Pressure Flame Research Articles

Study of energy transfer processes in CH as prerequisite for quantitative minor species concentration measurements

Energy transfer effects on CH A-X (0,0) laser-induced fluorescence spectra have been studied for the first time in rich methane and acetylene atmospheric-pressure flames using a picosecond laser system in conjunction with a streak camera. Results allowed determination of typical fluorescence lifetimes of levels in the A state populated by rotational energy transfer (RET), and of state-dependent quenching. These values are used to extend the LASKIN simulation program that has originally been developed to study energy transfer processes in OH, for modelling of spectra of the CH A-X (0,0) transition. Results show good agreement between experiment and simulation both in the spectrally as well as in the temporally-resolved regime.

Measurements of the indium hyperfine structure in an atmospheric-pressure flame by use of diode-laser-induced fluorescence.

We report on what we believe is the first demonstration of laser-induced fluorescence (LIF) in flames by use of diode lasers. Indium atoms seeded into an atmospheric-pressure flame at trace concentrations are excited by a blue GaN laser operating near 410 nm. The laser is mounted in an external-cavity configuration, and the hyperfine spectrum of the 5(2)P1/2 --> 6(2)S1/2 transition is captured at high resolution in single-wavelength sweeps lasting less than one tenth of a second. The research demonstrates the potential of diode-based LIF for practical diagnostics of high-temperature reactive flows.

Measurement of orientation and alignment moment relaxation by polarization spectroscopy: theory and experiment.

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump-probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Sigma+ -X 2Pi (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Sigma+ -X 2Pi (0,0) band with He as the collision partner. The decay of the experimental PS pump-probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions.

Cavity ring-down measurements of seeded NO in premixed atmospheric-pressure H2/air and CH4/air flames

Quantitative aspects of using cavity ring-down absorption spectroscopy near 226 nm for measurements of NO mole fractions in premixed atmospheric-pressure flames are discussed. Measurements in methane–air flames showed strong broadband absorption near 226 nm by hot CO2 molecules, precluding using the cavity ring-down method in these flames at atmospheric pressure. In hydrogen–air flames, the broadband absorption at this wavelength was substantially lower. Absorption cross sections derived from non-seeded cavity ring-down spectra suggest that absorption by water is the major contribution to the background in these flames. The detectability limit for NO by cavity ring-down measurements in hydrogen–air flames using the current setup is estimated to be ∼10 ppm. Effects of the cold boundary layer on the measured NO mole fraction were accounted for by measuring the radial distributions of temperature and NO mole fraction using coherent anti-Stokes Raman scattering and laser-induced fluorescence (LIF), respectively. Measurements performed in seeded stoichiometric and lean hydrogen–air flames showed no reburning at temperatures above 1750 K, demonstrating the adequacy of using these flames for calibration of LIF measurements. At lower temperatures, the mole fraction of NO in the hot gases was up to 30% lower than that expected from the degree of seeding in the cold gases.

Development of a sensor for temperature and water concentration in combustiongases using a single tunable diode laser

The water vapour spectrum in the 1–2 µmnear-infrared region is systematically analysed to find the bestabsorption transitions for sensitive measurement of H2Oconcentration and temperature in combustion environments using a single tunable diodelaser with typical distributed feedback single-mode scanning range (1 cm−1).The use of a single laser, even with relatively narrow tuning range, canoffer distinct advantages over wavelength-multiplexing techniques. Thestrategy and spectroscopic criteria for selecting optimum wavelengthregions and absorption line combinations are discussed. It should bestressed that no single figure of merit can be derived to simplify theselection process, and the optimum line pair should be chosen case bycase. Our investigation reveals that the 1.8 µmspectral region is especially promising, and we have identified 10of the best water line pairs in this spectral region for temperaturemeasurements in flames. Based on these findings, a pair of H2Otransitions near 1.8 µmwas targeted for the design and development of an initialsingle-laser sensor for simultaneously measuring H2Oconcentration and temperature in atmospheric-pressure flames. As part of thesensor development effort, fundamental spectroscopic parameters includingthe line strength, line-centre frequency and lower state energies of theprobed transitions were measured experimentally to improve the currentdatabases. We conclude with demonstration results in a steady and a forcedatmospheric-pressure laboratory combustor.

