Effects of acoustically driven oscillating flow on hydrodynamics of the flow around small spherical particles

Abstract

This theoretical study explores the effects of acoustically driven oscillating flows on the flow behavior around small spherical particles. Its application is in the area of enhancement of combustion of pulverized coal particles and coal water slurry fuel droplets via augmentation of heat and mass transfer to and from these particles that are entrained in the steady mean flow. The particle is 100 μm in diameter. The sound‐pressure levels are between 140–160 dB, resulting in peak acoustic velocities of 2.5–50 m/s. The acoustic Reynolds numbers based on peak acoustic velocity and particle diameter are 16–150. The phenomenon is examined at 50 and 2000 Hz corresponding to Strouhal number 0.0002 and 0.08 in order to investigate the effects of low and high frequencies. The velocity field around the particle, the separation angle, and wall shear stress are calculated by solving the two‐dimensional, unsteady laminar Navier‐Stokes equations with a numerical technique. From the results, it can be seen that at 50 Hz the flow field around the particle is governed by the particle curvature and magnitude of the free stream velocity whereas, at 2000 Hz, the acceleration of the flow with time (due to oscillation) is also an important factor. This results in different heat transfer behavior due to different separated regions at both frequencies under the same free stream velocity. For example, at a Reynolds number 16, steady flow does not separate; neither does the flow at 50 Hz. However, at 2000 Hz, a separated region is observed due to the free stream flow acceleration. These results are discussed in detail.