Measurements of absolute CH concentrations by cavity ring-down spectroscopy and linear laser-induced fluorescence in laminar, counterflow partially premixed and nonpremixed flames at atmospheric pressure

We report quantitative, spatially resolved measurements of methylidyne concentration ([CH]) in laminar, counterflow partially premixed and nonpremixed flames at atmospheric pressure by using both cavity ring-down spectroscopy (CRDS) and linear laser-induced fluorescence (LIF) in the A-X (0, 0) band. Three partially premixed (phiB = 1.45, 1.6, 2.0) flames plus a single nonpremixed methane-air flame are investigated at a global strain rate of 20 s(-1). These quantitative measurements are compared with predictions from an opposed-flow flame code when utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The LIF measurements of [CH] are corrected for variations in the electronic quenching rate coefficient by using predicted major species concentrations and temperatures along with quenching cross sections for CH that are available in the literature. The peak CH concentration obtained by CRDS is used to calibrate the quenching-corrected LIF measurements. Excellent agreement is obtained between CH concentration profiles measured by using the CRDS and LIF techniques. The spatial location of the CH layer is very well predicted by GRI 3.0; moreover, the measured and predicted CH concentrations are in good agreement for all the flames of this study.     

Phase-locked two-line OH planar laser-induced fluorescence thermometry in a pulsating gas turbine model combustor at atmospheric pressure

Two-line OH planar laser-induced fluorescence (PLIF) thermometry was applied to a swirling CH4/air flame in a gas turbine (GT) model combustor at atmospheric pressure, which exhibited self-excited combustion instability. The potential and limitations of the method are discussed with respect to applications in GT-like flames. A major drawback of using OH as a temperature indicator is that no temperature information can be obtained from regions where OH radicals are missing or present in insufficient concentration. The resulting bias in the average temperature is addressed and quantified for one operating condition by a comparison with results from laser Raman measurements applied in the same flame. Care was taken to minimize saturation effects by decreasing the spectral laser power density to a minimum while keeping an acceptable spatial resolution and signal-to-noise ratio. In order to correct for the influence of laser light attenuation, absorption measurements were performed on a single-shot basis and a correction procedure was applied. The accuracy was determined to 4%-7% depending on the location within the flame and on the temperature level. A GT model combustor with an optical combustion chamber is described, and phase-locked 2D temperature distributions from a pulsating flame are presented. The temperature variations during an oscillation cycle are specified, and the general flame behavior is described. Our main goals are the evaluation of the OH PLIF thermometry and the characterization of a pulsating GT-like flame.     

Characterization of a CH planar laser-induced fluorescence imaging system using a kHz-rate multimode-pumped optical parametric oscillator

The performance characteristics of a new CH planar laser-induced fluorescence (PLIF) imaging system composed of a kHz-rate multimode-pumped optical parametric oscillator (OPO) and high-speed intensified CMOS camera are investigated in laminar and turbulent CH4-H2-air flames. A multi-channel Nd:YAG cluster that produces up to 225 mJ at 355 nm with multiple-pulse spacing of 100 μs (corresponding to 10 kHz) is used to pump an OPO to produce up to 6 mJ at 431 nm for direct excitation of the A-X (0, 0) band of the CH radical. Single-shot signal-to-noise ratios of 82:1 and 7.5:1 are recorded in laminar premixed flames relative to noise in the background and within the flame layer, respectively. The spatial resolution and image quality are sufficient to accurately measure the CH layer thickness of ~0.4 mm while imaging the detailed evolution of turbulent flame structures over a 20 mm span. Background interferences due to polycyclic-aromatic hydrocarbons and Rayleigh scattering are minimized and, along with signal linearity, allow semi-quantitative analysis of CH signals on a shot-to-shot basis. The effects of design features, such as cavity finesse and passive injection seeding, on conversion efficiency, stability, and linewidth of the OPO output are also discussed.     

Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition

This study quantifies the range of premixed flame conditions for which CH fluorescece diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios (phi) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at phi = 1.25 and phi = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as afactor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-MECH 3.0 chemistry. Sources experimental uncertainties are discussed.     

