Development of lean-premixed combustion technology with low emissions and stable operation in an increasingly wide range of operating conditions requires a deep understanding of the mechanisms that affect the combustion performance or even the operability of the entire gas turbine. Due to the relative wide range of natural gas composition supplies and the increased demand from Oil&Gas customers to burn unprocessed gas as well as liquified natural gas (LNG) with notable higher hydrocarbons (C2+) content, the impact on gas turbine operability and combustion related aspects has been matter of several studies. In this paper, results of experimental test campaign of an annular combustor for heavy-duty gas turbine are presented with focus on the effect of fuel composition on both emissions and flame stability. Test campaign involved two different facilities, a full annular combustor rig and a full-scale prototype engine fed with different fuel mixtures of natural gas with small to moderate C 2 H 6 content. Emission trends and blowout for several operating conditions and burner configurations have been analyzed. Modifications to the burner geometry and fuel injection optimization have shown to be able to reach a good tradeoff while keeping low NO x emissions in stable operating conditions for varying fuel composition.

A numerical investigation of pollutant emissions of a novel dry low-emissions burner for heavy-duty gas turbine applications is presented. The objective of the work is to develop a robust and cost-efficient numerical setup for the prediction of NOx and CO emissions in industrial gas turbines and to investigate the pollutant formation mechanisms, thus supporting the design process of the novel burner. In the first part of this work, a RANS approach is adopted to characterize the global flame behavior. Then, unsteady simulations exploiting the Scale Adaptive Simulation (SAS) approach have been performed to assess the prediction improvements that can be obtained with the unsteady modelling. For all simulations, the Flamelet Generated Manifold (FGM) model has been used. However, FGM typically faces issues predicting flame emissions, such as NOx and CO, due to the wide range of time scales involved. Specific models are typically used to predict NOx emissions, introducing additional transport equations. Also CO prediction, especially at part-load operating conditions could be an issue: as the load decreases and the extinction limit approaches, a super-equilibrium CO concentration appears in the exhaust gases. To overcome this issue, a specific CO burn-out model, following the original idea proposed by Klarmann, has been implemented. In order to support the design process, an in-depth CFD investigation has been carried out, evaluating the impact of an alternative burner geometrical configuration on stability and emissions and providing detailed information about the main regions and mechanisms of pollutants production.

The aim of this work is the experimental investigation of the effects of the addition of water and urea into jet fuels, on the reduction of nitrogen oxides (NO x ) emissions and eventually improvement of the lean flame stability in aeroengine combustors. Experiments have been carried out using a 300-kW liquid-fueled swirling combustor. Various urea and/or water concentrations have been tested at the same fuel/air ratio. In order to study the flame behavior, noninvasive optical diagnostic techniques, as charge-coupled device (CCD) cameras in different spectral ranges (visible and UV ranges, with different optical filters), have been adopted to analyze the shape and the brightness of the flame structure. Measurements of exhaust emissions (NO x , SO 2 , carbon monoxide (CO), CO 2 , and O 2 ) have also been performed in order to evaluate the impact of emulsification on the entire combustion process. Finally, the thermal efficiency losses with respect to the neat jet test case were also analyzed for each emulsified fuel condition.

Uncertainty quantification (UQ) is becoming an essential attribute for development of computational tools in gas turbine combustion systems. Prediction of emissions with a variety of gaseous fuels and uncertain conditions requires probabilistic modeling tools, especially at part load conditions. The aim of this paper was to develop a computationally efficient tool to integrate uncertainty, sensitivity, and reliability analyses of CO and NO x emissions for a practical swirl-stabilized premixed burner. Sampling-based method (SBM), nonintrusive polynomial chaos expansion (NIPCE) based on point collocation method (PCM), Sobol sensitivity indices, and first-order reliability method (FORM) approaches are integrated with a chemical reactor network (CRN) model to develop a UQ-enabled emissions prediction tool. The CRN model consisting of a series of perfectly stirred reactors (PSRs) to model CO and NO x is constructed in Cantera. Surrogate models are developed using NIPCE-PCM approach and compared with the results of CRN model. The surrogate models are then used to perform global sensitivity and reliability analyses. The results show that the surrogate models substantially reduce the required computational costs by 2 to 3 orders of magnitude in comparison with the SBM to calculate sensitivity indices, importance factors and perform reliability analysis. Moreover, the results obtained by the NIPCE-PCM approach are more accurate in comparison with the SBM. Therefore, the developed UQ-enabled emissions prediction tool based on CRN and NIPCE-PCM approaches can be used for practical combustion systems as a reliable and computationally efficient framework to conduct probabilistic modeling of emissions.

