Subproject A02 Tropea/Roisman
Droplet/spray impact onto a wall film of different fluids
Fuel mixture generation and emissions in modern combustion engines are influenced by the interaction of fuel spray droplets impacting onto lubricating oil films in the cylinder. A further influence on emissions is found in SCR DeNOx systems, in which urea-water solutions are injected into catalytic converters and impact onto existing liquid films of varying urea concentration. In both cases the drop/wall interaction is affected by the fact that the drop and the liquid film are different liquids and may exhibit different degrees of miscibility. The composition of secondary drops resulting from the impingement and the state of the liquid film influence significantly the mixture preparation in either the engine or the converter. It is therefore of importance, also for the correct simulation of this physical phenomenon, to understand the mechanisms of spray impact with differing liquids.
The goal of this work is to better understand and to model the impact of a single droplet onto a liquid film of a different fluid. Specific aims are to measure the splashing threshold, secondary droplet diameter, and the composition/dilution of the remaining wall film. These properties are determined by the impact parameters and combinations of liquid properties (viscosities and surface tensions). The measurements and observations of the drop impact will be used as basis for the development of a theoretical model. The model should be adapted for further numerical simulations of spray impact.
In order to conduct the experiments, a test facility as pictured in Fig. 1 will be designed and constructed. The experimental setup consists of a droplet generator to generate steady train of monodisperse droplets, a tilted surface made of transparent material to allow optical access to the film, a collection container including a balance, a nozzle to create an optional gas wall jet, diverse optical instruments including a high-speed camera, a Chromatic-Confocal-Sensor and Laser Induced Fluorescence as well as a system of liquid supply and control.
The wall film is generated through a slot in the plate and the thickness is determined by the volume flow and the tilt angle of the plate. Using the collection container the total volume flow as well as the mixture ratio, in case of immiscible fluids, can be determined. A high-speed camera allows the observation and characterization of crown formation and splash as well as the determination of the size of the secondary droplets.
To characterize the wall film, two techniques are used: Chromatic-Confocal-Sensor (CHR) to measure the film thickness and Laser Induced Fluorescence (LIF) to measure the fluid ratio within the film.
The alignment of the droplet generator and tilt angle of the impact plate allows variation of the impact angle between 0° and 90°. Furthermore, the Reynolds and Weber numbers, and therefore the impact parameters, as well as the wall film thickness, can be varied. Different liquid combinations are used, for example isooctane/oil (Shell Helix Ultra VS 5W 30) and an urea – water solution of different concentration.
On the theoretical side a model will be developed to predict the thickness of the crown formation as well as the film thickness of both fluids within the crown wall film.