Page 157 - Mechanical Engineers' Handbook (Volume 4)

146 Heat-Transfer Fundamentals

e entrainment, evaporator section
eff effective
ƒ fin
i inner
l liquid
m mean, maximum
n nucleation
o outer
0 stagnation condition
p pipe
r radiation
s surface, sonic or sphere
w wire spacing, wick
v vapor
spectral
free stream
axial hydrostatic pressure
normal hydrostatic pressure

Transport phenomena represents the overall field of study and encompasses a number of
subfields. One of these is heat transfer, which focuses primarily on the energy transfer oc-
curring as a result of an energy gradient that manifests itself as a temperature difference.
This form of energy transfer can occur as a result of a number of different mechanisms,
including conduction, which focuses on the transfer of energy through the direct impact of
molecules; convection, which results from the energy transferred through the motion of a
fluid; and radiation, which focuses on the transmission of energy through electromagnetic
waves. In the following review, as is the case with most texts on heat transfer, phase change
heat transfer, i.e., boiling and condensation, will be treated as a subset of convection heat
transfer.

1 CONDUCTION HEAT TRANSFER
The exchange of energy or heat resulting from the kinetic energy transferred through the
direct impact of molecules is referred to as conduction, and takes place from a region of
high energy (or temperature) to a region of lower energy (or temperature). The fundamental
relationship that governs this form of heat transfer is Fourier’s law of heat conduction, which
states that in a one-dimensional system with no fluid motion, the rate of heat flow in a given
direction is proportional to the product of the temperature gradient in that direction and the
area normal to the direction of heat flow. For conduction heat transfer in the x direction this
expression takes the form

T
q kA
x
x
is the heat transfer in the x direction, A is the area normal to the heat flow, T/ x
where q x
is the temperature gradient, and k is the thermal conductivity of the substance.