Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH4, CO, CO2, H2 and N2, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities. © 2009 Elsevier Inc. All rights reserved.

The chaotic optical communication systems are based on the synchronization between the transmitting and receiving chaotic laser diodes. The performance of the system is determined by the bit error rate which can result from either the synchronization deviation or the desynchronization bursts. The time required for resynchronization results in loss of bits. The synchronization recovery time is studied for the different chaotic synchronization schemes. Also, the effect of the parameters mismatch between the transmitting and receiving laser diodes on the recovery time is investigated. ©2009 IEEE.

One of the most recently applied laser-based techniques in combustion environment is the laser-induced breakdown spectroscopy (LIBS). The technique has been extensively and successfully applied to elemental concentration measurements in solids and liquids. The LIBS signal is much weaker in gases and hence more work is required for quantitative measurements in flames. In the present work we used two orthogonal Nd:YAG lasers that operate at the fundamental wavelength with laser pulse energy of about 100 mJ/pulse. A Princeton-Instruments IMAX ICCD camera attached to a PI-Echelle spectrometer was used for signal detection. The lasers are focused using two 5-cm lenses. Several calibration points have been collected in well defined and homogeneous mixtures of air and fuel in order to be used as references for the measurements in turbulent partially premixed flames. This work shows that the application of the LIBS technique in a turbulent combustion environment is feasible and signal is enhanced by applying an orthogonal dual-pulse arrangement for air-fuel. © 2009 Elsevier B.V. All rights reserved.

The structure and dynamics of a turbulent partially premixed methane/air flame in a conical burner were investigated using laser diagnostics and large-eddy simulations (LES). The flame structure inside the cone was characterized in detail using LES based on a two-scalar flamelet model, with the mixture fraction for the mixing field and level-set G-function for the partially premixed flame front propagation. In addition, planar laser induced florescence (PLIF) of CH and chemiluminescence imaging with high speed video were performed through a glass cone. CH and CH2O PLIF were also used to examine the flame structures above the cone. It is shown that in the entire flame the CH layer remains very thin, whereas the CH2O layer is rather thick. The flame is stabilized inside the cone a short distance above the nozzle. The stabilization of the flame can be simulated by the triple-flame model but not the flamelet-quenching model. The results show that flame stabilization in the cone is a result of premixed flame front propagation and flow reversal near the wall of the cone which is deemed to be dependent on the cone angle. Flamelet based LES is shown to capture the measured CH structures whereas the predicted CH2O structure is somewhat thinner than the experiments. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Low swirl burner provides stable lifted flames for fundamental studies of flame structure and turbulence/chemistry interaction in well defined boundary conditions. In the present study the stability characteristics of the burner have been investigated with four tangential jets at the same stoichiometry as the main jet. Two different burner nozzles with 40 mm and 53.5 mm diameters have been used for the stability measurements. In addition, a combined two-dimensional Rayleigh/LIPF-OH technique has been applied for simultaneous measurements of temperature and OH-radical for reaction zone and flame front investigation. Three flames have been selected near extinction for detailed measurements. The data show that the relation between of the main jet velocity, U, and the velocity of the four tangential jets, u, is linear. For the present data set with the nozzles investigated the linear trend can lead to an almost constant ratio of UD/u as 5.08 mm with D as the nozzle diameter of the burner. The flame structure varies from corrugated to highly wrinkle according to the turbulence level. © 2007 Elsevier Inc. All rights reserved.

Flame kernels in spark ignited combustion systems dominate the flame propagation and combustion stability and performance. They are likely controlled by the spark energy, flow field and mixing field. The aim of the present work is to experimentally investigate the structure and propagation of the flame kernel in turbulent premixed methane flow using advanced laser-based techniques. The spark is generated using pulsed Nd:YAG laser with 20 mJ pulse energy in order to avoid the effect of the electrodes on the flame kernel structure and the variation of spark energy from shot-to-shot. Four flames have been investigated at equivalence ratios, φj, of 0.8 and 1.0 and jet velocities, Uj, of 6 and 12 m/s. A combined two-dimensional Rayleigh and LIPF-OH technique has been applied. The flame kernel structure has been collected at several time intervals from the laser ignition between 10 μs and 2 ms. The data show that the flame kernel structure starts with spherical shape and changes gradually to peanut-like, then to mushroom-like and finally disturbed by the turbulence. The mushroom-like structure lasts longer in the stoichiometric and slower jet velocity. The growth rate of the average flame kernel radius is divided into two linear relations; the first one during the first 100 μs is almost three times faster than that at the later stage between 100 and 2000 μs. The flame propagation is slightly faster in leaner flames. The trends of the flame propagation, flame radius, flame cross-sectional area and mean flame temperature are related to the jet velocity and equivalence ratio. The relations obtained in the present work allow the prediction of any of these parameters at different conditions. © 2007 Elsevier Inc. All rights reserved.

