Abstract Conventional Brayton cycles have demonstrated to be significantly less efficient than alternative propulsion systems (spark ignition, diesel, fuel cells, etc.) for low power output applications, such as for small size UAVs. The gas turbine performance could be enhanced through the introduction of heat exchangers, with the consequent increase of the overall engine weight. Semi-closed cycles have documented advantages of higher thermal efficiency and degree of compactness than traditional intercooled-recuperated open cycles. This paper discusses advantages and applicability of semi-closed cycles to a small gas turbine, designed for a medium altitude UAV mission. In particular, size and altitude effects have been accounted in the performance evaluation of two different semi-closed cycle arrangements designed for an output shaft power of 100 hp (74.57 kW). Resultant performance has been compared with equivalent simple recuperated and intercooled-recuperated open cycles. Furthermore, a final engine performance comparison has been made with data obtained from a similar analysis performed on a larger engine, with a power output of 300 hp (223.71 kW) and designed for an extremely high altitude UAV application. While promising results have been obtained for the larger case study, where semi-closed cycles have demonstrated superior performance and higher engine compactness than conventional solutions, similar trends have not been displayed for the smaller engines, as consequence of the strong size effects observed in the turbomachinery performance. For the 100 hp engine the semi-closed cycles are slightly outperformed by the open cycle engines.

Abstract Methane (CH 4 ) and bio alcohols have different ignition properties. These have been extensively studied and the resulting experimental data have been used to validate chemical kinetic models. Methane is the main component of natural gas, which is of interest because of its relative availability and lower emissions compared to other hydrocarbon fuels. Given growing interest in fuel-flexible systems, there can be situations in which the combustion properties of natural gas need to be modified by adding biofuels, such as bio alcohols. This can occur in dual fuel internal combustion engines or gas turbines with dual fuel capabilities. The combustion behavior of such blends can be understood by studying the auto ignition properties in fundamental combustion experiments. Studies of the ignition of such blends are very limited in the literature. In this work, the auto ignition of methane and bio alcohol fuel blends is investigated using a shock tube facility. The chosen bio alcohols are ethanol (C 2 H 5 OH) and n-propanol (NC 3 H 7 OH). Experiments are carried out at 3 atm and 10 atm for stoichiometric and lean mixtures of fuel, oxygen, and argon. The ignition delay times of the pure fuels are first established at conditions of constant oxygen concentration and comparable pressures. The ignition delay times of blends with 50% methane are then measured. The pyrolysis kinetics of the blends is further explored by measuring CO formation during pyrolysis of the alcohol and methane-alcohol blends. The resulting experimental data are compared with the predictions of selected chemical kinetic models to establish the ability of these models to predict the disproportionate enhancement of methane ignition by the added alcohol.

Abstract Two different numerical techniques for chemistry acceleration are examined with Large Eddy Simulation of a commercial swirl industrial gas turbine combustor operating at 3 bar. This work presents the results for SGT-100 Dry Low Emission (DLE) gas turbine provided by Siemens Industrial Turbomachinery Ltd. The related experimental study was performed at the German Aerospace Centre, DLR, Stuttgart, Germany. LES with detailed chemistry calculations is an attractive tool to study turbulent premixed flames in industrial gas turbine combustors, because it can help understand turbulence-chemistry interactions, detailed flame characteristics and pollutant formation. Detailed chemistry can capture kinetically dominated processes such as ignition, extinction and pollutant formation. However, computational resources required for such calculations are often prohibitive due to the computational costs of transporting and integration of a large number of species with a wide range of chemical time-scales. Chemistry acceleration techniques can substantially reduce run-time with ideally a small loss in accuracy. Therefore, the purpose of this work is to quantify the relative increase in performance and potential loss in accuracy with two chemistry acceleration techniques namely Clustering, Dynamic Mechanism Reduction (DMR) and their combination. The results show that the different chemistry acceleration techniques do not compromise the time averaged flow statistics. However, there are some differences in NO and CO emissions. Chemistry acceleration techniques yield up to ∼3 times speed-up of the simulation.

