The development and study of a virtual orifice synthetic jet

Monique  Catherine Embury

doi:10.7273/000004059

A synthetic jet is a zero-net-mass transfer jet, meaning it operates entirely from its working fluid which is also the ambient fluid, ejecting the fluid on the outflow and using the same fluid during its suction flow, producing a time-averaged jet flow. Most synthetic jets use constant-diameter orifices, but recent experiments with a contracting diameter demonstrated a significant increase in momentum flow rate, making the contracting diameter promising for thermal applications. Along with its benefits came its pitfalls, miniaturization and fatigue strength. To avoid these issues, a virtual orifice was developed, which siphons fluid via a bypass slot and redirects it radially inward to the primary flow as it exits the orifice. The virtual aperture reduces the complexity of the design and volume needed. Computational fluid dynamics simulations were used to optimize the geometry and operating conditions for the design, selecting dimensions for optimal exit speeds. The device was then manufactured and tested. We used a centimeter-scale device driven by a piston operating at 4 Hz and a resistive heater of similar size mounted 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 2 to 8 jet diameters and considered both a virtual orifice and constant-diameter synthetic jet. The virtual orifice design did not prove as promising as originally hoped, only providing a 3% increase in heat transfer. Particle image velocimetry (PIV) was implemented to assess the fluid behavior of the virtual orifice synthetic jet over a constant-diameter synthetic jet. The impingement plate was held at distances ranging from 2 to 8 jet diameters, in order to observe how the flow impinged the plate as well how it behaved without. 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 to be visualized, analyzed, and compared to CFD results. The image analysis indicated that the virtual orifice did increase exit velocity 8% over the constant-diameter synthetic jet which mirrored CFD results.

The development and study of a virtual orifice synthetic jet

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