Quote Originally Posted by Erasmus354 View Post
So we are assuming that the air reaches water temp by the time it leaves the radiator. On any curve on the chart you have a constant airflow and a variable flow rate. You should as you said reach a point where the curve plateau's and it shouldn't go back down. At this point you have reached the limit of what the air can absorb. Since the airflow is constant, air in is constant (20C for arguments sake), air out is constant (30C - water temp), you should have constant temperature dissipation. You have the same flow of air through the same area with the same change in temperature. Yet obviously your measurements show that the water temperature is not decreasing by the same amount. It is decreasing less, so how is the air increasing by the same number of watts but the water is decreasing by less and less as the flow rate increases? You are gaining energy somewhere, it just doesn't make sense.
This can be answered by that balance I posted:

h*A*(T2-T1) this is the air side heat transfer. IF, the air flow rate is the same, then h is constant, A and (T2-T1) are both constant as you mentioned.

This has to be balanced by

mdot*Cp*(T2-T1), which is the water side heat transfer.

mdot is your mass flow rate. Cp is going to be pretty constant at these small temperature changes. So, as mdot slows (T2-T1) (larger temperature difference) has to go up to balance the equation..or the other way around...as mdot gets higher your (T2-T1) (smaller temp difference) gets smaller..