Mechanism of novel streamlined roughness element enhancing the flow and heat transfer performance from arrays of impinging jets

The numerical investigation of the flow and heat transfer characteristics from arrays of impinging jets with the roughed target surface is presented in this paper. Three novel streamlined roughness elements are proposed: similar round protuberance, similar trapezoidal straight rib, and similar trapezoidal curved rib. The jet Reynolds number ranges from 15000 to 30000, the protuberance height is h/H=0.36, the rib height is h/H=0.3, 0.5, and 0.7 respectively. The results show that the protuberance can shorten the nozzle to the stagnation point distance, increasing the heat transfer at the stagnation point by nearly 40%. The rib has a remarkable effect of guiding the flow in span-wise direction and away from the target surface, weakening the cross-flow effect. The straight rib gets a better guidance performance, the curved rib provides a larger surface area. An appropriate increase of rib height can improve local heat transfer, continuing to increase mainly enlarge heat transfer area. The decent aerodynamic shape decreases the flow discharge coefficient by only 2% at the rib height of 0.3H.