Abstract
In this study, the combined impact of carbon nanotube volume fractions and the electric field on the enhancement of dielectric oil heat transfer is investigated. The geometry studied is an annular layer filled with the dielectric oil and bounded by two eccentric or concentric cylinders. A temperature gradient and an electrical potential difference are applied between the two cylindrical walls. The goal is to analyse the flow behaviour and the heat transfer improvement using a double enhancement technique by combining an active (electric field) and passive (nanoparticles) approach. An in-house numerical simulation code using the finite volume method was developed to solve the set of coupled equations describing the conservation laws of the electro-thermo-hydrodynamic flow associated with the oil. A parametric study, outlining the influence of different relevant variables has been performed. The parameters investigated are the thermal Rayleigh number (2000 ≤ T ≤ 100 000), the electrical Rayleigh number (100 ≤ T ≤ 600), the radius ratio (0.1 ≤ Γ ≤ 0.9), the eccentricity (0 % ≤ e ≤ 90%), the nanotubes volume fraction (0 % ≤ Φ ≤ 0.5%)and the injection level (1 ≤ C ≤ 10) respectively. The effect of these variables on the heat transfer intensification is examined in detail. According to the adopted configuration, heat transfer intensifications between 3% and 77% have been recorded essentially when an electric field was applied. Also, the addition of a 0.5% volume fraction of carbon nanotubes provides a heat transfer improvement of around 27%. Furthermore, for a first time a multiparameter correlation has been developed. This correlation can be useful to better understand the impact of various physical parameters on the heat transfer in annular spaces and also to predict exact values of the Nusselt number.