CFD simulations of a reverse flow combustion chamber

Computational Fluid Dynamics (CFD) simulations provide advantages to identify the physical behaviors of flow inside combustor and to define the combustion characteristics. The simulation analysis becomes necessary to reduce time dependence and cost issue to determine initial design data of a combustor before experimental tests. In this thesis, a 1/13 sector of a reverse flow annular combustor, intended to be used in a 1000 hp turbo-shaft helicopter engine, is investigated for reacting flow CFD simulations. The real geometrical dimensions of combustor were used without any scaling factor to create computational domain. The computational domain was simulated for different degree of fineness of computational mesh to check for mesh independency. The 3D generated mesh has roughly 10 million unstructured polyhedral elements. The CFD model was proposed to simulate the spray combustion for a reverse flow annular combustor. The numerical studies consist of two reacting flow cases, and one non-reacting liquid fuel injection case. The CFD simulations were performed in order to determine the reacting flow characteristics of combustor by using FLUENT v.14 software in a 3D steady-state fashion. All simulations were carried out Reynolds Averaged Navier-Stokes (RANS) and with the Realizable k-ϵ turbulence model. The first CFD approach for combustion model is based on the transport equation for each species. The combustion has been simulated with the finite rate chemistry model and is modelled using one-step reaction mechanism for heptane fuel (C7H16). Eddy-Dissipation Concept (EDC) model was used for the turbulence-chemistry interaction. The EDC model can be described as a detailed description of the dissipation of the turbulent eddies on the concept of interaction between turbulence and chemistry of flame. The second CFD approach for combustion model is the steady diffusion flamelet concept with the Probability Density Function (PDF) closure method for turbulence-flame interaction. With this concept, the thermochemical properties of an adiabatic turbulent flame can be defined by using the mixture fraction and the scalar dissipation rate at stoichiometric value. The β-PDF method was used to generate a look-up table database for turbulent combustion. For chemical reaction, heptane (C7H16) fuel mechanism which was derived by the University of California San Diego, is used for chemical reaction process. It has 58 species and 325 reactions. Spray combustion has several application areas in industry. For liquid-fuel combustion in gas turbine combustors, the pressure swirl atomizer has been found to be the cheapest and the most reliable type of atomizer and the pressure swirl atomizer will be described in this study. The pressure swirl atomizer accelerates the liquid through swirl ports into a central swirl chamber. The swirling liquid pushes against the walls of the swirl chamber and develops a hollow air core. It then emerges from the orifice as a thinning sheet, which is unstable, breaking up into ligaments and droplets. The progression from the internal atomizer flow to the external spray consists of two steps as film formation sheet breakup, atomization. FLUENT® software uses the Linearized Instability Sheet Atomization (LISA) model for the pressure-swirl atomizer. The Kelvin-Helmholtz Rayleigh-Taylor model (KH/RT) will be described for secondary breakup mechanism. The combustion characteristics, the flow characteristics, and the atomization model will be investigated for the results section of this study. The comparing of two different combustion concept will be discussed in conclusion chapter. The thesis covers all the details of these models and their approaches.