Bow Shock Effect on Particle Transport and Deposition in a Hypersonic Impactor

Abstract

Supersonic/hypersonic impactors are used as a collector and/or size separator of nano- and micro-particles. Thin film and fine line pattern deposition by aerosol jets are other applications of deposition of supersonic/hypersonic impactors. At extremely low backpressures, the exiting flow from a nozzle forms a supersonic free jet. The supersonic jet forms a strong normal shock in the front of the impactor plate. The stagnation pressure, backpressure and distance between the nozzle exit and the impactor plate affect the flow field. Due to the rather complicated flow in the impactor, studies of particle motions in supersonic impactors are rather scarce. In this study the airflow and particle transport and deposition in a supersonic/hypersonic impactor are numerically simulated. The axisymmetric compressible Navier-Stokes equation is solved and the flow properties are evaluated. It is assumed that the particle concentration is dilute, to the extent that the presence of particles does not alter the flow field. Deposition of different size particles under different operating conditions is studied. The importance of drag, lift and Brownian forces on particle motions in supersonic impactors is discussed. Sensitivity of the simulation results to the use of different expressions for the drag force is also examined. A strong bow shock on the flowfield has much effect in drag forces on particles. It is shown that the Stokes-Cunningham drag with variable correction coefficient is most suitable for computer simulation studies of nano-particles in supersonic/hypersonic impactors. The computer simulation results are shown to compare favorably with the experimental data.