Effect of jet inflow angle on heat transportation in an impinging jet interfering with a Couette flow

Abstract

The effect of the incoming angle of a jet that interferes with a Couette flow is studied using numerical simulations. When the jet flows against the direction of the Couette flow, the instantaneous vortex structure and heat transfer on the jet-impinging surface become complex and irregular. In contrast, when the jet flows in the same direction as the Couette flow, the instantaneous vortex structure has a strong and distinct periodicity about the main flow direction as a vortex street with a sine wave velocity fluctuation. This periodicity also affects the heat transfer on the jet-impinging surface. The Strouhal number calculated from these time periodicity increases with increasing jet angle. In the time-averaged flows and temperature fields, the Nusselt number exhibits a few characteristic distributions. The heat transfer concentrates around the jet impingement region at a negative jet angle whereas the area associated with significant heat transfer coefficient separates symmetrically about the flow in spanwise direction at a positive jet angle and the value of heat transfer coefficient on the center axis is low. Furthermore, a large and strong counter-rotating vortex pair is formed when the jet flows in the same direction as the Couette flow. As reported in our previous study, this pair vortex involves a main flow fluid and results in a characteristic Nusselt number distribution. Comparing the overall heat transfers on the jet-impinging surface, the lowest time-averaged heat transfer is obtained when the jet is orthogonal to the Couette main flow. Generally, the heat transfer over the surface is higher when the jet flows against the Couette flow than when the jet flows in the same direction as the Couette flow. However, since the pressure loss also increases drastically at the same time, it is found that the ratio of heat transfer over pressure loss increases with increase in jet angle. Considering the temperature and velocity vector distributions, and plots of turbulent heat flux, when the jet flows against the Couette main flow, the effect of jet impingement is dominant on heat transfer. However, when the jet flows in the same direction as the Couette main flow, most of the heat transfer is carried by the pair vortex. Although the contributing vortex type is different from similar previous studies conducted, the mechanism of generating this pair vortex is based on time-averaging the legs of the periodic vortex street.