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A simple heuristic model was developed for estimating the entropy of a solute molecule in an ideal solution based on quantum mechanical calculations with polarizable continuum models (QM/PCMs).

This study highlights the importance of heat release rate in low-temperature oxidation (LTO) on non-uniform end-gas autoignition and strong pressure wave generation, which are substantially relevant to knocking combustion. The simulations are conducted using the compressible Navier–Stokes equations with detailed transport and chemical kinetics models in a one-dimensional constant-volume reactor. Four fuel/air stoichiometric mixtures, n-butane, i-octane, n-heptane, and dimethyl ether (DME)/air mixtures, are simulated. The results show that larger knocking intensities are produced with n-heptane and DME in their negative temperature coefficient (NTC) regimes because of the stronger non-uniformity of end-gas autoignition. The non-uniformity of end-gas autoignition is enhanced by a pressure wave disturbance that is caused by the rapid temperature rise of the end-gas region in LTO. In particular, the high heat release rate with the DME/air mixture generates a distinct pressure wave disturbance in the reactor, which considerably enhances the non-uniformity of end-gas autoignition through the reflection of the wave at the wall. In contrast, the heat release rate in the n-heptane case is milder than that in the DME case, and therefore, the knocking intensity in the n-heptane case is smaller compared to that of DME due to less enhancement of the non-uniform end-gas autoignition. No large knocking intensities are produced with n-butane and i-octane, which have weak NTC, because of the absence of a temperature rise in LTO. Thus, this study concludes that the high heat release rate in LTO and the generated pressure wave disturbance play a significant role in the generation of large knocking intensities through the enhancement of non-uniform end-gas autoignition.

The present study addresses effects of pressure wave disturbance on end-gas autoignition behavior and subsequent pressure wave development, which are of particular interest for understanding a fundamental process of knocking combustion in spark-assisted ignition engines. Such pressure wave disturbance is initiated by a compression wave generated from spark-like ignition and continuously exists in a combustion chamber. The investigation is conducted through a direct numerical simulation with a 1-D constant volume reactor, where the compressible Navier–Stokes equations are solved with detailed reaction mechanisms of n − C 7 H 16 and n − C 4 H 10 /air mixtures. The width of an ignition kernel parametrically changes in order to control the strength of an initial compression wave and thus pressure wave disturbance. The result showed that stronger compression waves with larger kernel widths enhance the progress of the autoignition process at the wall through the transient increase in pressure and temperature due to the wave reflection. Consequently, stronger pressure waves such as a developing detonation are generated with the enhanced inhomogeneous end-gas autoignition starting from the wall. In contrast, weak pressure wave disturbance produces no strong pressure waves due to the lower inhomogeneity of autoignition timing in an end-gas region. An analysis with temperature variation in an end-gas region indicates that cool flame generation helps maintain the superiority of the wall for earlier autoignition through the heat release at an earlier stage. Therefore, a stronger pressure wave associated with inhomogeneous end-gas autoignition is likely generated around 650 K in the case of an n − C 7 H 16 /air mixture, which corresponds to a distinct peak generation of knocking intensity in the negative temperature coefficient regime. For lower temperature conditions such as 500 K, the end-gas autoignition first takes place in a region away from the wall. This is because the initially induced temperature inhomogeneity in the end-gas region is considerably weakened due to wave interactions and dissipation effects during longer ignition delay times. Thus, multiple end-gas points including the wall can become a preferred point for earlier end-gas autoignition, resulting in a variation of end-gas autoignition locations. The result for an n − C 4 H 10 /air mixture is finally shown and the difference from that for the n − C 7 H 16 /air mixture is highlighted.

