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.?

You can try using the average of bottom plate and air temperature.

dear friend thx, actually my project is to make the correction of an experimental analysis by CFD. in the experiment there are two horizontal plates insulated from their outer surface and the bottom plate is heated by resistance(so there is a heat flux through the upper plate ) and capt in a constant temp.in the experimental calculations, surrounding air temp is used in the equation q=h(Twall-Tref). so in fluent i've tried to do the same but i exactly got the half of the experimental results.(all heat flux, heat transfer coefficient and nusselt number is the half of the expected)(is it because there are two faces of the plate so i've to multiple the results by two or something else. Why results are like this you think?). but after your advice i've tried the bulk temp and get the correct nusselt and heat transfer coefficient but heat flux is the same. so to make the correction by experimental result i need a twice of this heat flux. pleeeease help why is it the half of the expected result? it is so urgent!!!

anyone can help me please response to my problem . its really urgent and necessary