Investigation Of Enhanced Flow Boiling Heat Transfer In Minichannels By Direct Metal Laser Sintering With Nanofluid

With the development of technology,traditional single minichannel have barely to meet the actual needs of heat transfer.For further improve the heat transfer performance of minichannel heat exchanger,this study Direct Metal Laser Sintering technology for Metal 3D printing is used to fabricate minichannels heat exchangers and the micro-cavity enhanced heat transfer technology,nanofluid enhanced heat transfer technology and sound field enhanced heat transfer technology in minichannel are applied and the flow boiling characteristics of minichanne are researched.It provides a theoretical basis for the high heat flux transport of high-tech equipment such as aerospace,so the research of this project has important scientific significance and broad application prospects.Around this objective,the main research work finished are as follows:?1?A flow boiling experimental platform is built,and the minichannel heat exchangers are manufactured by Direct Metal Laser Sintering technology of metal 3D printing technology.R141b is used as working fluid.The boiling nucleation mechanism of DMLS minichannel is analyzed.Meanwhile,the heat transfer performances of ordinary wire-cut minichannel and minichannel fabricated by DMLS are compared and studied,it is found that the heat transfer performance improve by 32%.The boiling nucleation mechanism of heat exchange plane microstructure was analyzed.Based on Hsu's activation principle and activation conditions,“V”type,inverted“?”type and“cylinder”type microcosmic cavity structures are artificially applied to the bottom of minichannel heat exchanger.The results show that a series of cylindrical bubbles appeared in the artificial micro-cavity by high-speed camera,which increased the activation core density per unit area.This is also the most important reason for micro-cavity to enhance heat transfer.The main reason is that the heat transfer coefficient of mini-channel heat exchanger with micro-concave arrangement is 2.62 higher than that of conventional DMLS,but the corresponding pressure drop also increases.The design parameters of cavity are studied on the heat transfer performance,and it is found that the number of micro-concave cavity,the diameter of the entrance of the concave cavity and the shape characteristics of the micro-concave cavity have significant effects on the heat transfer performance.The heat transfer coefficients of inverted"?"and"V"activated micro-concave mini-channel heat exchangers are 46.4%and 46.2%higher than those of"cylindrical"micro-concave mini-channel heat exchangers.?2?Graphene/R141b with different mass concentration are prepared by two-step method.The factors affecting the stability of graphene/R141b are studied.The results show that the ultrasonic oscillation time,dispersant and particle mass concentration have significant effects on the stability of graphene/R141b.The flow boiling of DMLS mini-channel enhanced by graphene/R141b is studied.The boiling heat transfer coefficients of 0.01%⁰.1%graphene/R141b are larger than pure refrigerant R141b.The increase ratio of heat transfer coefficients shows a non-linear trend with the increase of concentration.The optimal concentration is 0.03%.A maximum increase of 35.4%for the average boiling heat transfer coefficients of graphene/R141b fluids are higher than pure refrigerant R141b.The weights of particle mass concentration in heat transfer and pressure drop are 0.509 and 0.491,respectively,by the entropy method.Based on the comprehensive evaluation,the optimum concentration of nanoparticles is 0.015wt%,which means that the heat transfer coefficient is the best and the total pressure drop is the smallest.The heat transfer performance of flow boiling in DMLS microchannels enhanced by conventional nanofluids Al2O3/R141b and graphene/R141b is also studied.At the same conditions,the heat transfer coefficient of graphene/R141b is 39.8%higher than that of Al₂O₃/R141b,respectively.?3?Based on the superposition principle of heat transfer,the minichannel boiling heat transfer of nanofluids is divided into particle convection heat transfer and flow boiling heat transfer.Combined with existing empirical formulas,nanofluid flow boiling heat transfer coefficients theory model in mini-channel is put forward,the study found that boiling heat transfer coefficient of nanofluids between the experimental and theoretical model prediction of the average deviation is from 5.83%to 17.0%,which explain the theory prediction model can be a very good prediction in minichannel flow boiling heat transfer performance.?4?Study on enhancement of boiling heat transfer in mini-channel by two different ways acoustic field.are designed:applying sound field outside the channel entrance and locally in the boiling region.It is found that the ultrasonic vibrating rod is placed at the entrance of the flow field in the mini-channel.The force of the sound field in the boiling zone is not obvious,and the heat transfer coefficient is increased by 6.89%and 6.39%respectively.Through high-speed photography,the results show that the main function is the cavitation of sound field,which makes the heat exchange medium produce micro bubbles before the inlet.The average heat transfer coefficient increases by 25.5%when sound field is applied locally in the boiling zone,and the heat transfer enhancement effect is obvious in the low heat flux region,while the enhancement effect is weakened in the medium and high heat flux regions.Effect of sound field on the heat transfer performance of graphene/R141b under is discussed.The average heat transfer coefficient is increased by 32.0%respectively.The effect of sound field could activate the nanoparticles in graphene/R141b nanofluids,effectively alleviate the deposition of graphene/R141b nanofluids on the channel surface during the boiling process,and delay the arrival of"inflection point"which deteriorated the heat transfer performance.