Abstract Integrated Gasification Combined Cycle (IGCC) is a technology that integrates the coal gasification and combined cycle to produce electricity efficiently. Due to the fact that the heating value of syngas from coal gasification process is typically lower than that of the natural gas, the conventional gas turbine will have to be adapted for syngas. The nozzle adjustment is the key to the successful transformation since the ignition properties are different between syngas and natural gas which have totally different compositions. The nozzles suitable for natural gas have been prone to partially melting around the flame stabilization holes on sidewalls of the nozzle in real operation. Thus a computational fluid dynamics (CFD) model was constructed for the syngas nozzles as well as combustion chamber of the gas turbine for low heating value syngas to study the thermostability of the nozzle. The detailed structure of the syngas nozzle, the combustion characteristics of syngas, as well as the actual operation condition of the gas turbine were all employed in the CFD model to improve the simulation accuracy. The reason of partially melting of the nozzles suitable for natural gas can be attributed to that the syngas leaked from the flame stabilization holes into the mainstream air can quickly mix with air, adhere to the sidewalls of the nozzles and then ignite around the holes which result in temperatures high enough to melt the material of the nozzle around the holes through CFD simulation. Finally, a new structure of the syngas nozzle was proposed and validated by CFD simulations. The simulation result shows that the flames caused by the syngas leaked from the flame stabilization holes are no longer adhering to the nozzle sidewalls and local high temperature can be lowered by about 30% which will not be able to melt the nozzle material.

Abstract Low emission gas turbine engines, operating under fuel lean conditions, are susceptible to light-around issues. Traditionally, gas turbine manufacturers rely on experimentation and testing to understand the relight characteristics of a combustor. However, since the last decade, numerical simulations are gaining popularity in performance evaluation of the light-around characteristics of the gas turbine combustors. In the present work, assessment of the Flamelet Generated Manifold (FGM) combustion model is carried out to understand its performance for capturing the correct ignition sequence in a linear multi-burner methane-air swirl combustor designed by COmplexe de Recherche Interprof essionnel en Aérothermochimie (CORIA) in the context of Knowledge for Ignition, Acoustics, and Instabilities (KIAI) project. The present work uses linear five, four and two swirled injector configurations for the validation of the simulation results. Non-reacting and reacting Large Eddy Simulations (LES) are performed for three injector arrangements to predict the main flow structure, mixing, flame propagation and ignition sequence. Non-reacting time-averaged flow quantities such as mean axial and radial velocities are data-sampled and compared with the experimental results. The predicted results show a good comparison between simulation and experimental data. Ignition sequence and timing predicted from the reacting LES for all the three configurations studied in this work, also compare well with the experimental data. This numerical investigation confirms that the FGM combustion model used in the LES framework can be successfully employed for the prediction of the relight characteristics of the gas turbine engines.

Abstract Three Flamelet Generated Manifold reaction source term closure options and two different reactor types are examined with Large Eddy Simulation of an industrial gas turbine combustor operating at 3 bar. This work presents the results for the SGT-100 Dry Low Emission (DLE) gas turbine provided by Siemens Industrial Turbomachinery Ltd. The related experimental study was performed at the German Aerospace Centre, DLR, Stuttgart, Germany. The FGM model approximates the thermo-chemistry in a turbulent flame as that in a simple 0D constant pressure ignition reactors and 1D strained opposed-flow premixed reactors, parametrized by mixture fraction, progress variable, enthalpy and pressure. The first objective of this work is to compare the flame shape and position predicted by these two FGM reactor types. The Kinetic Rate (KR) model, studied in this work, uses the chemical rate from the FGM with assumed shapes, which are a Beta function for mixture fraction and delta functions for reaction progress variable and enthalpy. Another model investigated is the Turbulent Flame-Speed Closure (TFC) model with Zimont turbulent flame speed, which propagates premixed flame fronts at specified turbulent flame speeds. The Thickened Flame Model (TFM), which artificially thickens the flame to sufficiently resolve the internal flame structure on the computational grid, is also explored. Therefore, a second objective of this paper is to compare KR, TFC and TFM with the available experimental data.

