Droplet Impact on a Curved Moving Liquid Film

Abstract

Abstract The European Commission has set targets to reduce the emissions of CO2 and NOx by 75% and 90% respectively, per passenger kilometre, by 2050. Computational modelling of elements within the aero-engine will play a key role in achieving this goal. Currently, the computational models used to characterise impact outcomes from droplets splashing on wall films in bearing chambers are based on correlations obtained for droplet impacts on plane films. The present study bridges that gap, being the first experimental study investigating droplet impact on a film moving on a curved surface. Experiments were conducted on a curved channel where the variation in the channel curvature provided the opportunity to investigate droplet impact at three different curvatures with three corresponding impact angles. Two droplet diameters of 2.34 mm and 1.90 mm were obtained using two different needle sizes attached to a syringe. The height of the syringe from the liquid film was varied to investigate the effect of droplet impact velocity. Five flow rates were considered to investigate the effect of film thickness on droplet impact. To investigate the dynamics of the droplet impact a high-speed imaging technique was used. Qualitatively, two different impingement outcomes were observed, namely, crown formation and crown splashing. The key difference between the two outcomes is in the production of secondary droplets from the crown in crown splashing. Quantitatively, it was observed that at lower angles of impingement (less oblique), the transition from crown formation to crown splashing agrees strongly with previous literature for impact on static films and films on an angled plane where the transition value of the splashing parameter is 2100 based on the absolute velocity of the droplet. At the higher angles of impingement (more oblique), the critical splashing parameter (K) value was found to increase, as with droplet impact on an inclined surface or liquid film, but to a significantly higher value (2800 at 38°) than was typically obtained for an inclined plane (2400 at comparable angle). This indicates that the transition from crown formation to splashing at the highest radius of curvature is different to that of inclined planes at higher angles. Investigating impact event duration, the present study suggests that with the increase in the impact energy, the time required for the film to come back to its original state increases.