[Springyuu]

Hi, I‘m doing a simulation about the natural convection in a square cavity, set the thermal conductivity of air at 293 K . when at post-processing, I need the Surface Nusselt number Nu= q*L/(T_wall-T_ref)/k. I get the T_ref = 290 K by the mass-weighted average of fluid's static temperature. I wonder whether the thermal conductivity k needs to use the value at 290 K rather than at 293 K? Thanks a lot.