Perturbation of a laminar boundary layer by a synthetic jet for heat transfer enhancement

An experimental investigation of a cross-flow interaction between a synthetic jet and a flat plate laminar boundary layer is reported. The synthetic jet uses a piezo-actuator for displacing the diaphragm, thus enabling flow control in terms of the excitation amplitude and the modulation frequency. The role of these parameters on heat transfer enhancement from the flat plate is investigated. Measurements are carried out using hotwire anemometry for the flow field while the heat transfer coefficient and jet spreading are imaged respectively by liquid crystal thermography and the laser schlieren technique. Results show that the average heat transfer coefficient increases with excitation amplitude and a maximum of 44% enhancement is observed. Amplitude modulation at low frequencies also increases the heat transfer coefficient. Overall, the study indicates the efficacy of a synthetic jet actuator for heat transfer enhancement with excitation amplitude and modulation frequency as control parameters. Visualization using liquid crystal thermography shows dual streaks over the flat surface indicating the footprint of vortical structures from the synthetic jet inside the laminar boundary layer. The role played by amplitude modulation in enhancing heat transfer is clearly demonstrated by schlieren visualization and further confirmed by hotwire measurements. The synthetic jet also increases the average turbulence content inside the boundary layer. Power spectra show an overall increase in the amplitude of the low frequency fluctuations arising from synthetic jet actuation. The time-averaged velocity profile behind the synthetic jet shows similarity to the wake profile behind a surface-mounted obstacle. Analogous to physical obstacles such as ribs, these results show that a synthetic jet can also be used as a device for heat transfer enhancement in a boundary layer.