A simple model based on a quantum chemical approach with polarizable continuum models (PCMs) to provide reasonable translational and rotational entropies for liquid phase molecules was developed. A translational term was evaluated with free-volume compensation for the Sackur–Tetrode equation. We assumed that the free-volume corresponds to the cavity volume in the PCM. A rotational term was modeled as restricted rotation of a dipole in the electrostatic field. Entropies were assessed for twenty species in the liquid-phase using the proposed model, and the computed values were compared with experimental values. Quantum chemistry calculations were conducted at the ωB97X-D/6-311++G(d,p) level with the conductor-like PCM method. Predicted entropies were in good agreement with the experimental entropies, and the root mean square deviation was 17.2 J mol−1 K−1. The standard enthalpy change of formation was then investigated for eleven specific species. The CBS-QB3//ωB97X-D method provides a reasonable standard enthalpy of formation for gasified species; however, improvement of the accuracy is required for liquid species. Finally, the dependence of the Gibbs energy on temperature was investigated for the eleven specific species. When the ideal gas treatment is used, the Gibbs energy trends for the gaseous and liquid phases are quasi-parallel for all of the species, although the Gibbs energy trends for liquids based on the proposed model intersected the gaseous trend (i.e. the intersection is the boiling point). However, the model significantly under or overestimated the experimental boiling points. The error of the boiling points was predominantly due to the inaccuracy of the enthalpy.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2017-11-03, 資料番号: PA1810069000

The present study deals with the mechanisms for the hot-spot formation and pressure wave development associated with end-gas autoignition during knocking combustion of n -heptane/air mixture. The discussion is based on a one-dimensional (1-D) direct numerical simulation, where the compressible Navier–Stokes equations are solved with a detailed chemical kinetic mechanism of n -heptane, involving 373 species and 1071 reactions. The result demonstrates that the first trigger for a hot-spot formation is a compression wave generated by forced autoignition of a hot kernel and its reflection at a wall. The wall reflection of the propagating compression wave periodically produces an instantaneous temperature increase, which leads to the production of a larger amount of chemical species compared to that of other end-gas points. This non-uniform progress of chemical reaction process continues to exist at the wall, although the temperature increase is transient, resulting in faster autoignition and pressure wave generation at the wall. Thus, an important aspect of observing chemistry behaviors rather than temperature is demonstrated on the mechanism of hot-spot formation. The present study further addresses the reactivity of hot-spots on the relevance to pressure wave developments, wherein a zero-dimensional (0-D) ignition problem with pulsed compression waves is introduced. The higher reactivity of n -heptane/air mixture against pulse waves is observed with the faster ignition delay times in lower and higher initial temperature conditions. Conversely, the result at initial temperatures of 750—800 K indicates the lower reactivity with no significant effects of pulse waves on the ignition delay times. This is connected with the fuel characteristics of a negative temperature coefficient. Thus, in the 1-D simulations, a hot-spot with the high reactivity enhances spatial temperature difference in the end-gas region, leading to strong pressure wave generations. In contrast, a hot-spot with the low reactivity suppresses the pressure wave development with little spatial variation in temperature. The result demonstrates a significant aspect of hot-spot formation and reactivity on pressure wave development during knocking combustion.

Accepted: 2016-07-11, 資料番号: SA1160313000

This work analyzed the thermal decomposition of ammonium nitrate (AN) in the liquid phase, using computations based on quantum mechanics to confirm the identity of the products observed in past experimental studies. During these ab initio calculations, the CBS-QB3//ωB97XD/6–311++G(d,p) method was employed. It was found that one of the most reasonable reaction pathways is HNO3 + NH4+ → NH3NO2+ + H2O followed by NH3NO2+ + NO3– → NH2NO2 + HNO3. In the case in which HNO3 accumulates in the molten AN, alternate reactions producing NH2NO2 are HNO3 + HNO3 → N2O5 + H2O and subsequently N2O5 + NH4+ → NH2NO2 + H2O. In both scenarios, HNO3 plays the role of a catalyst and the overall reaction can be written as NH4+ + NO3– (AN) → NH2NO2 + H2O. Although the unimolecular decomposition of NH2NO2 is thermodynamically unfavorable, water and bases both promote the decomposition of this molecule to N2O and H2O. Thus AN thermal decomposition in the liquid phase can be summarized as NH4+ + NO3– (AN) → N2O + 2H2O.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2016-06-14, 資料番号: PA1710033000

