Application of a hybrid RANS-LES CFD methodology to primary atomization in a coaxial injector

Non-zonal hybrid RANS-LES models, i.e. those which do not rely on user-prescribed zones for activating RANS or LES, have shown promise in accurately resolving the energycontaining and highly anisotropic large-scale motions in complex separated flows. In particular, the recently proposed dynamic hybrid RANS-LES (DHRL) approach, a method which relies on the continuity of turbulence production through the RANS-to-LES transition zone, has been validated for several different compressible and incompressible single phase flow problems and has been found to be accurate and relatively insensitive to mesh resolution. Time-averaged source terms are used to augment the momentum balance. An added benefit of the DHRL is the ability to directly couple any combination of RANS and LES models into a hybrid model without any change to numerical treatment of the transition region. In this study, an attempt is made to extend the application of this model to multiphase flows using two open literature coaxial two-stream injectors involving non-Newtonian liquids. For the first time, the new model has been successfully implemented in a multiphase framework, combining the SST RANS model with MILES LES approach. Favre averaging is used to ensure consistency between the momentum equations and the density fluctuations. It was found that the momentum source terms must be density weighted in order to ensure stability of the solution. Primary atomization findings with a stable model are encouraging. The spray character with the new model was somewhere between that of a RANS model and the LES result. Droplet sizes, which are indicative of the shear layer energy, for the RANS model were greater than the hybrid results, which were comparable to the LES result and matched the experimental expectation. Additionally, the new approach showed a liquid core breakup length close to that expected from the literature.