Time and phase average heat transfer in single and twin circular synthetic impinging air jets

This work presents an experimental investigation of impingement heat transfer in single circular synthetic jets and twin circular synthetic jets in phase opposition. All experiments have been performed at Reynolds number equal to 5100 and Strouhal number equal to 0.024 varying the jet axes distance and nozzle to plate distance. An IR camera is used as temperature transducer for both time average and phase average heat transfer measurements. Time average heat transfer maps show that single synthetic impinging jets have a behavior similar to that of continuous jets: at low nozzle to plate distance (up to 4 diameters) the heat transfer distribution shows an inner and an outer ring shaped region of maximum while for higher nozzle to plate distance such a feature disappears.While obviously the twin configurations produce an heat transfer enhancement due to the fact that two jets instead of one are impinging, the interaction is found in general to have a beneficial effect. The physical behavior is in common between single synthetic jets and twin configurations at jet axes distance equal to 3 and 5 diameters. The twin circular synthetic air jets, with jet axes distance equal to 1.1 diameter, shows a different behavior with respect to single synthetic jet for H/D equal to 2 but for values of H/D higher than 4 it starts acting like a single synthetic jet differently from the other twin configurations which behave as two separated synthetic jets. Phase averaged measurements allow for an accurate description of the heat transfer mechanism: at low nozzle to plate distances (2 and 4 diameters) the heat transfer is dominated by the unsteady impinging flow produced by the ring vortex that sweeps the wall and causes the formation of the inner and outer ring shaped regions. At higher nozzle to plate distance the heat transfer is due also to a steady and less coherent turbulent flow since the impingement occurs after the potential core and the saddle point.