I am a student working on my bachelor thesis. I need to make an energy balance for my model. I have heat source in the middle of the model and also solar radiation of intensity 800 W/m^2 at angle 90 degrees to the model. The problem is with net radiation rate (thermal) and leaving radiation rate (thermal). I need to know the rules to calculate them to compare the results.. can anyone help?!

build a control volume diagram on paper, you'll see that thing are pretty straight forward

here's the hint

net = leaving - arriving

but usually your control volume is in = out

so you need to arrange the equations appropriately

there should be a post on here where i did an example of an energy balance