Experimental Study On Condensation Heat Transfer Characteristics In Microchannels Under Mechanical Vibration

Microchannel heat exchangers are widely used in automotive condensers due to their compact structure and high heat flux density.Affected by engine vibration and uneven road surface during vehicle operation,the on-board condenser will inevitably vibrate,which will affect the flow and heat transfer characteristics of the condenser.At present,there is a lack of research on microchannel condensation heat transfer under vibration conditions at home and abroad,resulting in the influence of mechanical vibration on small-scale condensation heat transfer and the mechanism is still unclear.To this end,this paper conducts an experimental study on the effect of mechanical vibration on condensation heat transfer and frictional pressure drop in microchannels.The specific work is as follows.A vertical vibration test bench for microchannel condensation heat transfer was designed and built.The circulating working fluid is R134a,the experimental section is a circular copper tube with an inner diameter of 1mm,a flow pattern visualization window is set at the exit of the experimental section,and a micro-field high-speed camera is used to record the flow pattern image in real time,which solves the problem of shooting flow pattern images under high-frequency vibration conditions..The sensors are calibrated before installation,and the wall temperature measurement adopts the method of built-in measuring points and the combination of circumferential and axial multiple measuring points to ensure the measurement accuracy of wall temperature.The influence of environmental heat dissipation on the experimental measurement is theoretically analyzed,and the calculation formula of environmental heat dissipation is obtained by fitting the single-phase experimental data,and the influence of heat dissipation on the experimental heat transfer coefficient is corrected.The influence of the nonlinear change of axial temperature in the experimental section on the calculation of heat transfer coefficient is theoretically analyzed,and a new calculation formula of heat flux is deduced.The correlation between the classical single-phase heat transfer and frictional pressure drop in the tube is selected to verify the accuracy and reliability of the test bench,and the overall error is within10%.Condensation heat transfer coefficient and frictional pressure drop of R134a at saturation temperature 40℃,mass flow rate 200-400kg/（m~2s）,dryness range 0-1,vibration frequency 15-50Hz,amplitude 0-2.4mm,flow pattern image,pressure difference signal in experimental section,etc.were experimentally measured,and the pressure difference fluctuation signal of different flow patterns was denoised according to the composite index,and then the wavelet packet was used to analyze the energy of the signal.The results show that under static conditions,annular flow,intermittent flow and bubbly flow are observed,and the pressure difference signals of the three flow patterns are mainly concentrated in the first sub-band.Both the characteristics and the differential pressure signal changed,the fluctuation range of the interface became larger,the energy ratio of the first sub-band of the differential pressure energy signal decreased,and the energy ratio of the second and fourth sub-bands increased.The results of heat transfer coefficient and friction pressure drop analysis show that under static conditions,heat transfer coefficient and friction pressure drop increase with the increase of mass flow rate and dryness.The application of vibration enhances heat transfer in most cases,but also weakens heat transfer in some working conditions;however,the frictional pressure drop under vibration conditions is greater than that under static conditions,which indicates that the effect of vibration on condensation heat transfer and pressure drop has a different mechanism.Significantly different,this paper makes a preliminary analysis of its complex mechanism.Finally,the test results and the calculation results of the correlation between heat transfer and pressure drop in the literature are compared and analyzed,and the applicability of the correlation under different conditions is evaluated.The research in this paper can provide theoretical support for the understanding of the law and mechanism of condensation heat transfer and pressure drop in microchannels under vibration conditions,as well as the optimal design of condensers under vibration conditions.