and material structure

9 Energy losses and irreversibility

10 Variable material properties

11 Influence of ambient conditions

12 Variety of energy sources

Because of the time-dependent, multidimensional nature of typical systems, the governing equations are generally a set of partial differential equations, with nonlinearity arising due to convection of momentum in the flow, variable proper-ties, and radiative transport However, approximations and idealizations are used to simplify these equations, resulting in algebraic and ordinary differential equations for many practical situations and relatively simpler partial differential equations for others These considerations are discussed in Chapter 3 as part of modeling of the system However, the equations for a few simple cases are given here to illustrate the nature of the governing equations and the effect of some of these complexities.The simplest problems are those that assume steady-state conditions, with or without flow, while also assuming uniform conditions in each part of the system These problems lead to algebraic equations, which are often nonlinear for ther-mal systems This situation is commonly encountered in thermodynamic systems such as refrigeration, air conditioning, and energy conversion systems Then, the governing set of algebraic equations may be written as

f1 (x1, x2, x3,n) 0

f2 (x1, x2, x3,n) 0

in actual practice, more elaborate analysis would generally be needed.

If the time-dependent behavior of the system is sought, for a study of the dynamic characteristics of the system, the resulting governing equations are ordi-nary differential equations in time, if the assumption of uniform conditions within each part is still employed Then the governing equations may be written as

in the startup and shutdown of the systems, as well as in determining the effects

of changes in operating conditions like flow rate, pressure, and heat input

If the conditions in the different parts of the system cannot be assumed to

be uniform, the problem is referred to as distributed A time-dependent, dimensional flow with the assumption of constant fluid properties, as in a duct or over a heated body, is represented by the equations (Burmeister, 1993)

two-tt

u

T u

tt

u

tt

u y

1R

tt

p

tt

³µ´

v

T u

tt

v

tt

v y

1R

tt

p

tt

³µ´

2

2

v

where the first equation gives the conservation of mass and the other two give the

momentum force balance in the x and y directions, respectively Here, u, v are the

velocity components in the x, y coordinate directions, p is pressure, R is the density,

T is time, and N is the kinematic viscosity of the fluid The corresponding energy

equation for the temperature T in the fluid, with A as its thermal diffusivity, is

tt

T

T u

tt

T

tt

T

tt

³µ´

2

2

T

The corresponding three-dimensional equations may similarly be written by

adding the components in the z direction, including the z-momentum equation

The problem then becomes much more complicated, but many practical stances, such as environmental processes, cooling of electronic equipment, and manufacturing systems, require three-dimensional analysis for accurate results for use in design and optimization

circum-If there is no flow, for instance, in the circumstance of conduction in a tionary solid body such as the wall of a building or of a blast furnace, the energy equation, Equation (1.9), reduces to

sta-tt

T

T A

tt

³µ´

2

2

T

Here, the energy equations, (1.9) and (1.10), are linear because T appears in its first

power The flow is obtained from Equation (1.6) through Equation (1.8), which are

cou-pled with u, v, and p as the unknowns The momentum equations are nonlinear because

of the inertia terms u ∂u/∂x, v ∂u/∂y, etc., in which higher powers of the unknown

veloc-ity components appear Equation (1.6) through Equation (1.10) are partial differential equations and are written for the relatively simpler constant property, two-dimensional circumstance for the Cartesian coordinate system and a Newtonian fluid Even then, these are quite complicated In practical systems, we often encounter many additional complexities that make the analysis a very difficult and challenging affair

Inclusion of variable properties and/or radiative transport can give rise to ear mechanisms, the former due to the dependence of the properties on the dependent

nonlin-variable such as temperature T and the latter due to the variation of radiation heat transfer as T4 For example, if the thermal conductivity k, density R, and specific heat

at constant pressure C p vary with temperature T, Equation (1.10) becomes

p

t

tt

tt

Similarly, Equation (1.9) may be modified for variable properties Thus,

nonlin-earity arises due to nonlinear powers of T resulting from property variation with T.

