Experimental investigation of flow and thermal characteristics of synthetic jetissuing from sharp-edged orifices

The present experimental study reports the flow and heat transfer characteristics of a synthetic jet issuing from a sharp-edged orifice (diverging-shaped orifice). The experiments are carried out for a varied range of opening angles of sharp-edged orifices (theta = 0 degrees, 30 degrees, 60 degrees, 90 degrees, and 120 degrees), Reynolds number (Re = 3243-8143), different jet-to-surface spacings (z/d = 1-16), and for two different values of orifice thicknesses, t = 5 mm (t/d = 0.33) and 10 mm (t/d = 0.66). The hot-wire anemometry is used to study the flow characteristics of synthetic jet, while heat transfer characteristics are studied by using a thermal imaging technique. The time-averaged flow fields associated with sharp-edged orifices reveal that orifices with t = 5 mm and 10 mm exhibit saddle-backed and top-hat velocity profile shapes, respectively. The results show that for a square-edge orifice (theta = 0 degrees), the heat transfer rate decreases with an increase in orifice plate thickness from 5 to 10 mm, while the opposite trend in heat transfer is observed with sharp-edged orifice. The heat transfer rate with a 10 mm thick sharp-edged orifice is higher than the 5 mm thick sharp-edged orifice for all the tested opening angles. Furthermore, the results also show that for sharp-edged orifices, the heat transfer rate increases with the increase in opening angle from theta = 0 degrees to 60 degrees, while it decreases with further increasing from theta = 60 degrees to 120 degrees. The maximum value of average Nusselt number (Nu(avg)) is obtained for theta = 60 degrees for both the orifice thicknesses (t = 5 and 10 mm), and this effect is found to be more pronounced for t = 10 mm orifice. For sharp-edged orifice (theta = 60 degrees), the maximum enhancement in Nu(avg) is found to be 12.66% and 23% higher for t = 5 mm and 10 mm, respectively, compared to the equivalent square-edged orifice (theta = 0 degrees). The cause for variation in heat transfer rate with sharp-edged orifices is interpreted due to the effect of flow recirculation and mass flow rate. A correlation has been proposed for Nu(avg) as a function of different opening angles. The present finding is useful for the optimization of the synthetic jet geometrical parameters for the effective heat transfer rate.