Velocity and area ratio effects on a coaxial impinging jet

The formation of a circular water film by a coaxial liquid jet impinging onto a horizontal plate is a phenomenon largely unexplored in the literature. To investigate the effects of velocity ratio and area ratio on such coaxial impinging jets, a coaxial jet nozzle with an inner diameter of 2 mm and an outer diameter of 8 mm was designed. Experiments and numerical simulations were conducted using this nozzle. The experimental research employed high-speed photography to capture the circular water film formed by the coaxial jet impacting the plate. The influence of velocity ratios ranging from 0 to 0.28 was studied at a fixed inner jet flow rate. Numerical simulations were conducted with a constant outer-to-inner nozzle diameter ratio of 4, focusing on the effects of varying area ratio on the flow patterns of the coaxial impinging jet. The results demonstrate that the simulation strategy accurately predicts the radius of the circular water film, liquid film thickness, and wall shear stress. At a fixed inner jet flow rate, the water film radius increases with the velocity ratio, while the turbulence level within the film initially increases and subsequently decreases. Under a fixed total flow rate of 6 l/min, a larger area ratio leads to stronger interactions between the inner and outer jets, significantly impacting the coaxial jet structure, which is highly dependent on the velocity ratio. Similar to single jets, the coaxial jet exhibits good normalization and self-similarity after fully merged. The mutual interactions between the inner and outer jets of the coaxial impinging jet result in more bubbles in the water film, and a slower decay of wall shear stress than that of single jets, both of which are beneficial for surface cleaning.