Zhiwei Guo

Effects of Substrate Hardness and Spray Angle on the Deposition Behavior of Cold-Sprayed Ti Particles

In this study, finite element analysis combined with experimental observation was conducted to clarify the effects of substrate hardness and spray angle on the deposition behavior of cold-sprayed Ti particles. It is found that metallurgical bonding is highly possible to occur between the Ti particle and Cu substrate due to the intensive metal jet at the rim of the interface which helps to remove the cracked oxides. Because metallurgical bonding and large interfacial contact area can guarantee high adhesion strength, the thick Ti coating is achieved after deposition on the Cu substrate. As for the soft Al substrate, the first layer Ti particles are embedded in and then trapped by the soft substrate material, which results in the occurrence of mechanical interlock at the interface. As a consequence, the final coating thickness is also relatively large. When using hard stainless steel as the substrate, the essential conditions for forming the mechanical interlock are lacked due to the high hardness of the substrate material. In addition, the metal jet at rim of the interface is less prominent and also the interfacial contact area is smaller in comparison with the Ti-Cu case. Therefore, the particle-substrate bonding strength and the consequent coating thickness are relatively low. Besides, it is also found that the particle deformation and coating quality are significantly affected by the spray angle. The deformation of the particle localizes at only one side due to the additional tangential momentum. Also, such localized deformation becomes increasingly intensive with decreasing the spray angle. Moreover, the coating thickness is found to reduce with the decrease in spay angle, but the coating porosity shows a reverse trend.

Significant influence of carrier gas temperature during the cold spray process

Abstract Carrier gas is known as the medium to inject the cold sprayed powders into the main driving flow inside the nozzle. Hence, the properties and conditions of carrier gas should be of great importance to the particle motion behaviour and then particle deposition process. In this study, the effect of carrier gas temperature on the particle acceleration and deposition in cold spray process was investigated by both numerical and experimental methods. It is found that the supersonic driving gas flow and the consequent particle acceleration behaviour are significantly influenced by the carrier gas temperature, more specifically, higher carrier gas temperature results in higher particle impact velocity. In addition, because the carrier gas has additional heating effect on the powder particles before injection, the final impact temperature also increases with the carrier gas temperature, which leads to the reduction in the critical velocity. The increase in particle impact velocity and reduction in critical velocity enable the deposition efficiency and coating bonding strength to significantly improve as the carrier gas temperature increases. The particle impact velocity is found to be more influential than critical velocity reduction.

Effect of Spray Angle on Temperature Distribution within the Metallic Substrate in Cold Spraying

This study investigates the effect of spray angle on the temperature distribution within the metallic substrate in cold spraying. Computational fluid dynamic approach is employed in this work to achieve this objective. The simulated results show that spray angle significantly influences the temperature distribution within the substrate. For the perpendicular spraying, the temperature gradient contours present regularly annular shape, which means the temperature distribution is only dependent on the radial position. Furthermore, the peak value of the surface temperature is not at the geometric center (stagnation point) but at the position slight away from the center, which may result from the transition of the flow from laminar to turbulent state. Along the radial direction, there exists a secondary peak of the temperature arising from the expansion waves in the adherence jet. With decreasing the spray angle, the secondary peak in the uphill direction disappears. But in the downhill direction, the secondary peak still exits and the distance from the geometric center increases with the decrease in spray angle. In addition, the temperature distribution within the substrate indicates that the residual tensile stress inside the substrate mainly exists near the front zone, but for the angular spraying at the uphill direction, it appears near the back surface. Besides, the simulated results indicate that the best way to preheat the substrate at the angular condition is to move the nozzle from uphill toward the downhill direction.

Simulation of cavitation performance of an axial flow pump with inlet guide vanes

The cavitation performance of an axial flow pump with inlet guide vanes for different flow rates is studied in this article. The effects of inlet guide vanes on pump hydraulic performance and cavitation are investigated, where the total vapor fraction of impeller zone (Ftv) is calculated to predict the critical net positive suction head, which is compared with that predicted by efficiency criterion for different flow rates. The influences of the development of cavitation on internal flow in impeller zone are also investigated. The results obtained show that the cavitation performance of axial flow pump can be improved at off-design flow conditions by adjusting angles of inlet guide vanes to positive values at low flow rates and by regulating angles of inlet guide vanes to negative values at high flow rates. As the net positive suction head decreases, the vapor fraction first increases slowly and then increases greatly, clearly presenting cavitation process from inception to full development, which can be used to predict the required net positive suction head. When net positive suction head decreases to the value around required net positive suction head, the cavitation zone from tip of blade suction side close to leading edge and the cavitation zone from hub part of the blade suction side connect together. After the connection, the cavitation zones have great influence on the velocity flow, leading to the decrease in pump performance.

Three dimensional simulation of the arc inside an insulator-arrester with a multichamber system

Based on the thermo-physical properties of plasma air, the temperature and flow fields of an insulator–arrester with a multi-chamber system are investigated using three-dimensional computational fluid dynamics. The plasma flow is assumed to be steady incompressible laminar flow and in local thermodynamic equilibrium. The results obtained show that the arc inside the chamber bends downwards, indicating that the Lorentz force induced is directed away from the exit and pushes plasma air down. The outlet also provides passage for plasma flow both in and out of the chamber.

Stochastic Forcing of the Batchelor Vortex

The dynamics of the high‐Reynolds‐number vortices with axial flow continuously forced by a random excitation is investigated. Large response is possible, especially for the helical modes (with azimuthal wavenumbers |n| = 1). The general trends are complicated by a number of issues, including a long‐wavelength effect and a resonance effect, both of which were recently discovered for optimal perturbation and are found here to be present for stochastic forcing of swirling flow. Overall, the results suggest that strong response energy are present in the moderate‐ to high‐swirl regime of practical interest (swirl number q≤2). For axisymmetric (n = 0) and higher azimuthal wavenumbers |n|≤1, the axial flow contributes little to the response energy.