Efficacy of impinging air jets under various cross-flow conditions

The performance of a turbulent air jet impinging on a flat surface and subjected to cross flow is investigated. The study considers many different parameters, including Reynolds number Re, the normalized orifice-to-target spacing Z/D, and the ratio α of cross-flow to jet Reynolds numbers. As reported by other investigators, the optimum value of Z/D is 6. At this optimum Z/D, when the Reynolds number is raised from 5100 to 23 000, the stagnation Nusselt number Nu increases by 150%. A correlation is proposed to estimate stagnation point heat transfer. Velocity profiles are used to examine the cooling characteristics of jets with and without cross flow. Cooling in both the upstream and downstream regions is estimated and compared with the performance of a jet without cross flow. For Z/D = 6, the stagnation Nu decreases by 84% when α is increased from 0 to 2. The corresponding decrease in the stagnation Nu for Z/D = 2 is 75%. Cross flow significantly affects the flow pattern of fluid near the target surface, and anomalous flow behavior is observed in the stagnation region. Without cross flow, the radial velocity is close to zero at r/D = 0, and the fluid velocity increases in the outward direction and attains a maximum value at r/D = 1. With cross flow, the position of the peak velocity shifts. Average, stagnation point, and peak heat transfer coefficients are discussed in terms of the Stanton number. At low values of α, there is an increase in the peak Nu. With cross flow, the location of the peak velocity changes. The augmentation in cooling in the downstream region and the reduction in cooling in the upstream region depend strongly on Z/D and α. For Z/D = 2, at α = 1, the average heat transfer downstream increases by 15% and the corresponding decrease upstream is 35%.