Numerical analysis on the reductions of drag and heat transfer in a gas-solid two-phase pipe flow

Abstract

The present study deals with numerical analyses for the reduction mechanisms of momentum and heat transfers in a turbulent dilute gas-particle two-phase flow through a vertical pipe. In the first part of the present work, the "two-fluid model" has been incorporated with Lumley``s drag reduction model to analyze the mechanism of momentum transfer. The change of the effective viscous sublayer thickness by the presence of particles is modeled by Lumley``s theoretical model. The eddy viscosity of the gaseous phase is obtained by a turbulent kinetic energy in equilibrium and the eddy viscosity of the particulate phase is modeled by analysing the diffusive mechanism of particles in the turbulent flow. The numerical computations of the friction factor and the pressure drop in a fully developed pipe flow by the present theory are in good agreement with the corresponding experimental data for an average particle size of 15 ㎛. It is proved that Lumely``s model is successful in predicting the correct reduction behavior of the drag in the gas-particle flow has been found to be dependent on the particle relaxation time, Kolmogoroff time scale and the solids-gas loading ratio. In the second part, a "two-fluid model" using a thermal eddy diffusivity concept and Lumely``s drag reduction theory, is proposed to analyze heat transfer of the turbulent dilute gas-particle flow in a vertical pipe with constant wall heat flux. The thermal eddy diffusivity model is derived to be a function of the ratio of the heat capacity-density products $\overline{\rho}$Cp of the gaseous phase and the particulate phase and also of the ratio of thermal relaxation time scale to that of turbulence. At low loading ratio, the size of the viscous sublayer thickness is important for suspension heat transfer, while at higher loading the effect of the ratio $\overline{\rho_p}Cp_p/\overline{\rho_f}Cpf$ is dominant. The major cause of decrease in the suspension Nusselt number at low loading ratio is found t...