Rotational Raman scattering from premixed and diffusion flames

A combination of stimulated Raman pumping and rotational Raman spectroscopy is used to accomplish the first measurement of the polarizability anisotropy γ11,13 (355 nm) for the S11 (1) transition in molecular hydrogen H2. Saturation of the Q01(1) transition connecting the |X1 Σ+ g, v = 0, J = 1 > state to the |X1 Σ+ g, v = 1, J = 1 > state in H2 by stimulated Raman pumping is the critical element in this experiment. The observed intensities of the rotational Raman lines for these states allow an estimate of γ11,13 (355 nm) as 0.358 ± 0.004 Å3. A comparison of this value to that obtained from fundamental ab initio calculations in H2 also is possible for the first time.

Detailed kinetic modeling of C1 — C3 alkane diffusion flames

A computational study of oscillatory extinction of spherical diffusion flames

The rotational spectrum of 3-furonitrile has been collected from 85 to 500 GHz, spanning the most intense rotational transitions observable at room temperature. The large dipole moment imparted by the nitrile substituent confers substantial intensity to the rotational spectrum, enabling the observation of over 5600 new rotational transitions. Combined with previously published transitions, the available data set was least-squares fit to partial-octic, distorted-rotor A- and S-reduced Hamiltonian models with low statistical uncertainty (σfit < 0.031 MHz) for the ground vibrational state. Similar to its isomer 2-furonitrile, the two lowest-energy vibrationally excited states of 3-furonitrile (ν17, ν24), which correspond to the in-plane and out-of-plane nitrile bending vibrations, form an a- and b-axis Coriolis-coupled dyad. Rotationally resolved infrared transitions (30-600 cm-1) and over 4200 pure rotational transitions for both ν17 and ν24 were fit to a partial-octic, Coriolis-coupled, two-state Hamiltonian with low statistical uncertainty (σfit rot < 0.045 MHz, σfit IR < 6.1 MHz). The least-squares fitting of these vibrationally excited states provides their accurate and precise vibrational frequencies (ν17 = 168.193 164 8 (67) cm-1 and ν24 = 169.635 831 5 (77) cm-1) and seven Coriolis-coupling terms (Ga, GaJ, GaK, Fbc, FbcK, Gb, and Fac). The two fundamental states exhibit a notably small energy gap (1.442 667 (10) cm-1) and an inversion of the relative energies of ν17 and ν24 compared to those of the isomer 2-furonitrile. The rotational frequencies and spectroscopic constants of 3-furonitrile that we present herein provide a sufficient basis for conducting radioastronomical searches for this molecule across the majority of the frequency range available to current radiotelescopes.

This article studies the generation of entropy disturbances by laminar premixed flames. The total entropy generation equals the integrated ratio of the local heat release rate and the local temperature, namely, [Formula: see text]. Due to this path dependency, evaluating this integral requires an understanding of how the heat release is distributed in the temperature space. Several studies evaluate the local entropy generation as [Formula: see text], where [Formula: see text] refers to the burned gas temperature, implicitly assuming all the heat release occurs at [Formula: see text]. Such an approximation is motivated by the high activation energy nature of combustion chemistry. This work evaluates this assumption by comparing it to results from one-dimensional premixed flame calculations for hydrogen, methane, and propane-air flames over a range of pressures, equivalence ratios, and preheat temperatures, quantified via the ratio [Formula: see text]. We show that this assumption is quite reasonable for methane and propane-air flames (with errors ranging from 5% to 25%) but deviates significantly from the exact results for hydrogen flames (where errors can be as high as 50%). In general, the peak heat release moves to lower temperatures as preheat temperature is increased. Noting that the temperature sensitivity of heat release is directly related to the activation energy, we use Law's approach to extract global activation energies and show that the deviations of [Formula: see text] from unity can be approximately correlated with [Formula: see text]. Finally, we show that significant improvements in entropy generation calculations can be obtained by estimating [Formula: see text] using [Formula: see text], where [Formula: see text] is the temperature at which the reaction rate peaks. This estimation leads to predictions of ∼5% within the exact value for the hydrocarbon cases but can still be in significant error for hydrogen at certain conditions.

Nonresonant two-photon Raman transitions with $\mathrm{\ensuremath{\Delta}}J=\ifmmode\pm\else\textpm\fi{}2$ selection rules induce molecular alignment by altering the rotational quantum number $J$, whereas three-photon Raman transitions with $\mathrm{\ensuremath{\Delta}}J=\ifmmode\pm\else\textpm\fi{}1$ cause molecular orientation. Here, we perform a combined numerical and theoretical investigation to explore how these Raman transitions depend on laser intensity and to discern multiorder Raman transitions in laser-induced alignment and orientation going beyond two- and three-photon processes. Our results demonstrate the presence of higher-order transitions in molecular alignment and shed light on how alignment affects subsequent three-photon rotational Raman transitions as revealed by the alignment and orientation spectra and the Dyson series expansion of the unitary operator. As the laser pulse strength increases, multiorder transitions become directly observable, which can be utilized to enhance the alignment and orientation of molecules through the excitation of multiple rotational states.

Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

The increasing global demand for energy, the need to reduce green house gasses, and the depletion of fossil fuel resources have led for the need for renewable fuel sources such as biodiesel fuels. In the diesel engines, biodiesel fuels can also be used directly without comprehensive engine changes. Biodiesel relates to a diesel fuel that is based on vegetable oil or animal fat consisting of longchain of methyl, ethyl, or propyl esters. Methyl ester fuel burns more efficiently and has lower emissions of particulate matter, unburnt hydrocarbon, and carbon monoxide than fossil fuels. However, combustion of methyl ester fuel results in increased nitrogen oxides (NOx) emissions relative to fossil fuels. This study is concerned with characterizing the formation of NOx in the combustion of methyl formate under a counter diffusion flame. This was carried out in an Exhaust Gas Recirculation (EGR) system. Simulation of the process was done using Combustion Simulation Laboratory Software (COSILAB), and involved simulating the reactions of methyl formate fuel. The results obtained were compared to those of the methane/air diffusion flame, which is a well-characterized system. The extension validated the results obtained for the methyl formate/air diffusion flame. The reduction of NOx was found to be 26% and 14% in methane and methyl formate diffusion flame respectively from 0% to 29.5% of EGR. Increased EGR from 0% to 29.5% increased NOx reduction. Compared to methane/air diffusion flame, methyl formate/air diffusion flame with and without EGR had lower NOx emission. This was found to be true when examining the amount of other metrics viz. temperature, H, OH and N radicals associated with NOx. This showed that EGR system have an effect on NOx formation.

Single-laser-shot femtosecond rotational coherent anti-Stokes Raman scattering (fs-RCARS) temperature measurements are performed across a 6-mm line in a turbulent, sooting ethylene jet flame to characterize temperature gradients. A 60- fs pulse is used to excite many rotational Raman transitions in N 2 , and a 160-ps pulse is used to probe the Raman coherence. The spatial resolution of the measurements is 500 μm in the direction of beam propagation and 50 μm in the transverse directions. Measurements have been performed at multiple locations in the jet flame, and the measured temperature are similar to previously recorded point measurements. Future work will include performing simultaneous laser-induced incandescence (LII) measurements to measure soot volume fraction to perform joint statistical analysis of the sooting turbulent flame.

The relative intensities of O and S branch lines in the v=1←v=0 Stokes Raman spectrum of gaseous H2, D2, N2, O2, and CO have been measured in order to obtain a parameter χ= (α∥−α⊥)e/{re[∂ (α∥−α⊥)/∂r]e}. Values for χ reflect the detailed molecular electronic structure and are needed in various scattering theories and to correct rotational and vibrational–rotational Raman intensities for molecular nonrigidity and vibrational-rotational interaction. The values of χ obtained are +0.38±0.01 (H2), +0.38±0.01 (D2), +0.45±0.09 (N2), +0.23±0.07 (O2), and +0.27±0.13 (CO). By combining the measured values of χ with experimental values for 〈α∥−α⊥〉v=0 in the literature, the following values of the polarizability anisotropy and its first derivative at re are obtained: (α∥−α⊥)e=0.288 (H2), 0.282 (D2), 0.691 (N2), 1.080 (O2), and 0.525 (CO) Å3; [∂ (α∥−α⊥)/∂r]e=1.02 (H2), 1.00 (D2), 1.40 (N2) 3.89 (O2), and 1.72 (CO) Å2. These values are compared with literature theoretical values. Molecular polarizability is an experimentally accessible quantity which reflects in a rather direct way the electronic structure of simple molecules. For this reason the orientational anisotropy of molecular polarizability and its variation with internuclear distance are of fundamental interest in developing understanding of the detailed electronic structure of diatomic molecules. Polarizability anistropies and their derivatives are directly related to both absolute and relative intensities of vibrational and rotational Raman transitions.

The products of the ClO self-reaction have been studied in a flowing chemical reactor using submillimeter wave spectroscopy. The complete spectrum between 415 to 435 GHz has been measured as well as selected transitions in the range 285 to 415 GHz. The major products have been identified as the ClO dimer (Cl2O2) and chlorine dioxide (OClO). The observed rotational b-type spectra of the most abundant isotopic species35 ClOO35Cl and 37ClOO35Cl have been analyzed. The observed nuclear spin statistics for the main species, the relative abundance of the lesser species, and the structure determination demonstrate unambigiously that the ClO dimer must possess identical chlorine atoms in a peroxide structure. The rotational constants as well as a complete set of quartic centrifugal distortion constants have been determined. Structural parameters for the vibronic ground state have been calculated: rOO=142.59(21) pm, rClO=170.44(4) pm, ∠ClOO=110.07(1)° and dihedral angle=81.03(1)°. Rotational transitions in the first excited torsional state have been measured and analyzed for the isotopic species 35,35 and 35,37. An effective molecular structure for the first torsional excited state has been calculated. A torsional frequency has been derived from relative intensity measurements and the cis and the trans barrier heights estimated.

Extinction of near-stoichiometric flames diluted with nitrogen in a stagnation flow

The stability of a diffusion flame burning in products of combustion

THE hydroperoxyl radical has been postulated as an intermediate in numerous oxidation reaction mechanisms. In particular, mechanisms involving it are capable of giving detailed explanations of the explosion limits and reaction-rates in hydrogen–oxygen systems in static reaction vessels1,2. In discussing hydrogen-oxygen and hydrogen–air flames, however, reactions of HO2 are usually omitted, on the assumption that most of the flame reaction occurs at too high a temperature for the HO2 forming reaction (2) to compete with the chain-branching reaction (1). We have recently obtained evidence, arising from a detailed investigation of a slow-burning hydrogen–oxygen–nitrogen flame and from calculations involving two faster (higher temperature) flames, that this assumption may be invalid in many cases.