grid independence questions
 

Hello, I have some questions about grid independence, could someone help me please?
My model is a 2-D chamber filled with air,and the flow in the chamber is laminar,and the temperatures of the walls are constant. Now I want to do grid independence study, should I change the grid numbers of the four walls at the same time or one by one? which parameter should i monitor as the criterion of grid independence?
By the way,if two walls' temperature of the chamber are Tc, one wall's heat flux is contant,the last wall is adiabatic, how can i calculate the Ra?
Waiting for your help,thank you very much!