| Popis: |
A single impinging jet exhibits high heat transfer performance around an impingement point on a wall. However, the heat transfer performance deteriorates as it moves away from the impingement point. Consequently, multiple impinging jets are commonly introduced to overcome the shortcomings of a single jet: inhomogeneous heat distribution on the wall and a narrow heating area. However, inhomogeneous heat transfers still occur. Therefore, a new jet control is required to improve the uniformity of heat transfer. Meanwhile, blooming jets are produced by appropriate combinations of axial and helical excitations at the nozzle exit. Using appropriately selected excitations, a jet can split into two separate jets (bifurcating jet) or spread into a shower of toroidal vortex rings. Blooming jets exhibit good performances of mixing and diffusion, suggesting possible applications in flow control. However, studies regarding the heat transfer performance of blooming jets are non-existent. In this study, we conducted direct numerical simulations of blooming jets impinging upon a wall and investigated their flow characteristics and heat transfer performances. As control parameters, the impingement distance (the distance from the nozzle to the wall) and frequency ratio (the axial excitation frequency to the helical frequency) are varied. The vortex structures and velocity magnitude reveals flow modulations due to blooming control. With the time-averaged local Nusselt number, the heat transfer performance of the blooming jets is evaluated quantitatively. Compared with uncontrolled jets, the uniformity of heat transfer of blooming jets is better, suggesting their potential application for leveling the heat transfer of impinging jets. |