Cool-Flame Burning and Oscillations of Envelope Diffusion Flames in Microgravity

The two-stage combustion, local extinction, and flame-edge oscillations have been observed in single-droplet combustion tests conducted on the International Space Station. To understand such dynamic behavior of initially enveloped diffusion flames in microgravity, two-dimensional (axisymmetric) computation is performed for a gaseous n-heptane flame using a time-dependent code with a detailed reaction mechanism (127 species and 1130 reactions), diffusive transport, and a simple radiation model (for CO₂, H₂O, CO, CH₄, and soot). The calculated combustion characteristics vary profoundly with a slight movement of air surrounding a fuel source. In a near-quiescent environment (≤?2 mm/s), with a sufficiently large fuel injection velocity (1 cm/s), extinction of a growing spherical diffusion flame due to radiative heat losses is predicted at the flame temperature at ≈ 1200 K. The radiative extinction is typically followed by a transition to the “cool flame” burning regime (due to the negative temperature coefficient in the low-temperature chemistry) with a reaction zone (at ≈ 700 K) in close proximity to the fuel source. By contrast, if there is a slight relative velocity (≈?3 mm/s) between the fuel source and the air, a local extinction of the envelope diffusion flame is predicted downstream at ≈ 1200 K, followed by periodic flame-edge oscillations. At higher relative velocities (4 to 10 mm/s), the locally extinguished flame becomes steady state. The present 2D computational approach can help in understanding further the non-premixed “cool flame” structure and flame-flow interactions in microgravity environments.     

Line Raman, Rayleigh, and laser-induced predissociation fluorescence technique for combustion with a tunable KrF excimer laser

We have applied a line UV Raman, Rayleigh, and laser-induced predissociation fluorescence technique for measurement of turbulent hydrocarbon flames. The species concentration of CO(2), O(2), CO, N(2), CH(4), H(2)O, OH, and H(2) and the temperature are measured instantaneously and simultaneously along a line of 11.4 mm, from which the gradients with respect to mixture fraction and spatial direction are obtained. The technique has been successfully tested in a laminar premixed stoichiometric methane flame and a laminar hydrogen diffusion flame. In addition the technique has been tested in a highly turbulent rich premixed methane flame. The data show that the technique can be used to provide instantaneous measurements of local profiles that describe the local flame structure in highly turbulent flames.     

Analysis of laser-induced-fluorescence carbon monoxide measurements in turbulent nonpremixed flames

The influence of fluctuating concentrations and temperature on the laser-induced-fluorescence (LIF) measurement of CO in turbulent flames is described, under conditions in which the fluorescence and the temperature are measured independently. The analysis shows that correlations between CO concentration and temperature can bias the averaged mole fraction extracted from LIF measurements. The magnitude of the bias can exceed the order of the average CO mole fraction. Further, LIF measurements of CO concentrations in a turbulent, nonpremixed, natural gas flame are described. The averaged CO mole fractions are derived from the fluorescence measurements by the use of flame temperatures independently measured by coherent anti-Stokes Raman spectroscopy. Analysis of the fluctuations in measured temperature and fluorescence indicates that temperature and CO concentrations in flame regions with intensive mixing are indeed correlated. In the flame regions where burnout of CO has ceased, the LIF measurements of the CO mole fraction correspond to the probe measurements in exhaust.     

Influence of an acoustic field on flame development and transition to detonation

The work is devoted to experimental and numerical study of flame interaction with acoustic waves in closed and semiclosed pipes filled with preliminarily mixed gaseous mixtures. We analyze the influence of eigenfield (generated by the flame itself) and external acoustic field on the flame dynamics. We show that acoustic field affects the combustion process at all stages. The effect increases with any increase in the energy of initiation of combustion. At later stages of flame development, acoustic waves can initiate the transition to detonation or prevent it. Thus, it is possible to control the combustion modes using external acoustic field.     

Radiative, collisional, and predissociative effects in CH laser-induced-fluorescence flame thermometry

Laser-induced fluorescence of the CH radical is used to determine the flame-front temperature of an 8-Torr premixed CH(4)/O(2) flame. The A (2)D-X (2)Delta (0, 0) and B (2)Sigma- - X (2)II (0, 0) bands give values of 1960 +/- 60 and 1920 +/- 70 K, respectively. This is an improvement over a previous study that found discrepancies up to 20% between these bands. New rotational-level-dependent transition probabilities are the main reason for this improvement. The weaker off-diagonal bands A-X (0, 1) and B-X (0, 1) yield temperatures of 1930 +/- 90 and 1830 +/- 100 K, respectively. The influence of rotational transfer on the predissociated levels that have N' > in B (2)Sigma-, v'= 0 was investigated with fluorescence scans and a statistical power-gap law model of the relaxation; deviations up to 8% in temperature can occur because of this process.     

Simulation of a premixed turbulent flame with the discrete vortex method

The behaviour of a premixed turbulent flame is numerically studied in this paper. The numerical model is based on solving turbulent flow field by the discrete vortex method. The flame is considered to be of zero thickness boundary which separates burnt and unburnt regions with different constant density and propagates into the fresh mixture at a local curvature‐dependent flame speed. The flame front is located by means of level‐set algorithm. The flow turbulence is simulated through the unsteady vortex‐shedding mechanism. The computed velocity fields, turbulence scalar statistics as well as flame brush thickness for the turbulent V‐flame are well comparable to experimental results. The computed Reynolds stresses in the flame brush region based on unconditioned velocities are substantial, but the two conditioned Reynolds stresses are negligible. These results show that the intermittency effect is a major influence on turbulent statistics in premixed flame and should require careful consideration in numerical models. Copyright © 2000 John Wiley & Sons, Ltd.     

