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Introduction

Mechanical engineering design involves both mechanical and thermal designs. Mechanical design deals with mechanical strength and structural properties of materials; motion and dynamics; geometrical dimensions and tolerances. Mechanical design requires knowledge of engineering mechanics, materials and strength of materials, vibration, and machine design. Thermal design deals with the thermal aspects of the components, processes, and systems, and requires knowledge of thermal science subjects such as thermodynamics, heat transfer, and fluid mechanics. A design of a product may require thermal design analysis first followed by mechanical design and are often interrelated. A product design may not only require mechanical concepts design but may also require knowledge of thermal science concepts and thermal design analysis techniques. Often, the product design requires additional subject areas such as electrical engineering and biomedical engineering, and multiphysics analysis.

1.1 Thermal Engineering Design

A thermal engineering design process involves the applications of concepts from fundamental engineering science topics such as thermodynamics, heat transfer, and fluid dynamics, following some specified well-defined steps and in an iterative process. A successful design process may involve several steps as shown in Figure 1.1 and is described below:

1. Conception: Requires some intuition about the final end-product using one's creative sense.
2. Synthesis: Some vision of the way the end results might be achieved. Consideration of multiple options and multiple pathways is given before developing the design.
3. Analysis: Ways to realize the design by following well-defined methodologies like thermal analysis, computer simulation analysis, economic analysis, and cost estimation.

   Such knowledge bases can be learned. The analysis step leads to defining the ratings and specifications of the product.

4. Evaluation: This is the way to prove the functioning of a successful design. This involves testing of a prototype requiring iterations. Use of sophisticated simulation methods and design tools may reduce number prototypes to be made, hence reduce cost in the design, development, and production.
5. Communication: Present the design to others in the form of technical reports and oral presentations.

Figure 1.1 Design process.

1.2 Elements of Design Analysis of Thermal Systems

Various elements and steps generally used in the design analysis of thermal systems are demonstrated in Figure 1.2. Column one in the figure shows that a design process starts with a conceptual design along with some potential options and alternatives. This is followed by the selection of type of components, selection of ranges for some key variables and parameters, and setting any constraints. Thermodynamic analysis is carried out to establish the initial specification and ratings of the major components.

Figure 1.2 Flowchart showing detail elements of thermal design process.

The component-level analysis and design are next carried out to develop the detail specification of each component in the system. Cost estimation followed by an economic analysis are essential to check the feasibility of the design. Iterative refinement can be carried out by changing the set variables and parameters. A system simulation is required to determine the expected operating conditions and performance at offload or part-load conditions and is generally used in the design stage to provide an improved design. Optimization step is often carried out to ensure the feasibility of the concept based on either the performance or the cost or both.

1.2.1 Some Special Aspects of Thermal Design

A thermal system may be very large and have a single application. For example: A utility large thermal power plant that produces 1000 MW of electric power. It could also be some systems that are produced in large numbers. For example, refrigeration units, air-conditioning unit, fuel cells, solar water-heating system, or smaller solar thermal power generation units in the ranges of 1-10 kW. Thermal systems generally involve a large number of components in one design, and often these components can be categorized such as heat exchangers, condensers, boilers, cooling towers heat sinks, pumps, fans, etc. Another important aspect of the thermal system design process is that many parameters must often be set, either arbitrarily or in relation to other aspects of the design. The values of the parameters will, however, affect both capital and operating cost, including the energy cost, and hence will require iterative refinements of the parameter values assumed.

1.2.2 Design Types

Designs can be categorized into different types: nonfunctional, functional, satisfactory, and optimum.

• Nonfunctional: The device does not function. For example, the cooling device is designed, but produces no cooling effect, and even produces undesirable effects such as irreversible heating.
• Functional: The device performs in the expected manner as it is designed to do so. For example, a designed cooling device is capable of cooling a water stream.
• Satisfactory: A functional design that meets some assigned criteria. For example, a chiller is designed to transfer 25 kW of heat from an air stream and cool the air stream from 25 to 13 °C.
• Optimal: A design that is obtained based on some specific restrictions. For example, a solar collector is designed to supply thermal energy to run a domestic solar water heating system.

The collector is designed and fabricated with a restriction of minimum cost and/or minimum weight.

1.3 Examples of Thermal Energy Design Problems

Some typical thermal design projects are discussed in Sections 1.3.1-1.3.7. The objective is to understand some of the basic steps to be followed in the design of thermal systems.

1.3.1 Solar-Heated Swimming Pool

The swimming pools of most hotels in the USA are currently outdoors and heated by gas heaters. It is proposed to use solar energy to heat the pool throughout year as needed. It is also proposed to have flat-plate solar collectors that receive energy from the sun and use the energy to maintain the water at a comfortable temperature range year-round.

A basic solar water-heating system along with two additional optional systems, shown in Figure 1.3, are described here for consideration. The basic water-heating system for swimming pool consists of a solar collector array, a pump, a piping system of valves and fittings.

Figure 1.3 Solar-heated swimming pool. (a) Option 1: a basic system: without an auxiliary heater and without any thermal storage. (b) Option 2: with an auxiliary heater and without any thermal storage. (c) Option 3: with an auxiliary heater and a thermal storage.

Figure 1.4 Pool water heat load.

A few assumptions are made, and a few variables and parameters are set before starting the analysis. A typical design process is described below.

Understand the requirements and set known Data:

1. Geographical location: This is important to establish the available solar radiation and to select the design outdoor conditions. For example - Select Santa Barbara, CA.
2. Pool dimension: Select the pool dimension to establish the amount of water to be heated and rate at which water will be circulated through the system. For example, select the pool size as 12 m long × 8 m wide with water depth that varies in the lengthwise direction from 0.8 to 3.0 m.
3. Design operating conditions:
   • - A comfortable water temperature range for the pool
   • - Design outdoor air conditions such as the dry-bulb and wet-bulb temperatures.

Select optional systems:

1. Consider system with or without a thermal storage.
2. Consider system with or without an auxiliary gas or electric heater.

Basic steps for analysis and design:

1. Develop a thermal model of the pool water considering all components of energy transfer or interaction with the surrounding. The model should include all major heat loss and heat gain components from/to the pool water body by a number of means such as convection heat loss from the water to surface to ambient air (qloss, conv); radiation heat loss from the water to ambient air and sky (qloss, rad); evaporation heat loss due to mass transfer loss of water to ambient air (qloss, evap); ground heat loss from the water body by conduction through pool wall to ground (qloss, ground) Figure 1.4.

   The pool water heat load is the sum of all the heat losses from pool water and given as

   where

   qloss, conv = Convection heat loss from pool water surface due to the wind speed flow qloss,...