Centerline heat transfer coefficient distributions of synthetic jets impingement cooling

Abstract

The paper presents the results of experimental investigations of heat transfer coefficients, velocity and pressure distributions of synthetic jets for two cases: constant Reynolds number and variable Stokes number and vice versa. Measurements were made for synthetic jet Reynolds number in the range of Re=5440–11,475, dimensionless stroke length L0=7.49–38.5 and dimensionless jet-to-plate spacing x/d = 0–20. The synthetic jet time-averaged and momentum velocity as well as the pressure for each case are presented. The centerline turbulence intensity and the Euler number are also reported. The centerline heat transfer coefficient distributions are compared with the distributions of the various velocity and pressure parameters. A new correlation between the local Nusselt number and the local Reynolds number is presented. Under synthetic jet impingement cooling the maximum of the stagnation point heat transfer coefficient appears mostly at axial distance of x/d ≈ 5. The location of the maximum Nusselt number at x/d ≈ 5 is caused by the modification of the heat transfer distribution due to the recirculation of hot air. With the decrease of the heat power released into the control volume by the impingement plate (heater) this modifying effects becomes less pronounced thus the maximum of the heat transfer coefficient shifts towards the location of about x/d = 2. The maxima of various flow parameters are generally observed at about x/d = 2 what was also reported by other independent researchers. Thus, it may be concluded that the purely mechanical phenomena does not full coincide with thermal ones in the case of synthetic jet impingement cooling.