Engine manufactures are currently seeking to develop spark ignition engines that are more fuel efficient, more refined and produce lower amounts of polluting emissions. To achieve these objectives an improved understanding of the factors governing the combustion process is required. Engine in-cylinder fluid motion is known to fundamentally affect fuel–air mixture preparation and flame propagation. Therefore, characterisation and quantification of the in-cylinder flow is an important step in the process of achieving the conditions necessary for optimal combustion. This thesis reports the application of two-colour Particle Image Velocimetry (PIV) to measure extended velocity fields within the combustion chamber of a firing production geometry optical engine. Two-colour PIV was used to obtain high spatial resolution fluid velocity maps for a range of crank angles and engine conditions. PIV measurements were obtained in the unburned gas ahead of the propagating flame and a combustible seeding material was used to clearly define the burned gas region. Data is presented for both the normal 2-valve running conditions and with one inlet port deactivated for both open-valve and closed-valve fuel injection timing.

Lean premixed combustion has potential for reducing emissions from combustion devices without compromising fuel efficiency, but it is prone to instabilities which presents design difficulties. From emissions point of view reliable predictions of species formation rates in the flame zone are required while from the point of view of thermo-acoustics the prediction of spatial variation of heat release rate is crucial; both tasks are challenging but imperative in CFD based design of combustion systems. In this thesis a computational model for turbulent premixed combustion is proposed in the RANS framework and its predictive ability is studied. The model is based on the flamelet concept and employs strained laminar flamelets in reactant-to-product opposed flow configuration. The flamelets are parametrised by scalar dissipation rate of progress variable which is a suitable quantity to describe the flamelet structure since it is governed by convection-diffusion-reaction balance and represents the flame front dynamics. This paramaterisation is new. The mean reaction rate and mean species concentrations are obtained by integrating the corresponding flamelets quantity weighted by the joint pdf of the progress variable and its dissipation rate. The marginal pdf of the progress variable is obtained using β-pdf and the pdf of the conditional dissipation rate is presumed to be log-normal. The conditional mean dissipation rate is obtained from unconditional mean dissipation rate which is a modelling parameter. An algebraic model for the unconditional mean scalar dissipation rate is proposed based on the relevant physics of reactive scalar mixing in turbulent premixed flames. This algebraic model is validated directly using DNS data. An indirect validation is performed by deriving a turbulent flame speed expression using the Kolmogorov-Petrovskii-Piskunov analysis and comparing its predictions with experimental data from a wide range of flame and flow conditions. The mean reaction rate closure of the strained flamelets model is assessed using RANS calculations of statistically planar one-dimensional flames in corrugated flamelets and thin reaction zones regimes. The flame speeds predicted by this closure were close to experimental data in both the regimes. On the other hand, an unstrained flamelets closure predicts flame speed close to the experimental data in the corrugated flamelets regime, but over predicts in the thin reaction zones regime indicating an over prediction of the mean reaction rate. The overall predictive ability of the strained flamelets model is assessed via calculations of laboratory flames of two different configurations: a rod stabilised V-flame and pilot stabilised Bunsen flames. For the V-flame, whose conditions correspond to the corrugated flamelets regime, the strained and unstrained flamelets models yield similar predictions which are in good agreement with experimental measurements. For the Bunsen flames which are in the thin reaction zones regime, the unstrained flamelet model predicts a smaller flame brush while the predictions of the strained flamelets model are in good agreement with the experimental data. The major and minor species concentrations are also reasonably well predicted by the strained flamelets model, although the minor species predictions seem sensitive to the product stream composition of the laminar flamelets. The fluid dynamics induced attenuation of the reaction rate is captured by the strained flamelets model enabling it to give better predictions than the unstrained flamelets model in the thin reaction zones regime. The planar flames and laboratory flames calculations illustrate the importance of appropriately accounting for fluid dynamic effects on flamelet structure and the scalar dissipation rate based strained flamelet model seems promising in this respect. Furthermore, this model seems to have a wide range of applicability with a fixed set of model parameters.

