A theoretical investigation of acoustic enhancement of heat and mass transfer-I. Pure oscillating flow

Effects of an oscillating flow induced by a high intensity acoustic field on heat and mass transfer to and from particles and droplets such as pulverized coal particles and coal-water slurry fuel droplets are investigated. Numerical solutions of two-dimensional, unsteady, laminar conservation equations for mass, momentum and energy transport in the gas phase give the velocity and temperature fields around a particle for different oscillating flows without superposed steady component as a function of time. The local and space-averaged Nusselt numbers depend on the change of imposed oscillating velocity U due to body curvature and flow acceleration. At low frequency (~50 Hz), curvature effects are dominant, resulting in increasing values of Nusselt numbers with increasing U. The Nusselt numbers can be approximated by the quasi-steady analysis. The effects of flow acceleration increase with increasing frequencies (~2000 Hz). The combined effects of curvature and flow acceleration result in the maximum difference of 9% in the space- and time-average Nusselt number for frequencies of 50, 1000 and 2000 Hz for the acoustic Reynolds number varying between 10 and 100. The present results show about 290% increase in the space- and timeaveraged Nusselt number for an acoustic Reynolds number of about 100 compared to that without an acoustic field.