Numerical And Experimental Study Of Bubble Growth And Flow Boiling Heat Transfer In Microchannels

To achieve the goals of carbon peaking and carbon neutrality,and realize sustainable economic and social development,developing energy-efficient utilization and transformation technologies has become a necessary choice.During the utilization and transformation of energy,how to effectively strengthen the heat exchange process is of vital significance for improving the efficiency of the entire system and reducing equipment investment and operating costs.Due to their compact structure and great heat transfer performance,microchannel two-phase heat sinks have developed rapidly in the past three decades,especially in recent years,various enhanced microchannels have upgraded the traditional microchannels.However,there are still some problems that need to be studied in depth.Due to the small size of the microchannels,the spatial and temporal precision of the experimental measurement is high,and the numerical simulation method has been regarded as a powerful tool for a comprehensive understanding of the flow and heat transfer mechanism during flow boiling in microchannels.In this study,the research is mainly carried out by numerical simulations,and verified and supplemented by visual experiments.The author explores the influencing factors of the bubble growth and merger processes in a microchannel,as well as the conjugate heat transfer and bubble backflow mechanisms in the microchannel;compares and analyzes the flow boiling heat transfer and pressure drop characteristics of enhanced microchannels such as finned microchannels and manifold microchannels;explores the flow boiling heat transfer mechanism of manifold microchannels.The main contents are:(1)This paper integrates and develops a serious of numerical models based on the Computational Fluid Dynamics(CFD)method,solves five classical cases to verify the numerical models,and compares three different phase-change models.The Continuum Surface Force(CSF)model is used to calculate the surface tension component in the tangential direction,which can accurately simulate the driving effect of thermocapillary force.Among the three phase-change models,the Hardt model has the highest accuracy in calculating the heat and mass transfer process,which is suitable for the study of the bubble growth mechanism in the microchannel;the Yang model consumes the least calculation time,and can accurately calculate the flow and heat transfer performance,which is suitable for the study of the simulation of flow boiling in microchannels.(2)Numerical studies are carried out on the bubble growth and merger processes in a microchannel,and the effects of wall superheat,contact angle,inlet Reynolds number,and surface tension characteristics of self-rewetting fluid and water are analyzed and discussed.The increase of wall superheat or the decrease of contact angle can accelerate the bubble growth and improve the heat transfer performance;when the inlet Reynolds number increases,the single-phase heat transfer is enhanced,but the growth rate of the bubble is slowed down.Compared with water,the self-rewetting fluid has a significant heat transfer enhancement in the process of bubble growth and merger.Due to the Marangoni effect,thermocapillary force in self-rewetting fluid drives the liquid towards the three-phase contact line,which results in a smaller dryout area and a larger thin liquid film area near the contact line,enhancing the heat transfer process.Compared with water,the self-rewetting fluid has the best heat transfer enhancement during two bubbles’ merger along the flow direction,which exceeds 50%.(3)To further study the solid-fluid thermal coupling problem,the author conducts numerical research on the conjugate heat transfer and bubble backflow mechanisms in the microchannels,analyzes the local and overall heat transfer characteristics,and suppresses the backflow phenomenon by various methods.Increasing the solid domain thickness and thermal diffusivity causes more heat to flow toward the thin liquid film region at the bottom of the bubble,resulting in an acceleration of the bubble growth.For phase-change applications with a non-uniform heat transfer process,there is an optimum thickness for optimum heat transfer performance that decreases as the thermal diffusivity of the solid domain increases.Compared with the Traditional Microchannel(TMC),the function of inhibiting the backflow phenomenon of the Combination of Microchannel and Microgap(CMC)method starts to work after the bubble reaches the downstream gap,while the elongated bubble in the Finned Microchannel(FMC)passes through the secondary channels to adjacent channels,thus balancing the flow distribution between channels and inhibiting backflow phenomenon.(4)Numerical studies are carried out on the single-phase and flow boiling processes in Traditional Microchannel(TMC)and two kinds of enhanced microchannels: FMC and Manifold Microchannel(MMC).From the comparison of different microchannel structures,the flow path in the microchannels of U-MMC is shorter,which destroys the thermal boundary layer,strengthens the convective heat transfer,and makes it easier for bubbles to leave the microchannels.Therefore,compared with TMC,the two-phase thermal resistance of U-MMC is reduced by 33%at most,and the temperature distribution uniformity is better.From the comparison of different MMC types,the Z-,C-,and H-MMC with small outlets have much uneven flow distribution and higher thermal resistance compared to the U-,ZU-,and HU-MMC.The open outlets of U-,ZU-,and HU-MMC and the inertial force in Z-MMC discharge the bubbles in time,avoiding bubbles blocking the channel and causing the backflow phenomenon.(5)The flow boiling experiments and numerical simulations in a manifold microchannel heat sink are carried out to further verify the numerical models,and analyze the flow boiling heat transfer mechanism based on the flow patterns and heat transfer characteristics.The flow patterns in MMC are mainly bubbly flow in microchannels and elongated bubbly or annular flow in outlet manifold channels,which deduces the dominant heat transfer mechanism as the combination of nucleate boiling and convective evaporation.However,since the experimental manifold layer used is made of glass,nucleate boiling in the microchannels dominates the heat transfer process of the MMC heat sink.The two-phase heat transfer coefficient increases with the increase of heat flux but does not vary with mass flux.To obtain a new heat transfer coefficient correlation,the constant term in the existing heat transfer coefficient correlation is corrected by fitting the experimental data.The Mean Absolute Error(MAE)of the modified Chen’s and Fang’s correlations are 6.66% and 5.23%,respectively,and most of the data points are within the error range of-10% to 10%.In summary,the bubble growth mechanism in microchannels is analyzed,and the flow boiling heat transfer mechanism of enhanced microchannels is explored.The results of this study will not only be beneficial to develop the flow and heat transfer theory of flow boiling in microchannels but also will be of practical value to the design of two-phase microchannel heat sinks.