Effects Of Fluid Properties On Liquid Film During Flow Boiling In Micro-channels-A Parametric Study

Modern micro-scale basic research promotes the development of system miniaturization and integration.Small high integration and high frequency speed performance trend are increasing in need with the rapid popularization of modern electronic technology,which lead to a dramatic increase of the electronic components calorific value of per unit area.High heat flux heat dissipation has become a bottleneck restricting the development of electronic equipment.Micro-channel phase-change thermal technology has the advantages of light and compact structure,high heat flux and high heat transfer coefficient,which develop rapidly in the thermal management of electronic devices.However,there is still no good theory to quantify the boiling heat transfer performance of micro-channel flow accurately in the field of heat transfer.The impact of dynamic characteristics of liquid film on the boiling heat transfer is not significant in macro-scales.But in micro-scales,heat transfer capability is closely related to dynamics characteristics of the micro-liquid membrane.During micro-channel flow boiling with different working fluids,the different thermal physical properties may lead to different flow and heat transfer characteristics.Therefore,the present paper carried out relevant experimental measurements and studied the near-wall micro-liquid membrane of different working fluids,aiming at mastering the transient change rules and dynamic characteristics of the micro-liquid membrane in the flow boiling with different working fluids.In the experiment,three typical working media,i.e.,deionized water,anhydrous ethanol and FC-72,were used to cover the physical properties of most fluids used in engineering applications.High-speed cameras were used to collect images of flowing boiling bubbles,and a laser confocal displacement measuring instrument was used to measure the transient changes of flowing boiling micro-liquid film.The local wall surface temperature signals were collected synchronously.The main research results of this paper are as follows:(1)It is found that the initial liquid film thickness in the flowing boiling state conforms to the Taylor model,similar to that in the adiabatic two-phase flow.The Aussillous-Quere correlation fits well the initial liquid film thickness in flow boiling of all the three fluids.(2)By use of the measurement results,the empirical correlation between vapor slug time and liquid slug time of different working fluids in the micro-channel steam flow boiling and heat flux is found;the effects of experimental conditions such as heat flux density and flow rate on the boiling liquid film thickness in micro-channel flow with different working media are studied;the average convective heat transfer coefficients of different working media in micro-channel flow boiling are calculated.(3)The model of transient thickness change of the micro-liquid film is established based on the theoretical analysis of liquid film evaporation and mass transfer process as well as the effect of vapor-liquid interface shear force.This model can reveal the influence rules of working substance properties on the transient distribution of micro-liquid film,and can quantify the liquid film thickness change caused by flow boiling in micro-channel more accurately.This method expands the applicability of the working fluids and builds up a theoretical foundation for revealing the boiling heat transfer mechanism of the phase-change flow of different working fluids.