Page 156 - Mechanical Engineers' Handbook (Volume 4)

Heat-Transfer Fundamentals 145

Nu Nusselt number, Nu hL/k, Nu hD/k, dimensionless
D
L
Nu Nusselt number averaged over length, dimensionless
P pressure, N/m , perimeter, m
2
Pe Peclet number, RePr, dimensionless
Pr Prandtl number, C /k, dimensionless
p
q rate of heat transfer, W
q rate of heat transfer per unit area, W/m 2
R distance, m; thermal resistance, K/W
r radial coordinate, m; recovery factor, dimensionless
Ra Rayleigh number, GrPr; Ra Gr Pr, dimensionless
L
L
Re Reynolds Number, Re VL/ , Re VD/ , dimensionless
L
D
S conduction shape factor, m
T temperature, K or C
t time, sec
T s adiabatic surface temperature, K
T sat saturation temperature, K
T b fluid bulk temperature or base temperature of fins, K
T e excessive temperature, T T ,K or C
sat
s
T ƒ film temperature, (T T )/2, K

s
T i initial temperature; at t 0, K
T 0 stagnation temperature, K
T s surface temperature, K
T free stream fluid temperature, K
U overall heat transfer coefficient, W/m K
2
V fluid velocity, m/s; volume, m 3
w groove width, m; or wire spacing, m
We Weber number, dimensionless
x one of the axes of Cartesian reference frame, m
GREEK SYMBOLS
2
thermal diffusivity, k/ C ,m /s; absorptivity, dimensionless
p
coefficient of volume expansion, 1/K
mass flow rate of condensate per unit width, kg/m s
specific heat ratio, dimensionless
T temperature difference, K

thickness of cavity space, groove depth, m
emissivity, dimensionless
wick porosity, dimensionless
wavelength, m
ƒ fin efficiency, dimensionless
viscosity, kg/m s

kinematic viscosity, m /s
2
reflectivity, dimensionless; density, kg/m 3
2
surface tension, N/m; Stefan-Boltzmann constant, 5.729 10 8 W/m K 4
transmissivity, dimensionless, shear stress, N/m 2
angle of inclination, degrees or radians

SUBSCRIPTS
a adiabatic section, air
b boiling, black body
c convection, capillary, capillary limitation, condenser