No abstract

The behaviour of impinging droplets is a field that has been studied for over 100 years mostly focused on impingements upon solid substrates or static films. Few studies report cases when the moving film might influence the impact outcomes. In many situations where droplet impact is industrially relevant there is film motion and relying on observations and correlations for impacts on static films might not be reliable. The University of Nottingham’s Gas Turbine and Transmissions Research Centre is conducting a research program investigating impact outcomes and crater morphology for water droplets of around 2–3 mm falling under the influence of gravity and impacting on films created by water flow down an inclined plane. In the investigation reported here dimensionless film heights were in the range 0.77 to 1.8 and the plane was inclined at 10° to the horizontal. This paper details the investigation into the morphology of the crater formed during an impingement event. The properties of the impinging droplet are measured using a high-speed camera to provide a side-view of the impingement. Brightness-Based Laser-Induced Fluorescence (BB-LIF) is used to provide three-dimensional measurements of the crater during the self-similar inertial regime. This is accomplished by doping the fluid with Rhodamine 6G, and exciting the fluorescence with a 527 nm pulsed Nd-Yag laser. A second high-speed camera observes the impingement from below in order to provide information about the behaviour of the film. The development of cavity depth is compared to published models from work on both deep and thin static films. Further, the development of cavity width with time is compared against existing models from static film research. A modification to these models is proposed that includes the effects of film velocity. The effect of film movement on the cavity footprint is examined; both the equivalent radius and the cavity width are investigated and the differences compared to static film experiments are quantified. Some modifications to an established width model are suggested, and an effect of droplet diameter upon this cavity width is noted. The work shows that static film models are not universally applicable for moving films.

Aeroengine bearing chambers are geometrically complex, typically containing shafts, bearings, seals and stationary components. Oil is supplied for lubrication and cooling and so the chamber contains a highly rotating two-phase (oil/air) flow where the oil is typically present as droplets, ligaments, mist and films. These films may be thick or thin and film speed varies with chamber location. It is desirable to know a priori the outcome of a droplet-film impact event in terms of mass, momentum and energy transfer.There is a significant body of research on the interaction between droplets and static films. The experimental parameter space has been characterised on the basis of film thickness and impact parameter to predict the outcome of an impingement. The impingement of droplets on moving films has only begun to be investigated over the last decade and consequently models have not yet been developed and the parameter space has barely begun to be characterised.Within this paper results are presented from an experimental study in which water droplets of 3 mm and 3.8 mm at 20 • C falling under the influence of gravity impinged onto water films flowing down an inclined plane. Film temperature was 30 • C and film thicknesses were between 2.3 mm and 4.2 mm. High speed imaging was used to determine the impingement outcomes and cavity morphology. A high speed infrared camera was used to determine the extent of the thermally affected region and its temperature behaviour.We find that by using the resultant droplet velocity (combining droplet and film velocities) the film impingement outcomes can be characterised into regions very similar to those for static films. The data is presented as a function of splashing parameter and non-dimensional film thickness. It was observed that for these impacts on supercritical films (Fr > 1) there is less propensity for secondary droplet formation through jet breakup than on static and subcritical films (Fr < 1). Data was obtained for extent of the thermally affected region. It was found that the cooler droplet liquid spreads over the inside of the crater before heating up to film temperature. Development of crater shape and size was also studied and data compared to established models for droplet impact on deep static films. During the initial stages of an impact crater area increases similarly to that for static films although the crater shape itself is less similar and is asymmetrical due to the film motion.

Abstract. The effect of varying the film temperature from 15C to 60 C is investigated using high speed video imaging. It is demonstrated that increasing the temperature difference between the droplet and film liquid changes the size and frequency of the secondary droplets. The impact can be split into three regimes. In the crater expansion regime, the crater follows a self-similar behaviour. In the second stage, the crater becomes deeper and wider at higher temperatures possibly due to decrease in the viscosity or surface tension. It is seen that the crater collapse is less dependent on the temperature and occurs at fixed time for a particular Webber number.