Flow and heat transfer characteristics of blooming jets impinging upon wall using DNS

Abstract

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.

Direct Numerical Simulations of Free Jets Emerging from A Precessing Nozzle

“…Thus, the flow structure depending on the frequency ratio affects the mixing performance. Compared with the previous blooming results [13], the mixing performance is higher than that of the conventional blooming jets regardless of the frequency ratio, suggesting that the proposed method can achieve high mixing performance.…”

“…The Reynolds number, the Prandtl number are, Re = 1500 and Pr = 0.71, respectively. Similar properties for blooming jets were used in the literature [11,12,13]. Furthermore, as shown in Fig.…”

“…where α indicates the inclination angle of the nozzle, and β indicates the rotation angle expressed as β = 2π f r t. The symbols f a and f r denote frequencies of the axial excitation and the nozzle rotation, respectively. In this study, the frequency ratio R f = f a / f r = 2.4, 2.5, the excitation intensity A a = 0.15 and the Strouhal number St = f a D/V 0 = 0.45 are used as a parameter for blooming jets, with reference to the literature [13]. The inclination angle is set as α = 5 • .…”

“…Blooming jets have recently been studied using DNS [11,12], which yielded the detailed frequency characteristics of vortical structures and mean flow properties. The authors [13] further applied blooming jet to the inflow of impinging jets and mentioned blooming jets for heat transfer control. In the experiment [1], the axial mode is realized by driving the piston inside the upstream pipe, and the helical mode is realized by precessing the nozzle with multiple rods around the nozzle actuated.…”

“…In this research, to realize the blooming jet easily, we devise a relatively simple method of a rotating inclined nozzle. Based on the findings of the blooming jet [13], we calculated two typical frequency ratios and demonstrated that blooming jets could be realized by the proposed method.…”