The Investigation On The Characteristics Of Flow Condensation Heat Transfer Of Refrigerant Inside A Rectangular Microchannel

As the increasing power and descending size of devices in MEMS(Micro Electro-Mechanical Systems)field,in many applications,such as aerospace equipment,high-performance microelectronics,etc.,the heat dissipation of the devices is challenged.Traditional heat dissipation and mechanical control method have been unable to meet the requirements,resulting in the miniaturization of the heat dissipation equipment.The development of microchannel phase changing heat transfer technology will significantly improve the compactness of the heat exchanger,reduce the size and weight of heat exchangers,and provide extremely high heat transfer coefficient,as well as temperature uniformity.In this thesis,experimental system and physical mathematical models for flow condensation inside rectangular microchannel were established,to investigate the flow and heat transfer characteristics of flow condensation experimentally and theoretically,and reveal the heat transfer mechanism that is significantly different from the conventional scale.It is of great significance for overcoming the challenges on the heat dissipation of highly integrated devices with high heat flux,the development of new cooling technologies and the perfection of phase changing heat transfer theory.Experimental investigation on the condensation heat transfer of refrigerant R410a in a rectangular micro-channel with hydraulic diameter of 0.67mm was conducted.Annular flow,wavy-annular flow,intermittent flow and bubble flow were observed along the flow direction.The experimental results showed that the correlation developed based on conventional scale channels fails to predict the heat transfer coefficient of flow condensation in microchannels.The heat transfer coefficient increases with mass flux and vapor quality,however,it decreases with wall sub-cooled and saturation pressure.A one-dimension annular flow model was established to study the heat transfer characteristics inside micro-channel.The predicted values of heat transfer coefficient had great agreement with experimental data with average relative error 16.2%.Theoretical analysis showed that,at first,the radius of the meniscus increases parabolically along the flow direction,and the condensate film thickness increases linearly along the flow direction.Secondly,the increase of mass flux causes the increase of gas-liquid interface shear stress,which reduces the thickness of the condensate film,thereby increasing the condensation heat transfer coefficient.Finally,the reduction of the hydraulic diameter enhances the importance of surface tension,resulting in a decrease in the meniscus radius and the film thickness,which results in higher heat transfer coefficient.The profile of condensate inside microchannel during flow condensation was complicated but significant for revealing the heat transfer mechanism.A three-dimensional theoretical model for condensation annular flow was established.The importance of condensate convection in the meniscus region was validated by the great agreement between computational results and experimental data with average relative error 5.3%.The range of the thin film region gradually shrinks but the meniscus region enlarges along the vapor flow direction.At the same time,the thickness of the condensate film in the thin film region increases first and then decreases,the thickness of the condensate film increases along the flow direction.Theoretical analysis shows that,the pressure drop of condensate in thin film region results from sureface tension is the main cause of the suction effect in meniscus region.The existence of the suction effect causes the condensate in the thin film region to flow toward the meniscus region,which reduces the thickness of the condensate film in the thin film region and enhances the heat transfer coefficient.The study also found that the suction effect enhanced along the direction of vapor flow,which is beneficial to improve heat transfer efficiency of annular flow.However,the increasing mass flux weakens the suction effect.A facile chemical etching method for fabricating copper surface with hierarchical nanostructures and low surface energy coating was adopted,to increase the contact angle of low surface energy liquid R141b on the copper from 12.80to 21.6°.Experimental results have shown that the the decrease of the surface free energy on the microchannel wall surface results in the increase of the contact angle and the velocity slip of condensate liquid,which enhances the condensation heat transfer coefficient.Moreover,a designed composite channel with gradient wettability had a 16.67%enhancement in the heat transfer coefficient compared to the original one.