Abstract Premixed or partially premixed swirling flames are widely used in gas turbine applications because of their compactness, high ignition efficiency, low NO x emissions and flame stability. A typical annular combustor consists of about eighteen to twenty-two swirling flames which interact (directly or indirectly) with their immediate neighbors even during stable operation. These interactions significantly alter the flow and flame topologies thereby bringing in some discrepancies between the single nozzle (SN) and multi nozzle (MN), ignition, emission, pattern factor and Flame Transfer Functions (FTF) characteristics. For example, in MN configurations, application of a model based on SN FTF data could lead to erroneous conclusions. Due to the complexities involved in this problem in terms of size, thermal power, cost, optical accessibility etc., a limited amount of experimental studies has been reported, that too on scaled down models with reduced number of nozzles. Here, we present a detailed experimental study on the behavior of three interacting swirl premixed flames, arranged in-line in an optically accessible hollow cuboid test section, which closely resembles a three-cup sector of an annular gas turbine combustor with very large radius. Multiple configurations with various combinations of swirl levels between the adjacent nozzles and the associated flame and flow topologies have been studied. Spatio-temporal information of the heat release rate obtained from OH* chemiluminescence imaging was used along with the acoustic pressure signatures to compute the Rayleigh index so as to identify the regions within the flame that pumps energy into the self-excited thermoacoustic instability modes. It was found that the structure of the flame-flame interaction regions plays a dominant role in the resulting thermoacoustic instability. To resolve the flow and reactive species field distributions in the interacting flames, two-dimensional, three component Stereoscopic Particle Image Velocimetry (SPIV) and Planar Laser Induced Fluorescence (PLIF) of hydroxyl radical was applied to all the test conditions. Significant differences in the flow structures among the different configurations were observed. Simultaneous OH-PLIF and SPIV techniques were also utilized to track the flame front, from which the curvature and stretch rates were computed. Flame surface density which is defined as the mean surface area of the reaction zone per unit volume is also computed for all the test cases. These measurements and analyses elucidate the structure of the interaction regions, their unique characteristics and possible role in thermoacoustic instability.

Abstract The process of ignition in aero-engines raises many practical issues that need to be faced during the design process. Recent experiments and simulations have provided detailed insights on ignition in single-injector configurations and on the light-round sequence in annular combustors. It was shown that Large Eddy Simulation (LES) was able to reliably reproduce the physical phenomena involved in the ignition of both perfectly premixed and liquid spray flames. The present study aims at further extending the knowledge on flame propagation during the ignition of annular multiple injector combustors by focusing the attention on the effects of heat losses, which have not been accounted for in numerical calculations before. This problem is examined by developing Large Eddy Simulations of the light-round process with a fixed temperature at the solid boundaries. Calculations are carried out for a laboratory-scale annular system. Results are compared in terms of flame shape and light-round duration with available experiments and with an adiabatic LES serving as a reference. Wall heat losses lead to a significant reduction in the flame propagation velocity as observed experimentally. However, the LES underestimates this effect and leads to a globally shorter light-round. To better understand this discrepancy, the study focuses then on the analysis of the near wall region where the velocity and temperature boundary layers must be carefully described. An a-priori analysis underlines the shortcomings associated to the chosen wall law by considering a more advanced wall model that fully accounts for variable thermophysical properties and for the unsteadiness of the boundary layer.

Abstract Thermo-acoustic instabilities are an important consideration in the design of modern power generation gas turbine combustors. While the design process must consider many competing requirements, such as temperature profiles, emissions, robustness to auto-ignition and flameholding, thermoacoustics is one of the most challenging to predict, and therefore design for. This is particularly true in the case of liquid-fueled systems, where the phenomenon results from a complex system of coupled multi-physics phenomena: fuel atomization and transport, mixing, reactive kinetics and acoustics. Nevertheless, emissions-compliant liquid fuel capability is becoming increasingly important to GT operators, thus it is critical to be able to predict the thermoacoustic instabilities of these combustors. In this work we present an approach to model the thermoacoustic feedback loop for a realistic liquid fuel nozzle in a single burner configuration. The approach is based on an analytical liquid-fuel diffusion flame model to provide the fluctuating heat release response to inflow perturbations. This is coupled with a 3D FEM description of the acoustic response of the single burner rig through a time-domain Green’s function model to predict the growth and saturation of pressure oscillations. The necessary flame model parameters are calibrated based on a range of test data obtained from the single burner rig with a tunable combustor length. The results are shown to compare well with test data across a range of operating conditions, and for two different nozzle geometries.

