Effects of driving signal waveshape on the velocity field and heat transfer of impinging synthetic jets

The present work reports on the measurements of the velocity fields and the heat transfer of impinging synthetic jets driven by different waveshapes. The actuator consists of a loudspeaker oscillating inside a cavity provided with a circular nozzle. Sinusoidal input voltage signals with variable ejection duty cycle d (ratio of the duration of the positive part of the signal to the total period) are applied to investigate the influence of the waveshape on the jet dynamics and the thermal performance. A parametric study is carried by varying both d (0.3, 0.4, 0.5, 0.6, 0.7) and the Strouhal number St (0.062, 0.12, 0.19) at constant value of the Reynolds number (3,000) and fixed nozzle-to-plate distance (equal to 2 nozzle exit diameters). The phase and time-average evolution of the synthetic jets is characterized by means of planar Particle Image Velocimetry; simultaneous measurements of the heat transfer coefficients are carried out via infrared thermography using a heated thin foil as heat flux sensor. The present results show that d plays only a minor influence on the time averaged velocity structure for all the values of the Strouhal number investigated. Nevertheless, for the highest value of St increasing d has a detrimental effect on the heat transfer. The phase-averaged velocity measurements reveal that this behaviour is essentially related to the reduced strength of the synthetic jet vortices formed during the ejection phase, which in fact dominate the heat transfer in such a configuration.