論文

© 2019 SAE International. All Rights Reserved. To adhere to stringent environmental regulations, SI (spark ignition) engines are required to achieve higher thermal efficiency. In recent years, EGR (exhaust gas recirculation) systems and lean-burn operation has been recognized as key technologies. Under such operating conditions, reducing CCV (cycle-to-cycle variation) in combustion is critical to the enhancement of overall engine performance. Flow-field CCV is one of the considerable factors affecting combustion in engines. Conventionally, in research on flow fields in SI engines, the ensemble average is used to separate the measured velocity field into a mean component and a fluctuation component, the latter of which contains a CCV component and a turbulent component. To extract the CCV of the flow field, previous studies employed spatial filter, temporal filter, and POD (proper orthogonal decomposition) methods. Those studies used a constant- separation filter size for the whole crank angle, although the turbulence scales change rapidly during the intake and compression stroke processes. Hence the definition of filter size has some room to be explored in order to take account of these features. The objective of this research is to improve the method of separating the CCV component using turbulence scales. For this purpose, high-speed PIV measurement was conducted on the symmetrical vertical plane for an optical IC engine at repetition rates of 12 kHz (1 C.A. deg. resolution) for the whole in-cylinder area and 48 kHz (0.25 C.A. deg. resolution) for the plug position. The measured data were separated into CCV and turbulent components by using the proposed filter, whose size was selected adaptively considering the integral time scale of the turbulent flow. The effect of time resolution on the filter size was then elucidated.

© 2018 Elsevier B.V. Phosphor thermometry is a method used to measure temperature based on the temperature-dependent phosphorescence of phosphors and elucidate heat transfer phenomena, such as high-temperature gas flow. Although various rare earth-doped thermographic phosphors are in use, the effect of oxygen concentration on their phosphorescence has not been sufficiently explored. We explore herein the applicability of Dy-doped yttrium aluminum garnet (YAG:Dy), a well-known rare earth-doped phosphor with temperature sensitivity above 1000 K, in phosphor thermometry at different oxygen concentrations. A third-harmonic Nd: YAG laser excited the sample. Phosphorescence was measured using a photomultiplier tube for lifetime detection. A spectrometer was used to detect the intensity ratio between two emission lines. The chamber was filled with a nitrogen–oxygen mixture with a controlled concentration. The phosphorescence intensity ratio depended on temperature over a wide temperature range and varied with the oxygen concentration, especially above 1000 K. The YAG:Dy lifetimes could be detected over the entire temperature range and remained constant up to 1000 K. In addition, the lifetimes decreased with the increasing oxygen concentration, especially above 1000 K, confirming the oxygen quenching effect. Consequently, YAG:Dy is confirmed to be sensitive to oxygen concentration for determining the intensity ratio and lifetime, especially above 1000 K.

© 2019 The Combustion Institute The response of laminar methane/air counterflow premixed flames under sinusoidal equivalence ratio oscillation was investigated numerically. The timescales of the oscillation were chosen to be sufficiently longer than the flame timescale so that the flame responds quasi-steadily. The response of periodically stratified flame (SF) with a detailed reaction mechanism exhibited the “back-support” effect, in that the consumption speed Sc response deviated increasingly from Sc of steady homogeneous flames (HFs) at higher oscillation frequencies. It was shown that even when the imposed oscillation timescale is much longer than the flame timescale, the flame response can still be delayed under a sufficiently large equivalence ratio gradient. Subsequently, the above results were compared with those obtained with a global four-step mechanism that omits back-diffusion radicals into the reaction zone. As a result, SFs with the global mechanism displayed a much smaller back-support effect in both lean and rich mixtures. Further analysis with modified diffusion coefficients revealed the dominant roles of H2 and radical species diffusion in inducing the back-support effect. Contrary to the previous findings, variations in burned gas temperature were found to play a negligible role in modifying Sc. Additionally, the hysteresis of the back-support effect under periodical stratification was found to be more prominent on the richer side because of the presence of a larger H2 pool.

© 2020 The Combustion Institute. Propagation of a H2-added strained laminar CH4/air flame in a rich-to-lean stratified mixture is numerically studied. The back-support effect, which is known to enhance the consumption speed of a flame propagating into a leaner mixture compared to that into a homogeneous mixture, is evaluated. A new method is devised to characterize unsteady reactant-to-reactant counterflow flames under transiently decreasing equivalence ratio, in order to elucidate the influence of flow strain on the back-support effect. In contrast to the conventional reactant-to-product configurations, the current configuration is more relevant to unsteady stratified flames back-supported by their own combustion products. Moreover, since H2 distribution downstream of the flame is known to play a crucial role in back-supported CH4/air flames, the influence of H2 addition in the upstream mixture is examined. The results suggest that a larger strain rate leads to a larger equivalence ratio gradient at the reaction zone through increased flow divergence, which amplifies the back-support. Meanwhile, since H2 addition in the upstream mixture does not affect the downstream H2 content, the relative increase in the consumption speed, i.e. the back-support, is suppressed with larger H2 addition. Especially, when the upstream H2 content decreases with the equivalence ratio, the H2 preferentially diffuses toward the unburned gas, which mitigates H2 accumulation in the preheat zone and further weakens the back-support.

© 2020 The Combustion Institute. Published by Elsevier Inc. This paper presents a phenomenological investigation of a new pattern of lean-premixed hydrogen-flame and flowfield dynamics under thermoacoustic instabilities (TIs) observed inside a low-swirl combustor (LSC) with two combustion chamber lengths of 300 and 500 mm. In this study, the initial bulk inlet velocity was set to V m = 15 m/s. The fuel flow rate was linearly increased while the air flow rate was kept constant, causing a transition from stable operation to TI. Simultaneous time-resolved electronically excited hydroxyl radical (OH *) chemiluminescence imaging and two-dimensional particle image velocimetry were performed along with pressure-fluctuation measurements to investigate spatiotemporal relationships between fluctuating quantities, i.e., OH * chemiluminescence, velocity fields, and pressure, before and after TI onset. A typical TI pattern characterized by a lifted, inverted conical flame caused by diverging, decelerating flow near the injector exit was observed in the long combustor. In contrast, the flame/flow dynamics pattern under TI in the short combustor had strong pressure oscillations and anomalous LSC flame structures which were characterized by a radially flat flame region and counter-rotating vortex pair (CVP) flame structures. In this case, the radial profile of the axial velocity assumed an atypical top-hat-like shape. This feature resulted in thermoacoustic coupling that satisfied the Rayleigh criterion over an almost entirely flat flame region and further strengthened the TI. Thermal expansions of CVP structures served as "converging nozzles" constricting the central flow region. Periodic appearances of this Venturi-like effect caused an alternating diverging (decelerating) and converging (accelerating) flowfield, with a significant peak-to-peak velocity-oscillation amplitude of 15 m/s, equivalent to V m . These pioneering observations regarding LSCs provide further insights into TI mechanisms in hydrogen-fueled gas turbine combustors.

© 2020, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. This experimental study explored the influence of flame structural characteristics on direct combustion noise produced by a lean-premixed gaseous H2 /air low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten-kHz OH* chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuation, respectively, while two-dimensional particle image velocimetry (2D-PIV) measurements at 4 Hz were used to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of ϕ ≥ 0.45. Spectral orthogonal decomposition of the OH* chemiluminescence and OH-PLIF images revealed the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, while the secondary peak is attributed to periodically generated vortical flame structures near the flame boundary. The 2D PIV results suggest that vortical flame structures are probably generated by the interaction between the flame and the inner/outer shear layers.