Taichang Zhang

Conformation-Specific Pathways of β-Alanine: A Vacuum Ultraviolet Photoionization and Theoretical Study

We report a photoionization and dissociative photoionization study of beta-alanine using IR laser desorption combined with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Fragments at m/z = 45, 44, 43, and 30 yielded from photoionization are assigned to NH(3)CH(2)CH(2)(+), NH(2)CHCH(3)(+), NH(2)CHCH(2)(+), and NH(2)CH(2)(+), respectively. Some new conformation-specific dissociation channels and corresponding dissociation energies for the observed fragments are established and determined with the help of ab initio G3B3 calculations and measurements of photoionization efficiency (PIE) spectra. The theoretical values are in fair agreement with the experimental results. Three low-lying conformers of the beta-alanine cation, including two gauche conformers G1+, G2+ and one anti conformer A+ are investigated by G3B3 calculations. The conformer G1+ (intramolecular hydrogen bonding N-H...OC) is found to be another precursor in forming the NH(3)CH(2)CH(2)(+) ion, which is complementary to the previously reported formation pathway that only occurs with the conformer G2+ (intramolecular hydrogen bonding O-H...N). Species NH(2)CHCH(2)(+) may come from the contributions of G1+, G2+, and A+ via different dissociation pathways. The most abundant fragment ion, NH(2)CH(2)(+), is formed from a direct C-C bond cleavage. Intramolecular hydrogen transfer processes dominate most of the fragmentation pathways of the beta-alanine cation.

An Experimental and Theoretical Study of Pyrrole Pyrolysis with Tunable Synchrotron VUV Photoionization and Molecular-Beam Mass Spectrometry

The pyrolysis of pyrrole (6.46% pyrrole in argon) has been performed with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) technique. The experiment was carried out over the temperature range of 1260-1710 K at a pressure of 267 Pa. About 30 intermediates have been identified by near-threshold measurements of photoionization mass spectra, and the corresponding mole fractions versus temperatures have been obtained. Moreover, the isomers of some pyrolysis products have been identified by the measurement of photoionization efficiency spectrum. The major products are H(2), C(2)H(2), HCN, C(3)H(4) (propyne), and C(2)H(3)N (acetonitrile). Meanwhile, some new intermediates, such as C(4)H(4)N (cyanoallyl radical) and C(2)H(2)N (cyanomethyl radical), have been determined. The major pyrolysis channels have been provided with the high-level ab initio G3B3 calculation and are well consistent with the experimental observation. The formation pathway of HCN via the cyclic carbene tautomer has been proved to be the lowest formation pathway, which is in accordance with previous theoretical work. The potential pathways of the early formed C(4)H(4)N species together with their subsequent consumption to C(2)H(2)N and C(2)H(2) have been discussed in detail. Also, the formation pathways of the major products of C(2)H(3)N and C(2)H(2) have been investigated as well.

Blowout Limits of Cavity-Stabilized Flame of Supercritical Kerosene in Supersonic Combustors

Blowout limits of cavity-stabilized flame of supercritical kerosene were experimentally studied by using Mach 2.5 and 3.0 direct-connect supersonic model combustors operated under various air and fuel conditions. Specifically, the effects of the stagnation temperature and the stagnation pressure on the blowout limits were investigated for supercritical kerosene injected from the wall upstream of a cavity flameholder in a Mach 2.5 combustor. Experiments were performed under the same conditions for supercritical kerosene injected from the rear part of the cavity bottom to study the influence of the location of fuel injection. The blowout limits of supercritical kerosene injected from the wall upstream of the cavity were further investigated in a Mach 3.0 combustor. Besides the effects of the stagnation temperature and stagnation pressure, the effect of the divergence angle of the combustor on the lean-fuel blowout limit was studied. Results show that there exist two blowout limits corresponding to the lean- and rich-fuel conditions for a given stagnation temperature. The location of fuel injection has substantial influence on the blowout limits, whereas the influence of the stagnation pressure and the divergence angle of the combustor can be neglected in the range of interest.

Measurements of the Blowout Limits of Supercritical Aviation Kerosene in a Supersonic Combustor

Combustion and flame stability of supercritical kerosene were studied experimentally in a supersonic model combustor at various air and fuel conditions. The lean and rich blowout limits of supercritical kerosene injected from the bottom of a cavity flameholder were examined in Mach 2.5 airflow. It was found that the rich blowout limit was very sensitive to the stagnation temperature of the airflow while the lean limit was only slightly affected. Damkohler number was used to correlate the two limits. On the other hand, the lean blowout limits of supercritical kerosene injected upstream the cavity were investigated and compared at three different Mach numbers of 2.0, 2.5 and 3.0.

Characteristics of a Supersonic Model Combustor with Two-Staged Injections of Supercritical Kerosene

Staged fuel injection plays an important role in designing of a scramjet combustor with balanced performance of combustion efficiency, flame stability, engine unstart, heat release distribution and subsonic/supersonic combustion mode transition. In this paper, a Mach 2.5 model combustor with two-staged fuel injections has been tested in airflow of total temperatures of 1500 K and total pressures of 1.3 MP. Supercritical kerosene with temperature of about 760 K was injected through two integrated fuel injection/flame-holder cavity modules. The effects of fuel injection distribution (injector spacing and fuel ratio) on the combustion pressure rise, thrust increment, lean blowout limit, wall temperature distribution and engine unstart have been examined.

Blowout Limits of Supercritical Kerosene in Supersonic Combustors

Blowout limits of supercritical kerosene were experimentally studied using direct-connect supersonic model combustors at various air and fuel conditions. Effect of air stagnation temperature on the blowout limit of supercritical kerosene injected from the wall upstream of a cavity was firstly examined in Mach 2.5 airflows. Blowout limit of supercritical kerosene injected from another location, the rear part of the cavity bottom, was also investigated in Mach 2.5 airflows to study the influence of fuel injection locations. Since results demonstrate the injection from the wall upstream of the cavity is beneficial to a wider stable combustion range, the blowout limits of supercritical kerosene injected from the wall upstream of the cavity were furthermore investigated in Mach 3.0 airflows. Moreover, with fuel injected from the wall upstream of the cavity, effects of stagnation pressure on the lean-fuel blowout limit were investigated in wide stagnation pressure ranges at three air stagnation temperatures. Finally, effect of the diverging angle of the combustor on the lean-fuel blowout limit was studied.

Pulsed Combustion of Hydrocarbon Fuels in a Supersonic Model Combustor

Pulsed combustion of hydrocarbon fuels (ethylene and kerosene) in a Mach number 2.5 supersonic model combustor was experimentally investigated using a pulse spark igniter located in the center of a cavity. The results demonstrated that pure pulsed combustion of ethylene could exist at relatively low stagnant temperatures while pulse-induced stable combustion could be obtained at higher temperatures. Experiments also showed that no significant pulsed combustion of kerosene existed when ethylene was used as pilot flame because the combustion of pilot ethylene offers much more heat and radicals than that supplied by discharging of the spark plug. rights reserved.