The Effect Of Dynamic Thin Liouid Film On Flow Boiling Heat Transfer Mechanism In Microchannels

Miniaturization is an inevitable trend of the science and technology development.The miniaturization of mechanical devices rapidly increased heat dissipation rate per unit device area.Flow boiling heat transfer can provide very high heat transfer coefficients and are highly compact with a high surface to volume ratio,it has become increasingly important in many applications,including high performance computer,laser,micro-electronics,hybrid vehicle power electronics,satellite electronics and MEMS.Despite their practical cooling merits,there are big differences of the existing research results on flow boiling heat transfer,its physical nature is still ambiguous,there are still no reliable correlations for the heat transfer coefficient prediction.Flow boiling instability in microchannels also hinder the thermal performance of many cooling systems utilizing liquid-vapor phase change in microchannels.Although several methods have been developed to suppress flow boiling instability,the potential causes are still plagued with significant uncertainties.The dominant heat conduction through thin liquid film vaporization at the liquid-vapor interface is important in the heat transfer characteristics,flow boiling instabilities also depends on the thickness of the liquid film variation.Therefore,an experimental quantification of thin liquid film dynamics is conducted in order to provide new physical insight on the nature of heat transfer characteristics and to identify the film instability mechanism.Firstly,the liquid film thickness measurement was carried out for two phase flow under isothermal condition in single horizontal microchannel.Nitrogen,ethanol,water and FC-72 are used as working fluids.Circular tubes with different diameters,D = 0.5,0.75 mm,are used.Laser focus displacement meter is used to measure the thickness of the thin liquid film.The correlation between bubble velocity,relative slip velocity,bubble length,liquid slug length,void fraction and the liquid film thickness are also investigated.Film thicknesses predicted by previous work are compared to experimental results.Based on the hydrodynamics of Taylor flow and the theoretical relationship between liquid film thickness and the critical physical properties,we carried out the surface tension and viscosity for refrigerant FC-72 as well as the liquid mixture of water/ethanol.Flow boiling is a transient phenomena,the heat transfer coefficient is time dependent and varies with various stages of the bubble dynamic growth,as well as with the cyclical passage of elongated bubble,vapor slug and liquid slug.The study is enabled through development of a novel device which enables synchronized measurement of liquid film thickness,surface temperature,inlet and outlet pressure drop as well as flow regime observation.The liquid film thickness and surface temperature measurement can achieve high spatial resolution of 20 ?m and a temporal resolution in the kHz range.Experimental investigations on flow boiling phenomena in a horizontal microchannel with the inner diameter of 0.94 mm were carried out.The heat fluxes ranging from 20 to 120 kW/m2,and the mass fluxes ranging from 50 to 400 kg/m2s during two-phase flow boiling.An inlet fluid temperature of 90 ?and an inlet pressure of 101 kPa(abs)corresponding to an inlet subcooling of 10 K were maintained at the channel entrance.The outside surface of the microtube was coated with indium tin oxide(ITO)in order to generate a uniform wall heat flux and allow for flow visualization.The dynamic growth of the vapor bubbles was quantitatively visualized.The laser focus displacement meter was employed to measure the dynamic thickness of liquid films at various heat fluxes.The dynamic model of the liquid film thickness was developed and compared with the experiment results.Conditions at which onset of boiling occurred were identified and the subsequent bubble dynamics was recorded with a high-speed video camera,elongated bubble flow is observed for the tested conditions.The present study provides detailed insights into the transient heat transfer coefficient and pressure drop information during the cyclic passage of elongated bubble,vapor slug,partial-dry-out,and liquid slug during slug flow regime.Flow boiling instability were considered during three stages of partially confined bubble growth,fully confined bubble growth and vapor venting in a single microchannel cross-section.The mechanism regarding the probable factors responsible for pressure and temperature oscillations were clarified..Finally,an updated three zone model for predicting heat transfer coefficient was established based on the flow boiling mechanism.The prediction results demonstrate a stronger contribution to heat transfer by the liquid slug as well as thin film evaporation process at lower heat flux,while at higher heat flux the overall heat transfer coefficient decreases due to dryout of the thin liquid film.The proposed evaporation model predicts well with the the experimental data.