Two-photon polarization spectroscopy applied for quantitative measurements of atomic hydrogen in atmospheric pressure flames

Two-photon – polarization spectroscopy has been applied for the first time to determine absolute number densities and kinetic temperatures of atomic hydrogen via the 1S–2Stransition in flames at atmospheric pressure. This sensitive laser spectroscopic technique, so far mainly developed and applied for quantitative plasma diagnostics, is also well suited for real combustion processes, because the signal is directly related to the two-photon absorption itself and therefore not limited by quenching or photo-ionization. It can be applied with high spatial and temporal resolution in a wide range of pressure and temperature. Furthermore, a well-established calibration procedure allows for a precise determination of the absolute ground-state density of atomic hydrogen. The measurements also demonstrate that a certain potential of this method comes especially into its own in conjunction with laser radiation of highest spectral quality, i.e. pulsed single-longitudinal mode UV radiation.

Quantitative laser-induced fluorescence of CH in atmospheric pressure flames

Quantitative laser-induced fluorescence of CH in atmospheric pressure flames

Quantitative laser-induced fluorescence of CH in atmospheric pressure flames

Absolute number densities of the CH radical were determined in a partially premixed methane/air flame (equivalence ratio was 1.36) at atmospheric pressure by exciting a predissociating level via the CH B–X(1,0) transition using a quasi-linear laser-induced fluorescence scheme. The peak number density was (1.0±0.4)×1013 cm-3 or 2.4±1 ppm at 1900 K, with a flame-front width of 250 μm (FWHM). Rotational energy transfer must be considered for correct laser-induced fluorescence signal interpretation. Competition between optical pumping and rotational relaxation in both excited and ground states produces a signal that varies almost linearly with laser pulse energy even for large pumping rates. For these conditions, the population of the initial ground-state rotational level is depleted by optical pumping, and rotational energy transfer collisions rapidly repopulate the level during the laser pulse. Deviations from linear behavior are less than 20%. The effects of spatial resolution and polarization of the fluorescence on the absolute measurements are also discussed.

Infrared–ultraviolet double-resonance spectroscopy of OH in a flame

We have experimentally investigated three types of two-color techniques for the detection of the hydroxyl radical using an infrared laser to pump individual ground-state rovibrational transitions and an ultraviolet laser to probe electronic transitions from the pumped intermediate levels. Signal scaling relations and saturation studies are compared for the following techniques: two-color laser-induced fluorescence (TC-LIF), polarization spectroscopy (TC-PS), and resonant four-wave mixing (TC-RFWM). Collisional effects are observed, but the time resolution set by the 1.5-ns laser pulses is not sufficient to determine state-to-state energy transfer rates in an atmospheric pressure flame.

Suppression of secondary explosions in transformer rooms

Large-scale explosion tests in a 50 m 3 room have been performed to simulate real transformer room explosions. The scale and the room layout were chosen as realistic as possible, with a moderate vent area. Prior to ignition hydrogen gas and transformer oil was sprayed into the room. Reference tests were carried out to investigate how gas concentration (i.e. energy deposited in the electric arc) and amount of oil mist would influence the explosion strength. Thereafter 3 manufacturers were invited to mitigate and suppress the explosions. The manufacturers were Atex (superheated water), Fike (powder) and Kidde (water suppression with salts added). In the initial tests showed that the amount of gas is a very important parameter for the explosion load, an implication of this is that a good circuit breaker in the transformer is important. The amount of oil mist was found to influence the explosion loads less. The suppression tests showed that for weak explosions, with average gas concentrations about lower explosion limit, all concepts reduced the explosion loads significantly. For higher gas concentrations (twice as high, still far from worst-case), only a moderate pressure reduction, or no reduction at all, was seen. Some concepts reduced the pressure impulse and flame exposure time significantly. For the strong explosions, it is assumed that performance of most systems could be improved, and suggestions for improvements for all concepts are proposed.

Influence of length-to-diameter ratio and static activation pressure on pressure generation and flame propagation in vented gas explosions

On drafting a new European standard Gas Explosion Venting Protective Systems a lack of data has been identified for venting at low activation pressures. Therefore, experiments have been performed on vented gas explosions using bursting foils of different strength. Cylinders of different volumes and length-to-diameter ratios (L/D) have been used as explosion vessels. The vessels were made from glass to make the flame propagation observable by a high-speed video system. For the dependence of the reduced explosion pressure p red on L/D a maximum occurred for L/D about 3. This could be shown for vessels of different volumes and for different vent sizes. Using bursting foils of different strength it could be shown that the pressure evolution after the vent had opened, depended on the activation pressure only, when the vent size was to small to prevent a second peak in the pressure-time curve.

