Research On Flow Boiling Heat Transfer Characteristics Of Wavy Micro-channel Facing Air Precooler

The compact micro-channel precooler is one of the key technologies for developing a wide-speed range combined cycle engine and the realization of a single-stage orbital power system.The heat transfer characteristics of a single micro-channel in the precooler are essential to the design of the high-efficiency precooler.In this paper,based on the basic configuration of the smooth curve micro-channel,the channel configuration is optimized,and the wavy curve micro-channel is designed.Through the flow boiling heat transfer experiment and numerical simulation,the influence mechanism of the wavy structure on the boiling heat transfer of the micro-channel is analyzed.A micro-channel boiling heat transfer experimental system is built,the experimental process and data reduction methods are clarified,and the maximum uncertainties of key parameters are calculated.The heat transfer characteristics of the single-phase flow are verified by the heat transfer correlation,which proves the reliability of the experimental system.In the working condition range of G=20.81-41.62kg/（m²·s）and q=1.56-72.76k W/m²,the flow boiling heat transfer experiment of wavy curve micro-channel is carried out.Through single-phase flow heat transfer experiment,it is found that the flow characteristics in the smooth curve micro-channel is affected by curvature,and the local heat transfer coefficient firstly increases and then slowly decreases.After the introduction of the wavy structure,the local curvature increases dramatically,and the wall temperature and local heat transfer coefficient rise in oscillation.Under the same Reynolds number,the average heat transfer characteristics of the wavy curve micro-channel are significantly improved,but the wavy structure will cause greater flow loss.Therefore,the curvature correction factor C_r,fis introduced to improve the friction factor correlation.Through the flow boiling heat transfer experiment,the local heat transfer characteristics of the micro-channel under different heat flux densities,mass flow rates,hydraulic diameters,and wavy structure amplitudes are studied.It is found that at a micro scale,the flow pattern in the channel can easily be transformed into an elongated bubble flow.The curvature of the smooth curve micro-channel is small.At low vapor quality,the secondary flow has a weaker effect on nucleate boiling.When the flow pattern is the elongated bubbles with high vapor quality,the liquid film is also weakly affected by the centrifugal force,the local heat transfer coefficient gradually decreases,and there is a tendency for heat transfer to deteriorate.The introduction of the wavy structure makes the liquid phase between bubbles receive strong centrifugal force,and when it flows through the wave peaks and troughs,a recirculation zone is generated,which improves the intensity of nucleate boiling.In addition,the wall curvature of the wavy curve micro-channel is large,and the liquid film generates a series of small vortexes,which increase the thickness of the liquid film,reduce the superheat of the liquid film,strengthen local nucleate boiling,and delay the deterioration of heat transfer.Based on the analysis of the pressure drop characteristics of the flow boiling,two main frequencies of the pressure drop oscillation of the smooth curve micro-channel are identified:high-frequency oscillation corresponds to bubble growth and separation caused by nucleate boiling,and low-frequency oscillation corresponds to the expansion and merging between elongated bubbles.After introducing the wavy structure,centrifugal force generates a recirculation zone in the liquid phase,strengthening nucleate boiling and triggering a new high-frequency pressure drop oscillation.Analysis shows that the wavy structure suppresses the low-frequency oscillation caused by the merger of elongated bubbles.The boiling process is numerically simulated through the VOF multiphase flow model and the Lee mass transfer model,and the boiling gas-liquid two-phase flow pattern under the experimental conditions is obtained,which verifies the experimental law of boiling heat transfer characteristics.It is found that the two-phase flow pattern in the smooth curve micro-channel is weakly affected by the wall with small curvature.In the wavy curve micro-channel,the large curvature at the peaks and troughs causes a recirculation zone when the liquid phase between the bubbles passes by.The downstream liquid film of the channel moves with the gas at high speed,a secondary flow vortex appears in the liquid film.The generation of the recirculation zone produces compression and crushing effects on the elongated bubbles and prevents the merging of the elongated bubbles.The vortex in the liquid film supplements the thickness of the liquid film and delays the deterioration of heat transfer.At the same time,more isolated bubbles can be observed in the liquid phase between bubbles of the wavy curve micro-channel.This phenomenon effectively supports the analysis of the wavy structure to enhance the nucleate boiling heat transfer of the micro-channel in the experimental study.