Abstract
The flame in a gas turbine model combustor close to blow-off is studied using large eddy simulation with the objective of investigating the sensitivity of including different heat loss effects within the modelling. A presumed joint probability density function approach based on the mixture fraction and progress variable with unstrained flamelets is used. The normalised enthalpy is included in the probability density function to account for heat loss within the flame. Two simulations are presented that use fixed temperature boundary conditions, and use adiabatic and non-adiabatic formulations of the combustion model. The results are compared against the previous fully adiabatic case and experimental data. The statistics for the simulations are similar to the results obtained from the fully adiabatic case. Improved statistics are obtained for the temperature in the near-wall regions. The non-adiabatic flamelet case shows the average reaction rate values at the flame root are approximately 50% smaller in comparison to the adiabatic flamelet cases. This causes the lift-off height to be overestimated. The time series of the lift-off height and the volume integrated heat release rate show that including non-adiabatic flamelets causes the flame to be highly unstable. A higher enthalpy deficit is seen in the near-field regions when the flame root is not present and experiencing some lift-off, suggesting that the flame is more dynamic when including heat loss.
Highlights
Lean combustion is utilised in modern gas turbine combustors in order to reduce the production of pollutants
This suggests that when the heat loss effects are included in the canonical model, i.e. premixed flamelets, the opening angle of the swirl flame becomes slightly larger due to weakened reaction rates in the inner shear layer, which is shown later
The combustion closure is based on unstrained flamelets with a presumed joint PDF approach based on the mixture fraction, progress variable and the normalised enthalpy, where the latter is included in the PDF to introduce the heat loss effects
Summary
Lean combustion is utilised in modern gas turbine combustors in order to reduce the production of pollutants. The mechanisms leading to blow-off are not well understood and under such conditions, the flame heat release becomes weaker. Heat loss effects can influence the stabilisation of the flame. There have been a number of recent modelling studies on flame blow-off (Zhang and Mastorakos 2016; Ma et al 2019), but heat loss effects are seldom considered. It is of interest from a modelling perspective to observe how heat loss effects can influence the flame behaviour close to lean blow-off conditions
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