Ammonia is a promising energy carrier and carbon-free fuel for power generation using combined-cycle gas turbines. However, its use results in the generation of relatively large amounts of NOx in the combustor. To address this issue, we propose a combined-cycle configuration including exhaust gas recirculation (EGR), in which the gas turbine is operated under fuel-rich conditions and the uncombusted hydrogen is burned in the heat-recovery steam generator (HRSG). Thus, hydrogen in the flue gas of the gas turbine increases the output power and improves the thermal efficiency of the system. Furthermore, in the combined system with EGR, the exhaust gas does not contain O2 and the combustion temperature can be reduced without altering the equivalence ratio. The proposed system is evaluated by thermodynamic modeling, and we find that low NOx emissions can be achieved while maintaining high thermal efficiency. Cold EGR is likely to be required to maintain the turbine inlet temperature below a technically feasible level, and a tradeoff between thermal efficiency and the NOx concentration at the combustor outlet is observed. The ideal operating conditions for this process thus depend on the technically feasible turbine inlet temperature, EGR ratio, and the permissible NOx concentration in the exhaust gas.

The reactions of NO2 with cis-/trans-CH3NHNH, CH3NNH2 and CH2NHNH2 have been studied theoretically by quantum chemical calculations and steady-state unimolecular master equation analysis based on RRKM theory. The barrier heights for the roaming transition states between nitro (RNO2) and nitrite (RONO) isomerization reactions and those for the concerted HONO and HNO2 elimination reactions from RNO2 and RONO, affect the pressure dependences of the product-specific rate coefficients. At ambient temperature and pressure, the dominant product of the reactions of NO2 with cis-/trans-CH3NHNH and CH2NHNH2 would be expected to be HONO with trans-CH3NNH and CH2NNH2, respectively, whereas it is CH3N(NH2)NO2 for CH3NNH2 + NO2. The product-specific rate coefficients for the titled and related reactions on the same potential energy surfaces were proposed for kinetics modeling., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1610065000

A numerical simulation of the spontaneous ignition of high-pressure hydrogen in a duct with two obstacles on the walls is conducted to explore the spontaneous ignition mechanisms. Two-dimensional rectangular ducts are adopted, and the Navier–Stokes equations with a detailed chemical kinetic mechanism are solved by using direct numerical simulations. In this study, we focus on the effects of the initial pressure of hydrogen and the position of the obstacles on the ignition mechanisms. Our results demonstrate that the presence of obstacles significantly changes the spontaneous ignition mechanisms producing three distinct ignition mechanisms. In addition, the position of the obstacles drastically changes the interaction of shock waves with the contact surface, and spontaneous ignition may take place at a relatively low pressure in some obstacle positions, which is attributed to the propagation direction and interaction timing of two reflected shock waves., Accepted: 2015-01-17, 資料番号: PA1510061000

The electrostatic interactions between gas generating agent and organic electron withdrawing additives were studied to understand how the interaction affects the reactions of their thermal behavior. To explore the electronic interaction and its effect, the thermal behavior of a mixture of dinitrobenzene isomers [p-dinitrobenzene (pDNB) and m-dinitrobenzene (mDNB)] and 1, 2,4-triazole (TA) was analyzed. Density functional calculations and electron density analysis proved that both pDNB and mDNB in a 1:1 ratio withdraw electrons from TA. The analysis of pyrolysis behavior showed that TA and dinitrobenzene mixtures (TA/pDNB or TA/mDNB) react at lower temperatures than each precursor. Gas chromatography mass analysis and UV/Vis spectroscopy showed reasonable results that suggested that pDNB and TA make condensed products in the initial stage of the reaction according to the differential scanning calorimetry measurement. When the mixtures were ignited under pressurized conditions, TA/pDNB burned easily while TA/mDNB burned only partially. These results suggested that the interaction between triazole and dinitrobenzene affects the pyrolysis and combustion behavior of a TA/dinitrobenzene mixture.

Hydrazine (N2H4)/nitrogen tetroxide (NTO) co-flowing jets were simulated to explore the hypergolic ignition processes in N2H4/NTO bipropellant thrusters. The Navier−Stokes equations with a detailed chemical kinetics mechanism were solved by direct numerical simulation to reveal the influence of the distinct chemical reaction; i.e., hydrogen abstraction by nitrogen dioxide (NO2). In the ignition processes, the hydrogen abstraction sequence, especially the reaction of N2H3 and NO2, played a significant role in preheating the mixture gases and it was significant in the NTO-rich conditions rather than in the specifically stoichiometric conditions. Thus, in the N2H4/NTO co-flowing jets, the preheated regions correspond to the regions where the NTO-rich flows existed and contacted the N2H4 jet. The ignition eventually occurred in the region where NTO has sufficient concentration. Hence, it was found that the ignition timing and position strongly depend on whether the unsteady behavior of the NTO jet provides appropriate environments for the hydrogen abstraction reactions., 資料番号: PA1510040000

