Modern Engineering Thermodynamics

CHAPTER 19
Introduction to Coupled Phenomena
CONTENTS
19.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
19.2 Coupled Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
19.3 Linear Phenomenological Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765
19.4 Thermoelectric Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 767
19.4.1 The Seebeck Effect . ..........................................................767
19.4.2 The Peltier Effect . . ..........................................................768
19.4.3 The Kelvin Effect. . . ..........................................................769
19.4.4 The Fourier Effect . . ..........................................................769
19.4.5 The Joule Effect . . . ..........................................................770
19.4.6 The Ohm Effect ..............................................................770
19.5 Thermomechanical Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776
19.5.1 Zero Heat Transfer . ..........................................................779
19.5.2 Zero Temperature Gradient. ....................................................779
19.5.3 Zero Pressure Gradient . . ......................................................780
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783
19.1 INTRODUCTION
Physical phenomena are generally considered independent of each other. However, phenomenon can become
coupled whenever the presence of one physical phenomenon induces one or more other physical phenomena
to occur simultaneously. For example, the heat transport of energy through a system can induce a flow of electrical
energy under certain circumstances; similarly, the flow of electrical energy can induce the flow of heat. This
particular coupling is called the thermoelectric effect. Since energy is always conserved, in this case, some of the
thermal energy is converted directly into electrical energy by an internal mechanism. The energy conversion efficiency
from thermal energy directly into electrical energy is normally quite low; however, the reverse energy conversion
process (from electrical to thermal energy) can be 100% efficient. Some of the more common types of
direct energy conversion coupled phenomena are shown later in Tables 19.1 and 19.2. Notice that most of these
couplings have been known for a very long time.
19.2 COUPLED PHENOMENA
Coupled phenomena have the potential for future technological utilization as important direct energy conversion
processes. Today, they are used mainly in sensors to produce low-level electrical signals that are proportional
to the magnitude of the other phenomenon present. For example, the thermoelectric effect is commonly
used to produce a voltage proportional to the local temperature that can be read by an instrument or a computer.
Such a device is called a thermocouple, and it is used as a temperature sensor.
Modern Engineering Thermodynamics. DOI: 10.1016/B978-0-12-374996-3.00019-1
© 2011 Elsevier Inc. All rights reserved. 763