| In the progress of the space exploration of our nation,the development in spaceborne electronic devices poses greater urgency than the thermal systems under terrestrial conditions.Spray cooling has been considered a promising cooling technology in this area owing to its great advantages of high heat flux removal ability,tight temperature control,low temperature difference between the fluid and surface,no lag at onset of nucleate boiling(ONB),etc.Although spray cooling characteristics and mechanisms have been fundamentally investigated by researchers all over the world for past years,its application as a thermal control technology of electronic systems still needs further investigations.The present study mainly focuses on heat transfer characteristics and mechanism of spray cooling regarding the following topics:the surface temperature non-uniformity,dynamic control and effect of gravity.Firstly,considering the spatial resolved features of spray cooling,distributions of droplet diameter and spray volumetric flux were measured,increasing the measurement resolution of the spray volumetric flux.The relationship between spray characteristics and surface temperature non-uniformity was established,based upon which the trends of surface temperature non-uniformity under different experimental conditions were found.The visualization results revealed that the vapor film initializes in the region where locates the highest surface temperature and it propagates to the whole surface as heat flux increases to CHF.The surface temperature non-uniformity in two phase heat transfer regime was predicted with the mean absolute error of 15.7%.This correlation considers the effect of heat flux input and is more suitable to be generalized than other models.Secondly,in order to satisfy the requirements for a short-timescale response in the control of transient thermal load and to ensure that the spray droplet parameters remain unchanged during this process,intermittent spray cooling(ISC)was studied.The surface temperature non-uniformity in ISC was measured and it revealed the corresponding effect on the sequential function specification method(SFSM)and future times regularization.Effects of Duty Cycle and injection frequency on the heat transfer characteristics of ISC were investigated.Boiling curves shift towards the higher superheat degree of the surface as frequency decreases.However,this phenomenon is suppressed as the Duty Cycle increases.The utilization of residual liquid in the non-injection period was confirmed via observations of the dynamic behavior through visualization experiments.The mechanism and model based on the flow discretization were proposed,which established the relationship between ISC and continuous spray cooling.The Strouhal number was introduced to account for the relative effect of spray volumetric flux and characteristic time,based upon which correlations of single phase regime,two phase regime,and CHF were developed.Further,this model was successfully introduced to the prediction of CHF in cryogen spray cooling(CSC).Thirdly,experiments were performed to improve the understanding of surface orientation effects on the heat transfer performance of spray cooling.Combined with other investigations,a parametric range based on spray volumetric flux and droplet diameter has been determined,where CHF shows dependency on surface orientations.Surface temperature non-uniformity owing to surface orientations can be neglected.The ISC experiments were conducted and the results showed that surface orientations have no effect on heat transfer performance of single phase regime,two phase regime and CHF of ISC.Based on the analysis of evaporation efficiency,the interaction between the vapour escape and spray impact(or surface wall)was proposed to be the main reason causing the surface orientation effects.In addition,the two critical mechanisms in ISC,namely the vapor escape and flow discretization,were proposed and believed to interact with each other.Finally,a scaled free fall facility was built based on the time scale of droplet-wall impact and the surface fluid film flow.Under the terrestrial condition,the liquid film flow of HFE-7000 on the ultra-smooth surface(?0.5 nm)was investigated.Three phase contact line and wetted area were obtained using image post-processing.Velocity of isolated liquid film was determined.Thermocapillary force due to surface temperature non-uniformity would accelerate the isolated liquid film flow toward the edge on the surface.Under the low gravity condition,variation of surface liquid film flow cannot be found during the drop.Regarding the ultra-smooth surface,the surface tension suppresses the effect of gravity on liquid film flow. |
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