Heat transfer and fluid flow characteristics of impinging jet using combined device with triangular tabs and synthetic jets

The fluid flow and heat transfer characteristics of an impinging jet with a combined active-passive device were investigated. The combined device consisted of triangular tabs and a synthetic jet array that provided periodic disturbance to the jet's shear layer. The Reynolds number based on the nozzle diameter (D) was 3.9 × 104, and the nozzle-to-wall spacing (L/D) was 2. Four triangular tabs were attached to the nozzle outlet. The synthetic jets were issued at an angle of 30° against the primary jet. Under the triangular tab operation, the longitudinal vortices were generated and the transverse vortex of the primary jet collapsed. Moreover, the velocity of the primary jet increased because of contracted flow. Under the synthetic jet operation, the development of the transverse vortex was promoted due to the increased instability of the shear layer. The jet's periodic disturbances strengthened because of the use of synthetic jets. The combined device operation led to increased velocity magnitude and periodic disturbance of the primary jet; thus, the peak RMS value of the velocity fluctuation was enhanced and the peak local Nusselt number near the stagnation point increased by 35% compared with cases where the device was not used. Additionally, the area-averaged Nusselt number was 10% larger than that obtained without the device. The heat transfer was governed by the jet's velocity, RMS, and periodic velocity fluctuation. These results indicate that the fluid flow and heat transfer characteristics of an impinging jet could be controlled by optimizing the actuation conditions of the combined active-passive device.