Effects of the stroke length and nozzle-to-plate distance on synthetic jet impingement heat transfer

•At high L0/Ds, Nu distribution is similar to that of a continuous impinging jet.•Maximum stagnation Nusselt number is attained at lower H/Ds as L0/D decreases.•Inner ring-shaped regions of maximum Nu are due to the trailing jet.•Outer ring-shaped regions of maximum Nu are due to vortex rings.•Nu fluctuations decrease as L0/D decreases and/or H/D increases. This study focuses on the combined effect of the nozzle-to-plate distance and of the stroke length on the cooling performances of impinging synthetic jets. Infrared thermography is used as temperature transducer in conjunction with the heated thin foil heat transfer sensor to measure time- and phase-averaged convective heat transfer. All the experiments have been performed at a fixed Reynolds number equal to 5250, while different values of the dimensionless stroke length (L0/D equal to 5, 10 and 20) and nozzle-to-plate distance (H/D between 2 and 10) have been considered. At high L0/D, the heat transfer behaviour resembles that of a continuous impinging jet. It is characterized by a time-averaged stagnation Nusselt number maximum between H/D equal to 4 and 6 and inner and outer ring-shaped regions of Nusselt number maximum at short H/D. These two regions are replaced by a bell-shaped distribution at higher nozzle-to-plate distances. The existence of these regions is clearly observed through the phase-averaged heat transfer measurements. At short H/D, the heat transfer evolution reveals the simultaneous presence of two outer ring-shaped regions. The external outer region is ascribed to the strong coherence of the primary vortex ring, while the internal one is mainly due to the vortex rings generated by the Kelvin–Helmholtz instability along the trailing jet shear layer. At high H/D, the internal outer-ring shaped region disappears because of the weakening of the trailing jet Kelvin–Helmholtz vortex rings. In opposition, at short L0/D, the time-averaged stagnation point Nusselt number is found to have a maximum at lower values of H/D, and no inner ring-shaped region of heat transfer maximum is observed. This region is not present, at low dimensionless stroke lengths, because of the weakness and reduced extent of the trailing jet in the flow field. Indeed, the phase-averaged measurements mainly show the heat transfer caused by the impinging primary vortex ring. Despite the weakness of the trailing jet, the outer ring-shaped region of heat transfer maximum is observed at short H/D because of the presence of