If radiative heat loss occurs at a surface, the corresponding energy exchange rate q, per unit area, with a black environment at temperature T e may be written as

where E is the surface emissivity and S is the Stefan–Boltzmann constant This

equation is nonlinear in T due to the presence of temperature as T4 Thus, linear equations are frequently obtained, making the solution difficult Iterative methods are often needed to obtain the solution Nonlinearity also makes it dif-ficult to scale up the results from a laboratory model to the full-size system Many

non-of these considerations are discussed in detail in Chapter 3

The various other complexities mentioned earlier also complicate the analysis and design of thermal systems Complex geometry and boundary conditions arise

in most practical systems, making it necessary to use simplifications and versatile numerical techniques such as finite element and boundary element methods Tur-bulent flow is encountered in many important processes, particularly in energy systems and environmental transport Special numerical models and experimental procedures have been developed to take turbulent transport into account Phase change, coupling with material characteristics, time-varying ambient conditions, irreversibility, and different energy sources, such as lasers, gas, oil, electricity, and viscous heating, further complicate the analysis of thermal systems and pro-cesses Several of these aspects will be seen to arise in examples given in later chapters However, our focus is not on analysis but on design, even though analy-sis provides many of the inputs needed for design Therefore, only a brief outline

of the basic characteristics of thermal systems is given here Specialized books, such as Ozisik (1985) and Incropera and Dewitt (2001) in heat transfer, Fox and McDonald (2003) and Shames (1992) in fluid mechanics, Howell and Buckius (1992), Cengel and Boles (2002) and Moran and Shapiro (2000) in thermodynam-ics, among others, may be consulted for details on different analytical and experi-mental techniques as well as for results obtained on a variety of fundamental and applied problems

1.3.3 T YPES AND E XAMPLES

As mentioned earlier, thermal systems are important in a wide variety of neering fields and disciplines Let us consider some important examples, types, and applications of these systems Several different ways of classifying thermal systems may be employed because of their diversity A common method is in terms of the function or application of the system Using this approach, several important types of thermal systems, along with commonly encountered applica-tions and examples, follow

engi-Manufacturing and Materials Processing Systems

Examples include processes such as casting, crystal growing, heat treatment, metal forming, drying, soldering and welding, laser and gas cutting, plastic extru-sion and injection molding, powder metallurgy, optical fiber drawing, ceramics, and glass processing Also included are food processing systems as well as com-mon household appliances such as ovens and cooking ranges This is an important area and many diverse thermal systems are employed for the different manufac-turing processes used in practice We have already discussed ingot casting, as

sketched in Figure 1.3 Figure 1.10 shows the schematics for continuous casting, plastic extrusion, glass fiber drawing, and hot rolling processes.

In continuous casting, molten material is allowed to solidify across an face as the bulk material is withdrawn at a given speed through a mold, which is

δ(Solidified

skin thickness) Water sprays

Supporting and withdrawing rolls

FIGURE 1.10 A few manufacturing systems (a) Continuous casting, (b) plastic screw

extrusion, (c) optical fiber drawing, (d) hot rolling (Figure 1.10(a) adapted from Ghosh and Mallik, 1986; Figure 1.10(b) from Tadmor and Gogos, 1979.)

(d) Distance

Station 2 Station 1

Rollers

U

Hot material

Feed mechanism Glass rod (preform) Furnace Furnace

Fiber cooling distance

Fiber diameter

monitor

Accelerated cooling

Coating applicator

Coating concentricity monitor

Coating diameter monitor Coated fiber

Winding drum Drawing pulley

Curing furnace

or lamps

Fiber

down region

Neck-FIGURE 1.10 (Continued).