There are many non-linear interaction factors responsible for the performance of the laser cutting process. Identification of the dominant factors that significantly affect the cut quality is important. The present research aims to evaluate the CW ND:YAG laser cutting parameters (the gas pressure, laser power, and scanning speed) for 1.2 mm thick ultra-low carbon steel sheets. The effect of the cutting parameters on the cut quality was then investigated, by monitoring the variation in hardness, oxide layer width and microstructural changes within the heat affected zone (HAZ). Results revealed that good quality cuts can be produced in ultra low carbon steel thin sheets, at a window of laser scanning speed of 1100-1500 mm/min and at a minimum heat input of 337 W under an assisting O2 gas pressure of 5 bar. Higher laser power resulted in either strengthening or softening in the HAZ surrounding the cut kerfs. The oxide layer width is not affected by the energy density input, but is affected by the O2 gas pressure due to exothermal reaction. © 2007.

There are many non-linear interaction factors responsible for the performance of the laser cutting process. Identification of the dominant factors that significantly affect the cut quality is important. In the current research, the gas pressure, laser power and scanning speed were selected as the cutting parameters. Effect of the cutting parameters on the cut quality was investigated, by monitoring the variation in hardness, oxide layer width and microstructural changes within the heat affected zone (HAZ). Results revealed that good quality cuts can be produced in ultra low carbon steel thin sheets, using CW Nd:YAG laser at a window of scanning speed ranging from 1100-1500 mm/min at a minimum heat input of 337watts under an assisting O2 gas pressure of 5 bar. Higher laser power resulted in either strengthening or softening in the HAZ surrounding the cut kerf. The oxide layer width is not affected by the energy density input but rather affected by the O2 gas pressure due to exothermal reaction. © 2007 American Institute of Physics.

This work studies the blow-out limit of turbulent premixed flames stabilized on a low-swirl (LS) burner and provides detailed velocity measurements on the burner exit by using the Particle Image Velocimetry (PIV) technique. It is found that the blow-out limit can be well characterized by the swirl number. Definitions of the relevant swirl numbers are discussed. With the help of PIV velocity measurements made on the burner exit, the relationship between the integral swirl number and the geometric swirl number is found for the investigated LS burner configuration. © 2007 Combustion Institute. All Rights Reserved.

Flame stability and mean structure of partially premixed flames have been investigated under the effect of the level of partial premixing and nozzle cone angle. The stability curves and maps of the mean flame structure based on temperature and CO and O2 concentrations measurements in some selected partially premixed flames in the thin reaction zones regime are presented and discussed. More radial and axial mean profiles of temperature and CO and O2 concentrations are also presented for another set of flames at the same equivalence ratio and several nozzle cone angles. The data show that partially premixed flames are more stable than non-premixed and premixed flames. An optimum degree of partial premixing was achieved in the present burner, beyond which the flames are less stable. This optimum level was achieved when the dimensionless mixing length normalized by the nozzle diameter is equal to 5. At this level of partial premixing the structure is likely to form three interacting reaction zones of lean, rich and diffusion with expected triple flame structure. In partially premixed flames a stabilization core has been observed close to the conical nozzle that provides more heat source at the nozzle exit. This is responsible for stabilizing the flames at high Reynolds number. The data also show that the cone angle has a great influence on the flame stability. Increasing the cone angle leads to more air entrainment, breaking the stabilization core and hence reduces the flame stability. The cone, in all cases, provides protected environment at the early stage of reaction near the nozzle exit where intense turbulence is expected. This leads to highly stable flames as compared to similar burners without cone.

