This ratio can be increased about 3.05%, 5.27% and 11.11% for roughness of 1.189 lm, 1.378 lm and 3.312 lm,respectively. Moreover, it can be clearly seen that this ratio decreases with increasing Reynolds number. It indicates that, under the testing conditions, the enhancement in heat transfer for a rough surface with respect to a smooth surface is greater than the pressure drop penalty; especially, at high roughness and low Reynolds number.