heat transfer project help

muhkiraz

I'm working on effect of distance on natural convection between two known sized horizontal copper plates (2mm thick) that are opened to environment. so that air can get in and out between plates. My bottom plate has a fixed temperature at 350K and top wall has a fixed temp. at 323.5K and the surrounding temp of air is 289.7K. My problem here is to calculate the heat flux and the heat transfer coefficient on the bottom plate. i've tried this but i couldn't find out what to enter for reference temp.? in the formula h=q"/(Twall - Tref ). what should i enter in the "reference value" part of the Fluent? is it the bulk temp. between plates or the surrounding temp.?