Rotational level dependence of ground state recovery rates for OH X 2Π(v″=0) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique

We report the first direct measurement of the rotational level dependency of the rate of recovery of initially depleted levels in the electronic ground state X 2Π(v″=0) of OH produced in different flame environments at atmospheric pressure. The initial depopulation of a specific rotational level is accomplished by an intense picosecond pump pulse at 308 nm to partially saturate the electronic A 2Σ–X 2Π(0,0) transition. The recovery of the depleted ground state population then is monitored by probing the same level via the (1,0) band at 283 nm using picosecond degenerate four-wave mixing (DFWM). Both laser wavelengths were derived from the pulse-amplified and frequency doubled output of two independently tunable distributed feedback dye lasers operated with Rh101 and Rh6G in ethanol, respectively, and pumped with the second harmonic of a frequency doubled ps-Nd:YAG laser. It is shown that the rate of repopulation of the depleted ground state levels decreases by 54% and 50% with increasing rotational quantum number, N″, ranging from 2–16 and 2–13 for stoichiometric CH4/air and H2/O2/He flames, respectively. Within experimental error their absolute values in both flames are equal and are not noticeably sensitive to an unequal depletion of the Zeeman sublevels, as created for different polarization configurations of the saturating pump beam and the DFWM probe beams. The rate of (1.8±0.4)×109 s−1 averaged over all rotational transitions investigated is smaller by a factor of 3 than the corresponding average rate of the temporal DFWM signal intensity decay determined by us previously. The rate also is smaller than total depopulation rates obtained in the excited A 2Σ+ state of OH for similar flame conditions.

Fluorescence-lifetime measurements in atmospheric-pressure flames using nanosecond-pulsed lasers

Measurements of fluorescence lifetimes are needed to quantify concentration measurements when using linear laser-induced fluorescence. However, lifetimes are only a few nanoseconds for many important species at atmospheric pressure. When using a typical Q-switched laser with a pulse width of about 10 ns, the fluorescence follows the shape of the laser pulse and the lifetime cannot be easily measured. In this paper, a technique is described for experimentally determining the fluorescence lifetime in atmospheric-pressure flames using a nanosecond-pulsed laser; that is, measurement of a lifetime an order-of-magnitude faster than the laser pulse itself. This technique relies on an observable temporal shift in the fluorescence signal as a function of the lifetime. Simulations show the efficacy of this approach, and data in liquid samples and in an atmospheric-pressure flame show excellent agreement with prior picosecond measurements. This technique is successful because only the temporal shift is examined and details of the fluorescence profile are ignored.

Interline transfer CCD camera for gated broadband coherent anti-Stokes Raman-scattering measurements

Use of an interline transfer CCD camera for the acquisition of broadband coherent anti-Stokes Raman-scattering (CARS) spectra is demonstrated. The interline transfer CCD has alternating columns of imaging and storage pixels that allow one to acquire two successive images by shifting the first image in the storage pixels and immediately acquiring the second image. We have used this dual-image mode for gated CARS measurements by acquiring a CARS spectral image and shifting it rapidly from the imaging pixel columns to the storage pixel columns. We have demonstrated the use of this dual-image mode for gated single-laser-shot measurement of hydrogen and nitrogen CARS spectra at room temperature and in atmospheric pressure flames. The performance of the interline transfer CCD for these CARS measurements is compared directly with the performance of a back-illuminated unintensified CCD camera.

Energy transfer in combustion diagnostics: experiment and modeling.

Laser induced fluorescence (LIF) of OH (A 2 sigma +) is measured in several atmospheric-pressure flames using a short-pulse laser system (80 ps duration) in conjunction with an intensified streak camera. The two-dimensional signal-detection technique allows one to simultaneously monitor rotational and vibrational relaxation as well as electronic quenching. Rotationally-resolved LIF spectra affected by energy transfer are compared with the results of a rate-equation model and are found to be in reasonably good agreement. It is shown that a significant contribution of fluorescence detected by broad-band techniques is due to levels populated by vibrational energy transfer (VET). Implications for picosecond LIF techniques for the time-resolved, quench-free detection of OH are discussed. A detailed analysis is presented for fluorescence spectra originating from levels populated by VET after excitation of states in the OH (A 2 sigma +, v' = 2) level.

