Impingement heat transfer performance of a square turbulent jet modified by slotted orifice synthetic jets

Abstract

Active flow control using synthetic jets has gained significant attention in recent times for enhancing impingement cooling, primarily through jet vectoring, vortex manipulation, and mixing enhancement. However, the fundamental question of whether significant thermal enhancement necessarily requires jet vectoring remains largely unexplored. The present study undertakes an experimental investigation of a slotted synthetic jet-assisted steady jet for impingement heat transfer performance. This work introduces a control approach in which two zero-net-mass-flux synthetic jets, symmetrically placed above and below a square steady jet, contrary to the commonly anticipated vectoring behaviour, force the primary jet into its jet preferred mode. Most sandwiched jet arrangements reported in the literature operate at velocity ratios above one and aim to vector or focus the primary jet. Here, the synthetic jets operate at velocity ratios at or below one, and the goal is to increase spreading rather than deflection for impingement heat transfer. Both synthetic jets are driven in phase at 185 Hz; the natural jet's preferred Strouhal number at Reynolds number of 5000 is 0.25. Synthetic jets are operated at Strouhal numbers of 0.025, 0.032, and 0.051, with velocity ratios of 1, 0.8, and 0.5, respectively, at 185 Hz. Results show that actuating synthetic jets produces broader, more uniform Nusselt number distributions, improving the area-averaged Nusselt number and potentially reducing thermal gradients and associated thermal stresses on the impinging system. The highest velocity ratio yielded the largest area-averaged Nusselt number but also reduced the stagnation Nusselt number by about 25% relative to the baseline configuration. However, the lower velocity ratio (VR = 0.8) showed better performance, as indicated by the comparison parameter (PCP), which accounts for the input forcing momentum and the enhanced Nusselt number. In practice, this arrangement is well-suited for cooling compact electronics and power modules, where a broad, uniform heat-transfer footprint at low actuation power is more useful than a single high peak at the stagnation point.