Numerical prediction of flow and heat transfer characteristics of water-fly ash slurry in a 180° return pipe bend

A three-dimensional numerical simulation is performed to predict the thermofluidic transport characteristics of water-fly ash slurry in an 180° return bend. U pipelines of diameter 53 mm with radius ratios of 2.98 and 5.6 are considered that may replicate a shell and tube type heat exchanger. The pressure drop and heat transfer characteristics are predicted and the effects of Dean, Nusselt and Reynolds numbers on the vortex structure formation and heat transfer are studied. The numerical simulation is carried out by deploying the granular Eulerian multiphase model following a finite volume approach. The turbulent transport is addressed using the RNG k−ε turbulence model. The results revealed that the heat transfer coefficient of pipe bends of smaller radius ratio is 53.28% more than the larger radius ratio for the solid concentration of 10% and velocity of 1 m/s. Its value increases with increase in the particle concentration and velocity due to the presence of a secondary flow in the bends. The Dean number increases with decreasing the radius ratio and the average Nusselt number increases with increasing the Reynolds number. With increasing Dean Number, the Nusselt number increases with decreasing the radius of curvature for the same particle concentration. When the particle concentration increases, the average Nusselt number also increases. The average Nusselt number in the return bend appears to be higher than that in the inlet and outlet pipes due to the presence of the secondary flows.