William Kendal Bushe
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - April 2022)
A universally applicable user-friendly tool has been developed for numerical simulations of turbulent combustion in gas turbines and is being evaluated in this work. Ultimately the goal is to reliably predict emissions of interest at low computational cost while maintaining a large degree of flexibility with regards to the fuel type and the complexity of the chemical kinetic mechanism as well as the type of combustion that may be present. Premixed, non-premixed as well as partially-premixed combustion could potentially be present in a technical application. Maintaining this versatility gives freedom of design to the user. In this work the criteria necessary for a combustion model suitable for such a tool will be defined and measures of success identified. The Uniform Conditional State (UCS) model allows for the chemistry to be computed a priori and disconnected from physical space using so-called conditioning variables; variables describing specific characteristics, which are deterministic for the combustion process. With that the computational cost is very low and a large degree of flexibility can be achieved compared to other modelling approaches. In theory, the recently developed model of UCS promises to satisfy the required criteria and thus an extensive study was conducted putting this model into practice for the first time. Initially a number of parameters within this modelling approach were defined based on weak assumptions and needed to be evaluated and verified as part of this work as well. With that a valuable report was completed to outline the ability of UCS to serve as a reliable predictive model of turbulence-chemistry interaction in its current state. For instance, while the computational cost indeed proved to be only 20% of that of a comparable simulation in one of the test cases, the accuracy of predictions of emissions such as NOx remains below the aspired target in another test case.The evaluation of this new approach was carried out based on a non-premixed swirl-stabilized model aero engine combustor, a premixed swirl-stabilized flame series and a partially-premixed lifted jet flame, while exploring different modelling parameters and limitations of the modelling approach.
Combustion technology has been applied in human society for millennia and, since the industrial revolution, has become an integral part of most energy supply chains. Simulation is an important tool in modern combustor design; this thesis aims to improve the quality of combustion simulation tools, and thereby facilitate the design of improved combustors. More specifically, it aims to examine how generalizing and/or relaxing the definition of conditional filtering – a common technique in turbulence-chemistry interaction modelling – can produce novel turbulence and combustion models. The work begins with an extension of the Conditional Source-term Estimation (CSE) model for turbulence-chemistry interaction modelling. A novel variation of the algorithm, termed CSE with Geometric Conditioning Variables (CSE-GCV) is proposed as a method of circumventing the theoretical and practical issues associated with traditional CSE ensemble division. In CSE-GCV, the concept of the conditional filter is generalized by introducing geometric (position-based) variables as conditioning variables. CSE-GCV is tested and found to be workable; a theoretical analysis demonstrates that CSE-GCV also generally has the advantage of reduced computational complexity compared to traditional CSE. In a separate study, the stabilization procedure employed in traditional dynamic sub-filter modelling for Large Eddy Simulation (LES) is re-interpreted as a form of conditional filtering based on position. This re-interpretation is used as the starting point for a "conditional dynamic" sub-filter model in which the stabilization procedure is based on filtering conditionally on scalar fields. Both the traditional and conditional models are applied to a turbulent flame; results suggest that the two models perform similarly, although performance of both is sub-optimal in the case considered. The final, two-part, study is based around the suggestion that, with sufficient conditioning, conditionally-filtered fields should be independent of position. It is found that assuming this uniformity produces a novel turbulence-chemistry interaction model, termed the Uniform Conditional State (UCS) model, in which the conditional scalar dissipation model is the key un-closed parameter. The UCS model is applied to a series of turbulent non-premixed flames, and is found to predict their properties to good accuracy, with details showing some sensitivity to the conditional scalar dissipation model.
Computational fluid dynamics (CFD) is indispensable in the development of complex engines due to its low cost and time requirement compared to experiments. Nevertheless, because of the strong coupling between turbulence and chemistry in premixed flames, the prediction of chemical reaction source terms continues to be a modelling challenge. This work focuses on the improvement of turbulent premixed combustion simulation strategies requiring the use of presumed probability density function (PDF) models. The study begins with the development of a new PDF model that includes the effect of turbulence, achieved by the implementation of the Linear-Eddy Model (LEM). Comparison with experimental burners reveals that the LEM PDF can capture the general PDF shapes for methane-air combustion under atmospheric conditions with greater accuracy than other presumed PDF models. The LEM is additionally used to formulate a new, pseudo-turbulent scalar dissipation rate (SDR) model. Conditional Source-term Estimation (CSE) is implemented in the Large Eddy Simulation (LES) of the Gülder burner as the closure model for the chemistry-turbulence interactions. To accommodate the increasingly parallel computational environments in clusters, the CSE combustion module has been parallelised and optimised. The CSE ensembles can now dynamically adapt to the changing flame distributions by shifting their spatial boundaries and are no longer confined to pre-allocated regions in the simulation domain. Further, the inversion calculation is now computed in parallel using a modified version of an established iterative solver, the Least-Square QR-factorisation (LSQR). The revised version of CSE demonstrates a significant reduction in computational requirement — a reduction of approximately 50% — while producing similar solutions as previous implementations. The LEM formulated PDF and SDR models are subsequently implemented in conjunction with the optimised version of CSE for the LES of a premixed methane-air flame operating in the thin reaction zone. Comparison with experimental measurements of temperature reveals that the LES results are very comparable in terms of the flame height and distribution. This outcome is encouraging as it appears that this work represents a significant step towards the correct direction in developing a complete combustion simulation strategy that can accurately predict flame characteristics in the absence of ad hoc parameters.
