T.S. Cheng

Simultaneous temperature and multispecies measurement in a lifted hydrogen diffusion flame

This paper reports that ultraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined and applied to a lifted hydrogen jet diffusion flame. Simultaneous, temporally and spatially resolved point measurements of temperature, major species concentrations (H[sub 2], O[sub 2] N[sub 2], H[sub 2]O), and absolute hydroxyl radical concentration (OH) are obtained with a single excimer laser for the first time. For OH measurements, the use of LIPF makes quenching corrections unnecessary. Results demonstrate that fuel and oxidizer are in a rich, premixed, and unignited condition in the center core of the lifted flame base. In the lifted zone, combustion occurs in an intermittent annular turbulent flame brush and strong finite-rate chemistry effects result in nonequilibrium values of temperature, major species, and OH radicals. Downstream in the slow three-body recombination zone, the major species concentrations are in partial equilibrium, the OH concentrations are in superequilibrium, and the temperatures are in subequilibrium. Far downstream in the flame, equilibrium values of temperature, OH radical, and major species are found.

Raman measurement of mixing and finite-rate chemistry in a supersonic hydrogen-air diffusion flame

Abstract Uhraviolet (UV) spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) from a KrF excimer laser are combined to simultaneously measure temperature, major species concentrations (H2, O2, N2, H2O), and OH radical concentration in a supersonic lifted co-flowing hydrogen-air diffusion flame. The axisymmetric flame is formed when a sonic jet of hydrogen mixes with a Mach 2 annular jet of vitiated air. Mean and rms profiles of temperature, species concentrations, and mixture fraction are obtained throughout the supersonic flame. Simultaneous measurements of the chemical species and temperature are compared with frozen chemistry and equilibrium chemistry limits to assess the local state of the mixing and chemistry. Upstream of the lifted flame base, a very small amount of reaction occurs from mixing with hot vitiated air. Downstream of the lifted flame base, strong turbulent mixing leads to subequilibrium values of temperature and OH concentration. Due to the interaction of velocity and temperature in supersonic compressible flames, the fluctuations of temperature and species concentrations are found to be higher than subsonic flames. Farther downstream, slow three-body recombination reactions result in superequilibrium OH concentrations that depress temperatures below their equilibrium values.

Raman scattering measurements in flames using a tunable KrF excimer laser.

Using a narrow-band tunable KrF excimer laser as a spontaneous vibrational Raman scattering source, we demonstrate that single-pulse concentration and temperature measurements, with only minimal fluorescence interference, are possible for all major species (O(2), N(2), H(2)O, and H(2)) at all stoichiometries (fuel-lean to fuel-rich) of H(2)-air flames. Photon-statistics-limited precisions in these instantaneous and spatially resolved single-pulse measurements are typically 5%, which are based on the relative standard deviations of single-pulse probability distributions. Optimal tuning of the narrow-band KrF excimer laser (248.623 nm) for the minimization of OH A(2)Sigma-X(2)II and O(2)B(3)Sigma(u)(-)-X(3)Sigma(g)(-) fluorescence interference is determined from fluorescence excitation spectra. In addition to the single-pulse N(2) Stokes/anti-Stokes ratio temperature measurement technique, a time-averaged temperature measurement technique ispresented that matches the N(2) Stokes Raman spectrum to theoretical spectra by using a single intermediate sta frequency to account for near-resonance enhancement. Raman flame spectra in CH(4)-air flames are presented that have good signal-to-noise characteristics and show promise for single-pulse UV Raman measurements in hydrocarbon flames.

An experimental investigation of the blowout process of a jet flame

An experimental study was performed to investigate the roles of triple flames and flame front instabilities in the blowout transient process. Two-dimensional laser-induced predissociative fluorescence (LIPF) OH and particle image velocimetry (PIV) diagnostic techniques were used for measurements of instantaneous flame structure and velcotiy data. Initial conditions were aligned by external acoustic excitation and triggering. The blowout transient process can be divided into four regions: the pulsating, onset of receding, receding, and extinction regions, according to the dynamic characteristics of the flame. In the pulsating region, the flame base is basically pulsating at two specific heights with jittering. Flame from instability may play a role in leading to the onset of blowout process. Both LIPF OH image and PIV results show the possible existence of the triple- (or edge-) flame structures in the flame base in the pulsating and onset regions. High strain rate, higher than the extinction strain rate, encountered by the flame base in the onset region should be cousidered as a prominent factor for the blowout process.

Single pulse vibrational Raman scattering by a broadband KrF excimer laser in a hydrogen–air flame

Spontaneous vibrational Raman scattering (VRS) is produced by a broadband excimer laser at 248 nm (KrF) in a H(2)-air flame and VRS spectra are recorded for lean, stoichiometric, and rich flames. Except at very lean flame conditions, laser-induced fluorescence (LIF) processes interfere with VRS Stokes lines from H(2), H(2)O, and O(2). No interference is found for the N(2) Stokes and N(2) anti-Stokes lines. In a stoichiometric H(2)/air flame, single-pulse measurements of N(2) concentration and temperature (by the VRS Stokes to anti-Stokes ratio) have relative standard deviation of 7.7 and 10%, respectively. These single pulse measurement errors compare well with photon statistics calculations using measured Raman cross sections.

