I simulated a flow in a vehicle passenger compartment . My final target is to obtain temperature field. Thus I wish to validate my code in solving temperature field. What is the most proper test problem? Could you give me answer? Thank you

Just find a thermometer and climb into a car , then turn on the air conditioner and start measuring the temperature at several different places. I think this real world approach is easier and better because it is not easy to get everything modelled in the simulation. Have a friend to measure the temperature when you are driving. ( you don't have to follow my suggestion, instead of turning on the air conditioner, you can turn on the heater instead.)