usually water cooled In plastic screw extrusion, the solid plastic is fed through a hopper, melted by energy input, and conveyed by the rotation of the screw, with

an associated pressure and temperature rise, finally pushing the molten material through a die to obtain a desired shape The material may also be injected into a mold and solidified for a process known as injection molding Similarly, a special glass rod, typically 5–10 cm in diameter and known as a preform, is heated in a furnace and pulled to sharply reduce the diameter to 100–125 Mm, yielding an opti-cal fiber in glass fiber drawing In hot rolling, the material is heated and reduced

in thickness by pushing it through two rollers that are at a given distance apart Several sets of rollers may be used to obtain the desired decrease in thickness or diameter Similarly, new thermal processes have been developed for the fabrica-tion of nanomaterials through chemical vapor deposition and other approaches.Heat transfer is very important in these processes because the temperature determines the forces needed, the withdrawal speed, and the quality of the final product Further details on these and other processes may be found in specialized books on manufacturing, such as Ghosh and Mallik (1986), Doyle et al (1985), and Kalpakjian (1989) Some of these processes will be considered again later in the book as examples With the development of new and improved materials, the design of thermal systems for materials processing has become crucial for manu-facturing new products and for meeting international competition

Energy Systems

Examples of energy systems include power plants, solar energy utilization, thermal energy systems, energy storage, solar ponds, and conventional and non-conventional energy conversion systems

geo-This is one of the most frequently mentioned areas for thermal energy siderations Different types of thermal systems arise depending on the nature of the energy source, such as nuclear, oil, gas, solar, or wind energy Most of these systems are covered in thermodynamics courses and are often treated as steady, lumped systems Figure 1.11 shows sketches of typical solar and nuclear energy systems In both cases, the energy collected or generated is used to run the tur-bines, which are then used to generate electricity A considerable literature exists

con-on thermal systems of interest in this field because of the tremendous importance

of power generation in our society; see, for instance, Howell et al (1982), Hsieh (1986), Van Wylen et al (1994), and Duffie and Beckman (1991)

Cooling Systems for Electronic Equipment

Systems that are of interest in this area include air cooling, liquid immersion, heat pipes, heat sinks, heat removal by boiling, and microscale systems

This is one of those areas where thermal considerations are extremely tant for the satisfactory performance of the system even though the main applica-tion is in a different area Thus, electronic systems, such as computers, televisions, digital multimeters, and signal conditioners, are used for a variety of applications,

impor-Containment vessel Steam generator

Power output

Condenser

Secondary loop pump

Primary loop pump

pressure turbine

Low- pressure turbine

High-Nuclear

reactor

(b)

Power output Bypass line

Solar boiler Solar

flux

Turbine

Storage

Pump Solar energy

collection system

(a)

Condenser

FIGURE 1.11 Power systems based on (a) solar energy and (b) nuclear energy (Adapted

from Howell and Buckius, 1992.)

most of which are not directly connected with fluid flow and heat transfer But the

cooling of the electronic system in order to ensure that the temperature T c of the various components, particularly of the chips or semiconductor devices, does not

exceed the allowable temperature level Tmax, that is, T c a Tmax, is often the most crucial factor in the design and operation of the system Further size reduction

of the system is frequently constrained by the heat transfer considerations Figure 1.12 shows typical air cooling and liquid immersion systems for electronic equipment The energy dissipated by the electronic components is removed by the fluid flow, thus allowing the temperatures to remain below the specified limit Figure 1.2 showed a sketch of a heat pipe for enhanced cooling of an electronic chip Many books, such as those by Steinberg (1980) and Kraus and Bar-Cohen (1983), have been written in response to the growing importance of this area Photographs

Air flow

Wall

(a)

Electric circuit boards

Relief valve Coolant in

(b)

FIGURE 1.12 Cooling systems for electronic equipment (a) Forced air cooling, (b) liquid

immersion cooling.

of typical electronic equipment with air cooling by means of a fan or a blower are shown in Figure 1.13, indicating the complexity of such systems in actual practice.