Hydrazine (N2H4)/nitrogen tetroxide (NTO) co-flowing plane jets were simulated to explore the hypergolic ignition processes and flame structures in N2H4/NTO bipropellant thrusters. The Navier–Stokes equations with a detailed chemical kinetics mechanism were solved in a manner of direct numerical simulation to reveal the influence of the distinct chemical reaction, i.e., hydrogen abstraction by nitrogen dioxide (NO2) and the thermal decomposition of N2H4. In the ignition processes, the hydrogen abstraction sequence played a significant role in preheating the mixture gases. Further, the ignition eventually occurred in the region where both N2H4 and NTO were sufficiently supplied for preheating. Hence, the ignition position and delay strongly depended on the fluid-mixing conditions. After the flames reached a steady state, the combustion flames uniquely comprised two types of flames, the outer diffusion flame and the inner decomposition flame. The outer diffusion flame came from the oxidization by NTO. The inner decomposition flame was caused and maintained by the heat transfer from the outer diffusion flame and a high rate of heat release from the thermal decomposition of N2H4. Because of the decomposition flame, the decomposition products such as NH3 and H2 were the major constituents of the downstream combustion gases., 資料番号: PA1510062000

A robust explicit time integration method for stiff chemical reaction equations is proposed and applied to zero-dimensional ignition and one-dimensional combustion flow problems. The proposed method based on a multi-time scale method significantly improves the robustness of the original method by introducing two new strategies: automatic adjustment of time step size for each characteristic group using a quasi-steady-state assumption and automatic reset of base time step size using two appropriate criteria. The results for several zero-dimensional ignition problems demonstrate the robustness and accuracy of the proposed method compared to existing explicit and implicit integration methods. The present study also provides a computational cost estimation of various terms in the governing equations using a one-dimensional combustion problem (knocking simulation), where the Navier−Stokes equations are coupled with the chemical reaction equations. As well as the zero-dimensional problems, the robustness and capability of the proposed method are demonstrated. While the proposed method alleviates the occupancy of chemical reaction part in the total computational cost compared to an implicit time integration method, it is found that the transport properties calculations relatively increase with considerable amounts, suggesting efficient modeling of transport properties calculations for multi-dimensional combustion problems., 資料番号: PA1510039000

This work examined the reaction pathways involved in the initial decomposition of aqueous hydroxylamine solutions via the overall reaction, 2NH2OH → NH3 + HNO + H2O, using quantum chemistry calculations incorporating solvent effects. Several possible decomposition mechanisms were identified and investigated: three neutral–neutral bimolecular, two water-catalyzed, one neutral trimolecular, two ion-neutral bimolecular, and one cation-catalyzed. Optimized structures for the reactants, products, and transition states were obtained at the ωB97XD/6-311++G(d,p)/SCRF = (solvent = water) level of theory, and the total electron energies of such structures were calculated at the CBS-QB3 level of theory. The cation-catalyzed reaction 2NH2OH + NH3OH+ → NH4+ + HNO + H2O + NH2OH (maximum energy barrier (ΔE0‡) = 53.6 kJ/mol) and the anion–neutral bimolecular reaction NH2OH + NH2O– → NH3 + 1NO– + H2O (ΔE0‡ = 79.0 kJ/mol) were both found to be plausible candidates for the dominant step in the initial decomposition. The res...

The kinetics and mechanisms of H atom abstraction reactions from CH3NHNH2 by NO2 (R1) and related reactions have been investigated theoretically by using ωB97X-D and CCSD(T)-F12 quantum chemical calculations and the steady-state unimolecular master equation analysis based on Rice–Ramsperger–Kassel–Marcus (RRKM) theory. For reaction (R1), both dissociation and isomerization steps between intermediate complexes were found to be important for the distribution of the dissociated bimolecular products. The dominant products of (R1) were found to be cis-CH3NHNH and HONO at lower temperature. The branching ratios for CH3NNH2 formation paths increased with increasing temperature. On the same reaction potential energy surface, six reactions including isomerization reactions between CH3NNH2 and cis-/trans-CH3NHNH catalyzed by HONO were suggested to compete with the reverse reaction of (R1). The temperature- and pressure-dependent rate expressions are proposed for kinetic modeling.

