Experimental Study On Flow Boiling Heat Transfer Characteristics Of Inclined Multi-microchannels Under Electric Field

Flow boiling,based on convection and interaction heat transfer of gas-liquid,has been widely encountered in a plethora of applications such as aerospace,nuclear reactors and bioengineering due to the significant advantages of strong equipment compatibility,huge thermal dissipation potential/efficiency,low cross-contamination and working medium recycling.The increasingly mature and abundant applications and growing heat dissipation problems have put forward new requirements for the design of high-performance compact cooling equipment,mainly including the advanced enhancement technology using passive or active method.Hereinto,electric field which could generate electrophoresis and electrostriction phenomenon caused by the net free charge and physical properties of fluid is one of the techniques.It is cost-saving,regulated and steady-running to deal with the problems above.Furthermore,more experiments should be carried out to explore the effect of orientation on flow boiling in microchannels.In this paper,the key issues in the current research on the mechanism of heat transfer characteristics of micro-scale flow boiling electric field and orientation are explored.The main research contents and results are as follows:（1）Multi-microchannels with hydraulic diameter of 2 mm for flow boiling are designed and fabricated.An experimental platform with the electrical and flow-rate regulation system is built.The electrode fabricated with enameled copper wires is placed above the medium and energized to generate an electric field between it and the heated surface with the applied voltage from 0 V to200 V.The thermal performance,with/without electric field,under various orientations from 0°to90°,is evaluated at an inlet mass flux of 51.58 kg/（m~2·s）and initial temperature of 299.15 K.By comparing the heat transfer evaluation index and flow pattern evolution law,the accuracy of data was verified by the repeated experiment.（2）Slow-type and alternate heat transfer enhancement phenomenon are found by adjusting the electric field or orientation separately,respectively.Results show that the enhancement of heat transfer has a threshold,and the onset of nucleation boiling is delayed by effective electric field.Electric field could bring more complex behaviors of bubbles which contributes to advance the flow pattern transition.Under available electric field,elongated bubbles burst and some vortexes appear in liquid film.Overall,the electric field could remarkably promote the HTC in lower vapor quality regions,and the maximum enhancement ratio is obtained as 1.48 under present conditions.However,the microchannels prematurely dries out due to the electric field.In this research,flow boiling HTCs are sensitive to orientation.With a constant applied voltage,the performance declines between 0°～90°while a special transition happens between 45°～60°（3）An HTC prediction model of inclined multi-microchannels under electric field is developed for the first time.Based on the Gungor-Winterton correlation,and revised with the exponential function as well as a non-dimensional number representing the electric field force,a new HTC asymptotic model is developed by first considering the effects of the electric field with an error of±15%to the experimental data.