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22.4.1 Introduction

Spray-wall interaction is an important part of the mixture formation process in port fuel injected (PFI) engines. A fuel spray impinges on a surface, usually at the intake port near the intake valve, as well as at the intake valve itself, where it splashes and subsequently evaporates. The evaporated mixture is entrained into the cylinder of the engine, where it is mixed with the fresh charge and any residual gas in the cylinder. The mixture that is compressed and burned, finally exits through the exhaust port. The process repeats itself between 200 and 8000 times per second, depending on the engine.

Several cycles worth of fuel remain in the intake tract due to film formation on the walls. This in turn makes the film important in hydrocarbon emissions for PFI engines. Additionally, film can form inside combustion chambers of direct injection (DI) types of engines. In a direct injection engine, fuel is injected directly into the combustion chamber, where the spray can impinge upon the piston if the injection event is early or late in the cycle. The modeling of the wall-film inside a DI engine, especially in diesel engines, is compounded by the presence of carbon deposits on the surfaces of the engine. This carbon deposit absorbs the liquid film as it impinges upon it. It is believed that the carbon deposits adsorb the fuel later in the cycle, however this phenomena is very complex and is not well understood.

DPM particles are used to model the wall-film. The wall-film model in FLUENT allows a single component liquid drop to impinge upon a boundary surface and form a thin film. The model can be broken down into four major subtopics: interaction during the initial impact with a wall boundary, subsequent tracking on surfaces, calculation of film variables, and coupling to the gas phase. Figure  22.4.1 schematically shows the basic mechanisms considered for the wall-film model.

Figure 22.4.1: Mechanisms of Splashing, Momentum, Heat and Mass Transfer for the Wall-Film

The main assumptions and restrictions for the wall-film model are as follows:

If you wish to model a spray impacting a very hot wall, the wall-jet model may be more appropriate as the assumption in the wall-jet impingement model is that there is a vapor layer underneath the drops which keeps them from making direct contact with the boundary surface. This may be a more accurate assumption for in-cylinder diesel computations at typical operating conditions.

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