LDV measurements of particle fluctuation velocities in dilute gravity-driven gas-particle flows

Abstract

Particle instantaneous velocities were measured by a laser-Doppler velocimeter (LDV) in a vertical channel, where particles were supplied from the top of the channel and gas superficial velocity was close to zero. According to kinetic theory of granular flows, particle instantaneous velocities are decomposed into two components, a local Eulerian velocity that represents bulk motion of neighboring particles and a residual component, by three different local averaged methods. Results indicate that the local averaged method based on inter-particle collision time fails to accurately estimate the rapid variation of local Eulerian velocity. In contrast, the local Eulerian velocity estimated by both the simple moving averaged method and the robust local regression method could well represent the collective motions of neighboring particles. Moreover, the magnitude of the residual component (i.e., granular temperature) estimated by the robust local regression method was less sensitive to the number of neighboring data points used in the decomposition process. Overall trends of granular temperature and particle-phase turbulent kinetic energy in the wall normal direction are different from each other, indicating that the granular temperature and particle-phase turbulent kinetic energy must be treated separately to accurately predict energy balance. The probability density function of the residual component is compared with a Gaussian distribution. At the center of the channel, it follows a Gaussian distribution, while in the vicinity of the wall it retains a long tail.