Large Eddy Simulation Of The Two-phase Combustion In A Multi-swirling Combustor

In order to study the two-phase combustion process of multi-swirling LPP combustor,an in-house code DELTA based on Large Eddy Simulation method was developed to solve the turbulent two-phase reacting spray flows.The code can be applied to predict the three-dimensional unsteady flow field and two-phase spray combustion in a multi-swirling LPP combustor,and to provide numerical support to the study of multi-swirling LPP comboustor's performance,pollutant emission and combustion instability.Based on the large eddy simulation program of the research group,a two-phase combustion parallel computing module was developed and implemented.The delveloped DELTA code was first evaluated by the KIAI n-heptane spray combustor and then applied to predict the two-phase combustion of the LPP combustor.A coupled Eulerian(for the continue gas-phase)and Lagrangian(for the discrete liquidphase)approach is adopted for the two-phase spray combustion.The Smagorinsky sub-grid model is adopted in the present work.The sub-grid-scale fluctuation's contribution to the dispersion and evaporation of the droplets are taken into account by means of a sub-grid stochastic model.A steady flamelet combustion model with detailed chemical mechanism is adopted to get the details of hot flow field and flame structure.In order to validate the accuracy of the two-phase combustion model,the KIAI n-heptane spray combustor provided by CORIA laboratory was selected as the validation case.LES was conducted for both non-reacting and reacting cases and the results were compared with the experimental results.The results show that:(1)The LES results of gas phase velocity and RMS agrees quite well with the experiment data for both non-reacting and reacting cases.(2)With the stochastic model for sub-grid evaporation and motion,the droplets diameter distribution and velocity distribution of simulation also have a good agreement with experiment.(3)The flame structure predicted by LES has the similar shape with experiment,consisting by an inner lean premixed flame,an outter diffusion flame and a hot zone between the two flames.According to the geometric structure of multi-swirling LPP combustor,a Solid-shielded method coupled with Levelset function was adopted to identify the solid region of swirler blades in the LPP combustor,and thus simplified the work of generating multi-block structured mesh.LES was conducted to simulate the non-reacting case and two-phase reacting case of the LPP combustor.The results were compared with the PIV experimental results.The results show that:(1)The LES results of non-reacting case are in good agreement with the PIV measurements and successfully capture the central recirculation zone,the lip recirculation zone and the recirculation zone.The radial distribution of axial velocity,radial velocity and tangential velocity at different axial positions are in good agreement with the experimental results as well.(2)A large-scale vortex structure generated by the Kelvin-Helmholtz instability of the swirling shear layer at the outlet of the primary swirler is noticed,and at the same time the cross-winding vortex tube structure is produced by the pilot swirler.(3)The distribution of fuel droplets in the combusor can be seen in the results of the reacting case,and it aslo can be noticed from the results that the larger diameter droplets are mainly concentrated on the outer side of the spray cone and the smaller diameter droplets are much dispersed by the swirling entrainment distribution.