Thomas A. Litzinger

Planar laser Rayleigh scattering for quantitative vapor-fuel imaging in a diesel jet

Abstract Quantitative images of vapor-phase fuel concentrations were obtained in an evaporating and combusting diesel jet using planar laser Rayleigh scattering. The diagnostic has been calibrated, evaluated, and successfully applied to an optically accessible direct-injection diesel engine for fired and nonfired operating conditions. The measurements were obtained in the leading portion of the diesel jet (the zone beyond 27 mm from the injector nozzle), where the fuel is entirely evaporated, and which corresponds to the main combustion zone in this engine. The technique was shown to be effective for quantitative imaging of the fuel-vapor concentration before ignition, with high spatial and temporal resolution. Additionally, images of the fuel-vapor concentration were further reduced to imagers of the equivalence ratio using an adiabatic mixing assumption to model the local temperature of the evaporating diesel jet. This procedure also yielded temperature distribution images. The results show that, at 4.5° crank angle (0.63 ms) after the start of injection, which corresponds to the time just before the first indicated heat release, the fuel and air are relatively well mixed in the leading portion of the diesel jet. At this crank angle, the equivalence ratio in the majority of the jet ranges from 2 to 4. The edges of the jet are well defined, with the signal level rising sharply from the background level up to levels corresponding to equivalence ratios in the jet. The temperature of the richest mixture regions in the jet is as low as 700 K, with the ambient air temperature at 1000 K. Finally, comparisons of Rayleigh images of the reacting and nonreacting jet show that the initial breakdown of the fuel, indicated by a significant decrease in the Rayleigh signal intensity, occurs throughout the cross section of the leading portion of the diesel jet.

Effects of oxygenated additives on aromatic species in fuel-rich, premixed ethane combustion: a modeling study

Abstract The motivation for the work described in this paper was conflicting results from diesel engine research on the question of whether the structure of an oxygenated compound blended into diesel fuel can affect the level of reduction of particulate emissions. A constant-pressure reactor model (SENKIN) was used to investigate the effect of oxygenated additives on aromatic species, which are known to be soot precursors, in fuel-rich ethane combustion. 5% oxygen by mass of the fuel was added to ethane using dimethyl ether (DME-CH3OCH3) and ethanol (C2H5OH). A significant reduction in aromatic species relative to pure ethane was observed with the addition of both DME and ethanol, but DME was more effective in reducing aromatic species than ethanol. One reason for the greater effectiveness of DME was found to be its higher enthalpy of formation, compared to ethanol, which led to a higher final temperature. However, with initial temperatures adjusted to achieve the same final temperature for all fuels, DME was still more effective than ethanol in reducing aromatic species compared to the base case of pure ethane. A reaction flux analysis was conducted to determine the mechanism of aromatic species reduction by the oxygenated compounds and the cause of the greater effectiveness of DME.

The oxidation of methane at elevated pressures: Experiments and modeling

Abstract A detailed chemical kinetic model has been developed for methane oxidation which is applicable over a wide range of operating conditions. A reaction mechanism, originally developed for high-temperature methane oxidation, was expanded and extended to include reactions pertinent to the lower temperature, elevated pressure conditions encountered in the flow reactor experiments performed in the study. The resulting 207-reaction, 40-species mechanism is capable of reproducing the experimental species concentrations for each of the cases studied. The concentration profiles of reactant, intermediate, and product species, including CH4, CH2O, CH3OH, H2, C2H6, C2H4, CO, and CO2, were obtained in the High Pressure Optically Accessible Flow Reactor (HiPOAcFR) facility for temperatures ranging from 930 to 1000 K and pressures of 6 and 10 atm. Based on the model, no appreciable change in reaction pathway was observed over the pressure and temperature range studied, with HO2 providing the major route for CH3 oxidation. CH2O was found to be a vital intermediate for all of the CH4 oxidation paths. In addition, inclusion of trace amounts of CH2O measured at the initial sampling location into the model initial conditions greatly reduced the predicted time to onset of fuel disappearance and enhanced the model agreement. This result is consistent with past engine studies which have found that CH2O is a significant pro-knock additive when added to a methane base fuel. The expanded reaction mechanism was also tested against shock-tube ignition delay and laminar flame speed data and was found to be in good agreement with the relevant experimental data.

