Alain deChamplain

Performance Prediction of a Ducted Rocket Combustor Using a Simulated Solid Fuel

The ducted rocket is a supersonic flight propulsion system that takes the exhaust from a solid fuel gas generator, mixes it with air, and burns it to produce thrust. To develop such systems, the use of numerical models based on computational fluid dynamics (CFD) has been increasing, but to date only simplified treatments of the combustion within ducted rockets have been reported, likely due to the difficulties in characterizing and accurately modeling the partially reacted, particle-laden fuel exhaust from the gas generator. Through a careful examination of the governing equations and experimental measurements, a CFD-based methodology that properly accounts for the influence of the gas generator exhaust, particularly the solid phase, has now been developed to predict the performance of a ducted rocket combustor using a simulated solid fuel. It uses an equilibrium-chemistry probability density function combustion model with two separate streams, one gaseous and the other of 75-nm-diam carbon spheres, to represent the exhaust products from the gas generator. After extensive validation with direct-connect combustion experiments over a wide range of geometries and test conditions, this CFD-based method was able to predict, within a good degree of accuracy, the combustion efficiency of a ducted rocket combustor.

Unsteady RANS and scale adaptive simulations of a turbulent spray flame in a swirled-stabilized gas turbine model combustor using tabulated chemistry

Purpose – The purpose of this paper is to numerically investigate the three-dimensional (3D) reacting turbulent two-phase flow field of a scaled swirl-stabilized gas turbine combustor using the commercial computational fluid dynamic (CFD) software ANSYS FLUENT. The first scope of the study aims to explicitly compare the predictive capabilities of two turbulence models namely Unsteady Reynolds Averaged Navier-Stokes and Scale Adaptive Simulation for a reasonable trade-off 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(NEQ) effects caused by turbulent strain such as flame stretching. Design/methodology/approach – Standard Eulerian and Lagrangian formulations are used to describe both gaseous and liquid phases, respectively. The computing method includes a two-way coupli...

Scale-Adaptive and Large Eddy Simulations of a Turbulent Spray Flame in a Scaled Swirl-Stabilized Gas Turbine Combustor Using Strained Flamelets

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.Copyright

Large Eddy Simulation of a Turbulent Swirling Jet-A1 Spray Flame Incorporating Chemical Non-Equilibrium Effects Through the Flamelet Model

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.Copyright

Large eddy simulation of spark ignition of a bluff-body stabilized burner using a subgrid-ignition model coupled with FGM-based combustion models

Purpose Safety improvement and pollutant reduction in many practical combustion systems and especially in aero-gas turbine engines require an adequate understanding of flame ignition and stabilization mechanisms. Improved software and hardware have opened up greater possibilities for translating basic knowledge and the results of experiments into better designs. The present study deals with the large eddy simulation (LES) of an ignition sequence in a conical shaped bluff-body stabilized burner involving a turbulent non-premixed flame. The purpose of this paper is to investigate the impact of spark location on ignition success. Particular attention is paid to the ease of handling of the numerical tool, the computational cost and the accuracy of the results. Design/methodology/approach The discrete particle ignition kernel (DPIK) model is used to capture the ignition kernel dynamics in its early stage of growth after the breakdown period. The ignition model is coupled with two combustion models based on the mixture fraction-progress variable formulation. An infinitely fast chemistry assumption is first done, and the turbulent fluctuations of the progress variable are captured with a bimodal probability density function (PDF) in the line of the Bray–Moss–Libby (BML) model. Thereafter, a finite rate chemistry assumption is considered through the flamelet-generated manifold (FGM) method. In these two assumptions, the classical beta-PDF is used to model the temporal fluctuations of the mixture fraction in the turbulent flow. To model subgrid scale stresses and residual scalars fluxes, the wall-adapting local eddy (WALE) and the eddy diffusivity models are, respectively, used under the low-Mach number assumption. Findings Numerical results of velocity and mixing fields, as well as the ignition sequences, are validated through a comparison with their experimental counterparts. It is found that by coupling the DPIK model with each of the two combustion models implemented in a LES-based solver, the ignition event is reasonably predicted with further improvements provided by the finite rate chemistry assumption. Finally, the spark locations most likely to lead to a complete ignition of the burner are found to be around the shear layer delimiting the central recirculation zone, owing to the presence of a mixture within flammability limits. Research limitations/implications Some discrepancies are found in the radial profiles of the radial velocity and consequently in those of the mixture fraction, owing to a mismatch of the radial velocity at the inlet section of the computational domain. Also, unlike FGM methods, the BML model predicts the overall ignition earlier than suggested by the experiment; this may be related to the overestimation of the reaction rate, especially in the zones such as flame holder wakes which feature high strain rate due to fuel-air mixing. Practical implications This work is adding a contribution for ignition modeling, which is a crucial issue in various combustion systems and especially in aircraft engines. The exclusive use of a commercial computational fluid dynamics (CFD) code widely used by combustion system manufacturers allows a direct application of this simulation approach to other configurations while keeping computing costs at an affordable level. Originality/value This study provides a robust and simple way to address some ignition issues in various spark ignition-based engines, namely, the optimization of engines ignition with affordable computational costs. Based on the promising results obtained in the current work, it would be relevant to extend this simulation approach to spray combustion that is required for aircraft engines because of storage volume constraints. From this standpoint, the simulation approach formulated in the present work is useful to engineers interested in optimizing the engines ignition at the design stage.