Application of drag reduction model accounting for droplet number density to a diesel spray

DDM (Discrete Droplet Model) is widely used for the numerical analysis of a fuel spray. In order to achieve highly accurate calculations using DDM, many mathematical sub-models such as breakup, drag force and collision are employed with DDM. In the conventional DDM, an isolated droplet (parcel) is assumed and the influence of surrounding droplets is not directly considered. However, it is well known that the state of the wake flow of droplet or the vortex behind droplet changes by the distance between droplets, and the drag coefficient is influenced. Especially, this effect is important in a dense spray. In this study, a newly developed drag force model, containing the drag reduction effect with the droplet spacing as a parameter is adopted and also, the effect on the spray tip penetration and the spray volume is examined. As a result, the spray shape and Sauter mean diameter were found to depend on the mesh size that defines the droplet spacing. The drag force reduction by the droplet number density tends to increase the spray volume. The calculation of a diesel spray at the injection pressure of 133 and 55 MPa indicated better agreements with experiments compared to the result using a conventional model.