| The heat gathered on the per unit area of device is constantly accumulated,which due to the rapidly improvement of the integration of electronic components and reduction of volume of equipment.To achieve efficient methods of heat dissipation in narrow dimension has become a problem to be settled urgently.The arrangement of micro pin-fin arrays settled in microchannel has strengths of small volume,high aspect ratio and outstanding heat transfer feature.This study utilized phase transition of working fluid in micro pin-fin arrays,where massive heat of microchannel could be taken away and the process of heat transfer enhanced obviously during flow boiling,aiming to obtain splendid heat transfer performance.Six shapes of micro pin-fin arrays have been designed and manufactured in this research,which are circle,ellipse,diamond,square,droplet and non-closed droplet.An experimental system with visualization setup have been established to analyzed flow boiling characteristics of micro pin-fin array.Deionized water was taken as working fluid to investigate the influence of the shape of micro pin-fins,heat fluxes,flow rate and inlet temperature of working fluid on flow and heat transfer performance.In addition,critical heat flux(CHF)characteristics of non-closed droplet micro arrays under different working conditions were analyzed.The transformation of flow pattern was captured by high-speed camera visually,and the relationship between the flow and heat transfer characteristics of the working fluid in different boiling processes was discussed.At the same time,the growth behavior of bubbles close to the micro pin-fins was studied.The flow and heat transfer of working fluid experienced four flow regimes successively with the increase of heat flux.When the heat flux is low,no bubble exist and working fluid is in single-phase convection state;With the gradual accumulation of heat,bubbles begin to appear on both sides of the microchannel,and the working fluid demonstrates subcooled boiling state;As the heat flux continues to increase,bubbles generate intensively between the micro pin-fins,and working fluid turns into nuclear boiling state;Finally,with the gradual thinning of the liquid film of annular flow in nucleate boiling,the working fluid is evaporated to dry up when the heat flux reaches critical value and enters film boiling state.The pressure drop between the inlet and outlet of the micro arrays fluctuates gently at the beginning and then risesviolently with the growth of heat flux.The trend of flow resistance is similar to pressure drop,which increases rapidly after annular flow appears during nucleate boiling state.The temperature of the bottom plate of micro pin-fin array increases greatly before reaching the saturation temperature of the working fluid,and then tends to be gentle when the wall temperature exceeds 100 ?.Besides,the heat transfer performance of boiling rises with the increase of flow rate.The heat transfer performance of flow boiling is exceptional when the inlet temperature of working fluid is high,however,the flow resistance is higher than other operation conditions.At the same time,the pressure drop and flow resistance of the non-closed micro pin-fin array which choose open cavity as incident flow direction are worse than that flow in from the opposite direction,but the heat transfer coefficient of boiling is higher.The CHF of the working fluid in the micro arrays is related to the flow rate and the inlet temperature of working fluid.The annular flow is evaporated and the bottom of microchannel is dried up when the heat flux reaches the critical value,in which case the friction resistance and capacity of heat transfer decreases and the wall temperature shows an upward trend.The heat generated on the bottom of microchannel can not dissipate in time when the flow rate is small,which results in the lower value of CHF.In addition,the decrease of inlet temperature of working fluid can distinctly reduce the flow resistance.The increase of flow rate and the reasonable setting of inlet temperature of working fluid can delay the occurrence of CHF and increase the value of CHF.The working fluid presents three states successively in the micro pin-fin arrays with the increase of Reynolds number under high temperature of sidewall.When the temperature of bottom plate is high and the flow rate is small,the working fluid will be evaporated immediately after entering the channel,which is film boiling state.With the ascent of Reynolds number,bubbles appear among micro pin-fin arrays intensively and the working fluid turns into nucleate boiling state,which includes annular flow,slug flow and bubbles.Finally,the number of bubbles gradually decreases as the Re continues to increase,and the working fluid presents single-phase convection state.The temperature of bottom of micro arrays decreases gradually,and the convective heat transfer performance soars with the increase of Reynolds number.By comparing the flow and heat transfer performance of six kinds of micro pin-fin arrays,it can be found that the construction of the cavity at front and the streamline ofthe tail in non-closed micro pin-fin array can effectually decline the flow resistance.However,the heat transfer performance of non-closed droplet is inferior due to the large feature size of the non-closed pin-fin and the increase of the effective heat transfer area.The growth and pattern of the bubbles in the vicinity of the non-closed micro pin-fin array are diverse.The waiting time and growth time of the bubbles in the region II at the back of the pin-fin and the region III which between the adjacent pin-fins fall with the increase of the heat flux,while the departure frequency of the bubbles increases.The growth time of the bubbles and the departure diameter of the bubbles in No.1 area of the tail of micro pin-fins can be reduced obviously as the flow rate of working fluid increasing.The waiting time and growth time of the bubbles increase with the growth of Reynolds number while the departure frequency reduces gradually.The departure diameter of the bubbles in the three regions of the micro pin-fin is affected by the flow pattern,showing a trend of climbing first and then dropping. |
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