development of spray and combustion models for the simulation of gas turbine combustion systems
A tool for the simulation of gas turbine combustors during oil operation is build within OpenFOAM by modelling the main processes involved.
A primary atomization model for sprays in cross flow has been further developed for Large-Eddy simulations. Cylindrical droplets with the size of the injector hole are introduced in the domain and atomized as they penetrate into the cross flow. Classic two-way coupling methods require a high cell-to-droplet volume ratio, otherwise stability issues or non-physical results can potentially arise. To avoid this limitation, a novel coupling method is developed to distribute the source terms to the cells that are within a specific distance from the parcel. Those cells are also used for the interpolation of gas phase properties at the droplet position. After a calibration and a mesh sensitivity study, the combined primary atomization and coupling models are validated for kerosene jet A-1 sprays at several operation conditions.
An evaporation model for multicomponent fuels and emulsions has also been implemented within the Euler-Lagrangian formulation. The model is suitable for pressures typical of heavy duty gas turbines and assumes an increased evaporation rate of the most volatile component when the droplet temperature reaches its boiling temperature. Evaporation of emulsion droplets is modelled following a shell approach, assuming that the water in the droplet core does not evaporate. A diffusivity model is introduced to account for the transport of the dispersed phase from the core to the outer shell. Experimental data for n-heptane and n-decane droplets under several ambient temperatures and pressures are used for validation.
The combustion model is based on the Flamelet Generated Manifold approach. The mixture fraction is calculated from the species mass fractions to couple it with the evaporation model. Flamelet tables are created for different enthalpies and mixture fractions so that they can be used in non-premixed simulations. The thermo-chemistry model considers up to two fuel components by using 4-D tables. One-dimensional results, in terms of laminar flame speed and main species profiles, from the combustion model are compared with the detailed chemistry solutions for hexadecane-water-air mixtures at two pressure levels.
Following the separate evaluation of each model, coupled simulations are performed to analyse their interaction and to verify that they can be used in the simulation of partially pre-vaporized spray flames. Evaporating sprays with different momentum ratios determine the impact of spray effects on the amount of evaporated mass, while 1-D spray flames show the influence of the spray statistics on the flame position. Effect of water addition is also investigated for both configurations.
Numerical simulations of a jet tube combustor at pressurized conditions are performed for different fuel mixtures, including liquid fuels. Experimental data measured at the DLR Institute of Combustion Technology is used as a reference. For gaseous fuel mixtures, simulation results are compared with OH-chemiluminescence images and PIV measurement data. Both flame length and lift are qualitatively well predicted, and the change in flame intensity from 0 to 40% propane mass content is properly captured. Velocity profiles within the chamber are in good agreement with the experiments. For liquid fuel cases, simulations are qualitatively compared with OH-chemiluminescence and Mie scattering images. As in the experiment, a low amount of non-vaporized mass flow at the entrance of the combustion chamber is observed. Compared to gas operation, the flame is more stable and intense. The addition of water leads to a slower spray evaporation, a greater spray-wall interaction, a reduced gas phase temperature and a less intense flame. A higher air inlet temperature increases the evaporation rate and the flame is more stable.
Subsequently, a forced response approach that uses mono-frequency excitations at the inlet boundary is employed to determine the acoustic response of each model individually. The atomization and evaporation processes are enhanced by acoustic waves due to their dependency on the relative velocity. The mean laminar speed and thickness of a premixed flame are not significantly affected by acoustics, having the former a larger response. Forcing is also applied to coupled simulations, so that acoustic response of evaporating sprays as well as 1-D spray flames is evaluated. Following this, the response of the jet tube combustor operating with both liquid and gaseous fuels is analysed.
