The comparison between heat transfer coefficient
obtained from the rough surface and that obtained from the
smooth surface (0.936 lm) shows that the heat transfer coefficients
increase by about 4.46%, 8.18% and 17.95% for surface roughnesses
of 1.189, 1.378, and 3.312 lm, respectively. This is because
the high surface roughness leads to high turbulence between plate
surface and recirculation flow at the trough which causes the disturbance
between recirculation flow and main flow to increase.
This results in the increase of the turbulence intensity and the heat
transfer coefficient of water flowing inside the channel