Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

Numerical simulation of premixed H2–air cellular tubular flames

The detailed flame structure of laminar premixed cellular flames in the tubular domain is simulated in 2D using a fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Numerical results for H₂/air flames are presented and compared against spatially resolved experimental measurements of temperature and chemical species including atomic H and OH. The experimental results compare well for flame structure and cell number, despite the numerical model under-predicting the peak temperature by 200 K. Numerical experiments were performed to assess the ability for cellular tubular flames to impact experimental and numerical investigations of practical flames. The cellular flame structure is found to provide a highly sensitive geometry that is useful for validating diffusive transport modelling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. the Soret effect) that is suitable for practical combustion codes.     

A single pulse shock tube study of pentene isomer pyrolysis

A single-pulse shock tube study of the four pentene isomers is carried out at 2 ± 0.16 bar and 900–1600 K. C₁ to C₆ species profiles were recorded using gas chromatography mass spectrometry analyses. The species are identified using mass spectrometry and quantified by flame ionization detection. High-pressure limiting and pressure-dependent rate constants for 2M1B, 2M2B and 3M1B + ? were calculated using RRKM theory with a Master Equation (ME) analysis using the Master Equation System Solver, MESS. A mechanism was formulated based on rate rules and theoretical calculations. Comparisons between experimental results and model simulations are provided for all of the five pentene isomers investigated with satisfactory agreement. Furthermore, an insight is provided into the influence of molecular structure on the reactivity of pyrolysis chemistry. Interestingly, it is found that the HACA mechanism is much less prominent for benzene formation compared to the role of cyclopentadienyl radical recombination with methyl radicals and also the recombination of propargyl radicals.     

Computational and experimental study of oxygen-enhanced axisymmetric laminar methane flames

Three axisymmetric laminar coflow diffusion flames, one of which is a nitrogen-diluted methane/air flame (the ‘base case’) and the other two of which consist of nitrogen-diluted methane vs. pure oxygen, are examined both computationally and experimentally. Computationally, the local rectangular refinement method is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids. The model includes C2 chemistry (GRI 2.11 and GRI 3.0 chemical mechanisms), detailed transport, and optically thin radiation. Because two of the flames are attached to the burner, thermal boundary conditions at the burner surface are constructed from smoothed functional fits to temperature measurements. Experimentally, Raman scattering is used to measure temperature and major species concentrations as functions of the radial coordinate at various axial positions. As compared to the base case flame, which is lifted, the two oxygen-enhanced flames are shorter, hotter, and attached to the burner. Computational and experimental flame lengths show excellent agreement, as do the maximum centreline temperatures. For each flame, radial profiles of temperature and major species also show excellent agreement between computations and experiments, when plotted at fixed values of a dimensionless axial coordinate. Computational results indicate peak NO levels in the oxygen-enhanced flames to be very high. The majority of the NO in these flames is shown to be produced via the thermal route, whereas prompt NO dominates for the base case flame.     

Template matching for improved accuracy in molecular tagging velocimetry

In 2D molecular tagging velocimetry (MTV), tags are written into a fluid flow with a laser grid and imaged at discrete times. These images are analyzed to calculate Lagrangian displacement vectors, often by direct cross correlation. The cross correlation method is inherited from particle imaging velocimetry, where the correlated images contain a random pattern of particles. A template matching method is presented here which takes advantage of the known geometry of laser written tag grids in MTV to achieve better accuracy. Grid intersections are explicitly located in each image by correlation with a template with several linear and rotational degrees of freedom. The template is a continuous mathematical function, so the correlation may be optimized at arbitrary sub-pixel resolution. The template is smooth at the spatial scale of the image noise, so random error is substantially suppressed. Under typical experimental conditions at low imaging resolution, displacement uncertainty is reduced by a factor of 5 compared to the direct cross correlation method. Due to the rotational degrees of freedom, displacement uncertainty is insensitive to highly deformed grids, thus permitting longer delay times and increasing the relative accuracy and dynamic range of the measurement. In addition, measured rotational displacements yield velocity gradients which improve the fidelity of interpolated velocity maps.     

Decay of 1+ states as a new probe of the structure of 0+ shape isomers

The nuclides 98Mo and 100Mo have been studied in photon-scattering experiments by using bremsstrahlung produced from electron beams with kinetic energies from 3.2 to 3.8 MeV. Six electromagnetic dipole transitions in 98Mo and 19 in 100Mo were observed for the first time in the energy range from 2 to 4 MeV. A specific feature in the two nuclides is the de-excitation of one state with spin J = 1 to the 0+ ground state as well as to the first excited 0+ state, which cannot be explained in standard models. We present a model that allows us to deduce the mixing coefficients for the two 0+ shape-isomeric states from the experimental ratio of the transition strengths from the J = 1 state to the 0+ ground state and to the 0+ excited state.     

High precision measurements of theg J-factors of the alkalis using the atomic beam magnetic resonance method

Theg _J-factors of Li⁶, Li⁷, Na²³, Rb⁸⁵, Rb⁸⁷ and Cs¹³³ in their² S _1/2 ground states have been measured relative to theg _J-factor of K³⁹ with a precision of a few parts in 10⁷ using the atomic beam magnetic resonance method.     

Two-photon predissociative fluorescence of H2O by a KrF laser for concentration and temperature measurement in flames

Two-photon laser-induced predissociative fluorescence (LIPF) of H₂O is examined as a potential measurement technique of H₂O concentration and temperature in flames. Two-photons of 248 nm light from a narrowband KrF laser excite H₂O to the highly predissociative state in a hydrogen-air flame. The subsequent bound-free emission is observed from 400–500 nm in the flame at temperatures of 1000–2000 K and is found to be free of fluorescence interference from other flame species. This LIPF signal is not affected by collisional quenching due to the short lifetime of the predissociative state (2.5 ps). Broadband laser dispersion spectra, narrowband laser dispersion spectra, laser excitation spectra and probability density functions of the H₂O fluorescence are obtained in the hydrogen flame. The H₂O LIPF signal is found to be temperature sensitive and a two-line LIPF technique is needed for concentration and temperature measurement. The accuracy of a two-line LIPF technique for H₂O concentration and temperature measurement is determined.