噴涂機(jī)器人設(shè)計—機(jī)身系統(tǒng)設(shè)計,噴涂機(jī)器人設(shè)計—機(jī)身系統(tǒng)設(shè)計,噴涂,機(jī)器人,設(shè)計,機(jī)身,系統(tǒng) Simulation of the Effect of Process Parameters on Particle Velocity in Cold Spray Using Laval Nozzle with Nine Holes Chuanshao Liua, Yaohui Jinb and Jianxin Zhengc Henan Polytechnic University, Jiaozuo 454003,P.R.China , , Keywords: Cold spray; Simulation; Laval nozzle with nine holes; Particle velocity. Abstract. Simulations of the supersonic flow field inside and outside of the Laval nozzle with single hole and nine holes were carried out based on the computational fluid dynamics method. The effects of different standoff distance and particle diameter on impact velocity of Cu particle spraying from single hole and nine holes were investigated firstly. The results show that the particle velocity obtained with the nine holes nozzle is higher than that of the single hole nozzle under the same standoff distance, and the smaller the standoff distance, the higher the particle velocity may be obtained with the nine holes, and the higher particle velocity may be obtained with smaller particle diameter for particles with diameters of 1 15 m. Furthermore the effects of different spraying pressure and temperature on particle velocity of Cu particle spraying from the nine holes nozzle were also studied. And the simulations indicate that the higher the spraying pressure and temperature may make the particle spraying with greater velocity. Introduction The standoff distance (SoD) between the nozzle and the substrate is one of the important parameters in the cold spray process, and which influences the particle impact velocity directly. Many scholars have focused on this problem. Pattison 1 found that a bow shock was formed at the impingement zone between the supersonic jet and the substrate when the SoD was small, and the bow shock was detrimental to the process performance as it reduced the particle impact velocity. His study also showed that the deposition efficiency was closely related to the SoD, and the bow shock reduced deposition efficiencies by as much as 40% under the SoD is less than 60mm when using a custom-made helium nozzle, operating at 2.0 MPa and 20oC. Alkhimov 2 found that the thickness of the compressed layer which formed between the bow shock and substrate depended on SoD when spraying air and helium, and the smaller the SoD, the thicker the compressed layer. His research also showed that aluminum particles less than 5m in diameter could be decelerated obviously in the compressed layer. Gilmore 3 and Dykhuizen 4 also found that the particles less than 5m in diameter could be decelerated and even deflected away from the substrate by the bow shock. The Laval nozzle with nine holes was used in this study in order to reduce the adverse effects of bow shock on particle impact velocity so as to obtain the better spraying effects under the same conditions of the nozzle exit area compared with the Laval nozzle with single hole. Theoretical Models Mathematical model. Compressible flow is a very complex and comprehensive phenomenon, and the actual flow of the nozzle is non-constant isentropic flow in the actual conditions. The flow inside of the nozzle is considered as a steady isentropic flow in theory so as to simplify the simulation. The governing equations used to describe the process are as follows 5. Continuity equation: 0)( =+ ut ; Momentum equation: +=+ puuut )()( ; Advanced Materials Research Vols. 314-316 (2011) pp 78-81 Online available since 2011/Aug/16 at (2011) Trans Tech Publications, Switzerland doi:10.4028/ All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, . (ID: 222.88.196.201-26/08/11,01:28:10) Energy equation: )()( Tkupete D +=+ ; Equation of state: p=RT Where u, p, and T represent the flow velocity, pressure and temperature; , , and k represent the flow density, viscous stress tensor and thermal conductivity, respectively; e, D are the stagnation internal energy per unit volume and viscous dissipation, respectively. Particles can be considered as discrete phase in the continual gas flow, and the acceleration of a spherical particle by the gas flow can be expressed by