Stewart Cant

Solution adaptive CFD simulation of premixed flame propagation over various solid obstructions

Abstract This paper presents the results of a number of calculations carried out in order to simulate combustion past obstacles of different shape and blockage ratio. The obstacle shapes considered are circles, squares, triangles and flat plates. Two-dimensional simulations are carried out with the McNEWT code. The code solves the reacting flow field with a laminar flamelet model on an unstructured mesh. Adaptive mesh refinement is applied so that the flame front is accompanied by mesh refinement throughout the calculation domain. A transition from laminar to turbulent combustion induced by passage past the obstacle is seen in the simulations. Evidence for the transition is found in the change in flame shape, flame speed and pressure. The simulations are compared with experimental data and there is good agreement between experiment and simulation.

Forced and Self-Excited Instabilities From Lean Premixed, Liquid-Fuelled Aeroengine Injectors at High Pressures and Temperatures

Measurements of unsteady pressure and chemiluminescence during flow forced operation of aeroengine lean direct injection fuel spray nozzles were made, with a goal to determine the response of the flame, subject to a range of air fuel ratios, fuel flow splits between pilot and mains injectors, and cooling flows. A rotating shutter installed at the downstream choked nozzle provided the excitation for forcing the mass flow rate between 100 to 600 Hz, at normalized intensities of 0.1 to 0.7 relative to the mean velocity at the injector. The experiments were performed at inlet conditions of 800 K and 5.7 bar.Self-excitation created by the coupling between the flame and the combustor cavity was observed, in the form of a broad peak around 275 Hz. Numerical studies indicate that the peak is associated with an entropy spot (a region of non-uniform temperature) travelling from the flame to the choked nozzle, followed by the ensuing expansion wave towards the injector and amplification of the excitation. Investigation of previous studies suggests that similar phenomena may have been present in other studies at high pressure. The main impact of the self-excitation is the significant amplification of the velocity fluctuations from 0.1 of the mean velocity away from the self-excitation frequency to around 0.7 at the peak. The flame response, represented by the ratio of the fractional fluctuations in OH* chemiluminescence to the fractional velocity fluctuations at the injector, can be determined under conditions where the self-excitation heat release contributes only a small portion of the forced heat release, based on the measured background. The flame response shows a significant dependence on both air fuel ratio and fuel splits, with a decreasing gain towards higher frequency.The results show that it is possible to generate high amplitude fluctuations on the flow using this method, and demonstrate the role of entropy spots during normal operation in lean direct injection systems. Finally, the results suggest that there is an interaction between the forcing frequency and the self-excitation, which may behave in a non-linear manner, and which deserves further investigation.Copyright

RANS and LES Modelling of Premixed Turbulent Combustion

Premixed combustion is becoming more common in practical combustion systems in response to increasing regulatory pressure to reduce unwanted emissions. The inherent ability of premixed flames to propagate into the unburned mixture leads to a more active response to turbulence by comparison with non-premixed flames. It is clear that the sheet-like reaction zone within the premixed flame is highly resistant to disruption by turbulent eddies. Hence flamelet structure is prevalent in premixed combustion over a broad range of turbulent velocity fluctuation magnitudes and turbulent eddy length scales. Modelling of turbulent transport in premixed flames is rendered more difficult by the occurence of countergradient transport in the presence of strong heat release. Modelling of the mean turbulent reaction rate has involved a variety of approaches involving either algebraic expressions or additional transport equations. A brief review of current modelling practice is presented, covering some simple models together with the flame surface density approach, the G equation and the more recent scalar dissipation rate model. The emphasis is on models that are applicable in the context of both Reynolds-averaged Navier Stokes (RANS) and Large Eddy Simulation (LES).

