Tham khảo tài liệu 'design and optimization of thermal systems episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 22 Design and Optimization of Thermal Systems 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 complicated than what might be expected from a study of thermal sciences alone. Analysis 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 simplified 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 properties 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 .
