What's phi equation?

I read source_phi.f. Its purpose is to determine source terms for phi eq. So what’s phi equation?

Given that source_phi is called from solve_energy_eq, I’m guessing that phi is the specularity coefficient. Also, given the comment note in the source_phi.f file, specifically “use the average boundary cell values to compute phi (sof, Aug 23 2005)” I’m betting it’s pointing to the paper, “Evaluation of boundary conditions used to model dilute, turbulent gas/solids flows in a pipe,” by S. Benyahia, M. Syamlal and T. O’Brien in Powder Technology, 156 (2005) pp 62-72. Check out equations 35 and 36. It follows similar logic for setting a value between 0 and 1.

Thank you! I’ll read the paper later.