Characterization of Gasoline Spray-Wall Interactions

Doktorsavhandling, 2004

Interest in spray-wall interactions has intensified recently because of the development of direct injection spark ignition engines. In this type of engine, impingement of the spray on the piston and cylinder walls leads to increased emissions of hydrocarbons and soot. The high level of hydrocarbon emissions is one of the major drawbacks of the direct injection spark ignition engine, so it is important to obtain detailed knowledge about the processes involved in spray impingement on surfaces and their effects. Acquiring such knowledge was the fundamental aim of the thesis. A literature survey was done to examine recent advances in our understanding of drop impingement on surfaces. Experiments described in the literature indicate that the outcome of spray impingement is controlled by various spray parameters, surface conditions, and liquid properties. A disadvantage of the pervious experiments is that most were performed with water droplets or diesel sprays and in atmospheric pressure. In this study, which was a part of the EU-project DWDIE (5th framework program, contract no. ENK6-CT2000-00051), experimental investigations were carried out on real gasoline sprays impinging on a heated wall under realistic engine conditions. The sprays were examined experimentally using both back-lit visualization and Phase Doppler Anemometry in a high-temperature, high-pressure spray chamber. From the experiments, data regarding the spray behavior, and the dependence of the secondary spray's properties on the wall properties and the ambient conditions were obtained. The wall film build-up and evaporation during the impingement process were examined by measuring the wall film thickness with a device specifically developed for the project. The experimental database was used to validate some of the existing models for simulating spray impingement on walls. The models (one developed within the framework of the project and the rest found in the literature) were implemented into the 3-dimensional CFD-code AVL FIRE™, which has a subroutine related to wall films and associated phenomena. The performance of the different spray-wall interaction models were investigated using a calculation domain capturing the geometrical features of the experimental set-up. The wall film dynamics and the wall-to-film heat transfer were modeled using the standard models available in the CFD-code AVL FIRE™.