The Cambridge CFD grid for large-scale distributed CFD applications

The Cambridge CFD (computational fluid dynamics) Grid is a distributed problem-solving environment for large-scale CFD applications set up between the Cambridge eScience Centre and the CFD Laboratory in the Engineering Department at the University of Cambridge. A Web portal, the Cambridge CFD Web Portal (CamCFDWP) has been developed to provide transparent integration of CFD applications to non-computer scientist end users. In addition to the basic services provided of authentication, job submission and file transfer, the CamCFDWP makes use of XML (extensible markup language) techniques which make it possible to easily share datasets between different groups of users. A Web service interface has recently been implemented for a CFD database which could be integrated in the CamCFDWP in the near future. We also review how this Web service can be made secure using SSL, XML signatures and XML encryption.

The Cambridge CFD Grid Portal for Large-Scale Distributed CFD Applications

The Cambridge CFD (computational fluid dynamics) Web Portal (CamCFDWP) has been set up in the Cambridge eScience Centre to provide transparent integration of CFD applications to non-computer scientist end users who have access to the Cambridge CFD Grid. Besides the basic services provided as other web portals such as authentication, job submission and file transfer through a web browser, the CamCFDWP makes use of the XML (extensible markup language) techniques which make it possible to easily share datasets between different groups of users.

On the use of biorthogonal interpolating wavelets for large-eddy simulation of turbulence

Recent work in the field of turbulence modelling has demonstrated the benefits of the wavelet-based multiresolution analysis technique as a tool for the formulation of the large-eddy simulation (LES) equations. In this formalism, the LES equations are obtained by projecting the Navier-Stokes equations onto a hierarchy of wavelet spaces. This paper investigates the use of biorthogonal interpolating wavelets as a basis for this projection, placing special emphasis on the wavelet-based differential operators that define this mapping. A detailed analysis of their convergence properties is presented and compared to those of their orthogonal counterpart, the Daubechies wavelets. Based on this study, we highlight the weaknesses of the unlifted interpolating wavelet representation for LES sub-grid modelling. Finally, we establish a link between the unlifted framework and the sampling-based LES approach recently proposed in the literature.

Effects of the Biodiesel Fuel Physical Properties on the Swirl Stabilised Spray Combustion Characteristics

An increasin g interest in biofuel applications in modern engines requires a better understanding of biodiesel combustion behaviour. Many numerical studies have been carried out on unsteady combustion of biodiesel in situations similar to diesel engines, but very few studies have been done on the steady combustion of biodiesel in situations similar to a gas turbine combustor environment. The study of biodiesel spray combustion in gas turbine applications is of special interest due to the possible use of biodiesel in the power generation and aviation industries. In modelling spray combustion, an accurate representation of the physical properties of the fuel is a first important step, since spray formation is largely influenced by fuel properties such as viscosity, density, surface tension and vapour pressure. In the present work, a calculated biodiesel properties database based on the measured composition of Fatty Acid Methyl Esters (FAME) has been implemented in a multi-dimensional Computational Fluid Dynamics (CFD) spray simulation code. Simulations of non-reacting and reacting atmospheric-pressure sprays of both diesel and biodiesel have been carried out using a spray burner configuration for which experimental data is available. A pre-defined droplet size probability density function (pdf) has been implemented together with droplet dynamics based on phase Doppler anemometry (PDA) measurements in the near-nozzle region. The gas phase boundary condition for the reacting spray cases is similar to that of the experiment which employs a plain air-blast atomiser and a straight-vane axial swirler for flame stabilisation. A reaction mechanism for heptane has been used to represent the chemistry for both diesel and biodiesel. Simulated flame heights, spray characteristics and gas phase velocities have been found to compare well with the experimental results. In the reacting spray cases, biodiesel shows a smaller mean droplet size compared to that of diesel at a constant fuel mass flow rate. A lack of sensitivity towards different fuel properties has been observed based on the non-reacting spray simulations, which indicates a need for improved models of secondary breakup. By comparing the results of the non-reacting and reacting spray simulations, an improvement in the complexity of the physical modelling is achieved which is necessary in the understanding of the complex physical processes involved in spray combustion simulation. Copyright

On the Application of Wavelets to LES Sub-grid Modelling

The wavelet-based multi-resolution analysis technique is used to develop a novel approach to the modelling of the sub-grid terms in the large-eddy simulation equations. This new approach is called WaveLES. A numerical framework for the solution of the projected equations is developed for one and three-dimensional problems. The WaveLES method is assessed in a priori tests on an atmospheric turbulent time series, and a direct numerical simulation. A posteriori tests are carried out for the Burgers equation.

