I am trying to simulate the heat flow in a system and the temperatures that I get for the fluid in my system do not match what I expect them to be.

When I expect to see a temperature rise of 3 degree Celsius, I only see 0.6 degree Celsius. When I expect 120C, I see a 21C.

My system is pretty simple: There is a copper box (with adiabatic outer walls) in which there are two FR4 boards (facing each other) (the boards are 2 mm away from the inner walls of the copper box). Each board has 13 chips (chip material = Typical CeramicBGA) on it and they are defined as volume sources (12 of them = 0.5 W and 1 of them = 1W). There is an inlet lid through which water flows in with a volume rate of 0.001Kg/s and temperature 17C and an outlet lid which is set to the standard temperature (25C) and standard pressure. So, everything looks good.

When I run the simulation, I see the water temperature go from 17C (average inlet temp) to 17.48C (average outlet temperature). I should see a ~3.39C rise (17 to 20.39 approx.), but I dont see it!

Here is how I expect to see a 3.39 C rise: (theoretical calculation)

Q = m c T  ==>  Q = p V c T.

Taking the time derivative of both sides: dQ/dt = p dV/dt c dT.

So, dT = (dQ/dt) / (dV/dt) c.     (the p cancels as we are using the volume rate, the units are consistent here with Celsius).

Plugging in all the values, dQ/dt = 14W (from summing up all the powers of the chips), dV/dt = 0.001Kg/s, c = 4120 J/kg K, I get a 3.39 Celsius.  

(p = density, V = volume, m = mass, c = specific heat, T = temperature, Q = heat)

I am confident that the calculation above is correct.

I have tried everything that I could think of and find online: like changing the mesh size, turning on gravity, turbulent/laminar flows, etc. However nothing has helped in getting the simulation to match the theoretical results. Am I doing something wrong?