Premixed or partially premixed swirling flames are widely used in gas turbine applications because of their compactness, high ignition efficiency, low NOx emissions and flame stability. A typical annular combustor consists of about twenty swirling flames which interact (directly/indirectly) with their immediate neighbors. 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. 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. 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. These measurements and corresponding analyses elucidate the structure of the interaction regions, their unique characteristics and possible role in thermoacoustic instability.

With the push to curb dangerous atmospheric pollutant production, energy generation technologies that reduce greenhouse gas emissions, while still providing adequate electrical supply are of high importance. With major energy infrastructure already in place, developing enhanced pollutant reducing combustor systems for micro gas turbines (MGTs), that can utilize low calorific fuels from renewable resources, is a major goal. The current work focuses on the experimental testing of an optimized two-stage combustor designed to operate with various fuel types, including natural gas and syngas produced via biomass gasification. Atmospheric experimental tests were performed and the results indicate larger flame lift-off heights and slightly higher CO gas emissions levels, while displaying lower NOx gas emissions levels for all thermal loads and air-tofuel equivalence ratios tested, compared to that of the previous combustor designs. Additionally, steady state CFD simulations were conducted and the results are in general good agreement with the experimental data. Overall, the results indicate high fuel flexibility of the combustor, as well as the ability to comply with the NOx emissions limits for a larger range of operating points, compared to that of the previously tested combustors.

The conventional operation of a hydrogen internal combustion engine (ICE) under lean conditions results in low NOx emissions, however, at the cost of power generated. In the present study, the power output of a hydrogen-fuelled ICE was increased while maintaining the NOx emissions at low levels. The power output was increased by turbo-charging, relatively richer operation, and spark timing optimization whereas a combination of EGR and H2-SCR aftertreatment was used to reduce NOx emissions. Turbo-charging resulted in a maximum torque output of 168 Nm @ 3200 rpm as compared to 70 Nm @ 1600 rpm for the naturally aspirated operation. However, the turbocharger could not generate enough boost at low speeds and the equivalence ratio was increased to obtain a high power output which resulted in a substantial increase in the NOx emissions. The use of EGR resulted in an average reduction of 72% in the NOx emissions. Retarding of spark timing significantly reduced the NOx emissions too, but was limited by the adverse impact on the torque. Since hydrogen would be available onboard a hydrogen-fuelled vehicle, we for the first time report external injection of H2 for use as a reductant in the selective catalytic reduction unit. Even under extremely oxidizing conditions, the efficiency of aftertreatment was found to be 35.4% averaged over various speeds. A maximum of 83.7% overall reduction in NOx emissions was achieved by using the combined EGR and H2-SCR strategies.

Electrified aircraft propulsion (EAP) systems hold potential for the reduction of aircraft fuel burn, emissions, and noise. Currently, NASA and other organizations are actively working to identify and mature technologies necessary to bring EAP designs to reality. This paper specifically focuses on the envisioned control technology challenges associated with EAP designs that include gas turbine technology. Topics discussed include analytical tools for the dynamic modeling and analysis of EAP systems, and control design strategies at the propulsion and component levels. This includes integrated supervisory control facilitating the coordinated operation of turbine and electrical components, control strategies that seek to minimize fuel consumption and lessen the challenges associated with thermal management, and dynamic control to ensure engine operability during system transients. These dynamic control strategies include innovative control approaches that either extract or supply power to engine shafts dependent upon operating phase, which may improve performance and reduced gas turbine engine weight. Finally, a discussion of control architecture design considerations to help alleviate the propulsion/aircraft integration and certification challenges associated with EAP systems is provided.

In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry were constructed via Additive Layer Manufacturing (ALM) for use in a generic gas turbine swirl burner. Further post-processing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics when compared with the unprocessed ALM swirl nozzle insert or a third nozzle insert which has been manufactured using traditional machining methods. An evaluation of the influence of variable surface roughness effects from these swirl nozzle inserts is therefore performed under preheated isothermal and combustion conditions for premixed methane-air flames at thermal power of 25 kW. High-speed velocimetry at the swirler exit under isothermal conditions gives evidence of the change in near-wall boundary layer thickness and turbulent fluctuations resulting from the change in nozzle surface roughness. Under atmospheric combustion conditions, this influence is further quantified using a combination of dynamic pressure, high-speed OH* chemiluminescence, and exhaust gas emissions measurements to evaluate the flame stabilization mechanisms at the lean blowoff and rich stability limits. Notable differences in flame stabilization are evident as the surface roughness is varied, and changes in rich stability limit were investigated in relation to changes in the near-wall turbulence intensity. Results show that precise control of in-process or post-process surface roughness of wetted surfaces can positively influence burner stability limits and NOx emissions and must therefore be carefully considered in the ALM burner design process as well as CFD models.

