MORPHOLOGICAL EVOLUTION CHARACTERISTICS OF LIQUID SHEETS FORMED BY MICROJETS IMPINGEMENT

This paper employs high-speed cameras to capture the flow morphology of liquid sheets formed by the impingement of microjets from both frontal and lateral perspectives, aiming to gain an in-depth understanding of the physical mechanisms governing the morphology evolution of liquid sheet. The Weber number of the jet varied between 6.3 and 404.5. Experimental results indicate that as the velocity of microjets increases, the flow morphology transitions from laminar to turbulent. The resulting liquid sheet undergoes various flow morphologies, including liquid chain, closed liquid sheet, destabilized liquid rim, and undulating liquid sheet. From a lateral perspective, stationary capillary waves are observed near the jet impact point, with the wavelength exhibiting a trend of rapid decrease followed by a slow reduction with increasing Weber number. An interface instability state is observed at the top of the liquid sheet, where oscillations induce interface disturbances propagating downward along the sheet edge, forming liquid beads that can develop into liquid threads. The generation of liquid threads removes some fluid from the liquid sheet, causing tearing above the base of the thread. The appearance of turbulent disturbances in the jet excites surface oscillations in the liquid sheet. Intermittent disturbances result in rapid attenuation of sheet oscillations after the disturbance disappears, while continuous turbulent disturbances stimulate vigorous and sustained oscillations in the liquid sheet. This finding confirms that introducing finite-sized disturbances by turbulence is necessary for exciting liquid sheet oscillations. Furthermore, sheet oscillations accelerate the formation of liquid droplets from liquid sheet and influence the spatial distribution of filamentous liquid threads. The swinging of the liquid sheet causes the convergence of tearing points on both sides towards the sheet's center, forming downstream elongated liquid threads, completing the atomization process from sheet to threads and finally to droplets. The study also employs shadowgraphy to obtain droplet diameter data during the transition from laminar to turbulent flow, revealing a shift from a multimodal distribution to an unimodal distribution conforming to the Gamma function. The observed evolution patterns and mechanisms of liquid sheet formation due to microjet impact contribute theoretical support and quantitative insights for related applications.