Combustion at extreme conditions, such as a turbulent flame at high Karlovitz and Reynolds numbers, is still a vast and an uncertain field for researchers. Direct numerical simulation of a turbulent flame is a superior tool to unravel detailed information that is not accessible to most sophisticated state-of-the-art experiments. However, the computational cost of such simulations remains a challenge even for modern supercomputers, as the physical size, the level of turbulence intensity, and chemical complexities of the problems continue to increase. As a result, there is a strong demand for computational cost reduction methods as well as in acceleration of existing methods. The main scope of this work was the development of computational and numerical tools for high-fidelity direct numerical simulations of premixed planar flames interacting with turbulence. The first part of this work was KAUST Adaptive Reacting Flow Solver (KARFS) development. KARFS is a high order compressible reacting flow solver using detailed chemical kinetics mechanism; it is capable to run on various types of heterogeneous computational architectures. In this work, it was shown that KARFS is capable of running efficiently on both CPU and GPU. The second part of this work was numerical tools for direct numerical simulations of planar premixed flames: such as linear turbulence forcing and dynamic inlet control. DNS of premixed turbulent flames conducted previously injected velocity fluctuations at an inlet. Turbulence injected at the inlet decayed significantly while reaching the flame, which created a necessity to inject higher than needed fluctuations. A solution for this issue was to maintain turbulence strength on the way to the flame using turbulence forcing. Therefore, a linear turbulence forcing was implemented into KARFS to enhance turbulence intensity. Linear turbulence forcing developed previously by other groups was corrected with net added momentum removal mechanism to prevent mean velocity drift. Also, dynamic inlet control was implemented which retained flame inside of a domain even at very high fuel consumption fluctuations. Last part of this work was to implement pseudospectral method into KARFS. Direct numerical simulations performed previously are targeting real engines and turbines conditions as an ultimate goal. These targeted simulations are prohibitively computationally expensive. This work suggested and implemented into KARFS a pseudospectral method for reacting turbulent flows, as an attempt to decrease computational cost. Approximately four times computational CPU hours savings were achieved.

A large eddy simulation (LES) model has been developed and validated for turbulent non-premixed and partially premixed combustion systems. LES based combustion modelling strategy has the ability to capture the detailed structure of turbulent flames and account for the effects of radiation heat loss. Effects of radiation heat loss is modelled by employing an enthalpy-defect based non-adiabatic flamelet model (NAFM) in conjunction with a steady non-adiabatic flamelet approach. The steady laminar flamelet model (SLFM) is used with multiple flamelet solutions through the development of pre-integrated look up tables. The performance of the non-adiabatic model is assessed against experimental measurements of turbulent CH4/H2 bluff-body stabilized and swirl stabilized jet flames carried out by the University of Sydney combustion group. Significant enhancements in the predictions of mean thermal structure have been observed with both bluff body and swirl stabilized flames by the consideration of radiation heat loss through the non-adiabatic flamelet model. In particular, mass fractions of product species like CO2 and H2O have been improved with the consideration of radiation heat loss. From the Sydney University data the HM3e flame was also investigated with SLFM using multiple flamelet strategy and reasonably fair amount of success has been achieved. In this work, unsteady flamelet/progress variable (UFPV) approach based combustion model which has the potential to describe both non-premixed and partially premixed combustion, has been developed and incorporated in an in-house LES code. The probability density function (PDF) for reaction progress variable and scalar dissipation rate is assumed to follow a delta distribution while mixture fraction takes the shape of a beta PDF. The performance of the developed model in predicting the thermal structure of a partially premixed lifted turbulent jet flame in vitiated co-flow has been evaluated. The UFPV model has been found to successfully predict the flame lift-off, in contrast SLFM results in a false attached flame. The mean lift-off height is however over-predicted by UFPV-δ function model by ~20% for methane based flame and under-predicted by ~50% for hydrogen based flame. The form of the PDF for the reaction progress variable and inclusion of a scalar dissipation rate thus seems to have a strong influence on the predictions of gross characteristics of the flame. Inclusion of scalar dissipation rate in the calculations appears to be successful in predicting the flame extinction and re-ignition phenomena. The beta PDF distribution for the reaction progress variable would be a true prospect for extending the current simulation to predict the flame characteristics to a higher degree.