The objective of this study is to numerically investigate the effect of cryogenic intake air temperature on the in-cylinder temperature and formation of exhaust emissions in a CI engine. The experimental setup was consisted of a single-cylinder diesel engine. The intake air temperature was varied from 18 °C to 40 °C, which was controlled by cooler and heater. Submodels were applied for the simulations of physical/chemical phenomenon of spray and combustion behaviors. The intake air temperature in numerical condition was varied from −18 °C to 18 °C. The numerical results were validated with experimental results for the reliability of this work. The results of this work were compared in terms of cylinder pressure, rate of heat release (ROHR), indicated specific nitrogen oxide (ISNO), indicated specific carbon monoxide (ISCO), ignition delay, in-cylinder temperature distributions, equivalence ratio distributions, NO mass fraction, and CO mass fraction. When the intake air temperature was decreased in steps of 9 °C, the cylinder temperature and cylinder pressure were decreased in steps of about 14.5 °C and 0.05 MPa, respectively. In all cases, the area where the NO formed in the cylinder was identified with the area of the high equivalence ratio and temperature in the cylinder. The amount of CO generation shows the similar distributions in the cylinder according to the intake air temperature conditions. However, the oxidation rate of formed CO under the low intake air temperature was lower than those of the high intake air temperature.

Converting existing diesel engines to natural-gas spark-ignition (SI) operation can reduce the dependence on oil imports and increase energy security. NG-dedicated conversion can be achieved by the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. Previous studies indicated that lean-burn NG inside the traditional diesel chamber (i.e., a bowl-in-piston geometry) is a two-stage combustion (i.e., a fast burn inside the bowl followed by a slower burn inside the squish). However, a triple-peak apparent heat release rate was seen at specific operating conditions (e.g., advanced spark timing at medium load and engine speed), suggesting that one of the two combustion stages may separate again. Specifically, the burn inside the squish region divided in two events before and after top-dead-center. This was due to the different flow motion inside the squish during the compression stroke compared to the one in the expansion stroke, which affected the combustion environments. The result was the apparition of two close peaks in pressure trace, which suggest larger gradients in pressure and temperature than at a more delayed spark timing. In addition, the phasing and magnitude of three peaks of the heat release changed cycle-to-cycle. As an advanced spark timing is the usual strategy used in lean-burn SI combustion, understanding phenomena such as the one presented here can be important for reducing engine-out emissions and increase engine efficiency.

The growing share of wind and solar power in the total energy mix has caused severe problems in balancing the electrical power production. Consequently, in the future, all fossil fuel-based electricity generation will need to be run on a completely flexible basis. Micro Gas Turbines (mGTs) constitutes a mature technology which can offer such flexibility. Even though their greenhouse gas emissions are already very low, stringent carbon reduction targets will require them to be completely carbon neutral: this constraint implies the adoption of post-combustion Carbon Capture (CC) on these energy systems. Despite this attractive solution, an in-depth study along with a robust optimization of this system has not yet been carried out. Hence, in this paper, a typical mGT with exhaust gas recirculation has been coupled with an amine-based CC plant and simulated using the software Aspen Plus A rigorous rate-based simulation of the CO2 absorption and desorption in the CC unit offers an accurate prediction; however, its time complexity and convergence difficulty are severe limitations for a stochastic optimization. Therefore, a surrogate-based optimization approach has been used, which makes use of a Gaussian Process Regression (GPR) model, trained using the Aspen Plus data, to quickly find operating points of the plant at a very low computational cost. Using the validated surrogate model, a stochastic optimization has been carried out. As a general result, the analyzed power plant proves to be intrinsically very robust, even when the input variables are affected by strong uncertainties.

