| Micro/minichannels has relatively larger ratio of surface area to volume than that of conventional channel,which makes the microscale technology become one of the most potential cooling methods to cool the electronic chip of MEMS.However,the mirco-channel is easy to be blocked by the vapor core under the state of boiling flow because of its small cross-sectional area,leading to the instability in terms of high pressure drop fluctuation of the system composed of many mirco-channels.In this paper,experiments tailored to the pressure drop characteristics between import and export of the test section and the pressure drop characteristics inside some single-channels are carried out based on two kinds of experimental section with different the hydraulic diameter and channel numbers by using high purity deionized water as the working fluid.The experimental data under various work conditions inside 14 parallel rectangular microchannels having a hydraulic diameter of 187.5 μm are obtained,and the effects of the mass flux,the effective heat flux,the ratio of heat flux to mass flux(qeff /G)and the outlet thermodynamic equilibrium quality(xout)on the time-mean total pressure drop between mainfolds of the test section are analyzed respectively.It is found that the effect of mass flux is more important on the pressure drop variation of single liquid phase flow than that of dynamic viscosity for the case of low heat flux.For high mass flux condition,the effect of mass flux on pressure drop is still dominant.With respect to the low outlet quality condition,the increment of the pressure drop caused by the increase of the outlet quality is insufficient to compensate for the decrease of the pressure drop caused by the decrease of the mass flux.However,In contrast to low outlet quality case,the effect of the outlet quality on the pressure drop increases gradually,which is slightly stronger than the influence of mass flux on the pressure drop.Additionally,it is found that the effect of the outlet quality on the pressure drop is more significant than that of mass flux after the static flow instability occor(i.e.high qeff /G ratio).A new correlation for prediction of pressure drop in micro-channels,originating from Friedel correlation,is developed by considering the ratio of liquid phase to vapor phase fanning friction coefficient(fl /fv),which is impacted by mass flux,outlet quality,dynamic viscosity,etc.,and Weber number(We).The comparison of the results predicted by the new correlation and current experimental data and experimental data obtained from literature shows that the correlation has applicability and can well predict the pressure drop in micro-channels.Furthermore,the total pressure drop between mainfolds of the test section with 6 parallel rectangular microchannels having a hydraulic diameter of 526.2 μm,is measured.Whilst,the pressure drops inside the two individual single-channels are also measured to compare the fluctuation characteristics of the pressure drop inside the two individual single-channels and that of the whole test section.It is found that the oscillation period and the oscillation trend inside the two individual single-channels are basically the same,which are basically consistent with the total pressure drop oscillation trend between mainfolds of the test section.The frequency corresponding to the maximum amplitude of the total pressure drop between mainfolds of the test section coincides with the frequency referencing to the maximum amplitude of the pressure drop inside the two individual single-channels.Therefore,based upon the maximum amplitude of the total pressure drop and the corresponding frequency,and combined with the visualization phenomenon of reversal flow,the criterion of flow instability is preliminarily developed,and the experimental conditions can roughly be divided into three types,i.e.unstable flow experimental conditions,incomplete steady flow experimental conditions and fully stable flow experimental conditions. |
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