The study of impingement cooling effects using a variable diameter synthetic jet

Alexander John Zielinski

Synthetic jets produce a time-averaged jet flow using oscillating flow through an orifice. These devices use the same working fluid as the ambient, and when miniaturized they can reduce the cooling system volume for electronics. Most synthetic jets use constant-diameter orifices, but previous experiments with a contracting diameter demonstrated a significant increase in the momentum flow rate. Because of this benefit, we investigated the effectiveness of a variable-diameter synthetic jet for impingement cooling. We used a centimeter-scale device driven by a piston operating in phase with a closing iris, each moving simultaneously at 4 Hz. Then, we mounted a resistive heater of similar size to an impingement plate, which was positioned at various distances from the exit of the synthetic jet. We measured the heater temperatures and power dissipation for distances ranging from 0.5 to 10 jet diameters, and considered both fixed-diameter and variable-diameter operations. For jet distances from 1 to 3 diameters away, heat transfer was enhanced by 4 to 8 times that of natural convection, with the variable-diameter enhancement approximately 35% greater than the constant diameter. After finishing the heat transfer testing, particle image velocimetry (PIV) was implemented to assess why the variable-diameter produced more heat transfer from x/D = 1 to 3. The parameters of the flow profiles were modified with the variable-diameter in comparison to the fixed-diameter. An impingement plate was located 3 diameters downstream in order to characterize all of the flow characteristics occurring when the vortex rings formed by the variable-diameter produced more heat transfer than the fixed-diameter. A high-speed camera, synchronized with a high-powered laser, captures the particle displacement produced near the exit of the synthetic jet. Then, PIV software enables the velocities, vorticities, and circulation to be visualized and analyzed. The image analysis indicated that significant improvements to the centralized velocity, entrainment, and localized circulation of the main vortex ring and trailing jet were present in the variable-diameter's flow. These enhancements provide explanations for why the variable-diameter produced 35% more heat transfer, and they revealed that the improved impingement distance was so limited because the timespan when they occurred was very brief.

The study of impingement cooling effects using a variable diameter synthetic jet

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