Large eddy simulation of evaporating two-phase flows
The objective of this study is to develop a CFD tool for performing reliable large eddy simulation (LES) of the compressible evaporating two-phase turbulent flow in a gas turbine combustor. The KIVA-3V code originally developed by Los Alamos National Laboratory is used as a baseline code. The KIVA-3V code has been modified to facilitate LES calculations. Both the temporal and spatial accuracies of the original KIVA-3V code have been improved to second order. A one-equation subgrid scale (SGS) turbulence model is implemented to describe the unresolved turbulent subgrid effect. To ensure that there are sufficient particle numbers to capture the dynamic droplet dispersion process, the ETAB breakup model coupled with a new hybrid droplet-particle algorithm is also implemented into the code. Furthermore, the effect of the subgrid scale (SGS) velocity on the droplet dispersion is included. The SGS velocity is computed from the subgrid turbulent kinetic energy predicted by the one-equation SGS turbulence model. A new collision model based on the concept of "particle cloud" is proposed and implemented in the code. The new model greatly reduces the grid-dependence of the original O'Rourke model in a Cartesian mesh. The gas solver of the new LES version of KIVA-3V code, which will be referred as KIVA-LES hereby) is validated against large eddy simulations of natural and forced plane impinging jets. Predictions were carried out for different inflow conditions, which include a natural plane impinging jet with a random perturbation on the inflow plane and a forced plane impinging jet with a Strouhal number of 0.36, locked both in phase and laterally in space. The first simulation was performed to quantitatively study the mean flow and turbulence statistics. The computed field variables and turbulence intensity of streamwise velocity agreed well with the experimental results. The second simulation was performed to study the vortex structures of a forced plane impinging jet. The predictions captured the typical vortex structures of this kind of flow, such as spanwise rollers, successive ribs, cross ribs and wall ribs were reproduced by the simulation, which were also previously detected by the experiment of Sakakibara et al. (103) with digital particle image velocimetry (DPIV) system, but to our -best knowledge never wholly reproduced by numerical simulations to date. Moreover, the study has also led to some new findings related to the formation and evolution of successive ribs, cross ribs and wall ribs. The new collision model is tested against analytical solutions of simplified realistic collision problems in a box volume. The grid-dependence of the model is also checked against some spray test cases. The new collision scheme is computationally more efficient than the frequently used O'Rourke's (87) scheme since it abandons a sampling procedure to compute the collision number. The new model delivers sufficient accuracy in calculating the collision numbers in cases with uniformly distributed droplets although O'Rourke's model seems to perform better for these scenarios. However, for the prediction of a real spray in Cartesian gird, the new model has delivered much improved results. The predictions of the new model do not show any grid-dependent artefacts. KIVA-LES with the Lagrangian spray models is used to predict non-evaporating and evaporating diesel fuel sprays. The computed results are compared with the experimental data by Hiroyasu and Kadota (55) and Naber and Siebers (81), as well as the predictions of the original KIVA-3V. The predictions are in good agreement with the data. The large scale vortical structures are reproduced by the LES simulations, which cause "branch-like" spray shape and influence the spray penetration depth. The predictions have also captured the differences between the dense and dilute regions of the sprays. The LES analysis of diesel sprays has also demonstrated that SGS velocity has significant influence on the predicted spray angles. Most importantly, grid-convergent results, which were difficult to obtain with the original KIVA-3V, have been obtained in the present study. Finally, the validated code is used to study evaporating two-phase spray flow in a coaxial gas turbine model combustor. The predictions were compared with some published experimental data. This is a first step towards a more comprehensive numerical analysis of practical industrial combustors where multiple inlets and more complex combustor geometry are encountered. Good agreement with the data is achieved. The predictions have captured the "ring-like" vortex just downstream the annulus and "worm-like" streamwise vortical structure further downstream. The axial droplet mass flux and Sauter mean radius (SMR) are well predicted. Overall the present study has demonstrated the capability of KIVA-LES with the newly developed collision model to provide reasonably accurate predictions of evaporating two-phase flows in coaxial gas turbine combustors.