| Phase-change heat transfer is an efficient heat transfer method,not only as an important way to solve the heat dissipation problem of current micro-electro-mechanical systems,but also a form of heat exchange that often occurs in many micro-electronic devices.Devices such as micro heat pipes(MHP),capillary pump circuits(CPL),loop heat pipes(LHP),micro-grooves and so on.All these devices achieve efficient heat transfer by phase change heat transfer.Phase-change heat transfer also plays an important role in thermal control,such as high-power and short-pulse laser cooling,aerospace thermal control and other fields.It is well known that with the continuous miniaturization of scales,the effects of surface and interface on liquid working fluids are enhanced.The flow and heat transfer mechanism of working fluids in channels are significantly different from conventional scales,such as surface effects,capillary effects,and laminar flow effects.The rapid thermal conduction effect has an important influence on heat and mass transfer.Compared with the conventional scale,the phase change heat transfer process in the microchannel is become much more complex.This process is an issue that unsteady,multi-scale and multi-physics,and its mechanism has not been fully revealed,further revealing the thermal mechanism of phase change heat transfer in microchannel to achieve efficient heat transfer and rapid cooling has become one of the research hotspots in the field of heat transfer.Numerous experimental studies have shown that the thickness of the thin liquid film and the micro-flow of the working fluid in the thin liquid film are directly related to the thermal process of the oscillating phase transformation,so the accurate measurement of the thickness parameter of the thin liquid film and the micro-flow of the working medium in the thin liquid film are helpful to further understand the thermal process of the working fluid phase change heat transfer in the microchannel.The theoretical data and numerical simulations are further refined by data obtained from various measurement techniques.However,due to the large scale of observation and the limited accuracy of the traditional thickness measurement method and the visual observation method,the measured data is limited,it is difficult to meet the visual research on the liquid film near the working fluid gas-liquid interface in the microchannel,especially the three-phase contact line.Based on the above reasons,this study combines visual observation with thickness measurement,and uses the Total Internal Reflection Fluorescence Microscopy(TIRM)to form a test bench for oscillating thin liquid film visualization.Based on this,The Nanoparticle Tracking Velocity Measurement Technology(MnPTV)enables simultaneous visualization and liquid film thickness measurement.The main tasks include the construction of the experimental system,the customization of the test piece,and the resulting data error analysis.In this study,the MnPTV measurement system was used to quantitatively observe the flow in the vicinity of the three-phase contact line during the oscillating flow of working fluids in different sizes of square quartz microchannels.The thickness of the liquid film,the velocity of the liquid film and the micro-flow trajectory of the liquid film are obtained by this experimental equipment.The experimental results show that the velocity of the liquid film is not uniform and symmetrical during the oscillation process.When the oscillation frequency increases,the response time of the liquid film is significantly shortened.At the same time,the capillary coefficient has a direct relationship with the thickness of the liquid film.When the oscillation frequency is low,the capillary coefficient is small,and the liquid film thickness distribution is in good agreement with the Taylor’s law.As the oscillation frequency increases,the capillary coefficient increases significantly.The empirical formula has a large error.The relationship between the oscillation frequency of the liquid film and the acceleration and deceleration of the liquid film,and the variation of the internal microfluidic flow(Marangoni flow)of the thin liquid film at different oscillation frequencies are studied,which directly predicts and improves the heat transfer of the working fluid in the microchannel. |
