Analysis of heat transfer in a pipe carrying two-phase gas-particle suspension

Abstract

A 'two-fluid model' using the thermal eddy diffusivity concept and Lumley's drag reduction theory, is proposed to analyse 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 rhoBAR-C(p) of the gaseous phase and the particulate phase and also of the ratio of the thermal relaxation time scale to that of turbulence. Lumley's theory is applied to find the variation of the viscous sublayer thickness depending on the particle loading ratio Z and the relative particle size d(p)/D. 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 rho-pBAR-C(pp)\rho-fBAR-C(pf) is dominant. The major cause of decrease in the suspension Nusselt number at low loading ratio is found to be due to the increase of the viscous sublayer thickness caused by the suppression of turbulence near the wall by the presence of solid particles. Predicted Nusselt numbers using the present model are in satisfactory agreement with available experimental data both in the pipe entrance and the fully developed regions.