Experimental investigation of free convection from short horizontal cylinder to Newtonian and power-law liquids of large Prandtl numbers

Abstract

This paper reports the results of an experimental investigation of free convection from electrically-heated short horizontal circular cylinder of small aspect ratio (L/d=8) to five liquids of large Prandtl numbers. Three of these fluids are Newtonian and two are weakly power-law shear thinning fluids. Experiments are conducted, under constant heat flux, at various bulk temperatures (40–175°C) and heat fluxes (0.66–104kW/m2) that correspond to heat generation from about 0.7 to 113MW/m3. Effects of oil bulk temperatures, oil properties and heat generation parameter, on the free convection results, are investigated.The results indicated that Newtonian fluids attain higher free convection heat transfer coefficient than the power-law fluids by 16% or more. Nusselt number (Nu=hd/k) of the laminar free convection data is correlated in terms of the power-law index and in classical and non-classical forms. Correlations cover Grashof number (Gr∗=gβqd4/kυ2) from 33 to 1.68×107, Rayleigh number (Ra∗=gβqd4/kυα) from 6.1×104 to 6.88×108, Prandtl numbers (Pr=cpμ/k) from 30 to 1850, Prandtl numbers ratio (Prb/Prw) from 1.03 to 133 and heat generation parameter from 0.001 to 0.009. The developed non-classical correlation that is based on the presented experimental data is the first correlation to consider the heat generation parameter. It predicts the experimental Nu within ±5% whereas the classical form correlation predicts Nu with uncertainty over ±20%.