the following equation when the interaction between the particles and gravity is ignored 6: pp pp Dp uuuu d C dt du = )( 4 3 Where up, dp, p represent particle velocity, diameter and density; CD is drag coefficient and expressed for a smooth spherical particle by eeD RaRaaC / 321 += and a1, a2, and a3 are constants, Re is the Reynolds number and defined by: /uudR ppe = and is the fluid dynamic viscosity and this equation can be practically applied to a Re 50000. Geometrical model. The jet flow region in cold spray process is made up of the internal flow of the nozzle and free jet flow region. Due to the use of nine holes nozzle, a three-dimensional model is built up in this study. Fig.1 shows the exit section diagram of the nine holes nozzle, and there are eight small holes with diameter of 1.67 mm uniformly distributed and a center hole with diameter of 2.6mm in the circular face with diameter of 6.4mm. The distance L (as shown in Fig.1) between the small hole and center hole is 2.25mm, the total area of the nine holes equals the exit area of the single hole with diameter of 5.4mm. Other dimensions of the Laval nozzle with nine holes are identical to the single hole nozzle, Fig.2 shows the section diagram of the Laval nozzle with single hole and computational domain, and its main dimensions are shown in table 1. Boundary conditions and solving method Gas inlet Outlet SoDAxis Wall Exit Divergent sectionConvergent section Substrate Fig.2 Section diagram of Laval nozzle with single hole and computational domain Inlet diameter(mm) 8 Convergent length(mm) 23 Divergent length(mm) 40 Throat diameter(mm) 2.7 Exit diameter(mm) 5.4 SoD(mm) 20 L Fig.1 Exit section diagram of the Laval nozzle with nine holes Table 1 Main dimensions of the Laval nozzle with single hole Advanced Materials Research Vols. 314-316 79 Gas inlet as shown in Fig.2 is selected for the pressure inlet boundary condition, and outlet is selected for the pressure outlet boundary condition which is atmosphere pressure and room temperature. And the air is selected as the accelerating gas. The standard k- turbulence model is utilized to disperse turbulence flow of gas, and standard wall functions are used to deal with the near wall region. Second-order upwind discretization scheme is used for governing equations. The computation of discrete phase follows the continuous phase flow field. Results and discussion Effect of SoD on particle velocity. The impact velocities of Cu particles with diameter of 2m sprayed by the Laval nozzle with the single hole and the nine holes are shown in Fig.3 with different SoD when spraying pressure P is 2.5MPa and spraying temperature T is 700K. It is seen clearly that the particle velocities obtained by the Laval nozzle with nine holes is higher than that of by the Laval nozzle with the single hole in the same simulation conditions. And the smaller the SoD, the higher the particle velocity may be obtained by the Laval nozzle with nine holes. It is also seen that the optimum SoD is 40mm when the Laval nozzle with single hole operates at P=2.5MPa and T=700K. 400 500 600 700 800 10 20 30 40 50 Pa rti cle ve loc ity (m /s) SoD (mm) Single hole Nine holes 300 400 500 600 700 800 0 5 10 15 Pa rti cle ve loc ity (m /s) Particle diameter (m) Single hole Nine holes Fig.3 Effect of SoD on particle velocity Fig.4 Effect of particle diameter on particle velocity When using the Laval nozzle with single hole, a series of compress waves are produced by the supersonic gas flow due to sharp compression before the substrate. And the shock wave will occur when the compression waves stacking with each other. Supersonic gas flow becomes subsonic gas flow when it goes through the shock wave, and the pressure, density, temperature of the gas flow rises sharply, while Mach number drops rapidly. The particle velocity decreases continuously owing to shock waves effect. The intensity of shock wave before the substrate also increases consequently with the decreasing of SoD, so does the influence on particle velocity. The gas flow tends towards stability, and so is the particle velocity when using the Laval nozzle with nine holes. Effect of