| Popis: |
The accuracy and robustness of spray models and their implementation in current commercial CFD codes vary substantially. However, common features are that the resulting spray penetration and levels of spray-generated turbulence - two factors that strongly influence the rate of heat released during combustion - are to a great extent grid size-dependent. In the work presented here a new kind of spray model has been implemented and thoroughly tested, under various ambient conditions, in the open source code OpenFOAM. In addition, since the turbulence model applied in simulations is known to strongly affect spray penetration rates, results obtained using both the standard k-e and RNG k-e models have been compared. In the new spray model, designated VSB2, the traditional Lagrangian parcel has been replaced by a so-called stochastic blob containing droplets with a distribution of sizes, rather than a number of uniform-sized droplets. These blobs do not interact with the grid directly, but through bubbles of locally determined size. One advantage of the VSB2 spray model is its less grid size dependence due to the interaction with the gas phase in the bubbles instead of the entire grid cell it currently occupy. Another benefit of the model is its robustness, resulting from rigorous calculation of equilibrium values for momentum and thermodynamic (saturation and temperature) parameters affecting transfer rates between the phases ensuring a bounded solution. Results obtained using the code are compared here with experimentally acquired data regarding spray penetration under various ambient conditions. Qualitative comparisons of the evolving spray shapes are also presented. |
