In the foreseeable future, the combustion of liquid fuel is expected to remain prevailing in energy-conversion systems such as gas turbines. Therefore, accurate and affordable predictions of spray combustion processes will continue to be an important topic. The flamelet model is a promising solution to account for detailed chemistry in turbulent combustion modeling. However, the traditional diffusion flamelet is unable to capture the evaporative heat loss when applied to spray combustion, leading to significant over-prediction of temperature.  In this study, a model flame of the quasi-1D counterflow spray flame has been developed. The two-dimensional multiphase convection-diffusion-reaction (CDR) equations have been simplified to one dimension using similarity reduction under the Eulerian framework. This model flame is able to directly account for non-adiabatic heat loss as well as multiple combustion regimes present in realistic spray combustion processes. The model flame has been analyzed to show that the counterflow spray flame is extinguished at a lower strain rate or scalar dissipation rate, compared to the single-phase counterflow diffusion flame. Due to liquid evaporation, there exists a non-monotonic behavior of the temperature/species mass fractions and non-linear relation between the enthalpy and the mixture fraction, different from its gaseous counterpart. A spray flamelet library is then generated based on the model flame. To retrieve data from the spray flamelet library, the enthalpy is used as an additional controlling variable to represent the interphase heat transfer, while the chemical reaction process is mapped to the mixture fraction and the progress variable. This approach can be considered as a general extension to the traditional flamelet/progress-variable (FPV) method, in the sense that FPV is automatically recovered when there is no evaporative heat loss in the gaseous combustion. To address the issue of discrete interpolation in a higher dimension, the inverse distance weighting (IDW) method is implemented.

 

The spray-flamelet/progress-variable (SFPV) approach is evaluated with the result from direct integration of finite-rate chemistry as a benchmark, carried out in a CFD simulation of 2D laminar counterflow flame, representing the recirculation region in gas turbine combustors for ﬂame stabilization. The SFPV approach gives better performance in terms of temperature, while the FPV approach over predicts by nearly 20%. In terms of species mass fractions, there is no significant difference between the two, both showing good agreement with the laminar finite-rate model. This suggests that the inaccuracy of the gaseous diffusion flamelet, when applied to spray combustion, lies mainly in the prediction of interphase heat transfer. The SFPV approach with enthalpy as an additional controlling variable is adequate to describe this phenomenon, with only a slight increase in the computational cost (solution of one additional scalar transport equation).

 


Development of a Multi-Phase Flamelet Generated Manifold for Spray Combustion Simulations

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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