particle diameter on particle velocity. Fig. 4 shows the effect of particle diameter on particle velocity when the Cu particles pass through the Laval gun to the substrate at the simulation conditions of SoD=40mm, P=2.5MPa and T=700K. It is seen clearly that the higher particle velocity may be obtained with the smaller particle using different Laval nozzle, while the particle velocity obtained by the Laval nozzle with nine holes is higher than that of the Laval nozzle with single hole at the same conditions. The particle velocity decreases rapidly by the effect of shock wave before the substrate because the small particle has much low mass, low inertia and influenced by the gas easily. While using the Laval nozzle with nine holes, particle velocity has a little change as the intensity of shock wave is diminished. The greater particle has higher weight, higher inertia and can not be accelerated easily by the gas, so the variation of the particle velocity is not apparent by the effect of shock wave before the substrate. Therefore the Laval nozzle with nine holes is appropriate for the small particles. Effect of pressure on particle velocity. Fig. 5 shows the effect of pressure on particle velocity when the Cu particles pass through the Laval gun with nine hole to the substrate at the simulation conditions of SoD=40mm, T=700K and the diameter of the Cu particle is 2m. The distance x as shown in Fig.5 80 Advanced Manufacturing Technology is from nozzle inlet to the substrate. It is seen clearly that the particle velocity has a little change and tends towards stability between the exit of nozzle and the substrate. With the increment of spray pressure, the particle velocity also has little change inside of the nozzle and small increase outside of the nozzle. Therefore there is little effect of the pressure on particle velocity. 0 100 200 300 400 500 600 700 800 900 0 0.02 0.04 0.06 0.08 0.1 0.12 Pa rti cle ve loc ity (m /s) x (m) 2.5 MPa 2.0 MPa 1.5 MPa 0 100 200 300 400 500 600 700 800 900 0 0.02 0.04 0.06 0.08 0.1 0.12 Pa rti cle ve loc ity (m /s) x (m) 700K 500K 300K Fig.5 Effect of pressure on particle velocity Fig.6 Effect of temperature on particle velocity Effect of temperature on particle velocity. Fig. 6 shows the effect of temperature on particle velocity when the Cu particles pass through the Laval gun with nine hole to the substrate at the simulation conditions of SoD=40mm, P=2.5 MPa and the diameter of the Cu particle is 2m. It is seen clearly that the effect of the spray temperature on particle velocity is large, and the higher the temperature, the higher the particle velocity. Furthermore, the massive plastic deformation occurs easier in both the incident particles and the substrate with the higher temperature. Conclusions (1) The particle velocity obtained by the Laval nozzle with nine holes is higher than that with the single hole at the same standoff distance, and the smaller the standoff distance, the higher the particle velocity may be obtained by the Laval nozzle with nine holes. (2) The higher particle velocity may be obtained with smaller particles using the Laval nozzle with nine holes at the same conditions. (3) The higher the spraying pressure and temperature may make the particle spraying with greater velocity using the Laval nozzle with nine holes. References 1 J. Pattison, S. Celotto, A. Khan. Surface & Coating Technology, Vol. 202 (2008), p. 1443-1454. 2 A. P. Alkhimov, S. V. Klinkov, et al. Journal of Applied Mechanics Technical Physics, Vol. 38, No. 2 (1997), p. 324-330. 3 D. L. Gilmore, R. C. Dykhuizen, et al. Journal of Thermal Spray Technology, Vol. 8, No. 4 (1999), p. 576-582. 4 R. C. Dykhuizen, M. F. Smith. Journal of Thermal Spray Technology, Vol. 7, No. 2 (1998), p. 205-212. 5 Hidemasa Takana, Kazuhiro Ogawa, Tetsuo Shoji. Powder Technology, Vol. 185 (2008), p. 116-123. 6 Wen-Ya Li, Hanlin Liao, G. Douchy. Materials and Design, Vol. 28 (2007), p. 2129-2137. Advanced Materials Research Vols. 314-316 81 Advanced Manufacturing Technology doi:10.4028/ Simulation of the Effect of Process Parameters on Particle Velocity in Cold Spray Using Laval Nozzle with Nine Holes doi:10.4028/