Disk to disk (same center)
Function F3D_3(r_1, r_2, a) returns the view factor value of one disk (finite area) from another one (finite area) parallel to it which has its center along the same normal.
Inputs:
r_1 is the radius of disk 1 [m or ft]
r_2 is the radius of disk 2 [m or ft]
a is the parallel distance between both disks [m or ft]
Example:
$UnitSystem SI K Pa J deg
r_1=0.1 [m]
r_2=0.2 [m]
a=0.5 [m]
F=F3D_3(r_1, r_2, a)
{Solution: F=0.1339 }
Reference: Siegel and Howell, Thermal radiation heat transfer, 4th edition, p.845.