Dynamic measurements of thermal boundary layer and heat transfer rates along a heated flat plate under the influence of ultrasonic field

We report observations made through direct visualization of perturbations in the thermal boundary layer over a vertically heated flat plate under the influence of an externally applied ultrasonic field. Under the natural convection regime, the thermal field has been mapped using a gradients-based imaging technique, namely rainbow schlieren deflectometry (RSD). The spatial and temporally resolved temperature gradient field has been retrieved through the quantitative processing of the two-dimensional hue distribution captured in the form of schlieren images. Near wall temperature gradients have been subjected to energy balance to determine the local variations of heat transfer rates along the length of the heated plate at different instants of time upon the application of ultrasonic field. Exploiting the potential of RSD, a one-on-one correspondence has been drawn on local perturbations of the thermal boundary layer (quantified in the form of boundary layer thickness) and the resultant spatial and temporal variation of heat transfer coefficient. The evaluation of the applied ultrasonic field showed a reduction in the thickness of thermal boundary layer and a corresponding augmentation of heat transfer coefficient by $\approx 103.7\%$. To the best of our knowledge, the present experimental study, is the first attempt to simultaneously map the real-time perturbations of thermal boundary layer profile subjected to an ultrasonic field and the corresponding variations in the local heat transfer rates in the vicinity of a heated flat plate under natural convection regime.