Strip element dA_1 to plane perdendicular rectangle
FDiff_4(a, b, c) provides the view factor from a strip element to a rectangle in a plane that is 90 degrees to the plane of the strip.
Inputs:
a is the height of the rectangle above the plane of the strip [m or ft]
b is the width of the rectangle and of the strip [m or ft]
c is the distance from the rectangle to the strip in the plane of the strip [m or ft]
Example:
$UnitSystem SI K Pa J deg
"This example determines the view factor between a rectangle of dimensions a x b and a differentially wide strip perpendicular to the rectangle. The calculation is done in two ways: 1) by integrating the FDiff_3 for 0<x<b and 2) direct calculation using FDiff_4. The results are identical but the option of integrating allows temperature of other properties to vary with position."
a=1 [m]
b=2 [m]
c=0.5 [m]
F_strip=integral(FDiff_3(a, x, c),x,0,b) "integrate for 0<x<b"
F=FDiff_4(a, b, c)
{Solution: F_strip = F = 0.2153 [m]}
Reference: Siegel and Howell, Thermal radiation heat transfer, 4th edition, p.843.