FWF-Simulation d Flüssigkeitsz. - Simulation of liquid atomization and spray formation

Many processes of energy conversion or production of materials rely on the atomization of a liquid into a dispersed state in a gaseous environment. The sprays thus produced have been of interest both for the technical applications and for scientific research since a long time. The main goal of all research and development was to predict the size distribution of the droplets in the sprays produced, so as to enable a best possible layout of atomizers and process equipment for maximum efficiency and/or highest product quality. This aim has still not been reached to date. The present project is designed to yield a novel computational technique for predicting the spray drop size distribution using a turbulence modelling approach. Sprays may be produced by the disintegration of liquid jets or sheets, depending on the kind of atomizer and the kind of application at hand. In our project we concentrate on sprays produced by jet break-up. Jets may disintegrate into droplets by three different mechanisms: by a capillary (Rayleigh) instability, by dynamic interaction with the gaseous environment (Kelvin-Helmholtz instability), or by turbulent velocity fluctuations in the liquid phase, which destabilize the liquid system at high Reynolds and Weber numbers of the liquid flow. In 1936, v. Ohnesorge found that the above three break-up mechanisms can be identified with three regimes of the Reynolds and Ohnesorge (or Weber) numbers of the jet. In the present project a technique for modelling turbulent spray formation will be developed, which relies on the description of turbulent liquid motion and the simulation of the resultant two-phase flow using Large-Eddy Simulation (LES). The drop size distribution is computed using two different approaches – a presumed PDF, which is a generalised Gamma function, and a stochastic PDF model. Parameters needed to determine the PDF shall be provided by the LES solution of the flow field. Experiments in sprays produced by atomization of liquid jets will be performed by means of a spray test rig to be set up in the course of the project. The measuring technique used will be the phase-Doppler technique. The measurement results, which consist in PDFs of the drop size and velocity and their moments, will be used to validate the computational model results.