Abstract For the purposes of combustion analysis, n-dodecane is used as the surrogate or a surrogate component for biodiesel and jet fuel. In order to capture kinetic effects in computational combustion, detailed and reduced models of n-dodecane are therefore used. This paper presents a comparative analysis of selected detailed chemical kinetic models of n-dodecane as well as reduction of these detailed models to more compact skeletal versions. The selected models are compared based on their ability to predict ignition phenomena. Measured ignition delay times from the literature are used as references. Both low- and high-temperature ignition simulations are considered. To further facilitate future computational combustion analysis, the detailed models are reduced using the Alternative Species Elimination (ASE) approach reported by Akih-Kumgeh and Bergthorson (Energy & Fuels, 2316–2326, 2013). The resulting skeletal models are compared in terms of their retained species, ranked species sensitivities, and kinetic parameters of the key reactions. Furthermore, within the framework of this paper, another model reduction technique is explored. The aim of this method is to further decrease model reduction time since this is often considered as a weakness of the otherwise effective ASE method. The resulting models from this exploratory reduction approach are compared with those obtained from the ASE method in terms of species retained and the accuracy with which combustion properties from the detailed models are predicted. Further chemical kinetic analysis of the reduced models is carried out with the aim of explaining observed similarities and differences.

Abstract The demand for increased efficiencies in modern aeroengines drives designs to higher pressure ratios, temperatures and shaft speeds. Consequently, higher cycle temperatures and parasitic heat loads are likely to become limiting factors in the design of bearing chambers. These chambers are lubricated and cooled with oil and pressurized using air taken from the main gas path. Historically, the temperature and pressure of the sealing air has been low enough to exclude the risk of bearing chamber oil fires caused by oil auto-ignition. However, temperatures are being driven towards the limits set by current design rules. With respect to oil fire risk assessment, current design rules are very conservative as they pessimistically assume the oil is continuously exposed to the maximum temperature expected in operation when determining the minimum residence time required for oil auto-ignition. Improved bearing chamber designs capable of tolerating higher temperatures could therefore be developed by applying a more rational level of conservatism, based on a more physics-based approach (such as considering the effect of an oil droplet being transported through a varying temperature field). In this paper a numerical methodology is developed to provide a pragmatic approach to addressing conservatism in bearing chamber oil fire risk assessment, with respect to oil auto-ignition. An unsteady Eulerian CFD prediction is used to compute the aerothermal flow field within a stylized bearing chamber. A Lagrangian discrete particle method is then used to track the oil droplet trajectories. The time-dependent droplet temperature histories are then used to compute the fractional accumulation of auto-ignition delay time using an empirically derived relationship. Finally, by defining an ‘auto-ignition (AI) energy’ accumulation factor, the methodology assesses whether an individual oil droplet has satisfied the criteria for auto-ignition. The present contribution examines the effects of various modelling parameters on the ‘AI energy’ accumulation factor. These include one/two-way coupling between the air flow and oil droplets, stochastic turbulence modelling of the droplet behavior, and the effect of droplet size and distribution. The work highlights that two-way coupling is required to ensure the thermal effect of the oil is modelled, despite the increased computational demand. Stochastic modelling of interactions between particles and the flow field is also required to capture the spread of particle trajectories and the resulting distribution of particle temperatures. A representative range of droplet sizes must also be simulated as the propensity for oil AI is a function of droplet diameter; the highest risk occurs for the smallest droplets whilst the largest droplets have greater cooling effect on the air flow. Given the extent of model simplification required to allow the work to be completed with a mid-spec desktop computer and the overall scope of the project, a validation of the findings has not been completed. Instead, an experimental validation is proposed as part of future investigation. The authors imagine that with enough investigation and validation, the understanding developed by the work could be applied as part of a computationally efficient industrial design toolset to inform the early stages of product design.

