CFD modelling of gas turbine combustion processes

Stationary gas turbines manufacturers and operators are under constant scrutiny to both reduce environmentally harmful emissions and obtain efficient combustion. Numerical simulations have become an integral part of the development and optimisation of gas turbine combustors. In this thesis work, the gas turbine combustion process is analysed in two parts, a study on air-fuel mixing and turbulent combustion. For computational fluid dynamic analysis work the open-source CFD code OpenFOAM and STAR-CCM+ are used. A fuel jet injected to cross-flowing air flow is simplified air-fuel mixing arrangement, and this problem is analysed numerically in the first part of the thesis using both Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) methods. Several turbulence models are compared against experimental data in this work, and the complex turbulent vortex structures their effect on mixing field prediction is observed. Furthermore, the numerical methods are extended to study twin jets in cross-flow interaction which is relevant in predicting air-fuel mixing with arrays of fuel injection nozzles. LES methods showed good results by resolving the complex turbulent structures, and the interaction of two jets is also visualised. In this work, all three turbulent combustion regimes non-premixed, premixed, partially premixed are modelled using different combustion models. Hydrogen blended fuels have drawn particular interest recently due to enhanced flame stabilisation, reduced CO2 emissions, and is an alternative method to store energy from renewable energy sources. Therefore, the well known Sydney swirl flame which uses CH4: H2 blended fuel mixture is modelled using the steady laminar flamelet model. This flame has been found challenging to model numerically by previous researchers, and in this work, this problem has been addressed with improved combustion modelling approach with tabulated chemistry. Recognizing that the current and future gas turbine combustors operate on a mixed combustion regime during its full operational cycle, combustion simulations of premixed/partially premixed flames are also performed in this thesis work. Dynamical artificially thickened flame model is implemented in OpenFOAM and validated using propagating and stationary premixed flames. Flamelet Generated Manifold (FGM) methods are used in the modelling of turbulent stratified flames which is a relatively new field of under investigation, and both experimental and numerical analysis is required to understand the physics. The recent experiments of the Cambridge stratified burner are studied using the FGM method in this thesis work, and good agreement is obtained for mixing field and temperature field predictions

Non-premixed swirl flame modelling using the open source CFD package openfoam

In this work the Sydney swirl stabilized burner for a hydrogen:methane fuel mixture is numerically modelled using the OpenFOAM C++ library package. A non-reacting high swirl test case (N29S159) and a reacting low swirl test case (SMH1) were investigated using Large Eddy Simulations and the Steady Laminar Flamelet concept. For the non-reacting case the velocity field components are in very good agreement with experimental results. For the reacting flow case the velocity components are also in excellent agreement with experimental values, however the scalar quantities exhibit some under prediction. Possible reasons for the under prediction are discussed

Large eddy simulation of scalar mixing in jet in a cross-flow

Jet in a Cross-Flow (JICF) is a flow arrangement found in many engineering applications, especially in gas turbine air-fuel mixing. Understanding of scalar mixing in JICF is important for low NOx burner design and operation, and numerical simulation techniques can be used to understand both spatial and temporal variation of air-fuel mixing quality in such applications. In this paper mixing of the jet stream with the cross-flow is simulated by approximating the jet flow as a passive scalar and using the Large Eddy Simulation (LES) technique to simulate the turbulent velocity field. A posteriori test is conducted to assess three dynamic Sub-Grid Scale models in modelling jet and cross-flow interaction with the boundary layer flow field. Simulated mean and Reynolds stress component values for velocity field and concentration fields are compared against experimental data to assess the capability of the LES technique, which showed good agreement between numerical and experimental results. Similarly, time mean and RMS values of passive scalar concentration also showed good agreement with experimental data. In addition, LES results are further used to discuss the scalar mixing field in the downstream mixing region

A numerical study of dust explosion properties of hydrogen storage alloy materials

Hydrogen as a clean fuel has gained increased attention in the recent years and considerable research is being undertaken to develop hydrogen storage technologies. Hydrogen storage using metal hydride is one such technology. Hydride materials, used in hydrogen storage technologies, in powder form can be an explosion hazard and testing these materials using standard techniques is also difficult. Research reported in this paper is an attempt to develop numerical methods to obtain explosion properties of such materials. In this work a one dimensional transport-type model is presented to simulate the dust explosion process in a closed 20-L spherical vessel. Transport equations for energy, species and particle volume fraction are solved with the finite difference method, whilst velocity distribution and pressure are updated with numerical integration of the continuity equation. The model is first validated with experimental data and then applied to simulate the explosion process of an AB2-type alloy powder used for hydrogen storage. Two kinetic models accounting for the particle burning mechanism are investigated in the current study. One is based on an Arrhenius surface reaction law, the other is based on a simplified diffusion-type d2 law. The former is found to be better in terms of prediction of the deflagration indices. This work is of great significance in safety assessment of new hydrogen storage materials in the processes of their production, storage and transportation