Status Report - Droplet Size Measurements

Kurzfassung

MoPAA stands for Measurement of Prefilming Airblast Atomisation. This project is part of the Common Programme 2003 - Annex II (Boutier-Dankert-2002) for the ONERA-DLR cooperation, that formulates the following tasks: -understand the fuel film disintegration at elevated pressure -correlate the film disintegration characteristics with the spray characteristics -obtain a detailed set of measurements performed at elevated pressure for CFD initial conditions Previous work on this subject was performed by P. Berthoumieu (Berthoumieu-2001), investigating the macroscopic behaviour of a planar liquid sheet with visualisation methods. The present work comes as a quantifying complement to the qualitative measurements of Berthoumieu. Phase Doppler Anemometry (PDA) measurements on the spray provide quantitative information on particle size, distribution and velocity. Furthermore the spray expansion is investigated. Line-of-sight particle size measurements with a MALVERN instrument are used for a parametric analysis of droplet size distribution additional to the operating range investigated with the PDA. The following results establish the evolution of the Sauter Mean Diameter (SMD) and spray diffusion as a function of air inlet velocity, kerosene inlet velocity and air static pressure. The experiments are performed at room temperature. Due to the negligible effect of vaporization in the isothermal case, the droplet size distribution measured in the far field represents the final result of the atomisation for a given operating condition. A comparison is done between hydrodynamic instabilities previously observed at low velocity and pressure regimes on the surface of the film, and the resulting spray. The resulting discussion states that a deterministic approach to model the spray distribution as a function of the liquid sheet inlet characteristics as defined by previous experiments at atmospheric pressure (for instance the "wavy sheet motion" followed by "prompt atomisation" as described by Lefebvre (Lefebvre-90)) may not be extensible to real engine conditions. In other words, here the liquid sheet behaviour along the prefilm surface seems of little impact on the resulting atomisation. The influence of gas phase parameters (in terms of pressure, velocity and temperature), and the turbulence level in the air and liquid boundary layers along the prefilm have to be investigated further into details in order to establish the prominent driver of the atomisation in realistic operating conditions. The last part of this report closes the project MoPAA on this statement, and suggests a tentative program MoPAA II, which is due to begin in September 2003.