With increased interest in reducing emissions, the staged combustion concept for gas turbine combustors is gaining in popularity. For this work, the effect of CO 2 dilution on laminar burning velocities of premixed methane/air flames was investigated at elevated temperature through both experiments and numerical simulations. High temperature experiments were conducted in an optically accessible constant volume combustion chamber at 1 bar and 473 K. Laminar burning velocities of premixed methane/air flames with 0%, 5%, 10% and 15% CO 2 dilution were determined using the constant pressure method enabled via schlieren visualization of the spherically propagating flame front. Additions of 5%, 10% and 15% CO 2 dilution cause a 30-35%, 51-54% and 66-68% decrease in the laminar burning velocity, respectively. Numerical results were obtained with CHEMKIN using the GRI-Mech 3.0 and the San Diego mechanisms. The GRI-Mech 3.0 shows better agreement with the general trends of the experimental laminar burning velocities of methane/air/CO 2 mixtures at 1 bar and 473 K. Additionally, the dilution, thermal-diffusion, and chemical effects of CO 2 on the laminar burning velocities of methane/air mixtures were investigated numerically by diluting the mixtures with both chemically active and inactive CO 2 following the determination of the most important elementary reactions on the burning rate through sensitivity analysis. Lastly, it was shown that CO 2 dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length with the addition of diluent.

This paper presents a newly developed method using rapid prototyping to develop gas engine cylinder heads with optimized charge motion characteristics to adapt to lean-burn and Miller combustion process requirements. The geometry in close vicinity to the inlet valve seats was designed to increase swirl and flow performance . A 3D printer was used to realize a rapid prototyping concept for the testing of multiple designs, the effects of the different designs were measured using a static flow test bench and a laser-optical method to visualize flow patterns. The results of the static flow bench tests showed potentially higher flow and swirl performance, with one high-swirl version proving beneficial specifically for lean-combustion and one high-flow version matching the Miller combustion requirements. The two cylinder head versions were then manufactured and the lean-combustion version was tested for on-engine performance on a sewage-gas driven lean-combustion engine. It has been shown that the cylinder head generates higher swirl on the test bench but achieves only a slight increase in combustion speed on the test engine. The potential to increase engine efficiency by intensifying swirl is therefore considered exploited. Research has further shown the CoV was reduced by 0.3 to 1.2 percent. Charge exchange losses have also been demonstrated to decrease at all tested engine settings. It has further been found that higher swirl intensity has a positive impact on engine emission levels, as the engine out CO emission can be significantly reduced.

Biogas is a promising alternative fuel to reduce the consumption of petroleum-based fuels in internal combustion (IC) engines. In this work, the effect of various biogas compositions on the performance, combustion, and emission characteristics of a spark-ignition (SI) engine is investigated. Additionally, the effect of Wobbe index (WI) of various fuel compositions was also evaluated on the operational limits of the engine. While considering a wide range of biogas compositions (including bio-methane), the percentage of carbon dioxide (CO 2 ) (in a blend of methane and CO 2 ) was increased from 0 to 50% (by volume). A single-cylinder, water-cooled, SI engine was operated at 1500 rpm over a wide range of operating loads with compression ratio of 8.5:1. With the increase in WI of the fuel, both low (limited by coefficient of variation (COV) of indicated mean effective pressure (IMEP)) and high (limited by pre-ignition) operating loads were decreased; however, it was found that the overall operating range was increased. Results also showed that for a given operating load, with the increase of CO 2 percentage in the fuel, the brake thermal efficiency was decreased, and the flame initiation and combustion durations were increased. The brake thermal efficiency was decreased from 16.8% to 13.7%, when CO 2 was increased from 0% to 40% in methane–CO 2 mixture at 8 N·m load. Concerning to emissions, a considerable decrease was noted in nitric oxide, whereas hydrocarbon, carbon monoxide and carbon dioxide emissions were increased, with the increase in CO 2 percentage.

By enhancing the pre-mixing of fuel and air prior to combustion, recently developed lean-burn combustor systems have led to reduced NOx and particulate emissions in gas turbines. Lean-burn combustor exit flows are typically characterized by non-uniformities in total temperature, or so-called hot-streaks, swirling velocity profiles, and high turbulence intensity. Whilst these systems improve combustor performance, the exiting flow-field presents significant challenges to the aero-thermal performance of the downstream turbine. This paper presents the commissioning of a new fully annular lean-burn combustor simulator for use in the Oxford Turbine Research Facility (OTRF), a transonic rotating facility capable of matching non-dimensional engine conditions. The combustor simulator delivers an engine-representative turbine inlet condition featuring swirl and hot-streaks simultaneously. To the authors' knowledge, this simulator is the first of its kind to be implemented in a rotating turbine test facility.