Assessment of LES Subgrid-scale Models and Investigation of Hydrodynamic Behaviour for an Axisymmetrical Bluff Body Flow

This work is concerned with the investigation of fluid-mechanical behaviour and the performance of different subgrid-scale models for LES in the numerical prediction of a confined axisymmetrical bluff-body flow. Four subgrid-scale turbulence models comprising the Smagorinsky model, Dynamic Smagorinsky model, WALE model and subgrid turbulent kinetic energy model, are validated and compared directly against the experimental data. Two different mesh counts are used for the LES studies, one with a higher mesh resolution in the shear layer than the other. It is found that increasing the mesh resolution improves the time-averaged fluctuating velocity profiles, but has less effect on the time-averaged filtered velocity profiles. A comparison against experiment shows that the recirculation zone length is well predicted using LES. The accuracy of the four different subgrid scale models is then assessed by comparing the LES results using the dense mesh with the experiment. Comparisons with the time-averaged axial and radial velocity profiles demonstrate that LES displays good agreement with the experimental data, with the essential flow features captured both qualitative and quantitatively. The subgrid velocity also matches well with the experimental results, but a slight underprediction of the inner shear layer is observed for all subgrid models. In general, it is found that the Smagorinsky and WALE models are more dissipative than the Dynamic Smagorinsky model and subgrid TKE model. Comparison of the spectra against the experiment shows that LES can capture dominant features of the turbulent flow with reasonable accuracy, and weak spectral peaks related to the Kevin-Helmholtz instability and helical vortex shedding are present.

LES of Nonlinear Saturation in Forced Turbulent Premixed Flames

The mechanisms for nonlinear saturation of a bluff-body stabilised turbulent premixed flame are investigated using LES with the transported flame surface density (TFSD) approach to combustion modelling. The numerical simulation is based on a previous detailed experimental investigation. Results for both the unforced non-reacting and reacting flows are validated against experiment, demonstrating that the fundamental flow features and predicted flame structure are well captured. Key terms in the FSD transport equation are then used to describe the generation and destruction of flame surface area for the unforced reacting flow. In order to investigate the non-linear response of the unsteady heat release rate to acoustic forcing, four harmonically forced flames are considered having the same forcing frequency (160 Hz) but different amplitudes of 10 %, 25 %, 50 % and 64 % of the mean inlet velocity. The flame response is characterised using the Flame Describing Function (FDF). Accurate prediction of the FDF is obtained using the current approach. The computed forced flame structure matches well with the experiment, where effects of shear layer rollup and growth of the vortices on the flame can be clearly observed. Transition to nonlinearity is also observed in the computed FDF. The mechanisms leading to the saturation of the flame response in the higher amplitude case are characterised by inspecting the terms in the FSD transport equation at conditions when the integrated heat release is at its maximum and minimum, respectively. Pinch-off and flame rollup can be seen in snapshots taken at the phase angle of maximum integrated heat release. Conversely, intense vortex shedding and flame-sheet collapse around the shear-layer, as well as small-scale destruction of flame elements in the wake, can be seen in snapshots taken at the phase angle of minimum integrated heat release. The pivotal role of FSD destruction on nonlinear saturation of the flame response is confirmed through the analysis of phase-averaged terms in the FSD transport equation taken at different locations. The phase-averaged subgrid curvature term is found to concentrate in the cusps and downstream regions where flame annihilation is dominant.