I have a system with two gasses separated by a wall as in a heat exchanger. One of the gasses changes temperature by the action of an outside agent and I'm trying to estimate the heat flow between the two gasses over time. I know that thermal diffusivity is a factor in this, but I don't know how to apply the numbers given for it to my situation. The units for thermal diffusivity are something like mm2/sec. I can assume that the temperature over the surface of the wall is uniform, so I don't have to worry about diffusion in those dimensions, but I do need to account for diffusion through the thickness of wall. I would expect to need a factor in mm/sec, but I have mm2/sec. Also, I was thinking that an electrical analog might make this easier to analyze as I have access to programs for simulating electrical circuits, but I can't find a definite analog for thermal diffusivity.