function [F12, A1, F21, A2] = frustum2base(rb, rt, h)
%[F12, A1, F21, A2] = frustum2base(rb, rt, h)
%
% Description:
%
% View factor between the interior wall of a conical frustum and
% its base.
%
% Inputs:
%
% rb = radius of base
% rt = radius of top of conical frustum
% h = height of the conical frustrum
%
% Outputs:
%
% F12 = view factor from interior wall to base
% A1 = area of the interior wall
% F21 = view factor from base to interior wall
% A2 = area of the base
%
% Reference:
%
% A Catalog of Radiation Heat Transfer Configuration Factors,
% third edition, John R. Howell, The University of Texas at Austin
%
% http://www.thermalradiation.net/sectionc/C-111.html
%
%==========================================================================
if rb <= rt
error('frustum2base: Base radius = %f, must be greater than top radius = %f.',rb,rt)
end
A1 = pi*(rb + rt)*sqrt((rt - rb)^2 + h^2); % Area of the wall
A2 = pi*rb^2; % Area of the base
H = h/rt;
R = rb/rt;
X = 1.0 + R^2 + H^2;
F12 = (2*R^2 - X + sqrt(X^2 - 4*R^2))/(2*sqrt(X - 2*R)*(1 + R));
F21 = A1*F12/A2; % F12*A1 = F21*A2 reciprocity