真空度对真空冷喷涂气固两相流的影响
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摘要
目的真空度直接影响着真空冷喷涂时气体流动特性和颗粒撞击速度,研究真空度对气体和颗粒流动特性的影响。方法确定真空冷喷涂系统结构,采用FLUENT软件进行真空冷喷涂气固两相流研究,通过数值模拟研究真空度对流场和颗粒撞击速度的影响,并研究相同压力比下的气固两相流特性。结果当入口压力一定时,喷管内的气体轴线速度、密度和温度与环境压力大小无关;而在射流区,环境压力越小,则气体轴线速度波动越小、密度越低,但到达基板后的气体温度均接近喷管入口温度。环境压力对大粒径颗粒的撞击速度影响较大,颗粒撞击速度随环境压力增大而先增后减,最佳环境压力可根据气相云图和气体密度来确定。当进出口压力比相同时,喷管内和射流区域内的气相速度云图基本相同,气体轴线速度曲线基本重合,而基板前的颗粒速度不同,此时环境压力越低,颗粒速度越高,越有利于形成涂层。结论采用计算流体动力学分析方法厘清了真空度对真空冷喷涂气固两相流的影响,为涂层制备奠定了理论基础。
Objective The flow behavior of gas and the particle impact velocity are directly affected by the vacuum degree in the process of vacuum cold spray. The influences of vacuum degree on gas and particle flow properties were studied. Methods The structure of the vacuum cold spray system was determined,and the gas-solid flow in vacuum spray was studied based on FLUENT.The influences of vacuum degree on the flow field and particle impact velocity were explored through numerical simulation,and the gas-solid flow properties under the same pressure ratio were studied. Results The numerical research indicated that when the inlet pressure was constant,the axis velocity,the density and the temperature of gas in the nozzle were independent of the ambient pressure,whereas in the jet flow region,lower ambient pressure led to lower fluctuation of the gas velocities and lower gas density.However,when the gas reached the substrate,its temperature was close to the inlet temperature in all cases. The ambient pressure had obvious effect on the impact velocity of the particle with large diameter,and the impact velocity first increased and then decreased with increasing ambient pressure,and the optimal ambient pressure could be determined by the gas contour and its density.When the pressure ratios of the inlet and outlet of the nozzle were the same,the gas contours in the nozzle and the jet flow region were almost the same,and the curves of the gas axis velocities almost coincided. While the particle velocities before the substrate were different,the lower the ambient pressure,the higher the particle velocity,which was more beneficial to coating formation.Conclusion The influence of vacuum degree on the gas-solid flow in vacuum spray was clarified based on the CFD,which laid the ground work for the preparation of coating.
Objective The flow behavior of gas and the particle impact velocity are directly affected by the vacuum degree in the process of vacuum cold spray. The influences of vacuum degree on gas and particle flow properties were studied. Methods The structure of the vacuum cold spray system was determined,and the gas-solid flow in vacuum spray was studied based on FLUENT.The influences of vacuum degree on the flow field and particle impact velocity were explored through numerical simulation,and the gas-solid flow properties under the same pressure ratio were studied. Results The numerical research indicated that when the inlet pressure was constant,the axis velocity,the density and the temperature of gas in the nozzle were independent of the ambient pressure,whereas in the jet flow region,lower ambient pressure led to lower fluctuation of the gas velocities and lower gas density.However,when the gas reached the substrate,its temperature was close to the inlet temperature in all cases. The ambient pressure had obvious effect on the impact velocity of the particle with large diameter,and the impact velocity first increased and then decreased with increasing ambient pressure,and the optimal ambient pressure could be determined by the gas contour and its density.When the pressure ratios of the inlet and outlet of the nozzle were the same,the gas contours in the nozzle and the jet flow region were almost the same,and the curves of the gas axis velocities almost coincided. While the particle velocities before the substrate were different,the lower the ambient pressure,the higher the particle velocity,which was more beneficial to coating formation.Conclusion The influence of vacuum degree on the gas-solid flow in vacuum spray was clarified based on the CFD,which laid the ground work for the preparation of coating.
