Simulations of partially premixed turbulent ethanol spray flames using doubly conditional source term estimation (DCSE)

Abstract

Singly Conditional Source-term Estimation (CSE) is extended to Doubly Conditional Source-term Estimation (DCSE) to simulate three turbulent ethanol spray flames (EtF1, EtF3 and EtF4) for the first time. Reynolds-Averaged Navier Stokes equations are solved with a two-way coupling Eulerian-Lagrangian method to account for the gas-spray interactions. The objective of this work is to compare the performance of CSE and DCSE in simulating flames with varying levels of pre-evaporation leading to different levels of premixing and combustion modes. Tabulated detailed chemistry is included. The gas phase temperature and spray velocity predictions are compared with experimental data and previously published studies. Both CSE and DCSE can capture the temperature trends adequately. However, DCSE clearly improves the mean temperature predictions over CSE and the predicted temperatures are in close agreement with the experimental data. Close to nozzle exit and centreline, CSE and DCSE systematically predict lower temperatures compared to the experiments. Also noticed in other published studies, this may be due to larger experimental errors at this location, combined with modelling assumptions. Further analysis is conducted by examining the mean heat release rate and mean evaporation rate reproduced in CSE and DCSE. DCSE is shown to better capture some features of the complex structure of these flames. The progress variable predictions are consistent with the temperature predictions. The predicted mean droplets velocity is in good agreement with the experimental data at z/D=10 and z/D=20. However, farther downstream, the mean spray velocity was underpredicted for CSE and DCSE. The liquid volume flux is underpredicted near the centreline indicating higher evaporation rates in the simulations compared to the experiments. The predicted Sauter mean diameter also agrees well with the experiments with some discrepancies for the large droplets. Further investigation is needed to improve the currently available evaporation models.