Large eddy simulation of temperature-variation effect of impinging planar lobed synthetic jet on flat plate and the semi-cylindrical concave plate

Abstract

Temperature-variation becomes increasingly evident when an impinging synthetic jet is restricted and confined excessively by the target plate. However, the confinement effect of the target plate on the temperature-variation effect during impinging heat transfer by a synthetic jet has not been clarified. In this study, impinging heat transfer by a synthetic jet induced by a planar lobed orifice on a flat plate and a semi-cylindrical concave plate is investigated by means of Large Eddy Simulation. The planar lobed synthetic jet is characterized by the geometry of the lobed nozzle, and the velocity along its major diameter is larger than that along its minor diameter. As the mainstream jet moves downstream, the difference between the major and minor diameters decreases gradually. At dimensionless jet-to-surface distance of H/d = 8, the flow field attached to the flat plate along the major diameter is similar to that along the minor diameter. The results show that two types of mechanisms have induced the temperature-variation. For the flat plate, its temperature-variation comes from the suction of the wall jet near the stagnation region confined by the target plate, which becomes more conspicuous particularly at small dimensionless jet-to-surface distance. By contrast, the temperature-variation is less conspicuous at large dimensionless jet-to-surface distance because the inhaling region is distant from the target plate. For the concave plate, its temperature-variation is due to the large recirculation zone along the curvilinear direction. During the suction process of the cycle, the heated wall jet attached to the plate surface is re-inhaled into the cavity through the large recirculation zone, and the temperature of the air inside the cavity increases. During the ejection process of the cycle, the wall jet is entrapped into the jet mainstream and then returns to the target plate. Correspondingly, the mean velocity of the wall jet and the wall shear stress of the target plate are less than those of the flat plate. Time and frequency analyses had revealed the characteristics of inherent aggressive pulsation inside the planar lobed synthetic jet. The velocity in the primary frequency dominates the planar lobed synthetic jet, whereas higher-order velocities are negligible.