Axisymmetric wall jet development in confined jet impingement

Abstract

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layered wall jet structure. Experiments were conducted using stereo particle image velocimetry at three different confinement gap heights (2, 4, and 8 jet diameters) across Reynolds numbers ranging from 1000 to 9000. The rotating flow structures within the confinement region and their interaction with the surrounding flow were dependent on the confinement gap height and Reynolds number. The recirculation core shifted downstream as the Reynolds number increased. For the smallest confinement gap height investigated, the strong recirculation caused a disruption of the wall jet development. The radial position of the recirculation core observed at this small gap height was found to coincide with the location where the maximum wall jet velocity had decayed to 15% of the impinging jet exit velocity. After this point, the self-similarity hypothesis failed to predict the evolution of the wall jet further downstream. A reduction in confinement gap height increased the growth rates of the wall jet thickness but did not affect the decay rate of the wall jet maximum velocity. For jet Reynolds numbers above 2500, the decay rate of the maximum velocity in the developing region of the wall jet was approximately −1.1, which is close to previous results reported for the fully developed region of radial wall jets. A much higher decay rate of −1.5 was found for the wall jet formed by a laminar impinging jet at Re = 1000.

The influence of aspect ratio on the iso-thermal flow characteristics of multiple confined jets

“…These findings are attributed to the effect of jet impingement on the bluff endwall that influences the development of the resulting flow and are consistent with a previous study employing an unconfined single jet. 17 Hence, it can be concluded that the position of the ERZ exhibits a strong dependence on the aspect ratio L c /D c for low-swirl configurations (θ j = 5 • ), which is most significant for L c /D c < 2.…”

“…Similarly, the PIV measurements of Long et al 10,11 found a strong dependence on jet angles (α j and θ j ) of the instantaneous and mean flow-fields generated with multiple symmetric jets. However, to the best of our knowledge, little or no information is available for the key geometrical parameters (e.g., L c and D c ) of the confined space for multiple jet configurations, although the aspect ratio of geometry has been found to have a significant influence on the flow-fields of free jet impingement [17][18][19] and cavity flow. [20][21][22] Importantly, the existing experimental data provide insufficient information to adequately understand the effect of the aspect ratio on the multiple confined jets in which both α j and θ j are variables.…”

“…16,17,[23][24][25][26][27] It has been found that the distance between the jet exit and an end plate, termed "confinement height" (H c ), can significantly influence the position and strength of the recirculation regions, wall jet development, and jet velocity. 17,23,24,26 It has also been found that the influence of the aspect ratio of a confined space is significant for near-field but negligible for far-field within a swirled jet chamber. 27 However, while these previous studies provide useful insight, they are of limited value for model validation due to the lack of information available for the inflow and boundary conditions.…”

“…For example, as L c /D c reduces from 2.5 to 1.5, the location of the stagnation point moves upstream from x/L c = 0.65 to 0.5, while the decay of U c /U e decreases significantly for L c /D c = 1.5. This is possibly caused by the effect of the end-wall on the development of central resulting jet flow that reduces the entrainment rate between the jet and surrounding fluids 17,35. Importantly, large data fluctuations were found at x/L c ≈ 0.48, with RMS ≈ 0.037 for the case of L c /D c = 1.5, which is consistent with the strong gradients caused by the combined effects of a radial outlet flow, a bluff end-wall, and a cylindrical chamber.…”

Interaction of dual sweeping impinging jets at different Reynolds numbers

“…As shown in Fig. 1, this implies that the flow field can be divided into three distinct regions: the free-jet, a stagnation region, and the wall-jet region (Guo et al, 2017 andYadav andAgrawal, 2018). Terzis (2016) correlated the flow dynamics and enhanced heat transfer by using both particle image velocimetry (PIV) and thermochromic liquid crystals (TLCs) to measure the flow field and local heat transfer rate, respectively.…”