Background corrections for laser-induced-fluorescence measurements of nitric oxide in lean, high-pressure, premixed methane flames

An experimental technique is presented that both minimizes and accounts for the interference background when laser-induced-fluorescence (LIF) measurements are made of NO in lean, high-pressure, premixed, CH(4)/O(2)/N(2) flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. O(2) fluorescence interferences are particularly troublesome in lean premixed flames. An excitation-detection scheme that minimizes the effects of these interferences is identified. A procedure that corrects the resulting LIF signal so as to account for any remaining interference signal is then developed. This correction is found to vary from less than 10% of the overall NO signal at atmospheric pressure to over 40% of the overall signal at 14.6 atm for LIF measurements of NO in a series of worst-case flames (phi = 0.6, dilution ratio 2.2). The correction technique is further demonstrated to be portable over a useful range of flame conditions at each pressure.     

Influence of G-jitter on the characteristics of a non-premixed flame: Experimental approach

The combustion of a flat plate in a boundary layer under microgravity conditions, which was first described by Emmons, is studied using a gas burner. Magnitude of injection and blowing velocities are chosen to be characteristic of pyrolyzing velocity of solid fuels, and of ventilation systems in space stations. These velocities are about 0.1 m/s for oxidiser flow and 0.004m/s for fuel flow. In this configuration, flame layout results from a coupled interaction between oxidiser flow, fuel flow and thermal expansion. Influences of these parameters are studied experimentally by means of flame length and standoff distance measurements using CH* chemiluminescence’s and visible emission of the flame. Flow was also studied with Particle Image Velocimetry (PIV). Inert flows, with and without injection, and reacting flow in a microgravity environment were considered to distinguish aerodynamic from thermal effect. Thermal expansion effects have been shown by means of the acceleration of oxidiser flow. Three-dimensional effects, which are strongly marked for high injection velocities were studied. Three-dimensional tools adaptability to parabolic flights particular conditions were of concern. Flame sensitivity to g-jitters was investigated according to g-jitters frequency and range involved by parabolic flights. It appears that flame location (standoff distance), flame characteristics (length, thickness, brightness) and the aerodynamic field of the low velocity reacting flow are very much affected by the fluctuation of the gravity level or g-jitter. The lower the g-jitter frequency is, the higher the perturbation. Consequently it is difficult to perform relevant experiments for a main flow velocity lower than 0.05m/s. DNS calculations confirm the present observations, but most of the results are presented elsewhere.     

Expressions for determination of layer absorbances of a weakly absorbing double-layer sample through reflectance at the Brewster angles

A new technique for determining absorbances of interferenceless (incoherent) layers based on measurements of the ratio of the front and the back reflectivities of a double-layer sample at the Brewster angles is proposed. A double-layer stack must have at least one absorbing layer, and the two layers should be interferenceless and should be thicker than the wavelength of the incident light. We found that under these conditions the ratio of the front and the back reflectivities at the Brewster angle of a sample surface is directly related to layer absorbance. For a layer with a known thickness this means finding the extinction coefficient of the layer material. In comparison with the conventional method for measuring transmittance, the advantage of this approach is that it affords an opportunity to get rid of the influence of surface effects on the measuring volume absorption coefficient. For a thick layer with known thickness, it makes possible the determination of a small bulk absorption on a background with even greater surface effects. We trust that this technique will prove to be powerful for measuring the extinction coefficients of weakly absorbing materials.     

Condensation behavior of gaseous phase during transported in the near-substrate boundary layer of plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition(PS-PVD)has exhibited the potential ability to prepare columnar structures for advanced thermal barrier coatings(TBCs).The coating structure is nominally affected by operating parameters,but it is controlled by the type of deposition unit actually and essentially.In order to realize the columnar structure deposited by gaseous phase units,the transition behavior of gaseous phase units to clusters must be fundamentally understood.This work investigated the transport process of gaseous phase units in the PS-PVD near-substrate boundary layer along with the condensation behavior.The Monte Carlo method was used to examine the transport process and condensation behavior of gaseous phase units under different scale boundary layers.Simulation results show that it is easier to form more numerous larger clusters at the edge of the plasma jet than at the center.Based on the understanding of the changes in deposition unit caused by the condensation of gaseous phase in the near-substrate boundary layer of PS-PVD,an outlook towards TBCs with different structures is presented.And it is in good agreement with the experimental data.     

