Experimental Study On Flow Boiling Heat Transfer Characteristics In A Micro Channel With Micro Pin Fins

With the improvement of the integration degree of microelectronic equipment,the heat flow per unit space increases rapidly,which seriously affects the normal operation of micro electronic devices.As a high efficient micro thermal dissipation structure,the channel of micro pin fins has attracted much attention in the field of micro space electron cooling because of its high aspect ratio,compact structure and high heat transfer efficiency.In recent years,many scholars at home and abroad have extensively studied the flow boiling heat transfer characteristics of micro pin fins,but the flow boiling mechanism of micro fin fins is not clear,few studies involving different shapes on flow boiling of micro pin fins.In this paper,diamond,circular and elliptical channels of micro pin fins are designed and developed by machining technology.Experiments were carried out to investigate the boiling heat transfer,pressure drop and critical heat flux characteristics for deionized water in three different shapes of micro pin fins.The mass fluxes from96 to 224 kg/(m~2·s),the inlet subcooling from 20 to 50~°C and the effective heat fluxes ranging from 10 to 240 W/cm~2 in this experiment.It provides theoretical basis for the design and optimization of the micro cooling channel,and has important practical significance.In order to reveal the mechanism of flow boiling heat transfer in micro pin fins,the flow boiling heat transfer and pressure drop characteristics in a circular,diamond and elliptical micro pin fins are investigated firstly in this paper,moreover,the instability of flow boiling in microchannel is also analyzed.The experimental results showed that the dominant heat transfer mechanism is nucleate boiling in the low heat flux region,while it is convective heat transfer of a thin liquid film evaporation in the medium and high heat flux regions.With the increase in heat flux and outlet quality,the boiling heat transfer coefficient decreases,but the two-phase pressure drop increases linearly.The presence of micro fins inhibits the reverse flow of bubbles,which reduces the flow boiling instability and delays the occurrence of critical heat flux.The flow boiling instability is most noticeable for the micro channel with the circular pin fins which reduces successively in the microchannel with diamond and elliptical pin fins.Secondly,CHF as an important parameter for the upper limit of flow boiling heat transfer in micro channel with micro pin fins,is the research emphasis of this paper.CHF is caused by dryout at the wall near the outlet of the microchannels,which in turn is attributed to the flow reversal upstream of the microchannels.The occurrence of CHF is marked by an abrupt increase in wall temperature near the outlet and an abrupt decrease in pressure drop across the microchannels.In addition,it is found that the experimental parameters such as mass flow,inlet undercooling,and the shape of micro fin also have a great influence on CHF.The experimental results show that the critical heat flux of micro channel of micro pin fins is higher than a smooth in the same channel which in turn the existence of micro fin greatly reduces the flow reversal upstream and flow boiling instability,the CHF of the elliptical micro pin fins is the largest,and the circular micro pin fins is the smallest.The CHF increases with the increase of mass flow rate and inlet subcooling,but decreases with the increase of outlet quality.The experimental data are compared with the correlation proposed by Kosar et al.The results show that the correlation is in good agreement with the experimental data.In this paper,the heat transfer,pressure drop and CHF characteristics of micro channels with three shapes of micro pin fins are emphatically investigated by means of experiments.The influence factors of experimental working fluid under different working conditions such as mass velocity,entrance subcooling,dryness of outlet,heat flux density etc.were studied.It revealed the mechanism of flow boiling heat transfer of micro channel with micro pin fins combined with the visual means,and has made a useful exploration for further research on flow boiling characteristics of microchannel wih pin fins.