MC simulation of aerosol aggregation and simultaneous spheroidization

AbstractA companion article by Tandon and Rosner (1999) showed that Monte‐Carlo (MC) simulation methods can generate joint pdfs of particle volume, v, and surface area, a, for coagulating populations of suspended nonspherical particles simultaneously sintering at finite rates. For continuum‐regime Brownian coagulation at times longer than the characteristic coagulation time, a “self‐preserving” asymptotic pdf shape was shown to extend to these bivariate populations, using the rescaled volume v/v̄(t) and rescaled area a/ā(t) as “similarity” variables. This article considers the corresponding problem of coagulation in the “free‐molecule” limit, relevant to the atmospheric pressure flame experiments of Xing et al. This combination of coagulation‐ and sintering‐rate laws again leads to “self‐preserving” rescaled jpdfs, explicitly time‐independent for an isothermal environment, again dependent on a Damköhler‐like number, Damf (ratio of coagulation time to fusion time). While flames are more complex than our idealized simulations, our present pdfs and associated “mixed moments,” capture important features of experimental results as one approaches the flame, especially evolution of a mean particle “shape factor,” mean number of spherules per aggregate, and narrowness of the spread of spherule sizes. This agreement, coupled with the versatility of MC‐methods for multistate variable particle population balances, encourages extensions leading to a versatile, tractable, formalism for the developing field of sol reaction engineering.

Triacylglycerol analysis of potential margarine base stocks by high-performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry and flame ionization detection.

Several margarine base stock candidates have previously been prepared for the purpose of finding better, more oxidatively stable food components: high-saturate vegetable oils, randomized vegetable oils, vegetable oil-hard stock blends, and interesterified vegetable oil-hard stock blends. Here are reported the triacylglycerol compositions of these products, determined using reverse-phase high-performance liquid chromatography (HPLC) coupled with a flame ionization detector or a quadrupole mass spectrometer with an atmospheric pressure chemical ionization source. Triacylglycerol percent composition results for samples of known composition (randomized and interesterified samples) exhibited less average error by HPLC coupled with a quadrupole mass spectrometer with an atmospheric pressure chemical ionization source, after application of response factors, than the results by HPLC coupled with a flame ionization detector. The fatty acid compositions calculated from the mass spectrometric data exhibited less average error than the fatty acid compositions resulting from the flame ionization detector data. The average error of the fatty acid compositions by the mass spectrometer was lowest for interesterified blend samples, next lowest for randomized samples, then followed by high-saturated fatty acid oils, normal oils, and blends. Analysis of the vegetable oil-hard stock blends by mass spectrometer required special treatment for calculation of response factors.

Atomic oxygen detection in flames using two-photondegenerate four-wave mixing

We report a theoretical and experimental investigation of atomicoxygen detection in an atmospheric-pressure flame by two-photondegenerate four-wave mixing (TPDFWM). We present analyticalexpressions for the TPDFWM signal that are based on the diagrammaticperturbation theory. The description of several polarizationarrangements is given and used in our experiments. Theimportance of the polarization scheme is emphasized and the mostefficient scheme is found by investigating signal and noisecontributions. In this case, thermal grating contributions aregreatly reduced and this allows quantitative concentrationmeasurements in flame to be performed using TPDFWM.

Investigations of the gas phase mechanism of diamond deposition in combustion CVD

Two diamond combustion CVD systems have been investigated with respect to the reaction mechanism in the gas phase: an atmospheric acetylene–oxygen welding torch and a flat, premixed acetylene–oxygen–argon flame at 50 mbar. The atmospheric pressure flame was used to study the influence of important deposition parameters including stoichiometry, deposition time, substrate temperature and pretreatment as well as that of some chemical additives. In the low-pressure flame, the chemical composition in the gas phase was investigated in detail for some specific deposition conditions. A combination of laser techniques and molecular beam mass spectrometry was used to measure spatially-resolved temperature and species concentrations with and without a substrate present. The results for some stable and intermediate species are compared with recent numerical models of the gas phase chemistry. An overview is given of some recent experiments, and perspectives for further work are discussed.

00/00980 Dynamic light scattering in sooting premixed atmospheric-pressure methane propane ethene and propeneoxygen flames

00/00980 Dynamic light scattering in sooting premixed atmospheric-pressure methane propane ethene and propeneoxygen flames