Experimental study on of dynamics of particles in the flow filed with intensive gradients

Abstract

The dynamic response of particles in hyper/supersonic flow is one of the key points of techniques using tracer particles, such as particle image velocimetry (PIV). In the literature, it is validated by the single oblique shock response testing. However, particles suffer intensive variation of velocity, density and viscosity, when used to trace and measure the complex flow field in the high speed vehicle engine. To test and validate the dynamics of particles in such a flow field with intensive gradient, in this paper we conduct a series experiments dealing with this issue. The study includes the measurements on the velocity field at the exit of the wind tunnel nozzle to testify the performance of PIV system, the measurements on the oblique shock response of particles in Mach 4.2 and Mach 3.0 supersonic flows over a 10° wedge and a 15° wedge respectively, and measurements on the double oblique shock response of particles in the flow field which is designed to simulate the flow field inside the vehicle engine with gradients and without the influence of expansion wave. Based on the particle dynamic models, the relaxation time, relaxation distance, Stokes numbers of different cases can be gained. And the influence of unstable shock oscillation is analyzed and revised based on image method and statistic analysis. It can be found that the relaxation time and distance increase with the Mach number, given the same wedge degree. However, with the same incoming Mach number, the relaxation time and distance drop when the wedge degree increases. Due to the intensive variation of Reynolds number and viscosity, the results show that in a certain extent particles lose their following ability by 5.7%, while its Stokes number increases by 1%. In the flow condition herein, the Stokes number still meets the requirement of supersonic flow. However the decrease of particle following ability is worth being concerned, especially when using particles in the complex flow field with more intensive gradients.