In this paper, the three-dimensional (3D) reacting turbulent two-phase flow field of a scaled swirl-stabilized gas turbine combustor is numerically investigated using the commercial CFD software ANSYS FLUENT™-v14. The first scope of this study aims to explicitly compare the predictive capabilities of two turbulence models namely Scale-Adaptive Simulation (SAS) and Large Eddy Simulation (LES) for a reasonable compromise between accuracy of results and global computational cost when applied to simulate swirl-stabilized spray combustion. The second scope of the study is to couple chemical reactions to the turbulent flow using a realistic chemistry model and also to model the local chemical non-equilibrium effects caused by turbulent strain. Standard Eulerian and Lagrangian formulations are used to describe both gaseous and liquid phases respectively. The fuel used is liquid jet-A1 which is injected in the form of a polydisperse spray and the droplet evaporation rate is calculated using the infinite conductivity model. One-component (n-decane) and two-component fuels (n-decane + toluene) are used as jet-A1 surrogates. The combustion model is based on the first and second moments of the mixture fraction, and a presumed-probability density function (PDF) is used to model turbulent-chemistry interactions. The instantaneous thermochemical state necessary for the chemistry tabulation is determined by using initially the partial equilibrium assumption (PEQ) and thereafter, the detailed non-equilibrium (NEQ) calculations through the laminar flamelet concept. The combustion chemistry of these surrogates is represented through a reduced chemical kinetic mechanism (CKM) comprising 1 045 reactions among 139 species, derived from the detailed jet-A1 surrogate model, JetSurf 2.0. Numerical results are compared with a set of published data for a steady spray flame. Firstly, it is observed that, by coupling the two turbulence models with a combustion model incorporating a representative chemistry to account for non-equilibrium effects with realistic fuel properties, the models predict reasonably well the main combustion trends, with a superior performance for LES in terms of trade-off between accuracy and computing time. Secondly, because of some assumptions with the combustion model, some discrepancies are found in the prediction of species slowly produced or consumed such as CO and H2. Finally, the study emphasizes the dominant advantage of an adequate resolution of the mixing characteristics especially with the more demanding simulation of a swirl-stabilized spray flame.

In this work, a comprehensive CFD model was developed for pyrolysis to provide significant emission reductions. In wood stoves with natural draft, the airflow is driven by buoyancy to overcome the resistance to flow within the stove. This combustion model is tested to reproduce at best the physical and chemical phenomena taking place in a natural draft wood log stove for low power (18 kW) residential heating. The RANS approach was used to solve the aerothermodynami c equations. A body fitted hexahedron mesh was adopted and the k-ε turbulence model was used to ensure closure of the Navier-Stokes equations. The chemical reaction for combustion was modeled using species transport and rate of species production formulated as the EDDY DISSIPATION CONCEPT. The key parameters for validation are based on temperature inside the combustion chamber at the center plane and the [CO]/[CO2] ratio for emissions at the exit plane. Therefore, the developed modelling approach can be used for engineering analysis and optimization of existing stoves and for relatively-quick evaluations of new stove designs.

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The heat soak-back occurring in engines under post-shutdown conditions is a well-known phenomenon. This behavior is caused by the transmission of accumulated heat from hot parts or cavities during idle operation (such as turbines, contention rings, etc.) to colder ones (combustion chamber, injectors, etc.) when the airflow inside the engine approaches nullity. Then stagnant fluids in components such as the injectors (mainly TDE) and bearings become exposed to this heat which spreads by conduction, radiation and natural convection through the engine, and potentially leading fuel or oil to decompose and to form a build-up of carbon through a phenomenon called “coking”. Heat soak-back to engine components on shutdown, due to the thermal inertia of heated turbine parts, has the potential to cause deposits to build up in fuel injectors which can over time block the injectors. Blocked or partially blocked injectors must then be removed from the engine, inspected and sent for cleaning. Both soak-back and coking phenomenon have already been investigated by some motorists through experimental and structural (FEA) studies [1]. To the author’s knowledge however, no CFD model considering the airflow has yet been discussed, mainly because of the computing resources and the time it requires to simulate this unsteady phenomenon. As part of the present study and in order to fill in the gap on the availability of numerical data in the open literature for the heat soak-back occurring in a gas turbine combustor, the following investigation implies CFD simulation to predict the thermal behavior and magnitude of such a soak-back and its potential consequence on the fuel passages. A previous CFD simulation done by the authors showed that the use of a radiation model was required to provide some very reasonable results. As a follow up, the work to be presented in this paper will provide a more complete numerical soak-back procedure that can be used to predict the thermal behavior inside the combustor of a just shutdown gas turbine engine. Prior to the heat soak-back analysis, a non-premixed combustion model is run to simulate idle condition. Then these more realistic results for idle are used as initial conditions for the analysis of the transient heat dissipation occurring after shutdown. The following work includes a quick description of the experimental setup, and an introduction to the operational conditions for a simplified test rig. The full numerical procedure is then described. An analysis highlights the improved ability of the numerical model in predicting when the coking temperatures are reached using the adopted modeling techniques. It is observed that results obtained by the present model compare well with the experimental data to validate the simulation of this not so obvious natural convection phenomenon for a better understanding of this transient problem.

A large eddy simulation (LES) of a turbulent swirl stabilized jet-A1 flame is presented. The scope of the study is to incorporate a reduced chemistry model, as well as, coupling the turbulent flow characteristics to the chemical reactions and at the same time model the local chemical non-equilibrium due to the turbulent strain. Standard Eulerian and Lagrangian approaches are used to describe both gas and liquid phases, respectively. A joint presumed probability density function (PDF) is used to model turbulent-chemistry interactions in swirling jet-A1 spray flames. A one-component fuel, n-decane, is used as a surrogate for jet-A1. The combustion chemistry of the one component is represented through a reduced chemical kinetic mechanism (CKM) which comprises 139 species and 1 045 reactions, derived from the detailed jet fuel surrogate model, JetSurf 2.0. Numerical results of the gas velocity, the gas temperature and the species mole fractions are compared with a set of published experimental data of a steady flame. In addition to the overall reasonable agreement obtained with the experimental data, it is observed that, by combining a sufficiently realistic chemistry model with LES to simulate a jet-A1 spray flame, the prediction of major species is significantly improved while pollutants such as carbon monoxide (CO) and other species involved in slow reactions, are under predicted for reasons discussed in the paper.