Design and Optimization of Thermal Systems Episode 1 Part 6 pdf

Basic Considerations in Design 97

available in the literature (Incropera and Dewitt, 2001). The use of these correlations

brings in the dependence of the cooling rate on the physical variables in the prob￾lem. The fluid is the most important parameter and may be chosen for high thermal

conductivity, which yields a high heat transfer coefficient, low cost, easy availability,

nontoxic behavior, and high boiling point, if boiling is to be avoided in the liquid. If

boiling is allowed, the latent heat of vaporization becomes an important variable to

obtain a high heat transfer coefficient. Oils with high boiling points are generally

used for quenching. The temperature Ta is another variable that can be effectively

used to control the cooling rate. A combination of a chiller and a hot fluid bath may be

used to vary Ta over a wide range. Clearly, many solutions are possible and a unique

design is not obtained. Different fluids that are easily available may be tried first to

see if the requirement on the cooling rate is satisfied. If not, a variation in Ta may be

considered. Optimization of the system may then be based on cost.

2.4 COMPUTER-AIDED DESIGN

An area that has generated a considerable amount of interest over the last two

decades as a solution to many problems being faced by industry and as a precur￾sor to the future trends in engineering design is that of computer-aided design

(CAD). With the tremendous growth in the use and availability of digital com￾puters, resulting from advancements in both the hardware and the software, the

computer has become an important part of the design practice. Much of engineer￾ing design today involves the use of computers, as discussed in the preceding

sections and as presented in detail in later chapters. However, the term computer￾aided design, as used in common practice, largely refers to an independent or

stand-alone system, such as a computer workstation, and interactive usage of the

computer to consider various design options and obtain an acceptable or optimal

design, employing the software for modeling and analysis available on the system.

Still, the basic ideas involved in a CAD system are general and may be extended

to more involved design processes and to larger computer systems.

2.4.1 MAIN FEATURES

As mentioned above, a CAD system involves several items that facilitate the itera￾tive design process. Some of the important ones are:

1. Interactive application of the computer

2. Graphical display of results

3. Graphic input of geometry and variables

4. Available software for analysis and simulation

5. Available database for considering different options

6. Knowledge base from current engineering practice

7. Storage of information from earlier designs

8. Help in decision making

Thus, the system hardware consists of a central processing unit (CPU) for numeri￾cal analysis, disk or magnetic tape for storage of data and design information, an

interactive graphics terminal, and a plotter for hard copy of the numerical results.

98 Design and Optimization of Thermal Systems

The computer software codes for analysis are often based on finite-element

methods (FEM) for differential equations because this provides the flexibility and

versatility needed for design (Zienkiewicz, 1977; Reddy, 1993). Different configura￾tions and boundary conditions can be easily considered by FEM codes without much

change in the numerical procedure. Other methods, particularly the finite-volume and

the finite-difference method (FDM), are also used extensively for thermal systems

(Patankar, 1980). The software may also contain additional codes on curve fitting,

interpolation, optimization, and solution of algebraic systems. Some of the important

numerical schemes are discussed in Chapter 4. Analytical approaches may also be

included. Commercially available computer software, such as Maple, Mathematica,

Mathcad, and Mathlab, may be used to obtain analytical as well as numerical solu￾tions to various problems such as integration, differentiation, matrix inversion, root

solving, curve fitting, and solving systems of algebraic and differential equations.

The use of MATLAB for these problems is discussed in detail in Appendix A.

The interactive use of the computer is extremely important for design because

it allows the user or designer to try many different design possibilities by enter￾ing the inputs numerically or graphically, and to obtain the simulation results

in graphical form that can be easily interpreted. Iterative procedures for design

and optimization can also be employed effectively with the interactive mode.

A graphics terminal is usually employed to obtain three-dimensional, oblique,

cross-sectional, or other convenient views of the components.

The storage of data needed for design, such as material properties, heat transfer

correlations, characteristics of devices, design problem statement, previous design

information, accepted engineering practice, regulations, and safety features needed

can also substantially help in the design process. In this connection, knowledge￾based design procedures may also be incorporated in the design scheme. Besides

providing important relevant information for design, the rules of thumb and heu￾ristic arguments used for design can be built into the system. Such systems are

also often known as expert systems since expert knowledge from earlier design

experience is part of the software, providing help in the decision-making process as

well. Since knowledge acquired through engineering design practice is usually an

important component in the development of a successful design, knowledge-based

systems have been found to be useful additions to the CAD process. Chapter 11

presents details on knowledge-based systems for design, along with several exam￾ples demonstrating concepts that can substantially aid the design process.

2.4.2 COMPUTER-AIDED DESIGN OF THERMAL SYSTEMS

The main elements of a CAD system for the design of thermal processes and

equipment are shown in Figure 2.27. The various features that are usually

included in such CAD systems are indicated. The modeling aspect is often the

most involved one when dealing with thermal systems. The remaining aspects are

common to CAD systems for other engineering fields. Much of the effort in CAD

has, over recent years, been largely devoted to the design of mechanical systems

and components such as gears, springs, beams, vibrating devices, and structural