Simultaneous temperature and species measurements of the glycidyl azide polymer (GAP) propellant during laser-induced decomposition

Abstract Simultaneous temperature and species measurements were performed to experimentally investigate thermal decomposition of cured Glycidyl Azide Polymer (GAP). Experiments were conducted at atmospheric pressure in argon with heat fluxes of 50, 100, and 200 W/cm 2 delivered by CO 2 laser. A micro-probe/triple quadrupole mass spectrometer system was used to analyze species products, and thermocouples were used to measure temperature. The burning behavior was monitored by a high-magnification video system. The following major species, in descending order of abundance, were detected: N 2 , HCN, CO, CH 2 O, NH 3 , CH 3 CHO, CH 2 CHCHNH, CH 3 CHNH, H 2 O, CH 4 , and C 2 H 4 . The mole fraction of hydrocarbons was about 0.02, while the mole fraction of imines was 0.09. The decomposition of GAP appeared to be dominated by the condensed phase chemistry and few reactions occurred in the gas phase. A significant amount of fine solid powder was observed in the gas phase, which was believed to be imines. It was found that the species and temperature were insensitive to the heat flux level. The mole fractions of the observed species at a heat flux 100 W/cm 2 were almost the same as those at a heat flux of 200 W/cm 2 , and the surface temperature was approximately 1050 K at both heat fluxes.

A Report on a Four-Year Longitudinal Study of Intellectual Development of Engineering Undergraduates

As part of an investigation into the effects of curricular reforms in the undergraduate program in the College of Engineering, a series of Perry-style interviews were conducted over a 4-year period. The study was undertaken in an attempt to assess the impact of collaborative design-based engineering courses that were being implemented. Students who completed the collaborative design course in their first year were higher on the Perry scale than their peers who did not. However, this effect was not sustained through the rest of the curriculum, as more traditional courses dominated. No statistically significant change in Perry position was observed for students in their 1st and 3rd years; however, a growth of approximately one Perry position was observed between the 3rd and 4th years. The relationship of current curricula to this pattern of intellectual development is discussed, and arguments for altering the curriculum to support intellectual development are made.

Chemical kinetic study of HAN decomposition

Abstract A numerical analysis was conducted to deduce the Arrhenius-type reaction rates of a reduced reaction model for HAN. The reaction rates were obtained by an inverse-based analysis in a way that minimized an objective function, which consisted of the difference between the calculated concentrations from the numerical model and experimental data as well as the uncertainties in the experimental data. The experimental decomposition process was modeled by applying the species conservation equations to the condensed-phase and the gas-phase regions separately. The experimental data that were used to deduce the rate coefficient parameters were the gas-phase species concentrations observed during thermal decomposition of 13M HAN including HNO 3 , N 2 O, NO, and NO 2 . The energy equation was not considered in the numerical model since measured condensed-phase reaction temperatures were used as input data. The uncertainties in the deduced reaction rates were calculated using the standard deviations of the experimental data. With the best-fit rate constants of the reaction model, the species evolution profiles of 13M HAN were reasonably recovered, and the condensed-phase mass fractions were predicted. The reaction model was applied to the simulation of the species evolutions for solid HAN and HAN-water solutions including 10.7 and 9M HAN. The simulated gas-phase concentrations coincided well with the experimental data, indicating that the proposed global reaction mechanism captured most of the key reactions of HAN.

A study of the gas-phase chemical structure during CO2 laser assisted combustion of HMX

Abstract To study the chemical structure of the HMX flame, species and temperature profiles were measured in the gas phase at heat fluxes of 100 and 300 W/cm2. A microprobe/triple quadrupole mass spectrometer was used to measure quantitative species profiles, and fine wire thermocouples were used to measure temperature profiles. The flame and surface structures were observed using a high-magnification video system. The major species at the surface were H2O, CH2O, HCN, NO2, N2O, N2, CO, and NO at atmospheric pressure with both heat fluxes. There was no CO2 existing at the surface. The mole fraction of triazine was found to be approximately 2.5% at the surface, which has not been reported during combustion of HMX. The species could play an important role in the gas phase chemistry. The species profiles showed two-stage reaction zones. The species profiles also showed that increasing heat flux stretched the secondary reaction zone, but did not stretch the primary reaction zone. No plateau at a typical dark zone temperature was observed for either heat flux. Finally, the temperature profiles in the gas phase and species concentrations obtained at the surface with heat fluxes of 100 and 300 W/cm2 were used as inputs to a 1-D gas-phase flame model. Disagreement between the model and experiments for stable species was observed and investigated in detail.

