Turbulent fluid flow and heat transfer in annular passages with rough surfaces and moving cores.

Abstract

This study investigates the behaviour of fully developed fluid flows in concentric annuli with rough surfaces and moving cores, by analytically predicting the effects of surface roughness, core velocity and turbulent motion on the momentum and heat transfer characteristics of the flow, such as friction forces, velocity profiles, temperature profiles and heat transfer. The analytical predictions are produced by a mathematical model based on an adaptation of Prandtl's mixing length theories and on the data of previous studies of turbulent flow in annuli and rectangular channels which were artificially roughened. The computer program developed for this study employs an iterative process to match velocity and temperature profiles with force and energy balances and calculates the desired momentum and thermal characteristics. The results indicate excellent correlation with previous experimental data on rough annuli with static cores and smooth annuli with moving cores. From these results, the combined effects of surface roughness and moving cores are inferred. New equations are proposed for predicting the radius of maximum fluid velocity and the effects of core velocity on roughness, in terms of annulus radius ratio, roughness geometry, flow's Reynolds number and core velocity. The study demonstrates the advantages, under certain conditions, of using roughness elements to increase heat transfer and enhance the overall efficiency in the energy transfer processes involved.