Dynamics of Laser Ablation in Superfluid $$^4\hbox {He}$$

Pulsed laser ablation of metal targets immersed in superfluid \(^4\hbox {He}\) is visualized by time-resolved shadowgraph photography and the products are analyzed by post-experiment atomic force microscopy (AFM) measurements. The expansion dynamics of the gaseous ablation half-bubble on the target surface appears underdamped and follows the predicted behavior for the thermally induced bubble growth mechanism. An inherent instability of the ablation bubble appears near its maximum radius and no tightly focused cavity collapse or rebound events are observed. During the ablation bubble retreat phase, the presence of sharp edges in the target introduces flow patterns that lead to the creation of large classical vortex rings. Furthermore, on the nanometer scale, AFM data reveal that the metal nanoparticles created by laser ablation are trapped in spherical vortex tangles and quantized vortex rings present in the non-equilibrium liquid.     

New method for simultaneous gas and aerosol retrievals from space limb-scanning spectral observation of the atmosphere

This study concerns the development of a new inversion method for simultaneous gas and aerosol retrievals in the upper layers of the atmosphere from limb-viewing multiwavelength-transmission infrared measurements. In this method, concentrations of gas species such as O3, NO2, HNO3, N2O, CH4, and H2O, and spectral dependences of the aerosol extinction coefficient are retrieved simultaneously. When this is done, smoothness constraints on the desired spectral dependencies of the aerosol extinction coefficient are used as an a priori assumption. The method is used in the treating of synthetic transmission spectra of the Improved Limb Atmospheric Spectrometer, which is based on the solar occultation technique and was on board the Advanced Earth Observing Satellite. A set of numerical tests shows the efficiency of the method.     

Experiments concerning resonance-enhanced multiphoton ionization probe measurements of flame-species profiles

Resonance-enhanced multiphoton ionization (REMPI) detection of radical species in low-pressure laboratory flames is a promising tool for the development and refinement of combustion models. For accurate REMPI species concentration measurements in flame zones with inherently high background ionization levels, an understanding of the influence of plasma sheaths on REMPI probe response is required. Proper probe response is found to depend on careful control of probe-biasing and laserfocusing conditions. Only negatively biased probes are suitable, because of the influences of secondary ionization on the response of positively biased probes. In situ probe calibration procedures with the (2 + 2) REMPI of N(2) at 270.6 nm are described. Detection of O atoms in a stoichiometric 20-Torr methane-oxygen flame permits a precise comparison of both laser-induced fluorescence and REMPI measurements with flame-modeling results.     

Laser-induced fluorescence of formaldehyde hot bands in flames

Laser-induced fluorescence and excitation spectra of formaldehyde in the A-X 4(1)(0) band at 370 nm are recorded in the primary flame front of a Bunsen flame. An examination of partition functions shows that this excitation can minimize temperature bias for formaldehyde in situ diagnostic measurements.     

微胶囊红磷和聚苯醚对高抗冲聚苯乙烯的协同阻燃作用

利用微胶囊红磷(MRP)和聚苯醚(PPO)来提高高抗冲聚苯乙烯(HIPS)的阻燃性能, 通过熔融共混法制备了一系列不同组成的MRP-PPO/HIPS复合材料。采用水平燃烧、垂直燃烧、氧指数、锥形量热分析、高温热分解实验等方法研究了复合材料的阻燃性能。研究表明, 阻燃剂用量相同时, 在HIPS基体中同时加入MRP和PPO得到的复合材料比单独加入MRP或PPO得到的复合材料具有更好的阻燃性能。当MRP-PPO/HIPS的质量比为10:20:70时, 复合材料的氧指数为23.9%, 水平燃烧级别达到FH-1级, 垂直燃烧级别达到FV-0级, 阻燃性能达到最佳。MRP用量过多时, 复合材料的阻燃性能下降。研究认为, PPO和MRP对HIPS具有较强的协同阻燃作用。两者以适当比例并用时能够使复合材料在燃烧时的热释放速率和燃烧热大幅度减小, 降低了气相燃烧区的温度, 起到气相阻燃作用。同时, 复合材料在热分解和燃烧时能够生成连续和致密的炭层, 抑制了燃烧过程中的热量传递和物质交换, 起到凝聚相阻燃作用。因此, 复合材料的阻燃性能显著改善。     

Soot formation and oxidation in oscillating methane-air diffusion flames at elevated pressure

Comparisons with respect to the sooting tendency are made between stationary diffusion flames and diffusion flames with pulsations induced by oscillating fuel flow. Time-resolved measurements of the soot particle properties in the flames are obtained by combining Rayleigh-scattering, laser-induced incandescence, and extinction measurements into the RAYLIX method. Furthermore, flame luminosity at 590 nm and OH*-chemoluminescence signals at 310 nm are monitored to obtain data regarding the flame structure. Mean soot volume fractions of oscillating flames are significantly different from those of stationary flames with the same mean fuel flow rate; oscillations of the total amount of soot are phase shifted and asymmetric compared with fuel flow oscillations.