Experimental Study On Subcooled Flow Boiling Heat Transfer In A Narrow Rectangular Micro-channel

Microchannel flow boiling technology has very important applications in heat dissipation of high power density micro-electronic devices.In this study,experiments of subcooled flow boiling in a narrow rectangular microchannel 5.01 mm wide and 0.52 mm high with 30 mm heated length was conducted using deionized water as the working fluid.Flow visualization by high-speed camera was carried out to reveal the heat transfer mechanisms.The heat flux was 0-30 W/cm~2,mass flux was 200-500 kg/m~2s,and inlet subcooling was 5-20?.Four different flow orientations were set:0°(bottom-heated horizontal flow),90°(vertical upflow),180°(top-heated horizontal flow),and 270°(vertical downflow).Superhydrophilic nano-silica surface was further prepared on hydrophilic silicon surface by the plasma enhanced chemical vapor deposition(PECVD)method to investigate its enhancement effect on heat transfer.The main results of this research are as follows:(1)Significant onset of nucleate boiling(ONB)and critical heat flux(CHF)are observed in the boiling curve,without obvious nucleate boiling hysteresis at ONB.Due to the low temperature gradient near the two sides of the channel,bubble nucleation is more prominent here.The flow pattern is heat flux and time dependent.At low heat flux,it is mainly isolated bubbly flow,while at high heat flux,it is elongated bubbly flow as well as partial dry-out and rewetting.(2)The local heat transfer coefficient decreases first and then increases in the axial direction.This is because the thermal boundary layer is thinner at the inlet,and the fully developed bubble flow at the outlet strengthens the fluid disturbance,thus enhancing the heat transfer.The increase of heat flux produces more nucleated bubbles,thus promoting nuclear boiling.The increase of mass flux and inlet subcooling both increase ONB and CHF.They have similar effects on heat transfer characteristics but different influence mechanisms,and the effect of inlet subcooling is greater.(3)24 existing subcooled boiling heat transfer correlations are evaluated using experimental data.Kandlikar,Shah,Chen,and Liu and Winterton correlations are found to have best prediction performance,but they all ignore the effect of subcooling in low subcooling region.Considering the subcooling effect,the Shah correlation is improved to obtain a new correlation with higher prediction accuracy,the mean absolute deviation of which is 7.87%.(4)Early occurrence of CHF is observed for vertical downflow,compared to other flow orientations.This is because the buoyancy and flow inertia force are in the opposite direction,preventing bubbles from detaching and moving,making them easier to merge and elongate,and prolonging partial dry-out,which leads to heat transfer deterioration.At low mass flux,the total pressure drop of vertical downflow is the highest due to buoyancy hindering fluid flow;while at high mass flux,the total pressure drop of bottom-heated horizontal flow is the highest due to the large frictional pressure drop.The pressure drop of vertical downflow has the largest fluctuation,indicating the most severe flow instability.With the increase of mass flux,the effect of buoyancy is gradually weakened while the inertia force dominates,thus the effect of gravity orientation gradually decreases.(5)The super-hydrophilic surface can effectively alleviate partial dry-out,thereby increasing CHF.For vertical flow,the influence of gravity on the main flow(manifested as the interaction between buoyancy and inertia)dominates the influence mechanism on heat transfer performance,while the effect of surface wettability is suppressed and obscured,so the heat transfer and pressure drop characteristics of different surfaces are basically the same.For horizontal flow,the effect of surface wettability is more significant.Compared to the hydrophilic surface,the super-hydrophilic surface increases heat transfer coefficient by about 10%and increases pressure drop by about 37%for bottom-heated flow,while it decreases heat transfer coefficient by about 16%and increases pressure drop by about 15%for top-heated flow.