The effect of nozzle geometry on local convective heat transfer to unconfined impinging air jets

In an impinging jet, nozzle geometry markedly impacts heat transfer between jet and plate by affecting the velocity profile at the jet exit and thereby potentially modifying the behavior of the jet's vortex structures. This study analyzed the influence of three different injections: a tube used as a reference, a round orifice, and a cross-shaped orifice perforated on a hemispherical surface. They all possess the same free area Ao, and the equivalent diameter is D = 14 mm. Experiments have been conducted for Reynolds numbers 23,000 ⩽ Re ⩽ 45,000, for orifice-to-plate distances 1 ⩽ H/D ⩽ 5 and for the temperature of the jet T∞⩽Tj⩽50°C. Aerodynamic results indicate that the hemisphere produces a “vena contracta” effect, which is greater in the round than in the cross-shaped orifice. The velocity profile at the jet exit (X/D = 0.1) presents a parabolic shape for the tube and an inverted parabolic shape for the round orifice, while the presence of two shear layers renders the cross-shaped orifice more complex and three-dimensional. Thermal results also show that a round orifice on the hemisphere causes higher heat transfer rate than the other injections.