A study of the chemical and physical processes governing CO2 laser-induced pyrolysis and combustion of RDX

Abstract The flame structure of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) propellants under laser-assisted combustion was studied to better understand related chemical and physical processes in the gas phase. Experiments were conducted from 0.1 to 3 ATM in pressure with heat fluxes of 50 to 600 W/cm 2 . Gaseous products were extracted through the use of quartz microprobes and analyzed by a triple quadrupole mass spectrometer (TQMS). Temperature profiles were measured using micro-thermocouple techniques to investigate reaction zones in RDX flames. Flame behavior was observed using a high-magnification video system. Major species in RDX flames were identified as H 2 , H 2 O, HCN, H 2 CO, NO, HNCO, N 2 O, and NO 2 at low masses ( m / z ≤ 47). In addition to these species, H 2 CNH with m / z = 29 was found to exist in the near-surface reaction zone as an important minor species. Higher molecular weight species were found at m / z values of 47, 54, 56, 70, 81, and 97; with the daughter mode operation of TQMS, they were identified as HONO, C 2 H 2 N 2 , C 2 H 4 N 2 , C 2 H 2 N 2 O, C 3 H 3 N 3 , and C 3 H 3 N 3 O, respectively. Increasing heat flux and decreasing pressure stretched out the reaction zones and were useful for investigating reactions near the deflagrating surface. However, the conditions appeared to have no effect on major reaction pathways. Two-stage chemical reaction pathways in the gas phase were explicitly identified from the major species profiles at all experimental conditions. Also, the reactions of minor high-mass species occurred in the primary reaction zone. The decomposition of RDX at the surface showed evidence of the two competing branch reactions into H 2 CO + N 2 O and HCN + HONO, as well as two subsequent reactions: H 2 CO + N 2 O → H 2 O + CO + N 2 and 2HONO → H 2 O + NO + NO 2 . With the consideration of the previous four reactions, the branching ratio for the two decomposition pathways of RDX was estimated to be about 2:1. For all experimental conditions, temperature profiles had a near-surface region where temperature increased very slowly; the extent of this zone increased as the near-surface reaction zones expanded. After this region, the temperature profiles increased to final flame temperatures without any dark zone temperature plateau. Based on comparisons of species and temperature profiles, this near-surface region is believed to be related to the consumption of NO 2 , production of NO and H 2 O, and production and consumption of high-mass species.

Oxidation of Propane at Elevated Pressures: Experiments and Modelling

Abstract The oxidation of propane in air at elevated pressure was investigated in a chemical flow reactor and modelled with a comprehensive chemical kinetic model. Results are presented for pressures of 3.6. and 10 atmospheres, temperatures near 850 and 900 K, and equivalence ratio of 0.3. Gas samples were analyzed using gas chromatography with aldehydes additionally sampled using a dinitrophenylhydrazine/acetoni-trile(DNPH/ACN) procedure. Major product species observed include C3H6, C2H5. and CO: trace amounts of CH4 and C02 were detected, as well as H2 and oxygenated species including CH2O, CH3CHO, C3H60, and C2H5CHO, Fuel conversion was increased with increased pressure and temperature, and the product distribution was significantly shifted in favor of C3H6 over C2H4 with increased pressure and decreased temperature. Comparison between modelling and measured results for ethylene concentrations supported the use of Tsangs recent values for the rate of propyl radical decomposition. The model compared we...

A review of experimental studies of knock chemistry in engines

Abstract Early engine research related to the chemistry of knock produced results which complement and support modern work and, in many cases, provide intriguing data against which modern concepts and models of knock can be compared. This article presents a chronological summary of research in motored and fired engines conducted to understand the chemical processes associated with knock. Further, it attempts to highlight the superior and intriguing aspects of these works. Four basic research areas are covered: fuel effects, including fuel structure, additives and pre-oxidation, pre-combustion heat release, spectroscopic investigations and rapid sampling valve research.