Effects of the nozzle-to-plate distance and nozzle exit section shape on the heat transfer behaviour of impinging synthetic jets

The versatility and effectiveness of impinging synthetic jets (SJs) for cooling applications have pushed the scientific community to optimise the design of these devices by investigating the impact of various flow parameters, such as the stroke length, the Reynolds number and the shape of the driving electric signal. The current work delves into the influence of the nozzle exit section shape (NESS) on the heat transfer behaviour of SJs. In addition to the circular baseline case, three different shapes are comparatively assessed: the triangle, the square and the rectangular NESS. Phase-locked 2D infrared thermography (IRT) measurements are carried out for five nozzle-to-plate distances: H/D∈[2,4,6,8,10], whereas the Reynolds and Strouhal numbers are kept constant and equal to Re = 3500 and Sr = 0.068, respectively. It is shown that the heat transfer behaviour of the SJ is significantly modified by the shape of the nozzle exit section, especially for short impingement distances. Upon ejection, the coherent vortical structures are shaped as the nozzle from which it is issued. As the vortical structures are convected downstream, they lose their geometrical coherence due to inertial instabilities. Indeed, at small nozzle-to-plate distances, the time-averaged Nu maps show the signature of the impinging vortical structures in the thermal pattern. At high nozzle-to-plate distances, however, the SJ's thermal footprint on the foil is almost bell-shaped, with all NESSs achieving a heat transfer behaviour similar to the circular nozzle. The highest time-averaged heat transfer rates are achieved with either the circular or rectangular NESS, depending on the impingement distance; the circular configuration is favoured at low H/D, while the rectangular configuration performs better at high H/D. Additionally, phase-averaged measurements revealed the jet axis-switching phenomenon for the triangular and rectangular NESS at H/D=6.