Abstract Utilizing direct-fired sCO 2 oxy-combustion is attractive for power generation applications because of the cycle’s inherent carbon capture, high efficiency, and small machinery footprint. However, there is a large amount of uncertainty regarding the combustion process of natural gas in carbon dioxide diluent at supercritical pressures. One such area of uncertainty is in regards to the ignition system. The performance of most common ignition systems is not proven at the elevated pressures and densities typical of these cycles. This paper presents an evaluation and down-selection of potential ignition systems considered for a sCO 2 oxy-combustor igniter. The ignition systems considered include spark ignition, laser ignition, heating element auto-ignition, external preheat auto-ignition, ignition using solid or liquid fuels, and external torch ignition. After a preliminary review, spark ignition and laser ignition were chosen for system reliability and repeatability. To further quantify the practicality of each system, a spark igniter and laser igniter were lab-scale tested to determine breakdown energies associated with these igniters. The spark igniter was tested using gaseous CO 2 and SF 6 (to attain higher fluid densities). The laser igniter was tested using supercritical CO 2 and gaseous CO 2 . An additional round of testing was conducted using the laser igniter in a constant volume combustion chamber (CVCC). Natural gas was combusted with oxygen in varying levels of CO 2 dilution to determine the required laser power for stable, reliable ignition and to quantify the high dilution flammability limit. Based on these test results, a laser igniter was selected as the most practical option for high-pressure sCO 2 combustor ignition.

Abstract Film cooling jets behaviour in a combustor chamber is deeply affected by swirling flow interactions and unsteadiness; on the other hand, the jets behaviour has a direct impact on different phenomena such as cooling capabilities and ignition. For these reasons, an in-depth characterization of the film-cooling flows in the presence of a swirling mainflow, demands dedicated time-resolved analyses. The experimental setup consists of a non-reactive single-sector linear combustor simulator installed in an open loop wind tunnel. It is equipped with a swirler and a multiperforated plate to simulate the effusion cooling system of the liner. The rig is scaled with respect to the engine configuration, to increase spatial resolution and to reduce the characteristic frequencies of the unsteady phenomena. Time-Resolved Particle Image Velocimetry (TRPIV) was exploited for the investigation testing different values of liner pressure drop. In addition, numerical investigations were carried out to gain a deeper insight of the behaviour highlighted by the experiments and to assess the capability of CFD in predicting the flow physics. In this work, the Stress-Blended Eddy Simulation (SBES) approach implemented in ANSYS Fluent was adopted. Oscillations of the jets and intermittent interactions of the mainstream with the wall of the liner and hence with the film development have been investigated in detail. The results demonstrate how an unsteady analysis of the flow structures that characterize the jets, the turbulent mixing of coolant flows and the interaction between mainstream and cooling jets is strictly necessary to have a complete knowledge of the behaviour of the coolant which in turn affects combustor operability and life-time.

Forced ignition, the initiation of combustion processes by rapid and localized introduction of energy, is central to the successful operation of many combustion systems. It is therefore of interest to investigate this process, starting from the introduction of energy to the emergence of self-sustained flame or the quenching of an otherwise initialized flame kernel. Since the process is highly non-equilibrium and involves various complex kinetic phenomena, it is important to understand the key aspects that control failed or successful ignition. Detailed studies of the early phases of the ignition process can lead to knowledge of more general characteristics of the problem so that reduced models of the ignition process can be developed. These reduced versions can be used in less costly computational studies to assess various ignition events. This paper reports an experimental and numerical investigations of the early phase of laser ignition. The gas mixtures, air, methane/N 2 and methane/air are considered to bring out the effect of heat release on the early flow field. The mixtures are studied at three different energy levels and the Jones blast wave theory is used to deduce the energy responsible for the development of the attendant shock waves. This energy is also used to specify initial conditions for the simulations of air and methane/air processes. Additionally, interferometry is used to resolve the density field within the plasma kernel. For the methane/air simulation two chemical models are used, a global reaction model supplemented by an ignition model and a two-step mechanism. The sensitivity of the simulations to the initial geometry of the laser spark is also investigated. The blast wave and interferometry results show that in the reacting methane/air mixture the resulting shock wave is strengthened by early heat release. It is also shown that the shock wave trajectory is not strongly affected by the initial spark geometry, but it has an impact on the velocity field and on the distribution of thermodynamic properties.

