Impingement Cooling Using a Virtual Orifice Synthetic Jet

Electronic devices continue to become smaller, more sophisticated and faster performing, needing to dissipate the heat generated in smaller areas. Researchers have turned to unconventional methods to enhance the local heat transfer rate in small, confined spaces, such as synthetic jets. These jets use oscillating flow, which have a zero net mass flux, to produce jet flow through a given orifice. These devices have a significant momentum flux, creating vortex rings that increase mixing and the heat transfer rate. Previous experiments have shown that a contracting orifice diameter can produce a higher momentum flux, enhancing the heat transfer rate by up to 6 times that of natural convection. This used a mechanical aperture, which presented problems with miniaturization as well as fatigue strength. To avoid this issue, a virtual orifice is 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 the volume needed. Computational fluid dynamics simulations were used to optimize the geometry for the design, selecting dimensions for optimal exit speeds and impingement plate cooling. Simulations demonstrated that the virtual orifice synthetic jet produced a 5% increase in convective heat transfer rate compared to an ordinary synthetic jet with the same characteristic dimensions. The design was fabricated and used to cool a square heater mounted on an impingement plate. The heater temperatures were measured for distances ranging from 2 to 8 jet diameters, showing comparable performance to the simulations. The virtual orifice design did not prove as promising as originally hoped, only providing a 3% increase in heat transfer, and should be looked at for other applications such as flow control or propulsion.