Although Large Eddy Simulation (LES) has demonstrated its potential for modelling the reaction in simple academic combustors, it is more computationally expensive than Reynolds Averaged Navier-Stokes (RANS) which has been used widely for industrial cases. The aim of this research is to employ LES at minimal grid resolution and computational resource requirements to capture the main characteristics of the reacting flows in a helicopter combustor and exhaust plume with the focus on NOx emissions. Test cases have been carried out to validate the current LES code for non- reacting jet, non-premixed combustion and unstructured grids. Despite the moderate grid refinement and simple chemistry models employed, the findings from these test cases have demonstrated good capabilities of the current LES to capture the mixing, flame and flow characteristics. In a farther test case, a key gas-phase chemical reaction selected for the helicopter exhaust plume modelling has also been tested. The validated LES code is then employed in the numerical study of the reaction in the helicopter combustor. The LES predictions in terms of the temperature and EINOx agree generally well with the combustor design, analytical solutions, previous LES and test measurements. Subsequently, the potential application of LES for the calibration of simpler models has been assessed for the generic and helicopter combustors. The results obtained from LES are compared with those from a one-dimensional combustor performance and emissions code, HEPHAESTUS, developed within the Cranfield University Power and Propulsion Department. The discrepancies between the results are found to be primarily due to specific simplification and assumptions established in the HEPHAESTUS model which can be addressed. Finally, LES has been employed to model the transformation of NO to NO2 in the helicopter exhaust plume. The findings from this research have demonstrated that, even without the implementation of highly dense mesh or advanced reaction model, LES is able to provide results with an acceptable level of fidelity at relatively low computational costs. These advantages make it a powerful predictive tool for future design and emissions optimisation investigations, and calibration of other simpler modelling approaches.

The first part of this thesis intends to create a fuel processor model capable of generating 1kW power as output through the use of a solid oxide fuel cell system. The fuel processor system consists of a reformer, heat exchanger network, desulphurizer and an afterburner. Modelled in VMGSimTM, inlet diesel gas is provided at the mass flow rate of 0.2596kg/hour, with the oxygen to carbon ratio calculated at 0.31 and the steam to carbon ratio arbitrarily set to be 2.25. The diesel fuel is preheated and mixed with air and steam and then fed to the auto-thermal reformer. The higher hydrocarbons are broken down and converted into hydrogen. The outlet of the reformer is fed into the SOFC where H2 is converted to generate energy which, in this case is approximately 1200W. The off gas is fed to the afterburner; where the remaining H2 is burnt and the energy is used to provide for steam generation and pre-heating through the heat exchangers. The project also focuses upon performing basic sizing calculations on components of the system. The fuel cell efficiency was found to be 62% and the system efficiency was calculated to be approximately 41%, which falls within the range given in literature. For the second part of this work, a ceramic porous tail-gas burner using a non-premixed feed of anode exhaust and air was modeled using COMSOL™. The reaction kinetics were experimentally assessed on the basis of COMSOL™ limitations and accuracy of the comparative results. Three performance metrics were evaluated in the analysis: i) velocity profile, ii) temperature profile, and iii) concentration profile. These metrics confirm the combustion reaction at the outer boundary of the porous ceramic in the burner. The spike of temperature and decrease of mass fraction of hydrogen, carbon monoxide and methane to approximately zero in the outlet exhaust confirms this study. This study was further validated by comparing results with the experimental data collected at NRC-IFCI. The results of COMSOL™ model agreed with the experimental results of NRC-IFCI. / Thesis (Master, Chemical Engineering) -- Queen's University, 2013-10-29 17:49:32.266

