Simulation of synthetic jets with non-sinusoidal forcing functions for heat transfer applications

Abstract

Effective techniques for cooling electronic devices are essential for dealing with increasing heat loads associated with higher density manufacturing processes. Many conventional cooling techniques are reaching the limits of their effectiveness, and more novel methods of heat transfer are necessary to cope with these higher cooling demands. Devices based on zero net mass flux jets (or synthetic jets) show promise in this area and are being actively investigated by a number of researchers. Typically in these devices the piston or diaphragm creating the synthetic jet is driven by a sinusoidal motion. In the present work the effect of using a non-sinusoidal forcing function was investigated using Computational Fluid Dynamics (CFD). A synthetic jet consisting of a channel, cavity and moving piston was modelled in order to capture accurately the flow and heat transfer from a heated surface. The open-source CFD software OpenFOAM was used for the simulations, and in particular its moving dynamic mesh libraries. Custom modifications were implemented to create the non-sinusoidal motion of the piston. Within OpenFOAM, mesh motion is facilitated by applying suitable boundary conditions for the displacement(s) at each boundary, while at each timestep a Laplace equation is solved for the displacement of the internal mesh nodes based on the displacement of the boundary nodes.Within this framework the nonsinusoidal motion of the piston was modelled by creating a custom boundary condition to give the desired piston displacement at each timestep. The piston motion was described by a phase modulated sinusoidal function, with the operating frequency of the modulator and carrier kept the same. The modulator phase was selected so that the piston motion exhibited a rapid discharge stroke, followed by a slower intake stroke. This motion increased the momentum of the fluid that impinged on the heated surface, while keeping the operating frequency constant. Behaviour of the system with sinusoidal motion of the piston was also simulated to allow the heat transfer enhancement due to the non-sinusoidal motion to be determined. A two dimensional synthetic jet was considered, with the k-ω SST turbulence model used to account for turbulence in the flow. An additional transport equation was solved for the energy, and heat transfer, in the system. The synthetic jet system investigated had a cavity width of 10 mm and a mean depth of 10 mm. The orifice had both a width and depth of 1 mm, while the heated surface was located 10 mm from the orifice discharge. Oscillating frequencies of 250, 500 and 1000 Hz were investigated.Comparison of the heat transfer results between sinusoidal and non-sinusoidal operation showed heat transfer to be between 5 and 10 percent higher for non-sinusoidal results.