Conditional Source-term Estimation (CSE) is a closure model for turbulence-chemistryinteractions. This model is based on the conditional moment closure hypothesis for the chemical reaction source terms. The conditional scalar field is estimated by solving an integral equation using inverse methods. CSE was originally developed for - and has been used extensively in- non-premixed combustion. This work is the first application of this combustion model to predictive simulations of turbulent premixed flames. The underlying inverse problem is diagnosed with rigorous mathematical tools. CSE is coupled with a Trajectory Generated Low-Dimensional Manifold (TGLDM) model for chemistry. The CSE-TGLDM combustion model is used with both Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) turbulence models to simulate two different turbulent premixed flames. Also in this work, the Presumed Conditional Moment (PCM) turbulent combustion model is employed. This is a simple flamelet model which is used with the Flame Prolongation of ILDM (FPI) chemistry reductiontechnique. The PCM-FPI approach requires a presumption for the shape of the probability density function of reaction progress variable. Two shapes have been examined: the widely used beta-function and the Modified Laminar Flamelet PDF (MLF-PDF). This model is used in both RANS and large-eddy simulation of a turbulent premixed Bunsen burner. Radial distributions of the calculated temperature field, axial velocity and chemical species mass fraction have been compared with experimental data. This comparison shows that using the MLF-PDF leads to predictions that are similar, and often superior to those obtained using the beta-PDF. Given that the new PDF is based on the actual chemistry - as opposed to the ad hocnature of the beta-PDF - these results suggest that it is a better choice for the statistical description of the reaction progress variable.
This thesis presents the development of an experimental apparatus and methods to allow the application of gaseous Raman spectroscopy to the challenging and original application of a small-scale, high-temperature methane/steam reformer developed to be representative of the technologies used in solid oxide fuel cell (SOFC) applications. The research is placed in the context of global energy trends and SOFC’s, with specific reference to the challenges related to directly internally reforming medium-temperature SOFC’s and the case for the development of non-intrusive measurement techniques for gas species and temperature is made. The practical aspects of the development of the broadband 308 nm Raman system are examined and previous works in this area are highlighted. The excitation light source is evaluated, the use of a liquid potassium hydrogen phthalate filter as a means to reduce Rayleigh line effects is demonstrated, and background fluorescence suppression through polarization of the 308 nm light source is presented. The arrangements of the experimental set-up, gas supply, metering, and humidification are shown, as are the optical arrangements for laser sheet formation and light collection. A description of the calibration experiments, procedures, and methodologies that are used to define the normalised relative differential Raman scattering cross sections of the major species of interest in this study is presented. The observation of an unexpected leakage of air into the reformer is described and a hypothesis is presented to explain the ingress of air. Finally, results are presented that describe the response of the optically-accessed reformer to variations in; operating temperature, humidification factor, total volume flow rate, methane volume flow rate, and the methane residency time within the reformer channel. From these results it was possible to conclude that increased reformer temperature increased reaction rate, increased gas residency time in the channel increased hydrogen production, and reactant streams with higher inlet mole fractions of methane resulting in increased reaction rates and amounts of hydrogen production. The performance of the reformer rig and the suitability of optical diagnostic techniques to the application of a SOFC scale reformer are discussed.
