Experimental Study On Boiling Flow And Heat Transfer Characteristics Of Different Shaped Pin Ribs And Hydrophobic Microchannels

As an efficient heat transfer structure,pin-fin microchannel has the advantages of high aspect ratio,small volume and high heat transfer efficiency.It has received wide attention for the unique applications in the field of high efficiency heat dissipation in small space.In this paper,the flow and heat transfer test bench of pin-fin microchannel was designed and built.Six kinds of pin fin shape and hydrophobic surface microchannel single-phase and boiling flow heat transfer experiments were caried out.For the circular pin fin microchannel,the friction coefficient decreases with the increase of Reynolds number,while the convective heat transfer coefficient increases significantly with the increase of Reynolds number.Heating power has little effect on friction coefficient.The higher the inlet temperature is,the greater the friction coefficient is.For the boiling flow,the pressure drop and friction coefficient in the circular pin-fin microchannel are larger at low Reynolds number.The pressure drop increases at first and then decreases with the increase of Reynolds number.With the increase of heating power and inlet water temperature,the pressure drop increases obviously at the same Reynolds number,and the friction coefficient changes little.The flow pressure drop in microchannels with different shapes of pin fins increases with the increase of Reynolds number.The triangle and square pin fins have the greatest pressure drop.When the Reynolds number is small,the pressure drop of droplet and diamond pin fins are small,and that of circular pin tins is large.When the Reynolds number is large,the pressure drop of circular pin fins is close to that of diamond pin fins,while that of ellipse pin fins is small.And the pressure drop of ellipse pin fins is small when Reynolds number is large.The temperature at the bottom of heat sink decreases with the increase of Reynolds number,while the convective heat transfer coefficient and Nusselt number increase with the increase of Reynolds number.The triangle pin fin has the maximum heat transfer coefficient and the diamond one have the minimum heat transfer coefficient.The shape of pin fin has a great influence on the pressure drop of boiling flow in microchannel.When Reynolds number is small,the microchannel is under the state of membrane boiling.With the large amount of steam in microchannel,the steam has a great influence on the flow.With the increase of Reynolds number,the pressure drops of drop shape and diamond pin fin microchannels decrease rapidly,while those of triangle and square pin fin increase rapidly at low Reynolds number,and the friction coefficients of triangle,circle and square are small.The temperature at the bottom of heat sink decreases with the increase of Reynolds number.The higher the heating power and the inlet fluid temperature are,the higher the temperature at the bottom of heat sink is.The flow resistance of the triangle and square pin fins are high.When the Reynolds number is small,the boiling is intense and the temperature at the bottom of heat sink is high.When Reynolds number is large,the temperature at the bottom of heat sink of triangle and square pin fins are the lowest followed by ellipse and circle one.In the drop shape fin,fluid has little disturbance,and stable vapor films has been formed in the microchannel.The temperature at the bottom of heat sink is high.At low Reynolds number,the heat transfer coefficients of triangle,square and circle pin-fin microchannels are small,while those of ellipse,drop shape and diamond pin-fin microchannels are in partial membrane boiling,and the heat transfer coefficients are large.With the increase of Reynolds number,the convective heat transfer coefficients of triangle and square pin fin increase significantly,while the convective heat transfer coefficients of diamond and drop shape change little.With the increase of Reynolds number,the pressure drop of microchannel flow under different contact angles increases firstly and then decreases.As the Reynolds number continues to increase,and the pressure drop increases significantly.Hydrophobicity has a significant effect on the frictional resistance of micro-channel boiling flow.The frictional resistance coefficients of hydrophobic and superhydrophobic surfaces are significantly smaller than those of ordinary surfaces.Under different hydrophobicity,the surface temperature of the microchannel floor firstly decreases rapidly and then decreases gently with the increase of Reynolds number.The boiling and flow heat transfer of hydrophobic pin fin microchannels can be divided into membrane boiling zone,membrane to nucleate boiling transition zone,nucleate boiling zone and single-phase flow heat transfer zone.The hydrophobic coating causes bubbles to accumulate on the wall and gas films are formed on the surface,which has a great influence on the convective heat transfer coefficient.At low Reynolds number,the hydrophobic coatings on diamond,drop shape and ellipse microchannel hydrophobic surfaces promote fluid flow,and the heat transfer coefficient is higher than that on ordinary surfaces.Because of the boundary layer separation at the end of the square pin fin,the flow field is disturbed greatly,the heat transfer coefficient increases with the increase of Reynolds number,and the hydrophobicity has no obvious influence on h.It is observed by High Speed Camera that when the flow rate in the microchannel is small,water splashing produces droplets on the glass cover.With the increase of flow rate,the flow development is at the state of membrane boiling,transition from membrane boiling to nuclear boiling,nuclear boiling and subcooled boiling.With the increase of heating power,the growth of bubbles in microchannels is faster and the growth period is shorter.At the same time,the radius of bubbles in microchannels increases.The heating power has no effect on the maximum radius of the bubble,whose radius of the bubble detachment remains basically unchanged.The bubble detachment radius is about 0.08mm at the end of pin fin disturbance,and 0.0565 mm at the transverse clearance of pin fin in microchannel.With the increase of Reynolds number,the radius of bubble migration in the tail region of pin fin increases slowly and then increases rapidly,and finally decreases after reaching the peak value which corresponds to the transition zone from membrane boiling to nucleate boiling,nucleate boiling zone and subcooled boiling zone in microchannel.The peak of bubble detachment radius corresponds to the end of exuberant nucleate boiling in microchannel.With the increase of heating power,the Reynolds number which corresponding to the peak value of bubble migration radius increases,and the influence of heating power on the maximum radius of bubble migration is small.Hydrophobicity has no significant effect on the radius of bubble migration in the circle pin-fin microchannel during the transition from membrane boiling to nucleation boiling in the disturbance zone at the end of pin-fin,but has an effect on the bubble migration radius in the nucleate boiling region.The increasing hydrophobicity lead to the decrease of bubble migration radius.There are plenty of bubbles in the tail region of the pin fin.With the increase of Reynolds number,the radius of bubble migration increases first and then decreases,and the hydrophobicity increases.The radius of bubble migration in the tail region of the pin fin decreases obviously.