Heat transfer characteristics of impinging steady and synthetic jets over vertical flat surface

In this paper, heat transfer characteristics of single-slot impinging steady and synthetic jets on a 25.4-mm × 25.4-mm vertical surface were experimentally investigated. The experiments were conducted with a fixed nozzle width of 1 mm. For the steady jet study, the parameters varied in the testing were nozzle length (4 mm, 8 mm, 12 mm, 15 mm), Reynolds (Re) number (100–2500), and dimensionless nozzle-to-plate spacing (H/Dh = 5, 10, 15, 20). Correlations for average Nusselt (Nu) number were developed to accurately describe experimental data. The heat transfer coefficient over a vertical surface increases with increasing Re number. For a small nozzle-to-plate spacing (H/Dh = 5), the average Nu number is not only a function of the Re number, but also a function of nozzle length. For large nozzle-to-plate spacing (H/Dh ⩾ 10) and a nozzle length larger than 8 mm, the heat transfer coefficient is insensitive to H/Dh and nozzle length. An 8-mm × 1-mm synthetic jet was studied by varying the applied voltage (20–100 V), frequency (200–600 Hz), and dimensionless nozzle-to-plate spacing (H/Dh = 5, 10, 15, 20). Compared to the steady jet, the synthetic jet exhibited up to a 40% increase in the heat transfer coefficient. The dynamic Re number was introduced to correlate heat transfer characteristics between synthetic jets and steady jets. Using the dynamic Re number collapses the synthetic and steady jet data into a single Nu number curve.