Master's Student Supervision (2010 - 2021)
The experimental data from the Cambridge-Sandia burner working in nine different configurations are post-processed to explore the effects of coordinate, swirl flow ratio, and stratification factor on the conditionally-averaged reactive scalars. First, the mixture fraction and progress variable are employed to construct a two-dimensional conditional manifold by using all of the data – regardless of the coordinate in the physical domain, swirl ratio, and stratification factor. Moreover, one-point, one-time measurements are utilized to compute the exact joint Probability Density Function (PDF) of the conditioning variables at each point. Having the conditional averages of temperature and mass fractions of reactive scalars in addition to the joint PDF of the conditioning variables, the manifold is applied to calculate the unconditional averages for each of the scalars. The mean values are also obtained by ensemble averaging all the data available for each of the measuring points, and the discrepancies between the values calculated from the two approaches are reported in order to assess the validity of the assumptions underlying the low-dimensional chemistry representations. The results suggest that the two chosen conditioning variables are not sufficient to make the manifold independent of the real domain, so, the normalized total enthalpy is introduced as the third conditioning variable and the process repeated. Results obtained from the three-condition manifold demonstrate that the discrepancies for prediction of the reactive scalars decrease when using the third condition. Moreover, the experimental data are employed to obtain the marginal PDFs for the conditioning variables at various points in the reacting domain. The measurements are then combined from all positions in space to form conditional PDFs of the normalized total enthalpy for various values of the other two variables. The correlation coefficients between the conditioning variables are also investigated. Next, to consider the association between the conditioning variables for modeling, the copula concept is introduced, and the performance of three different copulas are tested. Furthermore, the statistical moments of the conditioning variables are computed from the experimental data at different points and are utilized for modeling the joint PDF of the conditioning variables from two different approaches which are compared.
Combustion of hydrocarbon fuels plays a major role in meeting world energy de-mand and is expected to continue to do so for the foreseeable future. Numericalsimulation of combustion is an important tool that can be used in the design ofcombustion devices. Combustion simulation is highly challenging due to the com-plexity of the turbulence and chemical processes that occur, and modeling thesephenomena as well as their interactions is a field of ongoing research. This workaims to improve the tools available for combustion simulation, so that cleaner, moreefficient engines can be developed in the future.The specific focus of this work is a model for turbulence-chemistry interactioncalled Uniform Conditional State (UCS), which is related to conditional momentclosure (CMC) methods. As with CMC, the UCS model uses conditional averagingto achieve chemical closure. However, the UCS model is unique in assumingthat with sufficient conditioning, the conditional fields are spatially homogeneous,and can therefore be solved for on a conditional domain separate from the spatialdomain.The two research chapters focus on the validation of the UCS model. A studyexamining the effect of using a model PDF when mapping between the conditionaland spatial domains in the context of non-premixed combustion is presented in thefirst of these chapters. It is demonstrated that the UCS model is largely insensi-tive to the model PDF used, and that a β-PDF can be used successfully for bothconditioning variables (mixture fraction and progress variable) in this context.The second research chapter applies UCS to a fully-premixed swirling methaneflame. The results for velocity and temperature generally agree with experimentaland theoretical values and a stable flame is obtained for most lean stoichiometries.When compared to experiment the flame height is greater in the UCS simulation.There are some discrepancies from the expected behaviour in the results for speciesconcentrations and power output, which can likely be attributed to the use of theβ-PDF for progress variable within a premixed context. Despite these issues, theUCS model shows promise for use across the premixed-non-premixed continuum,provided that appropriate PDF models can be identified.
Conditional Source-term Estimation (CSE) is a chemical closure model for the simulation of turbulent combustion. In this work, CSE has been explored for modelling combustion phenomena in a spark-ignition (SI) engine. In the arbitrarily complex geometries imposed by industrial design, estimation of conditionally averaged scalars is challenging. The key underlying requirement of CSE is that conditionally averaged scalars be calculated within spatially localized sub-domains. A domain partitioning algorithm based on space-filling curves has been developed to construct localized ensembles of points necessary to retain the validity of CSE. Algorithms have been developed to evenly distribute points to the maximum extent possible while maintaining spatial locality. A metric has been defined to estimate relative inter-partition contact as an indicator of communication in parallel computing architectures. Domain partitioning tests conducted on relevant geometries highlight the performance of the method as an unsupervised and computationally inexpensive domain partitioning tool.In addition to involving complex geometries, SI engines pose the challenge of accurately modelling the transient ignition process. Combustion in a homogeneous-charge natural gas fuelled SI engine with a relatively simple chamber geometry has been simulated using an empirical model for ignition. An oxygen based reaction progress variable is employed as the conditioning variable and its stochastic behaviour is approximated by a presumed probability density function (PDF). A trajectory generated low-dimensional manifold has been used to tabulate chemistry in a hyper-dimensional space described by the reaction progress variable, temperature and pressure. The estimates of pressure trace and pollutant emission trends obtained using CSE accurately match experimental measurements.