Mixed convection heat transfer enhanced by PCD electrical field in a rectangular channel

For the cooling of transformer in the grid, the heat transfer efficiency of mixed convection is relatively low due to the low thermal conductivity and large viscosity of transformer oil. Therefore, this paper aims to study the PCD (periodically changed direction) electrical field to enhance the mixed convective heat transfer of transformer oil. Then, a semi empirical correlation is developed to calculate the Nusselt number affected by electrical field. Specially, the influences of heat flux, electrical field intensity and switching period on the heat transfer characteristics are discussed experimentally. As a result, the result reveals that the mixed convective heat transfer is enhanced increasingly with the decrease of switching time and improvement of voltage. Thus, the PCD electric field can effectively enhance the mixed convection heat transfer. In addition, the mechanism analysis shows that the PCD electrical field leads to the continuous change of fluid mode, and the polarized molecules continuously vibrate and move within a certain range, which leads to the violent disturbance of internal fluid. Meanwhile, the secondary flow induced by the electric field results in the disruption of thermal boundary layer, which strengths the heat transfer. Finally, the predicted results of the semi-empirical correlation are in good agreement with the experimental results, and the effect of electrical field intensity and switching periods are comprehensively considered in the correlation. The present study is of great importance for the transformer cooling.