Temperature-variation effect of piston-driven synthetic jet and its influence on definition of heat transfer coefficient

This study focuses on the temperature-variation effects in the piston-driven synthetic jet impingement and its influence on the definition of heat transfer coefficient. For this purpose, three test runs are conducted, including temperature variation test in the free synthetic jet, temperature variation test in the presence of an impinging target, and heat transfer test. Five operational frequencies (ranging from f = 5 Hz to f = 25 Hz) and seven dimensionless jet-to-surface distances (ranging from H/d = 2 to H/d = 14) are considered in the current tests. Due to the friction heat generation from the actuator itself and the warm air suction from the 'synthesized' formation, the temperature of synthetic jet experiences apparent variation during its working process. At the worst, the temperature rise could reach up to 20 degrees C with respect to the ambient temperature. As a consequence, the selection of the reference temperature shows a significant influence on the Nusselt number definition for the synthetic jet impingement, especially under small jet-to-surface distances and high operational frequencies. At the worst, the Nusselt number defined in the jet temperature or the adiabatic wall temperature maybe twice times of that defined in the ambient temperature. As the jet temperature and the adiabatic wall temperature are affected by too many process parameters, therefore, the accurate determination of them is really a challenging problem in the applications, which remains further identifications. (C) 2020 Elsevier Ltd. All rights reserved.