引文
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[2]PATTISON J,CELOTTO S,KHAN A.Standoff Distance and Bow Shock Phenomena in the Cold Spray Process[J].Surface and Coating Technology,2008,202(8):1443—1454.
[3]钟厉,王昭银,张华东.冷喷涂沉积机理及其装备的研究进展[J].表面技术,2015,44(4):15—22.ZHONG Li,WANG Zhao-yin,ZHANG Hua-dong.Research Progress of Precipitation Mechanism and Apparatus of Cold Spray[J].Surface Technology,2015,44(4):15—22.
[4]CHUN Doo-man,CHOI Jung-oh,LEE C,et al.Effect of Standoff Distance for Cold Gas Spraying of Fine Ceramic Particles(<5μm)under Low Vacuum and Room Temperature Using Nano-particle Deposition System(NPDS)[J].Surface&Coatings Technology,2012,206(8/9):2125—2132.
[5]YANG Guan-jun,LI Chang-jiu,LIAO Kai-xing,et al.Influence of Gas Flow during Vacuum Cold Spraying of Nanoporous Ti O2Film by Using Strengthened Nanostructured Powder on Performance of Dye-sensitized Solar Cell[J].Thin Solid Films,2011,519(15):4709—4713.
[6]WANG Y Y,LIU Y,LI C J,et al.Electrical and Mechanical Properties of Nano-structured Ti N Coatings Deposited by Vacuum Cold Spray[J].Vacuum,2012,86(7):953—959.
[7]HE X L,YANG G J,LI C X,et al.Preparation and Characterization of Ti O2Scattering Layer for Plastic Dye-sensitized Solar Cells by Vacuum Cold Spray[C]//Thermal Spray2013:Proceedings of the International Thermal Spray Conference.[s.l.]:ASM International,2013:184—189.
[8]FAN Sheng-qing,LI Chang-jiu,LI Cheng-xin,et al.Preliminary Study of Performance of Dye-sensitized Solar Cell of Nano-Ti O2Coating Deposited by Vacuum Cold Spraying[J].Materials Transactions,2006,47(7):1703—1709.
[9]郑建新,金耀辉,刘传绍.真空冷喷涂铜颗粒加速特性数值研究[J].表面技术,2013,42(1):5—8.ZHENG Jian-xin,JIN Yao-hui,LIU Chuan-shao.Numerical Study on Accelerating Characteristics of Copper Particles in Vacuum Cold Spraying Process[J].Surface Technology,2013,42(1):5—8.
[10]SAHNER K,KASPAR M,MOOS R.Assessment of the Novel Aerosol Deposition Method for Room Temperature Pre-paration of Metal Oxide Gas Sensor Films[J].Sensors and Actuators B:Chemical,2009,139(2):394—399.
[11]李文亚,张冬冬,黄春杰,等.冷喷涂粒子碰撞行为和临界速度预测的数值模拟研究现状[J].中国表面工程,2014,27(1):1—11.LI Wen-ya,ZHANG Dong-dong,HUANG Chun-jie,et al.State-of-the-art of Particles Impacting Behavior and Prediction of Critical Velocity for Cold Spraying by Numerical Simulation[J].China Surface Engineering,2014,27(1):1—11.
[12]YIN Shuo,WANG Xiao-fang,LI Wen-ya.Computational Analysis of the Effect of Nozzle Cross-section Shape on Gas Flow and Particle Acceleration in Cold Spraying[J].Surface&Coatings Technology,2011,205(8/9):2970—2977.
[13]JEN Tien-chien,LI Long-jian,CUI Wen-zhi,et al.Numerical Investigations on Cold Gas Dynamic Spray Process with Nano-and Microsize Particles[J].International Journal of Heat and Mass Transfer,2005,48(21/22):4384—4396.
[14]WINNICKI M,MAACHOWSKA A,DUDZIK G,et al.Numerical and Experimental Analysis of Copper Particles Velocity in Low-pressure Cold Spraying Process[J].Surface&Coatings Technology,2015,268(25):230—240.
[15]GRUJICIC M,ZHAO C L,TONG C,et al.Analysis of the Impact Velocity of Powder Particles in the Cold-gas Dynamic-spray Process[J].Materials Science and Engineering A,2004,368(1/2):222—230.