Studies on heat transfer and flow characteristics of continuous and synthetic turbulent jets

The results from the numerical investigations of both steady and unsteady circular turbulent jets impinging orthogonally on a flat surface are presented. The inlet velocity waveforms analysed consist of steady jets, square jets, triangular jets, and sinusoidal jets. In the case of unsteady jets, both intermittent and synthetic jets are examined. Reynolds number (Re) ranges from 5100 to 23,000, the normalized distance from the nozzle to the target surface (Z/D) spans from 2 to 12, and the frequency (f) varies from 0 to 200 Hz. Strouhal number varies from 0 to 0.14. To ascertain the validity of the numerical approach, a rigorous validation process was carried out. Baseline studies were conducted on steady jets. Unsteady jets outperform steady jets at low Reynolds numbers; however, at higher Reynolds number, the heat transfer capabilities of unsteady jets is inferior to steady jet. For instance, at Z/D = 6 and Re = 5100 the stagnations Nusselt number increases by 13% when intermittent square jet is used instead of steady jet. However, at the same Z/D and Re = 23,000, the stagnation Nusselt number for the intermittent square jet is 5% lower than that of the steady jet. A threshold frequency exists, above which an unsteady jet exhibits better performance than its steady counterpart, with this value determined to be 100 Hz. Among the unsteady jets studied, the intermittent square jet provides the highest effectiveness, whereas the sinusoidal jet exhibits the poorest performance across all operating conditions. The secondary peaks observed in steady jets at low Z/D and high Re are absent in unsteady jets. The boundary layer thickness is maximum for steady jet and minimum for intermittent square jet. The thickness of the boundary layer rises as Z/D increases and diminishes with an increase in f. At higher Z/D, the jet expands more prior to striking the surface, resulting in a thicker boundary layer near the surface.