Experimental Study Of The Boiling Heat Transfer In Micro-channels With Expanding Cross Section

The boiling heat transfer in micro-scale channels has so good heat transfer characteristics that now it is more and more widely used in cooling design of electronic cooling system which is high heat flux and space-limited.The broad industrial and market application prospects make the flow boiling heat transfer characteristics in micro-scale channels become one of the hotspots of scholars at home and abroad.In this paper,the deionized water was used as experimental medium.Experiments are performed in 60 groups of micro-channels using spark discharge processing technic(inlet dH=0.7 mm,outlet dH=1.5 mm)with expanding cross section along radius direction in cylindrical aluminum transverse plane under the condition of different mass flux and heat flux to evaluate the flow boiling heat transfer characteristics with deionized water.Research in Micro-scale channels is aimed at studying the influence rule of flow and heat transfer process of boiling heat transfer characteristics.First of all,according to the cooling requirements of high-power IC devices,the structure of heat transfer channel is innovated and designed.A set of experimental devices for the experimental study of boiling heat transfer of the deionized water in micro-scale channels was designed and assembled.The experimental equipment,experimental method,experimental working fluid,extrapolation measurement method and direct measurement method were compared and selected.Experimental operation problems and solutions and experimental errors were introduced and analyzed in detail By measuring the mass flux,heating voltage,wall temperature and vapor qualities and calculating other data,the parameters such as heat flux,the boiling point,the average boiling heat transfer coefficient and boiling number was analyzed and compared under different operating conditions.Research results are as follows:(1)The vapor qualities increases with the increase of heat flux.The increase of mass flux has weakened and hindered the increase of vapor qualities.In the case of a larger mass flux,the change in the vapor qualities is relatively stable and is not prone to the phenomenon of dry-out.(2)The average boiling heat transfer coefficient obviously decreases with the increase of vapor qualities.The relationship between the average boiling heat transfer coefficient and vapor qualities was a negative exponential relationship.With the increase of heat flux,the average boiling heat transfer coefficient decreases.Relative to the heat flux,the mass flux has little effect on the average boiling heat transfer coefficient.Therefore,it can be considered that nucleate boiling plays a major role under the condition of the experimental parameters.When the mass flux increases to a certain degree,the heat transfer mechanism has a tendency to turn into the convective boiling.(3)In the figures of the relationship between the average boiling heat transfer coefficient and heat flux,the vapor qualities and boiling number are found that when the mass flux reaches 0.96 g·s-1,the change of curve is different from that of the curve with low mass flux.There is a sudden increase in the trend.The effect of mass flux on the boiling heat transfer coefficient is enhanced,and the increase of mass flux has a positive effect on the heat transfer.It is considered that the convective boiling begins to appear.Finally,the influence of boiling number,Webber number,the vapor qualities and hsp,l on the boiling heat transfer coefficient is considered synthetically.A new heat transfer correlation for the boiling heat transfer in micro-scale channels is proposed.The experimental results are compared with the predicted values of the new heat transfer correlation.The average absolute error MAE is 9.4%,and 89.6%of the experimental values is within the ± 25%range of the predicted values.To a certain extent,the new heat transfer coefficient correlation can be applied to the prediction of the boiling heat transfer coefficient in this experiment.