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The autoignition timing of 1-hexene was controlled in a Homogeneous Charge Compression Ignition (HCCI) engine by reacting the fuel with ozone-containing air prior to its combustion. The experiments were conducted in a single cylinder research engine instrumented with a cylinder pressure sensor. The fuel was chemically characterised using Nuclear Magnetic Resonance (NMR) spectroscopy before and after its reaction with ozone. The NMR analyses showed that this preliminary reaction produced several oxygenated products within the fuel, and was likely to have resulted in the formation of ozonide molecules having a peroxidic structure with several oxygen molecules in series. To understand how this would affect the ignition reactions, the ignition process was modelled numerically using a single-zone chemical kinetic reactor model. The modelling suggested that peroxide molecules decomposed during the compression of the reactants in the engine and advanced ignition timing by promoting early decomposition of the fuel through the formation of radicals.

This article presents an investigation of the swirl number effects on the stability limits of various syngas compositions in an atmospheric premixed variable-swirl burner. Lean blowout (LBO) and flashback (FB) experiments were performed using fuel mixtures containing varying amounts of H2, CO and CH4 at two swirl numbers. Reducing the swirl number from 0.66 to 0.53, reduced the flashback propensity of various syngas/air mixtures but it did not affect the LBO limits considerably. The flow-field in the combustor was studied at S = 0.66 and 0.53 using high-speed particle image velocimetry (PIV) for a mixture of H2/CH4 (50:50) at various equivalence ratios. At both swirl numbers, for the non-reacting flow and low equivalence ratios the flow-field consisted of an inner recirculation zone at the entrance of the combustor, an annular high velocity zone and an outer recirculation zones between the high-velocity zone and the bounding walls. Increasing the equivalence ratio towards the flashback limit, had varying effects on the flow-field, depending on swirl number. At S = 0.66, increasing the equivalence ratio did not have a significant effect on the general features of the flow-field. At S = 0.53 the flow-field consisted of an inner recirculation zone at low equivalence ratios, but as the equivalence ratio was increased, the high velocity zone extended radially towards the center and the recirculation zone disappeared from the flow-field. The high-speed OH*-chemiluminescence images recorded at the onset of flashback revealed significant differences between S = 0.66 and S = 0.53 in the flame stabilization mechanism prior to flashback and flame propagation in the premixing tube. Considering the velocity measurements together with the OH*-chemiluminescence images, it was concluded that at S = 0.66 flashback was caused by CIVB mechanism whereas at S = 0.53 flashback was initiated by the competition between the flame speed and flow velocity in the core flow.

This study examines the influence of vortex core precession on flame flashback of swirl-stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion around the central axis of the burner. The eccentric motion of the vortex generates a secondary flow, which is thought to reduce the angular velocity and tangential momentum available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame tip on flame flashback behaviour were conducted using high-speed sequences of OH*-chemiluminescence images. Temporal analysis of a large sample of images revealed the existence of a systematic rotational frequency of the flame tip around the central axis of the burner. Analysis of the radial position of the flame tip in relation to its axial propagation velocity showed that flashback velocity increased as the flame tip eccentricity approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. (Less)

Autoignition of hydrogen in air was studied in a turbulent flow reactor using OH*-chemiluminescence. High-speed imaging was used to visualise the formation of auto-ignition kernels in the flow, and to analyse the conditions under which temporary stabilisation of the flame kernels occurred. The experiments were carried out at temperatures of 800-850 K, pressures of 0.8-1.2 MPa and an equivalence ratio of phi = 0.25. Measurements of the autoignition delays yielded values in the range of tau = 210-447 ms. The autoignition delay results indicated that, over the range of conditions studied, ignition delays reduced with decreasing pressure. This observation contradicted homogeneous gas-phase kinetic calculations, which predicted an increase in autoignition delay with decreasing pressure. If the kinetic model was altered to include surface reactions at the reactor walls, the calculations could be qualitatively reconciled with the experimental data, suggesting that wall reactions had a significant influence on autoignition delays. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. (Less)

Autoignition of dimethyl ether in air was studied in a turbulent flow-reactor with optical access, at conditions relevant to micro gas turbine combustors. The ignition process was visualized using OH*-chemiluminescence imaging, showing the formation of multiple autoignition kernels along the central axis of the reactor. Ignition delays in the range of τ = 112–310 ms were measured at temperatures of T = 739–902 K, pressures of P = 0.2–0.4 MPa, equivalence ratios of ϕ = 0.225–0.675, and initial flow velocities of Ui = 8–46 m/s. The effect of adding nitrogen to the reactants as a diluent was investigated for mole fractions of additional nitrogen ranging from 0 < XN2 < 0.1. The experimental ignition delays were compared with homogeneous gas-phase chemical kinetic modeling. Comparison between the modeling and experiments showed significant discrepancies, but agreement was improved when heat transfer in the reactor was taken into account in the modeling.