Detection and imaging of multi-reaction zones is an essential tool for understanding the detailed structure of complicated flames. In this work a combined 4-camera technique is presented for multi-reaction zones imaging. The technique combines highly advanced laser-based diagnostics tools, namely Rayleigh scattering, laser-induced predissociation fluorescence (LIPF) of OH, LIF of PAH, and LIF of formaldehyde (CH2O). The application of this combined technique in turbulent non-homogeneous hydrocarbon flames is quite new. The technique shows its ability to detect simultaneously rich, lean and diffusion reaction zones. The 3 reaction zones can be spatially resolved, providing essential information about their interaction and overall flame stability. Therefore, the detection and study of triple flame structures in non-homogeneous turbulent flames becomes possible. An example of a triple flame structure in a turbulent lifted non-premixed methane flame is presented. The present work proves that the developed technique is a powerful tool for multi-reaction zone measurements in turbulent and laminar flames. © TÜBİTAK.

The structure and stabilization mechanism of turbulent lifted non-premixed hydrocarbon flames have been investigated using combined laser imaging techniques. The techniques include Rayleigh scattering, laser induced predissociation fluorescence of OH, LIF of PAH, LIF of CH2O, and planar imaging velocimetry. The geometrical structure of multi-reaction zones and flow field at the stabilization region have been simultaneously measured in 16 hydrocarbon flames. The data reveal the existence of triple flame structure at the stabilization region of turbulent lifted flames. Increasing the jet velocity leads to an increase of the lift-off height and to a broadening of the lift-off region. Further analysis of the stabilization criterion at the lift-off height based on the premixed nature of triple-flame propagation and flow field data has been presented and discussed. © 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The flow field and reaction zone structure at the stabilization point of turbulent partially premixed lifted methane flames have been investigated based on advanced laser-based imaging techniques. Planar imaging velocimetry technique, PIV, is used for the flow field and laser induced predissociation fluorescence technique, LIPF, is used for OH radical. The techniques are used to explore the stabilization mechanism at the base of three turbulent partially premixed methane lifted flames. The partially premixed mixture is created in a concentric tube burner. Velocity field and conditioned velocity at the flame base for different exit jet velocities and equivalence ratio have been investigated and discussed in this work. The data show that the flames stabilize at a position with an almost similar flow field structure. In addition, velocity profiles across the flame front at its base are quite similar to that of laminar triple flames. According to the present and our previous measurements in lifted turbulent partially premixed flames the stabilization mechanism is likely to be controlled by premixed flame propagation with triple flame structure at the base of the flames. This supports the application of triple flame structure model as a base for turbulent partially premixed flames at the stabilization position. © 2004 Elsevier Inc. All rights reserved.

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C2Hx. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point. © 2003 The Combustion Institute. All rights reserved.

Fractal analyses have been conducted to investigate flame-front wrinkling of turbulent premixed Bunsen flames. Emphasis is placed on the geometric interpretation of measured fractal parameters and their relationship with parameters used in current combustion models. The outer cutoff scale is found to be four times the integral length scale of flame-front wrinkling defined in the Bray-Moss-Libby model. A revised Gibson length scale is derived by taking into account the curvature effect. Dependency of the revised Gibson scale on the Karlovitz number agrees better with the bulk of available data for the inner cutoff scale than the original definition. The term "self-similarity dimension" is proposed for the fractal dimension measured at finite spatial resolution to highlight the self-similarity feature of local flame fronts. The self-similarity dimension is found to increase linearly with time, and correlates well with the turbulent flame brush thickness. © 2003 Elsevier Science Inc. All rights reserved.

Turbulent partially premixed methane flames stabilized in a concentric flow conical nozzle burner have been investigated and presented in this work. The flames are classified in the thin reaction zones regime. The flames were investigated experimentally based on simultaneous measurements of the velocity field and OH-radical. A combined planar imaging velocimetry (PIV) and laser-induced predissociation fluorescence (LIPF) technique was applied. The mean, rms, and instantaneous structure of four flames are presented and discussed in this article. High stability of the investigated flames is partially due to the partial premixing of the jet and mainly to the flow divergence created by the conical nozzle. This creates a stabilization core with lower mean velocity at the center and lower rms of the velocity fluctuations at the flame brush. No significant recirculation could be observed within the stabilization core above the conical nozzle tip. The reaction zone structure in the present partially premixed flames is likely to show double reaction zones in all flames that support earlier theory and experiments.

