Chapters 2 and 3 discussed the analytical and numerical solution of one-dimensional (1-D), steady-state problems. These are problems in which the temperature within the material is independent of time and varies in only one spatial dimension (e.g., x). Examples of such problems are the plane wall studied in Section 2.2, which is truly a 1-D problem, and the extended surface problems in Chapter 3 that are only approximately 1-D. The governing differential equation for these problems is an ordinary differential equation (ODE) and the mathematics required to solve the problem are straightforward.

Chapters 1 through 4 discuss steady-state problems, i.e., problems in which temperature depends on position (e.g., x and y) but does not change with time (t). Steady-state problems become progressively more difficult as the dimensionality of the problem increases from 1-D to 2-D (and even to 3-D, although this was not covered). This chapter begins the consideration of transient conduction problems, i.e., problems where temperature depends on time. This chapter specifically considers the simplest transient problem, one in which the temperature approximately depends only on time and not on position.

Chapters 7 through 10 discuss convection situations involving single-phase fluids. The thermodynamic state of the fluids in these problems is sufficiently far from their vapor dome that they do not undergo a phase change. In this chapter, two-phase convection processes are examined. Two-phase processes occur when the fluid is experiencing heat transfer near the vapor dome so that vapor and liquid are simultaneously present. If the fluid is being transformed from liquid to vapor through heat addition, then the process is referred to as boiling or evaporation. If vapor is being transformed to liquid by heat removal, then the process is referred to as condensation.

Chapter 2 considered problems that were truly one-dimensional. Energy transfer occurred only in one coordinate direction and therefore temperature varied only in that direction. In this chapter, we will examine problems that are only approximately one-dimensional, referred to generally as extended surfaces. Extended surfaces are typically thin pieces of conductive material that can be approximated as being isothermal in two dimensions and having temperature variations in only one direction. In an extended surface, energy is transferred laterally (i.e., across the thickness) but the temperature change induced by the energy transfer is sufficiently small that it can be neglected. The extended surface approximation greatly reduces the complexity of the problem and can often be applied with little loss in accuracy.

A heat exchanger is a device that is designed to transfer thermal energy from one fluid to another. Heat exchangers are everywhere in our modern society. Nearly all thermal systems employ at least one and usually several heat exchangers. The background material related to conduction and convection, presented in Chapters 2 through 11, is required to analyze and design heat exchangers. Section 12.1 reviews the applications and types of heat exchangers that are commonly encountered. The subsequent sections provide the theory and tools required to predict and understand the performance of these devices.

Heat transfer is the term used to describe the movement of thermal energy (heat) from one place to another. Heat transfer drives the world that we live in. Look around. Heat transfer is at work no matter where you currently are.

Chapters 2 through 6 consider heat transfer in a stationary medium where energy transport occurs entirely by conduction and is governed by Fourier’s Law. Thus far, convection has been considered primarily as a boundary condition for these conduction problems. Convection refers to the transfer of energy that occurs between a surface and a moving medium, most often a liquid or gas flowing through a duct or over an object. Convection processes include fluid motion and the related energy transfer. The additional terms in an energy balance related to the fluid flow often dominate the now familiar energy transport by conduction. The presence of fluid motion complicates heat transfer problems substantially and links the heat transfer problem with an underlying fluid dynamics problem. The complete solution to most convection problems therefore requires sophisticated computational fluid dynamic (CFD) tools that are beyond the scope of this book.

Chapter 8 provides correlations that can be used to solve external flow forced convection problems where an external flow is defined as one where the boundary layer can grow without bound. For flow over a flat plate located sufficiently far from any other surface, the boundary layer is never confined by the presence of another object and therefore continues to grow from the leading edge to the trailing edge. An internal flow is defined as a flow situation where the growth of the boundary layer is confined; that is, the boundary layers can only grow to a certain thickness before being constrained. Internal flows are often encountered in engineering applications (e.g., the flow through tubes or ducts).

From a thermodynamic perspective, thermal energy can be transferred across a boundary (i.e., heat transfer can occur) by only two mechanisms: conduction and radiation. Conduction is the process in which energy exchange occurs due to the interactions of molecular (or smaller) scale energy carriers within a material. The conduction process is intuitive; it is easy to imagine energy carriers having a higher level of energy (represented by their temperature) colliding with neighboring particles and thereby transferring some of their energy to them. Radiation is a very different heat transfer process because energy is transferred without the involvement of any molecular interactions. Radiation energy exchange is related to electromagnetic waves and therefore can occur over long distances through a complete vacuum. For example, the energy that our planet receives from the Sun is a result of radiation exchange. This chapter presents an introduction to radiation heat transfer with a focus on providing methods for solving radiation problems.

Chapter 5 discussed transient problems in which the spatial temperature gradients within a solid object can be neglected and therefore the problem is approximately zero-dimensional (0-D). In these lumped capacitance transient problems the solution is a function only of time. Lumped capacitance problems essentially ignore the process of conduction as being unimportant. This chapter discusses transient problems where internal, spatial temperature gradients related to energy transfer by conduction are nonnegligible (i.e., the Biot number is not much less than unity). The first section provides some conceptual tools that are not exact, but can be used to develop an understanding of transient conduction problems. More sophisticated analytical and numerical solutions that provide more exact solutions are presented in the remaining sections.