1,721,011 research outputs found
Fractal scaling of turbulent premixed flame fronts: Application to LES
No full textThe fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the "wrinkling factor" that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length i. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers. © 2014 Elsevier Inc
Aeroacoustic study of a slotted burner
No full textSlotted burner is an interesting application in the field of combustion research. In fact, this simple test case produces a two-dimensional laminar premixed flame, useful for understanding. In laboratory scale burner, it was observed that the tonal acoustic excitation of the flame changes the flame topology, the velocity and radical OH concentration field.1,2 Kartheekeyan et al.3 observed that tonal pressure fluctuation induces asymmetry and oscillations in the laminar flame front generated by slotted burner (SB). For this reason, during the design stage of a SB, an aeroacoustic study is of primary importance. Hence, in the present research, wall pressure fluctuations generated within a slotted burner have been studied experimentally for several mass flow rates. Wall pressure fluctuations have been measured through wall mounted microphones, providing single and multi-variate pressure statistics both in the time and in the Fourier domain. Furthermore the use of the wavelet based auto-conditioning permitted us to detect the signatures embedded into the pressure fluctuations. Finally in order to give an interpretation of the pressure signals, acoustic and aerodynamic investigations have been carried out as well. The analysis of the acoustic response of the slotted burner has been conducted numerically, whereas the aerodynamic study has been carried out by means of RANS simulation. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved
DNS of a variable density jet in the supercritical thermodynamic state
No full text[No abstract available
Turbulent consumption speed via local dilatation rate measurements in a premixed bunsen jet
No full textThe mean local reaction rate related to the average expansion across the front and computed from the mean velocity divergence is evaluated in this work. Measurements are carried out in a air/methane premixed jet flame by combined PIV/LIF acquisitions. The procedure serves the purpose of obtaining values of a turbulent flame speed, namely the local turbulent consumption speed SLC, as a function of the position along the bunsen flame. With the further position that the flamelet assumption provides a proportionality between turbulent burning speed normalized with the laminar unstretched one and the turbulent to average flame surface ratio, the proportionality constant, i.e., the stretching factor becomes available. The results achieved so far show the existence of a wide region along which the bunsen flame front has a constant stretching factor which apparently depends only on the ratio between turbulent fluctuations and laminar flame speed and on the jet Reynolds number. © 2013 The Combustion Institute
Direct numerical simulation of hydrogen-carbon monoxide turbulent premixed flame
No full text[No abstract available
Interplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation
No full textIn this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus-Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker, the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness of the flame curvature probability density function is identified as an unambiguous morphological marker for the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability. © 2016 American Physical Society
Chaotic analysis of the thermoacoustic instabilities of a trapped vortex combustor
No full textIn this paper, the dynamic instabilities of a Trapped Vortex Combustor (TVC) were studied experimentally. In particular, the recurrences in the pseudo-phase space of pressure and radiant energy uctuations were analyzed with a view to quantifying the recurrence plots and study the nonlinear behavior of the thermoacoustic instabilities. The present results reveal clearly three different regimes: combustion noise regime characterized by chaotic low-amplitude aperiodic oscillations, a transition regime towards instabilities comprised of intermittent high-amplitude periodic bursts that emerge among low-amplitude aperiodic oscillations, and the combustion instability regime formed by high-amplitude periodic oscillations. A quantification of the recurrence plots was performed by computing the recurrence rate index. The results show that the recurrence rate index fluctuated during the transition and dropped to a minimum in the proximity of instabilities. Therefore, the quantification of recurrence plots proved to be an useful method in order to define precursors that predict in advance combustion instabilities. © 2018 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved
Curvature effects in turbulent premixed flames of H2/Air: A DNS study with reduced chemistry
No full textData from a three-dimensional Direct Numerical Simulation of a turbulent premixed Bunsen flame at a low global Lewis number are analyzed to address the effects of the curvature on the local flame front. For this purpose, the chemical kinetics is modeled according to a reduced scheme, involving 5 reactions and 7 species, to mimic a H2/Air flame at equivalence ratio
Experimental investigation of Darrieus-Landau instability effects on turbulent premixed flames
No full textThe turbulent propagation speed of a premixed flame can be significantly enhanced by the onset of Darrieus-Landau (DL) instability within the wrinkled and corrugated flamelet regimes of turbulent combustion. Previous studies have revealed the existence of clearly distinct regimes of turbulent propagation, depending on the presence of DL instabilities or lack thereof, named here as super- and subcritical respectively, characterized by different scaling laws for the turbulent flame speed. In this study we present experimental turbulent flame speed measurements for propane/air mixtures at atmospheric pressure, variable equivalence ratio at Lewis numbers greater than one obtained within a Bunsen geometry with particle image velocimetry diagnostics. By varying the equivalence ratio we act on the cut-off wavelength and can thus control DL instability. A classification of observed flames into sub/supercritical regimes is achieved through the characterization of their morphology in terms of flame curvature statistics. Numerical low-Mach number simulations of weakly turbulent two-dimensional methane/air slot burner flames are also performed both in the presence or absence of DL instability and are observed to exhibit similar morphological properties. We show that experimental normalized turbulent propane flame speeds ST/SL are subject to two distinct scaling laws, as a function of the normalized turbulence intensity Urms/SL, depending on the sub/supercritical nature of the propagation regime. We also conjecture, based on the experimental results, that at higher values of turbulence intensity a transition occurs whereby the effects of DL instability become shadowed by the dominant effect of turbulence. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Fractal Analysis of Fluorescence Images to Assess Robustness of Reference-surface Positioning in Flame Fronts
No full textImages of laser-induced fluorescence (LIF) of hydroxyl radicals (OH) produced in premixed flames are here analyzed from the perspective of fractal theory applied to flame wrinkling. The results confirm that the signal isosurface showing the best correlation with the position of the maximum fluorescence gradient is in a strong correspondence with the inner layer within which the reference surface of maximum heat release can be found. Nonetheless, fluctuating maximum OH concentration or poor levels of signal-to-noise ratio could lead to a partial failure of either threshold or maximum gradient techniques of front extraction from LIF images. On the other hand, potential deviations from the reference surface can be tolerated provided that the chosen isosurface has a fractal dimension D2 ranging between 1.10 and 1.26, well below the limit for passive scalars (D2 = 1.37)
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