Autoignition of propane in air was visualised in a turbulent flow reactor using natural OH*-chemiluminescence imaging. The spatial and temporal development of autoignition kernels was studied in an optically accessible tubular section of the reactor. Kernel nucleation, movement and growth affected the location and movement of subsequent autoignition sites, and resulted in stagnation of the incoming flow and flashback. The autoignition delays of the reactants were measured under various conditions of temperature, pressure and equivalence ratio, relevant to micro gas turbines: Temperature T= 803-903 K, pressure p = 0.4-0.6 MPa, equivalence ratio phi = 0.2-0.6, mass flow rate of reactants m(r) = 8-21 g/s, with ignition delays tau between 191 and 498 ms. The effect of diluting the propane + air mixtures with CO2 was investigated for mole fractions of 0

An atmospheric, variable-swirl combustor was used to study the influence of syngas composition on the lean blowout (LBO) limit under various flow conditions created at different swirl numbers. The fuels used in the experiments consisted of generic mixtures of CO, H-2, and CH4. The results were compared to those for CH4 at the same swirl numbers. The effect of dilution on the LBO limit was studied by adding N-2 to the syngas mixture. The swirl number was varied by changing the ratio of axial/tangential flow through the combustor inlet and was determined using laser Doppler anemometry (LDA). A perfectly stirred reactor (PSR) model was used to test whether the experimental results could be explained by changes in chemical kinetics. The experiments showed that increasing the swirl number reduced the LBO equivalence ratio for a given fuel composition. At a certain swirl number, increasing the H-2/CO molar ratio of a binary mixture decreased the LBO equivalence ratio significantly. The addition of CH4 to a binary mixture shifted the LBO limit to higher equivalence ratios. N-2 dilution increased the LBO equivalence ratio of the syngas mixture; however, the impact was relatively small. The PSR model was able to predict the effect of adding CH4 reasonably well, but it underestimated the effect of the H-2/CO molar ratio on the LBO limit. (Less)

Water introduction in the micro gas turbine (mGT) cycle is considered the optimal route for waste heat recovery and flexibility increase of such a small-scale combined heat and power (CHP) unit. However, humidification of the combustion air in a mGT affects combustion stability, efficiency, and exhaust gas emissions. This can lead to a non-stable, incomplete combustion, which will affect the global efficiency negatively. Additionally, CO emissions will increase. The non-stable, incomplete combustion might result in an engine shutdown due to a flameout. To study the impact of humidification on the combustion of methane in a humidified mGT, we performed combustion experiments in an atmospheric, variable-swirl, premixed combustion chamber. The results of these experiments are summarized in this article. The effect of the humidification of the combustion air was simulated by adding steam to the combustion air. The impact of the steam injection on methane combustion has been studied at variable swirl number and steam fraction. Experimental results showed a linearly increasing lean blowout (LBO) equivalence ratio for methane combustion with increasing steam fraction. In addition, CO emission levels started to rise at higher equivalence ratio for higher steam fractions compared to combustion under dry conditions. The CO emission levels at stable combustion were however still the same order of magnitude as for the dry combustion. The swirl number has little effect on the LBO limit. Final results indicated the possibility to maintain complete and stable combustion under humidified conditions with low CO emissions at higher equivalence ratio compared to the dry combustion.

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H2/CH4 mixtures were visualized in an atmospheric variable-swirl burner using high speed OH* chemiluminescence imaging. The H2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown (CIVB) occurred at swirl numbers ≥0.53 for all of the tested fuel mixtures. Flashback in the boundary layer (BL) and flame propagation in the premixing tube caused by auto-ignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms.

Lean blowout (LBO) is one of the major instability problems of premixed combustion. LBO equivalence ratio is a function of inlet temperature and pressure, mass flow or aerodynamic loading, and fuel composition. All these, except the last, vary during startup and with load. Developing gas turbine combustors capable of operating within wider range of fuel compositions requires extensive knowledge about instability limits of the combustor at different operating conditions. In this work an atmospheric variable swirl combustor was used to study the influence of inlet temperature, mass flow, swirl number and dilution on lean blowout of methane. The equivalence ratio at LBO was investigated for methane at 3 different inlet temperatures at various swirl numbers. The swirl number was varied by changing the ratio of axial and tangential flow through the combustor inlet, and was determined using Laser Doppler Anemometry. The experiments showed that increasing the swirl number reduced the lean blowout equivalence ratio for a given inlet temperature and that increasing the inlet temperature reduced the lean blowout equivalence ratio at a certain swirl number. In order to study the effect of inlet mass flow rate on lean stability limit, blowout experiments were conducted at 7 different mass flow rates. The measurements showed that the total mass flow has a non-monotonic effect on the lean blowout limit. At total mass flow rates below 200 SLPM increasing the total mass flow extended lean stability limit whereas at mass flow rates higher than 300 SLPM the trend was reversed. The effect of fuel dilution on the LBO limit was also investigated by adding different fractions of N2 and CO2 to the fuel mixture. The results were compared with those for pure methane at the same swirl number. Dilution with either diluent reduced the LBO limit of methane. However at the concentrations lower than 50% the effect of dilution on LBO equivalence ratio was relatively small and no significant difference was observed between N2 and CO2 dilution.