Investigation of surface roughness effect on flow boiling characteristics in microchannels

In this PhD thesis, the effects of surface roughness in particular, hydraulic diameter, heat flux and mass flux on flow boiling heat transfer and pressure drop in multiple microchannels, were experimentally investigated and flow patterns with flow visualisation were revealed. The channel widths are 300 µm, 500 µm and 700 µm, while the channel height is 450 µm. Four different surface roughness values were used in the experiments. The surface roughness is characterized by the average surface roughness (Ra) while the studied surface roughness values are between 1-3.0 µm. In experiments, deionized water was used as working fluid. While the heat flux applied from the bottom of the microchannel heat sink is in the range 40-160 kW/m2, the mass flux range is 100-400 kg/m2s. According to the experimental results, it was seen that the increase in roughness at low heat flux positively affects the heat transfer coefficient. It is observed that the increase in roughness increases flow boiling heat transfer up to 42% by increasing the nucleation sites as the nucleate boiling regime is observed in the channel at low heat flux. The presence of elongated bubbly flow and annular flow in the channel at high heat flux suppresses the roughness effect, thus the effect of the roughness on the heat transfer becomes insignificant. For high mass flux values, it was observed that the pressure drop at low heat flux increased with surface roughness. Based on the database obtained as a result of the experiments conducted in that study, a new correlation is proposed, including surface roughness and surface enhancement factor for flow boiling heat transfer coefficient in microchannels. 87.5%, 97.7% and 100% of the data estimated by the proposed correlation with the MAE value of 10.9% are within the error bands of ± 20%, ± 30% and ± 40% respectively.