Experimental Study of Unsteady Laminar and Turbulent Flame Propagation in an Enclosure by Rayleigh Scattering PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages : 12
Book Description
The Rayleigh scattering technique has been used to investigate the flame structure of unsteady propane/air flames propagating under quiescent and turbulent conditions initiated by three ignition methods: (1) conventional spark plug (2) pulse jet combustion (PJC) and (3) Pre-Chamber ignition (PCI). The Rayleigh signal obtained for the laminar cases are all characterized by a sharp transition associated with the large density change which occurs as leading flame front of the expanding flame kernel crosses the measurement point. This indicates that the local flame structures are independent of the ignition source and may be characterized as flamelets. The maximum burning rates deduced from the pressure records show that PJC and PCI increases the burning rate from two to three times above that of conventional spark ignition. The Rayleigh scattering signal obtained for the turbulent cases are also characterized by sharp transition. The wrinkled laminar flamelet model, therefore, provides a valid description of the flame structures for all these unsteady flames. For a given equivalence ratio, the maximum turbulent burning rate deduced for the three turbulent cases with different ignition sources are similar. This suggests that while PJC and PCI enhances burning rate when conditions in the chamber are quiescent, the enhancement is not significant when turbulence fluctuations are present. The PJC and PCI, however, are capable of igniting leaner conditions than the spark plug. This may be due to the injection process which initially disperses ignition sites to a larger volume at a faster rate.
Author: Publisher: ISBN: Category : Languages : en Pages : 12
Book Description
The Rayleigh scattering technique has been used to investigate the flame structure of unsteady propane/air flames propagating under quiescent and turbulent conditions initiated by three ignition methods: (1) conventional spark plug (2) pulse jet combustion (PJC) and (3) Pre-Chamber ignition (PCI). The Rayleigh signal obtained for the laminar cases are all characterized by a sharp transition associated with the large density change which occurs as leading flame front of the expanding flame kernel crosses the measurement point. This indicates that the local flame structures are independent of the ignition source and may be characterized as flamelets. The maximum burning rates deduced from the pressure records show that PJC and PCI increases the burning rate from two to three times above that of conventional spark ignition. The Rayleigh scattering signal obtained for the turbulent cases are also characterized by sharp transition. The wrinkled laminar flamelet model, therefore, provides a valid description of the flame structures for all these unsteady flames. For a given equivalence ratio, the maximum turbulent burning rate deduced for the three turbulent cases with different ignition sources are similar. This suggests that while PJC and PCI enhances burning rate when conditions in the chamber are quiescent, the enhancement is not significant when turbulence fluctuations are present. The PJC and PCI, however, are capable of igniting leaner conditions than the spark plug. This may be due to the injection process which initially disperses ignition sites to a larger volume at a faster rate.
Author: Publisher: ISBN: Category : Languages : en Pages : 22
Book Description
A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.
Author: L. Talbot Publisher: ISBN: Category : Languages : en Pages : 7
Book Description
Studies of premixed turbulent flame propagation have been carried out using laser tomography, two-point Rayleigh scattering, and combined Rayleigh-LDV single point measurements. Probability density functions of intermediate states, correlation coefficients and correlation length scales within the reaction zone have been measured. A theoretical model has been developed which predicts these quantities, and very good agreement between theory and experiment is obtained.
Author: Sina Kheirkhah Publisher: ISBN: Category : Languages : en Pages :
Book Description
Characteristics of turbulent premixed flames were investigated experimentally. The investigations were performed using Mie scattering, Particle Image Velocimetry, Rayleigh scattering, and broad-band luminosity imaging techniques. Methane-air flames associated with a relatively wide range of turbulence intensities, fuel-air equivalence ratios, and mean bulk flow velocities were investigated. For a relatively moderate value of turbulence intensity, a new concept is introduced and utilized to provide a detailed description associated with interaction of turbulent flow and flame front. The concept pertains to reactants velocity estimated at the vicinity of the flame front and is referred to as the edge velocity. Specifically, it is shown that fluctuations of the flame front position are induced by fluctuations of the edge velocity. For a relatively wide range of turbulence intensity, several characteristics of turbulent premixed flames, namely, front topology, brush thickness, surface density, and consumption speeds are investigated. For the first time, several flame front structures are identified and studied. It is shown that, due to formation of these front structures, the regime of turbulent premixed combustion transitions from the regime of counter-gradient diffusion to that of the gradient diffusion. In addition to these, a comprehensive study is performed to investigate influence of flame configuration on several flame front characteristics. It is obtained that, although changing the flame configuration influences several flame characteristics, the trends associated with the effects of governing parameters on the characteristics are nearly independent of the flame configuration.
Author: Frank Tat Cheong Yuen Publisher: ISBN: 9780494608951 Category : Languages : en Pages : 0
Book Description
Turbulent premixed propane/air and methane/air flames were studied using planar Rayleigh scattering and particle image velocimetry on a stabilized Bunsen type burner. The fuel-air equivalence ratio was varied from & phis; = 0:7 to 1.0 for propane flames, and from & phis; = 0:6 to 1.0 for methane flames. The non-dimensional turbulence intensity, u'/ SL (ratio of fluctuation velocity to laminar burning velocity), covered the range from 3 to 24, equivalent to conditions of corrugated flamelets and thin reaction zones regimes. Temperature gradients decreased with the increasing u'/SL and levelled off beyond u'/SL> 10 for both propane and methane flames. Flame front thickness increased slightly as u'/SL increased for both mixtures, although the thickness increase was more noticeable for propane flames, which meant the thermal flame front structure was being thickened. A zone of higher temperature was observed on the average temperature profile in the preheat zone of the flame front as well as some instantaneous temperature profiles at the highest u'/SL. Curvature probability density functions were similar to the Gaussian distribution at all u'/ SL for both mixtures and for all the flame sections. The mean curvature values decreased as a function of u'/ SL and approached zero. Flame front thickness was smaller when evaluated at flame front locations with zero curvature than that with curvature. Temperature gradients and FSD were larger when the flame curvature was zero. The combined thickness and FSD data suggest that the curvature effect is more dominant than that of the stretch by turbulent eddies during flame propagation. Integrated flame surface density for both propane and methane flames exhibited no dependance on u'/S L regardless of the FSD method used for evaluation. This observation implies that flame surface area may not be the dominant factor in increasing the turbulent burning velocity and the flamelet assumption may not be valid under the conditions studied. Dkappa term, the product of diffusivity evaluated at conditions studied and the flame front curvature, was a magnitude smaller than or the same magnitude as the laminar burning velocity.
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Author: Ray E. Bolz
Author: L. Talbot
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Author: HANS RALPH MENKES
Author: Sina Kheirkhah
Author: Stanislav Kostka
Author: Frank Tat Cheong Yuen