The experimental data of the Nusselt number of cold water inside a smooth plate is compared with the previous correlation before the effect of roughness surface on heat transfer coefficient is investigated as depicted in Fig. 5. It can be seen in this figure that the tendency of predicted data and experimental results is similar. The predicted results of Khan correlation [11] and Okada correlation [12] give a mean absolute deviation (MAD) of 28.2% and 18.82% with experimental data.