Biogas is a renewable gaseous fuel with low calorific value and a low burning velocity. This burning characteristic makes stabilizing biogas flame difficult especially in high flow velocity applications, and hence presenting a real challenge for power generation systems. This thesis presents an experimental investigation of the effect of burner geometry (i.e., fuel nozzle geometry and swirl strength of the co-airflow) on the stability limits of a turbulent non-premixed biogas surrogate flame. Three different co-airflow swirl strengths (S = 0, 0.31, 0.79) were implemented using swirl generators with vane angle of 0º, 25º and 50º, respectively. Six different fuel nozzle geometries were used in order to study the effect of fuel jet centerline velocity on the stability limits of a low swirling (i.e., 25º) non-premixed biogas flame. Moreover, the biogas surrogate fuel composition was kept constant (60% CH4 and 40% CO2 by volume) using a mixture of pure methane and carbon dioxide gases. The results of the effect of co-airflow swirl strength on the stability limits of biogas flame revealed that the swirl plays an important role on both the flame mode and its stability limits for both attached and lifted flames. The experimental results revealed that at low swirl strength the attached flame lifts off and stabilizes at a distance above the burner, while at high swirl strength the flame remains attached but shortens and burns blue. Overall, the high swirl attached flame was found to stabilize over a wider range of flow conditions in comparison to the attached and lifted flame produced by low swirl. Importantly, the central fuel jet characteristics (induced by varying the fuel nozzle geometry) were found to drastically influence the upper and lower blowout limits of the low swirl biogas lifted flame, while multi-hole fuel nozzle geometry was found to significantly enhance the stability ranges. 2D PIV data was used to explain the stability limits and the experimental flame results were used to propose semi-empirical correlations capable of describing the turbulent biogas blowout stability limits. / October 2016

There are numerous numerical and experimental studies to find correlations of octane rating with fuel properties. This thesis was based on the hypothesis that quenching characteristics at ignition locations impact the flame development. Conversely, determination of quenching characteristics might serve as an effective measure to determine the fuel mixture octane number. This hypothesis was tested with premixed flame experiments using primary reference fuels (iso-octane and n-heptane) and commercial grade gasoline. Premixed flame experiments were conducted on a flat flame burner. Primary reference fuels of different ratios were taken and correlated to their respective thermal quenching condition by introducing co-flowing inert gasses at room temperature with fuel-air mixture. The inert gasses that were used in the experiment are nitrogen and helium and the results are analyzed using a camera and an imaging spectrometer. The experimental results support the hypothesis that flame quenching can be correlated to fuel mixture octane number, and holds potential as an alternative method to determine the octane number.

Various premixed flame phenomena are studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics is essential for all premixed combustion problems where multidimensional effects cannot be disregarded.The present thesis consists of six parts. The first part deals with the fundamental problem of curved stationary flames propagation in free-slip tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in tubes wider than some stability limit. The flame dynamics is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front.The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. Two-dimensional simulation of radially expanding flames displays a radial growth with 1.25 power law temporal behavior. It is shown that the fractal excess for 2D geometry obtained in thenumerical simulation is in good agreement with theoretical predictions.In third part the flame acceleration in tubes with non-slip at the walls is studied in the extremely wide range of flame front velocity. Flame accelerates from small initial velocity to supersonic speed in the laboratory reference frame. Flame acceleration undergoes three stages: 1) initial exponential acceleration in the quasi-isobaric regime, 2) almost linear increase of the flame speed to supersonic values, 3) saturation to a stationary high-speed deflagration velocity, which is correlated with the Chapman-Jouguet deflagration speed. The saturation velocity is in line with previous experimental results.In fourth part the role of viscous stress in heating of the fuel mixture in deflagration-to-detonation transition in tubes is studied both analytically and numerically. The developed analytical theory determines temperature distribution ahead of an accelerating flame. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. Viscous stress makes heating and explosion of the fuel mixture preferential at the walls.In fifth part we reveal the physical mechanism of ultra-fast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jet-flow, driving the acceleration. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.The last part of the thesis concerns the transition from deflagration to detonation. It is found that in sufficiently wide free-slip channels and for sufficiently fast flames Landau-Darrieus instability may invoke nucleation of hot spots within the wrinkled flame folds, triggering an abrupt transition from deflagrative to detonative combustion. Results on DDT in channels with non-slip at the walls are also presented. / QC 20100915

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