An experimental study of natural convection in vertical open-ended concentric and eccentric annuli

Abstract

The effects of eccentricity on the natural convection heat transfer from a vertical open-ended cylindrical annulus with diameter ratio of 1.63 and aspect ratio of 18:1 have been investigated experimentally. Measurements of transient and steady state wall temperature distributions for different eccentricities were collected to help understand the heat transfer processes and also to serve as benchmarks for the validation of future CFD studies. Particle image velocimetry was used to validate the calculated mass flow rate in the annulus. It was found that, whereas at very low eccentricities there was insensitivity to eccentricity on the overall heat transfer, a diminishing of the heat transfer occurred for higher eccentricities. Plots of the local azimuthal variation of the Nusselt number showed that at low eccentricities heat transfer improved on one side of the annulus but decreased on the other side. The Nusselt number was found to decrease with eccentricity, while the Rayleigh and Grashof numbers had an increasing trend. It was also found that the mass flow rate did not appear to be strongly influenced by varying eccentricity within the experimental range. The Reynolds number was calculated and the flow within the annulus was found to be transitional nearing laminar with increased distance from the inlet.