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Computational Dynamics May 11, 2001 06:52

Wall Film modelling
Wall Film Modelling

Liquid wall films occur in many industrial processes and in everyday experience such as rain on car windows and on aircraft wings. But perhaps the most challenging application for a CFD code is in engine design.

Liquid fuel wall films form in diesel and gasoline engines due to impingement of injected fuel sprays on pistons and intake valves and also onto the surfaces of ports and cylinders. Once formed, the film develops dynamically under the influence of gas flow and wall movement. At the same time, heat exchange with walls and the surrounding gas leads to evaporation, affecting the composition of the mixture and hence the complexity of the flow and combustion process. The presence of liquid fuel trapped on walls is, among other phenomena, blamed for increased soot formation and unburnt hydrocarbon emission, especially under cold start conditions.

CFD simulation can help engineers understand and optimise these highly complex processes, providing a powerful tool for improving efficiency and reducing pollutant emission in internal combustion engines.

From a CFD viewpoint, the challenge is to model an already complex set of processes and phenomena, together with the additional complication of needing to handle different various cell types in the dynamically changing geometry and mesh topology of an engine simulation.

The film model implemented in STAR-CD is based on work carried out by Professor Gosman's research group at Imperial College. It is linked with the existing Bai splashing model which predicts the behaviour of droplets hitting walls. These models simulate droplets as they "bounce", "stick" or "splash" and their contribution to a wall film as well as the dynamic development of the film itself. The functionality of this new capability of STAR-CD is as follows:

The model allows for convection in the film, mass transfer with the gas phase, as well as momentum and heat transfer with the walls and gas. These transfer processes are modelled with standard wall functions. The coupling between film and gas is realised through source terms and is completely embedded in the STAR-CD solution procedure. This is accomplished in both the sequential and parallel mode of operation. All the relevant film quantities, such as temperature, thickness, mass and velocity are available for post-processing.

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