The structure of premixed methane-air flames is analyzed using the "laminar flamelet concept". A new model based on one-dimensional set of transformed coupled second order differential conservation equations describing the species concentrations of CO2, CO, O2, CH4, H2O, H2 and N2 and the sensible enthalpy are presented in the present work. The equations are rigorously derived and solved numerically. In these equations, a reaction progress variable (c) is taken as the independent variable that varies from zero to one. A three-step chemical kinetic mechanism is adopted. This was deduced in a systematic way from a detailed chemical kinetic mechanism. The rates for the three steps are related to the rates of the elementary reactions of the full reaction mechanism. Calculations are made for different fixed values of the scalar dissipation rate (Χ) until the flamelet eventually reaches the extinction limit at different levels of pressure. Moreover, simultaneous and instantaneous 1-D measurements of CO2, O2, CO, N2, CH4, H2O, H2, OH and temperature have been carried out in a premixed laminar methane-air flame. A one-dimensional UV Raman-Rayleigh and Laser Induced Predissociation (LIPF) technique has been applied in the present work. The spatial and temporal resolutions are limited to the signal-to-noise ratio and the laser pulse duration. The results of the calculations are assessed against the measurements and previous predictions based on the asymptotic approach of C1 mechanism and a 4-step reduced mechanism. The burning velocity at different equivalence ratios was also deduced from the flamelet properties and assessed against available data. © 2000 OPA (Overseas Publishers Association) N.V.

A new burner design is presented in this work for highly stabilized partially premixed flames using gaseous fuel. The partially premixed flow is created in a concentric flow tube and the flames are stabilized by a relatively large diverging conical nozzle. The concentric flow conical nozzle burner (CFCN) creates highly stabilized flames at high Reynolds number, up to 60000. Three versions of the burner have been investigated and a maximum load of about 250 kW with 20 mm diameter nozzle has been achieved in partially premixed flame. Higher load is expected for larger burner size. The stability characteristics of the CFCN burner show that it is suitable for industrial applications. The burner turndown ratio is between 15 and 20. Fundamental research studies and modeling of the flames in CFCN are feasible because of the simple flow geometry of the burner. Therefore, four flames have been selected in the present work for detailed thermal reaction zone structure investigation and OH radical distribution using simultaneous two-dimensional imaging of Rayleigh scattering and Laser Induced Predissociation Fluorescence (LIPF). The reaction zone structure is relatively thin and may be classified in the thin reaction zones regime. The OH signal correlates well with temperature at the reaction zone. More fine structure in the preheat zone can be observed from the temperature images as compared with those of the OH radical. Stable flame structure with continuous reaction zone at high stretch conditions has been observed. © 2000 OPA (Overseas Publishers Association) N.V.

A method has been developed to accurately determine the turbulent burning velocity in planar turbulent premixed flames stabilized by a low swirl. Six lean methane/air flames have been investigated covering the flamelet as well as the thin reaction zones regime. The probability of finding the instantaneous flame front is measured together with the velocity field by combining simultaneously OH-laser-induced predissociative fluorescence with either Rayleigh thermometry or particle image velocimetry (PIV). It is demonstrated that the turbulent flame brush thickness is independent of ν/sL and that the turbulent burning velocity agrees with predictions from a model equation for the flame surface area ratio using the level set approach. The measurements of the two-dimensional flowfield show a recirculation zone downstream of the flame stabilization area. This exhibits a comparable flow pattern to stagnation point flames. However, mean strain rates are much lower since the flames stabilize close to the burner exit, and the flame is not influenced by the recirculation zone. A comparison of the root-mean-square velocities obtained from Laser-Doppler Amemometry measurements to those determined by PIV show a good agreement. It can be shown that turbulence is attenuated in the flame zone and only moderately increased behind it.