Constant volume combustion cycles for gas turbines are considered a very promising alternative to the conventional Joule cycle and its variations. The reason is the considerably higher thermal efficiency of theses cycles, at least for their ideal versions. Shockless explosion combustion is a method to approximate constant volume combustion. It is a cyclic process that consists of four stages, namely wave propagation, fuel injection, homogeneous auto-ignition and exhaust. A pressure wave in the combustion chamber is used to realize the filling and exhaust phases. During the fuel injection stage, the equivalence ratio is controlled in such a way that the ignition delay time of the mixture matches its residence time in the chamber before self ignition. This means that the fuel injected first must have the longest ignition delay time and thus forms the leanest mixture with air. By the same token, fuel injected last must form the richest mixture with air (assuming that a rich mixture leads to a small ignition delay). The total injection time is equal to the time that the wave needs to reach the open combustor end and return as a pressure wave to the closed end. Up to date, fuel stratification has been neglected in thermodynamic simulations of the SEC cycle. The current work presents its effect on the thermal efficiency of the cycle and on the exhaust conditions (pressure, temperature and Mach number) of shockless explosion combustion chambers. This is done by integrating a fuel injection control algorithm in an existing CFD code. The capability of this algorithm to homogenize the auto-ignition process by improving the injection process has been demonstrated in past experimental studies of the SEC. The numerical code used for the simulation of the combustion process is based on the time-resolved 1D-Euler equations with source terms obtained from a detailed chemistry model.

The capability of current turbomachinery-based engines limit the obtainable altitude and flight Mach number of modern aircraft. Maturing hypersonic technologies such as ram and scramjets allow greatly increased flight velocity but are not able to power themselves from the ground and, thus, rely on lift aircraft. Combined-cycle engines incorporate turbomachinery and ramjet technologies to allow both high and low flight Mach numbers. These high Mach capable systems are typically flown using specialty fuels such as JP-7 that are more applicable to hypersonic applications than conventional gas-turbine fuels such as JP-8/Jet A or JP-5. Potential issues exist, however, when operating a combined cycle that employs a legacy main combustor since these platforms were not designed for operation with these fuels. Of particular concern is the re-ignition performance of the turbomachinery core. The relight of the gas-turbine combustor occurs at high altitudes and relatively high vehicle speeds, yielding low pressure and temperature in the combustor as well as high combustor-dome velocities. All of these conditions are unfavorable for ignition. Additionally, heavy fuels require more energy for atomization and vaporization, which increases the probability that ignition will become a problem in a turbine-based combined-cycle (TBCC) system. Successful demonstration of legacy main-combustor technologies in hypersonic combined-cycle applications will eliminate the need for costly design of new main-burner technology. The literature does not provide information on the effects of running specialty fuels such as JP-7 and JP-10 in burners with conventional aerodynamic features. To fill that gap, a three-cup sector of a conventional fighter-class swirl-stabilized combustor configured to provide optical access through the sidewall was used in the study. Two test conditions representative of varying flight Mach number and altitude are evaluated. For each flight condition, the combustor pressure drop is varied to characterize ignition as a function of burner inlet velocity. Data are correlated by loading parameter and equivalence ratio at ignition, and at each test point the conditions are varied until ignition cannot be achieved.

In this study, syngas combustion was investigated behind reflected shock waves in CO 2 bath gas to measure ignition delay times and to probe the effects of CO 2 dilution. New syngas data were taken between pressures of 34.58–45.50 atm and temperatures of 1113–1275K. This study provides experimental data for syngas combustion in CO 2 diluted environments: ignition studies in a shock tube (59 data points in 10 datasets). In total, these mixtures covered a range of temperatures T, pressures P, equivalence ratios φ, H2/CO ratio θ, and CO 2 diluent concentrations. Multiple syngas combustion mechanisms exist in the literature for modelling ignition delay times and their performance can be assessed against data collected here. In total, twelve mechanisms were tested and presented in this work. All mechanisms need improvements at higher pressures for accurately predicting the measured ignition delay times. At lower pressures, some of the models agreed relatively well with the data. Some mechanisms predicted ignition delay times which were 2 orders of magnitudes different from the measurements. This suggests there is behavior that has not been fully understood on the kinetic models and are inaccurate in predicting CO 2 diluted environments for syngas combustion. To the best of our knowledge, current data are the first syngas ignition delay times measurements close to 50 atm under highly CO 2 diluted (85% per vol.) conditions.