In this study, the application of ultra-high fuel injection pressure (up to 300 MPa) is compared with that of a post injection strategy for the reduction of soot at medium load conditions with exhaust gas recirculation (EGR) rates greater than 40%. Emissions were predominantly studied at the engine's maximum brake torque speed of 1600 rpm. A 4.5-L, four-cylinder diesel engine with series turbochargers and a high-pressure EGR loop was used for all tests. Results indicate that, ultra-high injection pressures may not have large effects on hydrocarbons (HC) or CO emissions. Small soot reductions were achieved at the expense of increased NO x emissions. Post injections resulted in larger soot reductions for a small increase in NO x while allowing lower fuel pressures to be utilized. The increase in NO x emissions with a post injection was observed to be comparatively less at increased engine speeds. For operation at high EGR, post injections were observed to be more effective at reducing soot than ultra-high injection pressures. Both injection pressure and post injections were observed to have small to negligible effects on engine fuel consumption, leaving EGR and injection timing as the primary efficiency drivers at the conditions studied.

Heavy-duty compression–ignition (CI) engines converted to natural gas (NG) spark ignition (SI) operation have the potential to increase the use of NG in the transportation sector. A three-dimensional (3D) numerical simulation was used to predict how the conventional CI combustion chamber geometry (i.e., re-entrant bowl and flat head) affects the combustion stability, performance, and emissions of a single-cylinder CI engine that was converted to SI operation by adding a low-pressure gas injector in the intake manifold and a spark plug in place of the diesel injector. The G-equation based 3D computational fluid dynamics (CFD) simulation investigated three different combustion chamber configurations that change the size of the squish region at a constant compression ratio (CR) and a clearance height. The results show that the different flame propagation speeds inside and outside the re-entrant bowl can create a two-zone combustion phenomenon. Moreover, a larger squish region increased the flame burning speed, which decreased late-combustion duration (DOC). All these findings support the need for further investigations of the combustion chamber shape design for optimum engine performance and emissions in CI engines converted to NG SI operation.

According to current worldwide trends for homologation vehicles in real driving conditions is forced to test the engines in altitude and in highly dynamic driving cycles in order to approach nowadays and next future emissions standard. Up to now, there were two main options to perform this type of tests: round-robin tests of the whole vehicle or hypobaric chambers, both with high costs and low repeatability. In this paper a new device is described, which can emulate ambient conditions at whatever altitude between sea level and 5000 m high. Even it can be used to emulate ambient conditions at sea level when test bench is placed up to 2000 m high. The main advantages of the altitude simulation equipment are as follows: dynamic emulation of all the psychrometric variables affecting the vehicles during round-robin tests; lower space usage and low-energy consumption. The altitude simulator (AS) has been validated comparing with results from a hypobaric chamber at different altitudes. Previously a research about the dispersion in the measurements of both testing devices has been done for assessing the results of the comparison experiment. Final conclusion resulted in the same operating performance and emissions of the studied engine with both types of testing equipment for altitude simulation.

The present study investigates various measures to reduce pressure rise rates (PRRs) in a residual-affected homogeneous charge compression ignition (HCCI) engine. At the same time, the impact of those measures on efficiency and emissions is assessed. Experimental research was performed on a single cylinder engine equipped with a fully flexible valve train mechanism and direct gasoline injection. The HCCI combustion mode with exhaust gas trapping was realized using negative valve overlap (NVO) and fuel reforming, achieved via the injection of a portion of fuel during exhaust recompression. Three measures are investigated for the PRR control under the same reference operating conditions, namely: (i) variable intake and exhaust valve timing, (ii) boost pressure adjustment, and (iii) split fuel injection to control the amount of fuel injected for reforming. Variable exhaust valve timing enabled control of the amount of trapped residuals, and thus of the compression temperature. The reduction in the amount of trapped residuals, at elevated engine load, delays auto-ignition, which results in a simultaneous reduction of pressure rise rates and nitrogen oxides emissions. The effects of intake valve timing are much more complex because they include the variability in the amount of intake air, the thermodynamic compression ratio, as well as the in-cylinder fluid flow. It was found, however, that both early and late intake valve openings (IVOs) delay auto-ignition and prolong combustion. Additionally, the reduction of the amount of fuel injected during exhaust recompression further delays combustion and reduces combustion rates. Intake pressure reduction has by far the largest effect on peak pressure reduction yet is connected with excessive NO X emissions. The research successfully identifies air-path and injection techniques, which allow for the control of combustion rates and emissions under elevated load regime.