Experimental study of mixed convection heat transfer on the heated plate with the circular-nozzle synthetic jet

This study experimentally examined the mixed convection heat transfer characteristics of a horizontal heated plate with the impinging cooling of down-blowing (Model 1) or up-blowing (Model 2) synthetic jet. The synthetic-jet device was made of a loudspeaker and a cylindrical acrylic chamber. The signal function generator and amplifier imported electric voltage at different frequencies and amplitudes to the loudspeaker, so that the loudspeaker's membrane surface vibrated at different displacements and frequencies. Then, the 3 mm-diameter circular nozzle on the end face of cylindrical chamber generated synthetic jet. The ratio of nozzle diameter (d) to the heated-plate diameter (D) was 0.06. The variable parameters included mean jet Reynolds number (Rej = 230-4593), Grashof number (Gr = 1.25 × 105-5.51 × 105) and relative impingement distance (c/d = 1-11). The experimental results showed that at a specified Rej, the Nu of various models basically increased with c/d. However, some cases showed that Nu declined as c/d exceeded some critical value. In addition, when the impingement distance was short (c/d = 1) and in a small Reynolds number range (Rej ≦ 1155), Model 1 generated the cancelation effect (forward impinging jet and natural convection are opposite in direction) and enhancement effect (deflected wall jet intersects natural convection transversely) of mixed convection heat transfer at the same time. When c/d increased (e.g. c/d = 11), there was a large forward impingement contact area due to the diffusion effect of jet, and the cancelation effect of forced convection and natural convection of Model 1 was more significant. The enhancement effect of mixed convection heat transfer was formed in Model 2. When Rej was small, the Nu of Model 2 was larger than that of Model 1. When Rej ≧ 2105, both models had similar Nu, and it did not vary with Gr, meaning the overall heat transfer was dominated by forced convection. Finally, this study established an empirical Nu correlation of mixed convection in terms of Rej and Gr at various c/d for Model 1 and Model 2, applicable to heat transfer analysis and optimal design of synthetic-jet device.