I 've a question about sign convention in CFX. I made a simulation of steady state heat transfer of an insulated box, with internal T_in= 293 K, and outer of T= 213 K. I've to estimate the optimal thickness of insulating layer, so I created a boundary condition on the internal walls of heat transfer with heat source (energy). I've plotted the wall heat flux on a line across the layer, and noticed that heat flux varies with a peak in a range going from negative to positive values, whereas I expected that it decreases linearly, and in the best case it goes from 0 to negative flux equal to energy input source. Can you explain why this and what does it mean a negative heat flux? Thank you. Eng. Igor Di Varano

Fluxes into the computational domain are positive, out are negative. A negative wall heat flux means heat is going into the wall, out of the fluid domain.

-Robin

Thank you very much, I've found some information on help soon after my previous e-mail. Now there 's another point. I've plotted the wall heat flux through the thickness, but I have only a few points and I've been requested to enhance the number of points in the layer. I've tried to fit the mesh, but without success, as I have always the same points(7). I 've read on the handbook that wall heat flux is sensitive in proximity of wall surfaces, and in a lesser way within the wall. Is it true or with more elements I can expect the desired result? Thanks a lot. Igor