Environmental and Safety Systems

Examples of these systems include arrangements for heat rejection to ambient air and water, control of thermal and air pollution, cooling towers, incinerators, waste disposal, water treatment plants, smoke and temperature control systems, and fire extinguishing systems

The growing concern with the environmental impact of waste and energy disposal, including global warming and depletion of the ozone layer, has made

it essential to minimize the effect on our environment by developing new and

FIGURE 1.13 Typical electronic systems with air cooling by means of a fan (From

Stein-berg, 1980.)

improved methods for disposal Many thermal systems have been developed in response to this need These include systems based on fluids that would sub-stitute refrigerants like CFCs (chlorofluorocarbons) that adversely affect the ozone layer, improved incineration techniques for solid waste disposal, catalytic converters in automobiles to reduce harmful emissions, and scrubbers in power plants to reduce pollutants Figure 1.14 shows sketches of typical heat rejection systems from power plants, employing a lake as a cooling pond in the first case and a natural draft cooling tower in the second The effect on the local environ-ment, in terms of temperature rise, increased flow, and disturbance to natural yearly cycle, is of particular concern in these cases Safety is also a very impor-tant consideration Figure 1.15 shows a sketch of a room fire, indicating a hot upper layer containing the toxic and hot combustion products and a relatively

Temperature profile Lake

x

Cooled water Intake water

Hot water

Air flow

Cooling tower

FIGURE 1.14 Systems for heat rejection from a power plant (a) Natural lake as a cooling

pond, (b) natural draft cooling tower.

Temperature distribution

Fire

Plume

Opening Inflow Outflow Wake Ceiling

Stably stratified region

Walls

T

FIGURE 1.15 Flow and temperature due to fire in a room with an opening.

cooler and less toxic lower layer that is often safe until flashover occurs when everything in the room catches fire and the room is engulfed in flames Thus, the design of the system, which may be a building, ship, submarine, or airplane, for fire safety is clearly an important element in the overall construction and opera-tion of the system.

Aerospace Systems

Many thermal systems in aerospace applications are of interest here Some of the common ones are gas turbines, rockets, combustors, and cooling systems.This has been a particularly important area over the last three decades because of the space program Considerable progress has been made on the vari-ous thermal systems and subsystems that are needed Because of the large thrust needed at rocket launch and high cooling rates during reentry, much of the effort

in designing efficient systems has been directed at these two stages However, cooling, air conditioning, and electronic and energy systems during orbit, as well

as for a space station, have their own requirements and challenges

Transportation Systems

Most of the relevant systems in this area are thermal in nature These include internal combustion engines such as spark ignition and diesel engines; steam engines; fuel cells; and modern automobile, airplane, and train engines

This is an extensive field, closely associated with different kinds of thermal tems Though a traditional mechanical engineering field, this area has seen many significant changes in recent years, most of these being related to the optimization of existing systems New systems have also evolved in response to the need for higher efficiency, size that is more compact, greater safety, and lower costs Supersonic air transport has led to several interesting innovations in this field Figure 1.16 shows

sys-a few typicsys-al systems thsys-at sys-arise in trsys-ansportsys-ation Figure 1.16(sys-a) shows two designs for a jet engine, with hot gases being ejected from the nozzle to provide the thrust Figure 1.16(b) shows a spark ignition engine where the combustion process in the cylinder drives the piston, which moves the crankshaft and thus the wheels Figure 1.17 shows photographic views of gas turbine systems, indicating the intake, exhaust, and combustion chamber Similarly, Figure 1.18 shows sketches of engines for transportation, indicating a lightweight engine and a diesel engine that is turbo-charged for boosting power Clearly, the practical systems are extremely complicated and involve intricate flow paths, combustion processes, and control mechanisms For details on these and other systems, books on thermodynamics, such as those by Van Wylen et al (1994) and by Howell and Buckius (1992), and more specialized books

on various relevant topics, such as Heywood (1988), may be consulted

Air Conditioning, Refrigeration, and Heating Systems

Several different thermal systems are associated with this application, which is

of considerable interest to us in our daily lives These include vapor compression