Design and Optimization of Thermal Systems Episode 2 Part 5 docx

322 Design and Optimization of Thermal Systems

dimension, say the wall thickness of the oven, is small, the effort may be shifted

to other dimensions such as the height of the enclosed space. If the dimensions,

along with the configuration and the materials, are held constant, different heat￾ers and fans may be considered for the redesign. After each change, the design is

evaluated in terms of a chosen quantity or parameter that characterizes the design

to ascertain if the new design is an improvement over the previous one. If the

design appears to be becoming worse, the direction of the change is reversed.

In summary, the redesign procedure may be based on changing one design

variable, or several variables of a given type, for a given step, while the others

are held constant. The given constraints are taken care of in the selection of the

design variables. As the iteration proceeds, the effect arising from each change

is obtained and the sensitivity of the system performance to the different design

variables is determined. This allows one to focus on the most important variables

and thus converge to an acceptable solution more rapidly. The use of a design

parameter or characteristic quantity, which is based on the requirements for the

given problem, enables one to monitor the progress of the iteration and fine-tune

it for the problem under consideration.

5.4 DESIGN OF SYSTEMS FROM DIFFERENT

APPLICATION AREAS

We have considered the main aspects involved in the design of a thermal system,

starting with conceptual design and proceeding through initial design, model￾ing, and simulation to design evaluation, redesign, and convergence to an accept￾able design. It has also been seen that thermal systems arise in many diverse

Configuration

Materials

Dimensions

Components

Configuration

Formulation of

design problem

FIGURE 5.13 Priority for changing the design variables, considering the configuration,

materials, dimensions, and components as variables.

Acceptable Design of a Thermal System 323

applications and vary substantially from one application to another. The examples

considered thus far have similarly ranged from relatively simple systems, with a

small number of parts, to complex ones that involve many parts and subsystems.

In actual practice as well, the complexity of the design process is strongly depen￾dent on the nature and type of thermal system under consideration.

Among the simplest design problems are those that involve selecting different

components that make up the system and then simulating the system to ensure

satisfactory performance for given ranges of operating conditions. The governing

equations are generally nonlinear algebraic equations in such cases, and the vari￾ous numerical techniques outlined in Chapter 4 may be used for the simulation.

On the other hand, complex systems such as those in materials processing, aero￾space applications, and electronic equipment cooling generally involve sets of

partial differential equations that are coupled to each other and to other types

of equations that govern different parts of the system. A few examples from some

of the important areas of application are given here to illustrate the synthesis of

the various ideas and design steps discussed earlier.

5.4.1 MANUFACTURING PROCESSES

This is one of the most important areas in which thermal systems are of interest.

Though manufacturing has always been of crucial significance in engineering,

this area has become even more vital over the recent past because of the develop￾ment of new materials, applications, and processing techniques. Several impor￾tant manufacturing processes were mentioned earlier, including processes such

as plastic extrusion, heat treatment, casting, bonding, hot rolling, and optical fiber

drawing. The thermal systems associated with different manufacturing processes

are quite diverse, with different concerns, mathematical models, and governing

mechanisms. They are generally complicated and involve features such as

1. Time-dependent behavior

2. Combined transport modes

3. Strong dependence on material properties

4. Sensitivity to operating conditions

5. Strong coupling between the different parts of the system.

Other characteristics may also be important in specific applications, as discussed

in specialized books on manufacturing such as Ghosh and Mallik (1986) and

Kalpakjian and Schmid (2005).

The governing equations for manufacturing processes are typically partial dif￾ferential equations that are coupled through the boundary conditions and material

property variations. However, since the problem may vary substantially from one

process to another, it is very difficult to develop a general approach to modeling,

simulation, and design of these systems. A few examples of thermal systems in

manufacturing were discussed in earlier chapters and a few others will be consid￾ered in the presentation on optimization. Here, in the following example, we shall