Elevated pressure and temperature conditions are widely encountered during gas turbine operation. To avoid unexpected ignition and explosion of mixtures of fuel and air under these conditions, it is imperative to identify the flammability limits of relevant fuel mixtures. Common fuels include process gases such as natural gas, coke oven gas and IGCC syngas fuel. The flammability limits of pure fuels and common gas/air mixtures have been widely reported, however a significant lack of flammability data for fuel mixtures relevant for use in gas turbines as well as data at elevated pressure and temperature conditions is available. The objective of this study is to characterize the flammability limits of fuel/air mixtures and their dependence on initial temperature and pressure. Experimental studies of lean flammability limits (LFLs) for methane, hydrogen, and carbon monoxide, in addition to mixtures of these gases (i.e. CH 4 /H 2 , H 2 /CO, and CH 4 /CO 2 ) were performed at temperatures up to 200 °C and pressures up to 9 bar. ASTM Standard E918 (1983) provided the framework for tests using a one-liter pressure-rated test cylinder in which the fuel-air mixtures were prepared and then ignited. Flammability is determined using a 7% and 5% pressure rise criterion per the ASTM E918 and European EN 1839 standards, respectively. The LFLs for each gas and gas mixture are found to decrease linearly with increasing temperature for the temperature range tested. The LFLs of hydrogen and mixtures containing hydrogen are observed to increase with an increase in the initial pressure, whereas the LFLs of all other mixtures exhibit a negligible dependence on pressure. For mixtures, predicted LFL values obtained using Le Chatelier’s mixing rule (LC) are fairly consistent with the experimentally determined values near ambient conditions, however it is not recommended for use at elevated pressure and/or temperature. Finally, the experimental data presented in this study are compared with previous experimental studies, flammability limits calculated using numerical methods, and past studies of predicted LFL values for similar fuel/air mixtures. The purpose for characterizing the flammability limits for these gaseous mixtures is to extend the results to developing appropriate procedures for the safe industrial use of renewable gases, such as bio-derived methane, biogas composed mainly of methane and carbon dioxide, and renewably derived syngas which contains large quantities of hydrogen and carbon monoxide gas.

Burning leaner is an effective way to reduce emissions and improve efficiency. However, this increases the instability of the combustion and hence, increases the tendency of the flame to blowout. On the other hand, the ignition delay of a jet fuel is a crucial factor of the instability feedback loop. Shorter ignition delay results in faster feedback loop, and longer ignition delay results in slower feedback loop. This study investigates the potential effect of ignition delay on the lean blowout limit of a gas turbine combustion chamber. At the Low Carbon Combustion Centre of The University of Sheffield, a range of tests were carried out for a range of jet fuels on a Rolls-Royce Tay combustor rig. The ignition delay for each fuel was tested using Advanced Fuel Ignition Delay Analyser (AFIDA 2805). Lean blowout tests (LBO) was conducted on various air flows rates. High speed imaging was recorded using a high speed camera to give further details of the flame behavior near blowout limit for various fuels. The instability level was observed using the pressure, vibration and acoustic fluctuation. This paper presents results from an experimental study performed on a small gas turbine combustor, comparing Lean Blowout limit of different conventional, alternative and novel jet fuels with various ignition delay characteristics. It was observed that at higher cetane number, the blowout is improved remarkably. The Ignition plays an important role in determining the average instability level, and as result determines the Lean Blowout limit of a fuel.

In concepts of integrated design of combustor and turbine, an annular combustor model is developed and featured with multiple oblique-injecting swirling injectors to introduce gyratory flow motion in the combustion chamber. The ignition process is experimentally investigated to study the effects of introducing circumferential velocity component U c to the light-round sequence. Experiments are carried out with premixed propane/air mixture in ambient conditions. The light-round sequence is recorded by a high-speed camera, which provides detailed flame azimuthal positions during the sequence and gives access to the light-round time τ and the circumferential flame propagation speed S c . The results have also been compared with that obtained from a straight-injecting annular combustor. The effects of bulk velocity U b , thermal power P and equivalence ratio Φ are also explored. Due to the gyratory flow motion induced by oblique injection, the flame fronts only propagate along the direction of circumferential flow. Both of the circumferential flame propagation speed increase with increasing bulk velocity in two injection types. It seems mainly to depend on bulk velocity, regardless of Φ , in oblique-injecting combustor when compared with the straight one. It indicates that the circumferential velocity component would play a dominant role in light-round sequence when it is sufficient higher than the displacement flame speed.