A two-dimensional (2-D) simulation of spontaneous ignition of high-pressure hydrogen in a length of duct is conducted to explore ignition mechanisms. The present study adopts a 2-D rectangular duct and focuses on effects of the initial diaphragm shape on spontaneous ignition. The Navier–Stokes equations with a detailed chemical kinetics mechanism are solved in a manner of direct numerical simulation. The detailed mechanisms of spontaneous ignitions are discussed for each initial diaphragm shape. For a straight diaphragm, ignition only occurs near the wall owing to the adiabatic wall condition, while three ignition events are identified for a greatly deformed diaphragm: ignition due to reflection of leading shock wave at the wall, hydrogen penetration into shock-heated air near the wall, and deep penetration of hydrogen into shock-heated air behind the leading shock wave.

An interface-capturing method using a high-order central difference scheme is presented for simulations of compressible multicomponent flows. The present method adds consistent numerical diffusion terms to robustly capture interface discontinuities, while maintaining the velocity, pressure, and temperature equilibriums at interfaces. Analysis of the numerical errors generated at the interfaces leads to a proposal of new consistent numerical diffusion terms. The method solves a fully conservative form of the total mass, momentum, total energy, and species-mass, and an additional advection form of the specific heats ratio to preserve the pressure oscillation-free property at the interfaces. Several one- and two-dimensional problems are used to verify the proposed method.

Two-dimensional planar and three-dimensional round nitrogen jets under supercritical pressures are simulated with a wide range of conditions to explore their unique characteristics. A high-order method using a sixth-order compact scheme is applied. The present study parametrically covers two supercritical pressures of 4 and 8 MPa and three jet injection temperatures between a cryogenic jet of 82 K and a warmer jet of 133 K. Unique characteristics are found in both the mean temperature and temperature fluctuation profiles on the centerline (i.e., slower increase of jet temperature and relatively weak temperature fluctuation), only in the case of a near-critical pressure of 4 MPa and a cryogenic jet of 82 K. For the condition, the temperature profile on the centerline uniquely consists of four characteristic regions. The other conditions show no distinct features. The present study suggests that the distributions of the specific heat capacity at constant pressure help to explain the generation of the unique...

This study presents a strategy for simulations of cryogenic single-species jets under supercritical pressure conditions. In this strategy, a pressure evolution equation is introduced and numerical diffusion terms are consistently constructed in order to maintain the pressure and velocity equilibriums at fluid interfaces. By taking the idea of the equilibrium, the interfaces with high density and temperature ratio are robustly captured without the generation of spurious oscillations, while a high-order central differencing scheme resolves the flow fields. The present method preserves the mass and momentum conservation properties, while the poor energy conservation property is recognized. The one-dimensional interface advection and two-dimensional cryogenic jet mixing problems demonstrate the superiority and robustness of the present method over a conventional fully conservative approach.

A 3-dimensional computational fluid dynamics (CFD) simulation of a premixed hydrogen/air explosion in a large-scale domain is performed. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent burning velocity that characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent burning velocity is evaluated by analyzing the experimental data of hydrogen/air premixed explosion. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement. The effect of mesh size on simulated flame propagation velocity is also tested, showing that the numerical result agrees reasonably well with experiment when the mesh size is less than about 20 cm.

DME is thought to be a good alternative fuel due to its cleanliness and more excellent fuel economy. Although the prediction and loss prevention of flammability hazard is very important for safety of DME installations, the evaluation method with sufficient accuracy has not been established. In this study, a numerical combustion model is constructed and a 3-dimensional computational fluid dynamics (CFD) simulation of a premixed DME/air explosion in a large-scale domain is conducted. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent flame velocity which characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent flame velocity is evaluated by analyzing the experimental data of LPG/air and DME/air premixed explosions. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement.

