A numerical and experimental analysis of the methodology of thermal conductivity measurements in fluids by concentric cylinders

Abstract

A study of thermal conductivity measurements of fluids, using the technique of steady-state heat transfer in concentric cylinders, is presented. In order to evaluate the effect of convective flow on the measured value of conductivity, numerical results, which were obtained using computational fluid dynamics (CFD) in three dimensions, are compared with experimental data and analytical results of temperature profiles. This latter was obtained considering the hypothesis of heat transfer mechanism being entirely diffusive and solely in the radial direction. An experimental design was proposed aiming to analyze the effects of glycerol mass fraction (0%, 50 and 100%), annuli size (2.525, 4.525 and 6.525 mm) and heat rate (5, 10 and 15 W) in the formation of convective streamlines. The ratio between effective and absolute conductivities (kef/k) was used as a response to evaluate the convection intensity. The results were compared with empirical equations that correlate the ratio kef/k with the dimensionless numbers of Prandtl and Rayleigh, which are in the rate 4.37 ≤ Pr ≤ 3.8 × 103 and 9.0 ≤ Ra ≤ 4.4 × 104. An evaluation of the accuracy in measuring kef is showed based on the simulated data of temperature profiles in the axial, radial and angular directions.