Flow Dynamics and Heat Transfer Characteristics of Upward Impinging Jets

Upward, laminar, axisymmetric, submerged impinging jets, with water as the working fluid, are numerically investigated in detail, with the impingement surface subjected to high heating rates. The heating greatly changes the density, viscosity, and thermal conductivity of the fluid, which causes the post-impingement wall-jet to prematurely separate from the impingement surface at some radial distance from the center-line. Consequently, the local heat transfer rate deteriorates. The separated jet fluctuates, and it is found that the flow dynamics and heat-transfer properties strongly depend both on the heating rate and the inlet-based Reynolds number. The Nusselt number, the skin friction coefficient, the surface temperature, and the surface pressure are examined at the inlet-based Reynolds number of 600, for the three heating rates of 5, 10, and 20 kW/m2. When the area-averaged heat-transfer is compared with non-separated jet under identical conditions (which are realized by keeping the fluid properties independent of the temperature in the computation), it is found that the heat transfer efficiency on the impingement surface reduces by about 23%, 32%, and 35%, respectively. Afterwards, the Reynolds number is changed to 300 and 900, while keeping the heating rate constant at 10 kW/m2. In these cases, the efficiency reduces by about 40% and 9%, respectively.