The flow and heat transfer behavior of synthetic jets with star shaped orifice of different lobes

In the present experimental investigation, the flow and heat transfer characteristics of synthetic jet (SJ) issuing from different lobed (3-10) star-shaped orifices are studied. Tests are performed by actuating the SJ actuator at constant input power of 4Vrms, with varied range of orifice to surface spacings (1-16) and numerous values of Reynolds number (8692-10746). The synthetic jet's flow characteristics are studied using hot-wire anemometry, while the thermal characteristics are analyzed using an IR camera. Distinct peaks in turbulence intensity are observed in major and minor planes for different lobed (3-6) star orifices; this may be due to the development of two shear layers in such orifices, which consequently merges into a single shear layer for higher lobed (8-10) star orifice. The 6-lobed star orifice is found to exhibit maximum heat transfer performance at z/d varying between 2 and 14. At z/d = 2, the 6-lobed star orifice exhibit 7.6% and 10.1% improvement in average Nusselt number compared to circular and 10-lobed star orifice respectively, which may be due to formation of vortical structures along the lobed edges and subsequent switching of axes between the major and minor planes. To comprehend the energy associated with the issuing vortices, the power spectral density of the velocity signal at the orifice exit is examined. A unified correlation for the average Nusselt number is also derived for various star-shaped orifices.