Effects of fuel-air mixing on flame structures and NOx emissions in swirling methane jet flames

An experimental investigation is performed to study the effects of initial fuel-air mixing on NOx and CO emissions in swirling methane jet flames. The major parameters used to modify the initial fuel-air mixing ahead of the swirling flame are the swirl number, the fuel-air momentum flux ratio, and the fuel injection location. Two characteristic swirling combustion modes, the fuel jet-dominated (type-1) and the strongly recirculating (type-2) flames, are identified from flame visualization and 2-D laser-induced predissociative fluorescence imaging of OH by varying the fuel-air momentum flux ratio. Laser Doppler velocimetry (LDV) measurements show that the shear layer between the edge of the swirling recirculation zone and the external flow is a highly turbulent and rapid mixing region. The maximum mean flame temperature is located at the edge of the recirculation zone, indicating violent combustion and strong mixing of fuel, air, and hot products in this region. Strong and rapid mixing of the strongly recirculating flame, which increases mixture homogeneity and shortens the characteristic time for NOx formation, results in a lower NOx emission index than that in the fuel jet-dominated flame. Excess cold air entrained by the swirling flow may quench the combustion and the hot products, resulting in an increase of CO emission, indicating poor combustion efficiency. By modifying the fuel injection pattern with the annular fuel injector, which changes the fuel-air mixing pattern and properly smooths the rapid mixing leading to a higher flame temperature, the NOx emission level can further be reduced with a significant decrease in CO emission.

OPERATIONAL CHARACTERISTICS OF CATALYTIC COMBUSTION IN A PLATINUM MICROTUBE

Catalytic combustion of hydrogen in a platinum microtube, or subquenching diameter tube, is studied via theoretical analysis, experiments, and numerical simulation in terms of the major operation and design parameters. Fine-thermocouple, laser-induced fluorescence (LIF) and Raman scattering are used to measure the temperature and major species and OH concentration data at the tube exit. The experimental results show that the tube-exit temperature increases with fuel concentration, velocity, and tube size. For high fuel concentration and velocity cases in the 1000- and 500-µm tubes, an obvious gas-phase reaction behind the exit can be detected by thermocouple and LIF-OH images. Numerical simulation results show that smaller tube sizes and lower velocities would enhance the conversion ratio on the catalytic surface due to the enhanced diffusion of surface species of H2 and O2. Based on the current results and analysis, the characteristic operation regions of hydrogen catalytic combustion in microtubes are quantitatively identified in terms of parameters related to heat generation and heat loss characteristics, competition among the timescales, and tube size. Decreasing the tube size will shift the operation region toward the high-concentration and high-velocity portion of the domain with a smaller operation area.

Effects of partial premixing on pollutant emissions in swirling methane jet flames

Abstract Experimental measurements of visible flame heights, temperatures, and pollutant emission indices in partially premixed swirling flames with a broad range of central fuel tube equivalence ratios (Φ F ) are reported. Two cases of partially premixed swirling flames are studied; one with a constant fuel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the overall flame color changes from yellow to blue. Temperature measurements indicate that the flame structures become thinner and temperatures increase continuously with increasing partial premixing. Emission index, EINO x and EICO, values decrease with increasing levels of partial premixing and reach a minimum value at Φ F ≈ 3, followed by an increase as Φ F approaches the blowoff limits. The reduction in EINO x and EICO at optimum partial premixing (Φ F ≈ 3), as compared with that for the non-premixed swirling flame is at least 23% and 77%, respectively. The emission index for NO x scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predictions and measured results.

Characteristics of microjet methane diffusion flames

Characteristics of microjet methane diffusion flames stabilized on top of the vertically oriented, stainless-steel tubes with an inner diameter ranging from 186 to 778 μ m are investigated experimentally, theoretically and numerically. Of particular interest are the flame shape, flame length and quenching limit, as they may be related to the minimum size and power of the devices in which such flames would be used for future micro-power generation. Experimental measurements of the flame shape, flame length and quenching velocity are compared with theoretical predictions as well as detailed numerical simulations. Comparisons of the theoretical predictions with measured results show that only Ropers model can satisfactorily predict the flame height and quenching velocity of microjet methane flames. Detailed numerical simulations, using skeletal chemical kinetic mechanism, of the flames stabilized at the tip of d = 186, 324 and 529 μ m tubes are performed to investigate the flame structures and the effects of burner materials on the standoff distance near extinction limit. The computed flame shape and flame length for the d = 186 μm flame are in excellent agreement with experimental results. Numerical predictions of the flame structures strongly suggest that the flame burns in a diffusion mode near the extinction limit. The calculated OH mass fraction isopleths indicate that different tube materials have a minor effect on the standoff distance, but influence the quenching gap between the flame and the tube.

CHEMILUMINESCENCE MEASUREMENTS OF LOCAL EQUIVALENCE RATIO IN A PARTIALLY PREMIXED FLAME

Spatially resolved, time-averaged, multipoint measurements of flame emission spectra using two Cassegrain mirrors and two spectrometers are performed and the results are used to obtain the correlation of the intensity ratio of OH*/CH* and C2*/OH* to the equivalence ratio in the laminar flames over an equivalence ratio range of 0.8–1.4. Results show that a strong correlation exists between the intensity ratio and equivalence ratio. The calibration equations obtained from the laminar flame measurements are then applied to obtain the local equivalence ratio in a partially premixed swirling flame. Experimental results demonstrate that multipoint measurement of local equivalence ratio in the partially premixed swirling methane flames is feasible. However, this non-laser based chemiluminescence technique can only be applied to determine the local flame stoichiometry in the reaction zone of the flames. Further improvement of the measurement system and possibility of simultaneous measurements of equivalence ratio and temperature are discussed.