R. Stewart Cant

Direct Numerical Simulation of Turbulent Flame Kernels

A combustion DNS code has been developed to solve a fully compressible reacting flow and applied to studying the effects of a turbulent flame kernel. High accuracy numerical techniques have been employed which are 10th order explicit in space and a third order explicit Runge Kutta method in time. Parallel coding is achieved using the Message Passing Interface (MPI) and a performance test is presented showing efficiency and speed up factors. Turbulence is generated numerically for 64 independent simulations using the same laminar flame as an initial condition. Each initial turbulence field has been tested as a simulation of decaying isotropic turbulence without the inclusion of a flame. Initial results for the turbulent reacting simulations on a grid of 963 points are presented along with a laminar flame on a grid of 3843 points.

Flame transfer function for swirled LPP combustion from experiments and CFD

Combustion oscillations that arise in gas turbines can lead to plant damage. One method used to predict these oscillations is to analyse the acoustics using a simple linear model. This model requires a transfer function to describe the response of the heat release to flow perturbations. A transfer function has been obtained for a swirled premixed combustion system using experiments under atmospheric conditions and CFD. These results have been compared with analytical models. The experimental and computational transfer functions both indicate a low frequency zero. A time-delay spread model gives a good representation of the computational transfer function. The experimental transfer function is described well by a model that combines a time-delay spread with a constant gain.Copyright

Computational Modelling of Self-Excited Combustion Instabilities

It is well known that lean premixed combustion systems potentially offer better emissions performance than conventional non-premixed designs. However, premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems. Combustion instabilities (large-scale oscillations in heat release and pressure) have a deleterious effect on equipment, and also tend to decrease combustion efficiency. Designing out combustion instabilities is a difficult process and, particularly if many large-scale experiments are required, also very costly. Computational fluid dynamics (CFD) is now an established design tool in many areas of gas turbine design. However, its accuracy in the prediction of combustion instabilities is not yet proven.Unsteady heat release will generally be coupled to unsteady flow conditions within the combustor. In principle, computational fluid dynamics should be capable of modelling this coupled process. The present work assesses the ability of CFD to model self-excited combustion instabilities occurring within a model combustor. The accuracy of CFD in predicting both the onset and the nature of the instability is reported.© 2000 ASME

Modelling Combustion Instabilities Using Computational Fluid Dynamics

The drive for lower emissions has forced combustor designers to consider lean premixed combustion systems. Unfortunately, premixed combustion systems are particularly susceptible to instabilities, raising large periodic fluctuations in heat release and pressure, that may cause structural damage. A reliable computational tool for predicting the onset of these oscillations would be extremely useful during the design process.The work contained in this paper utilises computational fluid dynamics to model a simple premixed combustor, consisting of a bluff-body stabilised flame burning within a cylindrical duct. State of the art models are used to represent the combustion heat release and the turbulent transport within the combustor. Both forced oscillations and a nearly self-excited condition are modelled and compared with experiment.Copyright

Scalar transport and the validity of Damköhler’s hypotheses for flame propagation in intense turbulence

The turbulent burning velocity of premixed flames is sensitive to the turbulence intensity of the unburned mixture. Premixed flame propagation models that incorporate these effects of turbulence rest on either of the two hypotheses proposed by Damkohler. The first hypothesis applies to low-intensity turbulence that acts mainly to increase the turbulent burning velocity by increasing the flame surface area. The second hypothesis states that, at sufficiently high intensities of turbulence, the turbulent burning velocity is governed mainly by enhanced diffusivity. Most studies to date have examined the validity of the first hypothesis under increasingly high intensities of turbulence. In the present study, the validity of Damkohler’s second hypothesis is investigated. A range of turbulence intensities is addressed by means of direct numerical simulations spanning the “flamelet” and “broken reaction zones” regimes. The validity of Damkohler’s second hypothesis is found to be strongly linked to the behaviour o...

