Forced Laminar Flow Convection in a Horizontal Tube With Variable Viscosity and Free-Convection Effects

Abstract

The influence of free convection and variable viscosity on forced laminar flow of ethylene glycol initially at 32 deg F was investigated. The flow was horizontal in a circular tube with uniform wall heat flux and a fully developed velocity profile at the onset of heating. Local measurements were made for the calculation of Nusselt number, Reynolds number, Prandtl number, and Grashof number. Reynolds numbers ranged from 6 to 300, Grashof numbers ranged up to 2800, Prandtl numbers ranged from 26 to 500, and Graetz numbers ranged from 3 to 4800. The theoretical analysis established that the conditions for fully developed flow in the absence of free convection require an exponential variation of viscosity with temperature and that Nu and (dp/dx) are functions of Gz and viscosity ratio, μw/μb, only for both the developing and the fully developed region for this viscosity variation. Numerical results for flow and heat transfer which are in agreement with experiment were also obtained. Further substantiation of the free-convection parameter Ra1/4/Nu(Gz) was obtained.