Library Articles
A simple definition of heat transfer is energy in transit due to a temperature difference. If there is a temperature difference in a medium or between media, heat transfer must occur.
 
 
Commonly Used Symbols
The follow tables are a list of variables, their symbols, and the associative units commonly used in heat transfer expressions. All the equations present in the heat transfer section have the same quantity denoted by the symbols in the table.
 
Independent Parameters
Quantity
Symbol
Object
Units
Temperature
T
scalar
K
Time
t
scalar
s
Thermal conductivity
k
scalar
W/m · K
Specific heat
c
scalar
J/kg · K
Convection heat transfer coefficient
h
scalar
W/m2K
Emissivity
e
scalar
W/m2K
Density
r
scalar
kg/m3
Internal heat generation
qqen
scalar
kg/m3
 
Dependent Parameters
Quantity
Symbol
Object
Units
Heat flux
q"
vector
W/m2
1-D heat flux
q
scalar
W
 
Table of Dimensionless Parameters
As is common with fluid mechanics analysis, a number of dimensionless parameters are employed to describe convective heat transfer. A summary of these variables is included in the following tables:

General Convection (Forced and Free)
Parameter
Formula
Interpretation
Prandtl number
Ratio of fluid velocity boundary layer thickness to the fluid temperature boundary layer thickness.
Nusselt number

Ratio of heat transferred from surface to heat conducted away by fluid.

Forced Convection Only
Parameter
Formula
Interpretation
Reynold's number

Ratio of fluid inertia stress to viscous stress (for flow over flat plates).
Reynold's number

(Reynold's number for pipe flow)
Stanton number

 

Free Convection Only
Parameter
Formula
Interpretation
Grashof number

Ratio of fluid buoyancy stress to viscous stress.
Rayleigh number

 
Heat Transfer Coefficient

The heat transfer coefficient encompasses all of the parameters that influence convection heat transfer. It depends on conditions in the boundary layer, which are influenced by surface geometry, the nature of the fluid motion, and other fluid thermodynamic and transport properties. Convection heat transfer will frequently appear as a boundary condition in the solution of conduction problems. The table below is the typical values of the convection heat transfer coefficient.
 
Typical Values of Convection Heat Transfer Coefficient

Process
h
(W/m2 · K)
Free convection
 
Gases
2–25
Liquids
50–1000
Forced convection
 
Gases
25–250
Liquids
50–20,000
Convection with phase change
 
Boiling or condensation
2500–100,000

 

Some material excerpted from: Incroprera, Frank and De Witt, David P. Introduction to Heat Transfer. Second Edition. New York: John Wiley & Sons, Inc. 1985, 1990.

Copyright © 1985, 1990, by John Wiley & Sons, Inc.

This material is used by permission of John Wiley & Sons, Inc.