Multi-objective Numerical Investigation of a Generic Airblast Injector Design

© Copyright 2016 by ASME.In combustor design for aero-engines, engineers face multiple opposing objectives with strict constraints. The trend toward lean direct injection (LDI) combustors suggests a growing emphasis on injector design to balance these objectives. Decades of empirical and analytical work have produced low-order methods, including semi-empirical and semi-analytical correlations and models of combustors and their components, but detailed modeling of injector and combustor behavior requires computational fluid dynamics (CFD). In this study, an application of low-order methods and published guidelines yielded generic injector and combustor geometries, as well as CFD boundary conditions of parameterized injector designs. Moreover, semi-empirical correlations combined with a numerical spray combustion solver provided injector design evaluations in terms of pattern factor, thermoacoustic performance, and certain emissions. Automation and parallel coordinate visualization enabled exploration of the dual-swirler airblast injector design space, which is often neglected in published combustor design studies.

Aerodynamic Quenching and Burning Velocity of Turbulent Premixed Methane-Air Flames

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TOWARDS THE AUTOMATED OPTIMISATION OF LIQUID FUEL INJECTOR DESIGN FOR GAS TURBINE ENGINES

Given the trend towards leaner combustor primary zones and concurrent increases in injector air mass flow rates for emissions reduction, an automated fuel injector optimisation procedure is proposed for a generic aero-engine combustor. The modelling assumptions and the design of the toolset to be applied for the optimisation study, as well as preliminary results from the computational tools, are presented. The proposed configuration will enable the consideration of the following design parameters: the number of swirlers, the swirl number for each swirler, the air mass flow splits between the swirlers, and the fuel mass flow split for multiple prefilming surfaces. Results from the unsteady RANS spray combustion solver available through the OpenFOAM software package are combined with semi-empirical correlations in order to estimate and capture trends in emissions. Pattern factor and susceptibility to thermoacoustic oscillations are assessed directly through the simulation output. Due to computational costs, only the cruise condition is considered for optimisation, and off-design considerations have been limited to their impact on preliminary combustor sizing and design. A multi-fidelity optimisation strategy incorporating a multi-objective Tabu Search algorithm is also presented in light of the nature of the problem and the complexity of the design spaces constructed from CFD results.Copyright

Simulations of thermal upgrading methods for oil shale reservoirs

This paper analyzes reaction and thermal front development in porous reservoirs with reacting flows, such as those encountered in shale oil extraction. A set of dimensionless parameters and a 3D code are developed in order to investigate the important physical and chemical variables of such reservoirs when heated by in situ methods. This contribution builds on a 1D model developed for the precursor study to this work. Theory necessary for this study is presented, namely shale decomposition chemical mechanisms, governing equations for multiphase flow in porous media and necessary closure models. Plotting the ratio of the thermal wave speed to the fluid speed allows one to infer that the reaction wave front ends where this ratio is at a minimum. The reaction front follows the thermal front closely, thus allowing assumptions to be made about the extent of decomposition solely by looking at thermal wave progression. Furthermore, this sensitivity analysis showed that a certain minimum permeability is required in order to ensure the formation of a traveling thermal wave. It was found that by studying the non-dimensional governing parameters of the system one can ascribe characteristic values for these parameters for given initial and boundary conditions. This allows one to roughly predict the performance of a particular method on a particular reservoir given approximate values for initial and boundary conditions. Channelling and flow blockage due to carbon residue buildup impeded each methods performance. Blockage was found to be a result of imbalanced heating. Copyright 2012, Society of Petroleum Engineers.

Forced Flame Behaviour in a Spray Combustor

The stability of a liquid-fuelled annular aeroengine gas turbine was investigated using a parallelised three-dimensional uRANS code. The simulations were performed on a single sector of an idealised annular gas turbine geometry using a non-orthogonal body-fitted structured grid. A κ-e turbulence closure model was used and a Lagrangian fuel spray was solved using a Monte Carlo solution of Williams’ spray equation. Mass flow rate perturbations were imposed on the atomiser and the dilution ports of the combustor. The effect of the imposed perturbations on heat release was quantified using flame transfer functions. Excitation of either atomiser or dilution flows led to significant unsteady combustion. With careful design, fluctuations in the rate of combustion due to unsteady flow through the atomiser and dilution ports can cancel, leading to a more stable system.Copyright