Should use the table in:
H. C. Hottel. “Radiant Heat Transmission.” In Heat Transmission, ed. W. H. McAdams. 3rd ed. New York: McGraw-Hill, 1954.
cited both by Lienhard A Heat Transfer Textbook, 4/e, pdf.page.584, and by Cengel - Heat Transfer: A Practical Approach in Chapter 12 Radiation Heat Transfer, as the experimental data basis to do the calculation? I guess perhaps the intrinsic complicacy of the problem make it only amanable by direct experimental measurement, rather than calculation/simulation?
The Hottel graph as shown in Lienhard book is attached for reference:
Jerry, what is the application you are trying to study? Is this phenomena an important part of the system? The reason I ask is because in your previous post we discussed the radiation transparency of all fluids (https://forum.solidworks.com/thread/69503?tstart=0). If what you describe is important how are you planning on getting around the limitation? Unless you have a strategy there, your question seems moot.