Impingement heat transfer on flat and concave surfaces by piston-driven synthetic jet from planar lobed orifice

Abstract

The heat transfer characteristics of a single synthetic jet (SJ) issuing from planar lobed orifice are investigated experimentally in current study. Two specific planar-lobed orifices (petal-shaped orifice and arch-shaped orifice) and two typical targets (flat target and semi-cylindrical concave target) are taken into consideration. For each lobed orifice, three configurations are included. In the petal-shaped orifices, the lobe numbers are chosen as N=4, 6 and 8 in turns. While in the arch-shaped orifices, the lobe number is fixed as N=6 but the lobe aspect ratios are altered as AR=1, 2 and 3 respectively. All lobed orifices have the same exit area with the baseline round orifice. The experimental tests are performed under a series of operational frequencies (ranging from 10Hz to 25Hz) and dimensionless jet-to-target distances (ranging from 2 to 14 for flat target, and from 6 to 14 for concave target). Under the present conditions, SJ impingement heat transfer enhancement is mostly achieved by the planar lobed orifices when compared to the baseline round orifice, which depends tightly on the lobe shape and target shape. Among current petal-shaped orifices, the N=6 petal-shaped orifice is demonstrated to be a most promising orifice configuration. With regard to current arch-shaped orifices, a moderate lobe aspect ratio is demonstrated to be the superior. When compared to the flat target, SJ impingement heat transfer on the concave target is seriously reduced, regardless of orifice shapes. Interestingly, the relationship of SJ impingement heat transfer on the concave target, between the N=6 petal-shaped orifice and AR=2 arch-shaped orifice, is found contrary to that on the flat target. On the concave target, the N=6 petal-shaped orifice does not show its advantages on SJ heat transfer enhancement as appeared on the flat target. However, the AR=2 arch-shaped orifice still shows its positive role on SJ heat transfer enhancement obviously in comparison to the baseline round orifice.