The vaporization and burning of the N2H4 and NTO droplets were simulated with the interface-tracking method to accurately explore the auto-ignition processes and the flame structures. The N2H4 vapor plume developed behind the N2H4 droplet and reacted with the ambient NO2 gas through the hydrogen abstraction reactions. Thus, the N2H4 vapor and NO2 gas mixtures behind the droplet were preheated and reached the auto-ignition at a few ms. The auto-ignition occurred in the multiple points almost at the same time. After the ignition, the premixed flame developed around the droplet. Thus, the vaporization of the liquid N2H4 near the surface became significant. Then, the double flame structures which comprise the inner decomposition flame and oxidation flame appeared around the droplet. The NTO droplet was not auto-ignited in the computational time of the present study because little N2O4 vapor near a saturated temperature decomposed to NO2 gas which is necessary for the hydrogen abstraction reactions. When the ignition was forced, the double flames developed. The outer decomposition flame propagated to the boundaries of the computational domain, while the inner oxidation flame appeared near the droplet. Except for the propagation of the decomposition flame, the NTO droplet combustion was similar to that of the industrial fuels.

A three-dimensional (3D) simulation of N2 /H2 mixing for a coaxial injector under a supercritical pressure of 10 MPa is conducted using a highorder numerical method. Two outer H2 jets with injection temperatures of approximately 52 and 462 K are applied while an inner N2 jet with an injection temperature of approximately 97 K is applied. The mean and fluctuation properties and instantaneous flow fields are discussed in order to characterize the detailed mixing features for the two injection conditions. A clear dependence of dense-core length on the momentum flux ratio is also demonstrated.

This study proposes an approach for simulations of cryogenic fluid mixing under supercritical pressures using high-order schemes. In this approach, we introduce a pressure evolution equation and consistently construct numerical diffusion terms to maintain the velocity and pressure equilibriums at fluid interfaces. The interfaces with high density and temperature ratio are successfully captured without the generation of spurious oscillations, while a high-order central differencing scheme resolves the flow fields. The present method preserves the mass and momentum conservation properties, while the poor energy conservation property is recognized. The one-dimensional single and multi-species interface advection and two-dimensional cryogenic jet mixing problems demonstrate the superiority and robustness of the present method over a conventional fully conservative method.

In this study, we investigated the limits of classical molecular simulation on the estimation of thermodynamic properties of cryogenic hydrogen. Three empirical potentials, the Lennard-Jones (LJ) p...

We have calculated the potential energy surfaces for the singlet state of H2–H2 and for the quintet state of using the coupled-cluster theory with singles, doubles and perturbative triple excitations [CCSD(T)] and an aug-cc-pVQZ basis set. Resulting interaction potentials were expanded in spherical harmonics, separating the radial and the angular dependencies to obtain analytical expressions. Monte-Carlo simulations for the NVT ensemble were performed to evaluate pressures and heat capacities of dense H2 and O2 fluids. The results were compared with the available experimental data in NIST thermodynamic database.

An analytical method was proposed and validated for droplet diameters and size distributions. The method was developed based on the energy conservation law including surface free energy and Laplace pressure. Under several hypotheses, the law derived an equation indicating that atomization resulted from a kinetic energy loss. Thus, once the amount of loss was obtained, the droplet diameter was able to be calculated without any experimental parameters. When the effects of ambient gas were ignorable, injection velocity profiles of liquid jets were the essential factor for the reduction of kinetic energy. The minimum Sauter mean diameter produced by liquid sheet atomization was inversely proportional to the injection Weber number under the conditions of injection velocity profiles with laminar or turbulent. By applying the mean diameter model, a non dimensional distribution function was also derived assuming Nukiyama-Tanasawa's function. The validity of these estimation methods were favorably confirmed by comparisons with corresponding mean diameters and the size distributions, which were experimentally measured under atmospheric pressure.

The efficiencies of recently proposed, phenyl addition/cyclization (PAC), methyl addition/cyclization (MAC) and a popular hydrogen abstraction/acetylene addition (HACA) mechanisms have been examined experimentally by detecting the gas phase reaction products of pyrolysis of toluene with/without addition of benzene + acetylene by using vacuum ultraviolet (VUV) single photon ionization (SPI) time of flight mass spectrometry (TOFMS). Besides the observation of verities of large polycyclic aromatic hydrocarbons (PAHs), intense mass peaks of indene, phenylacetylene and propyne confirmed a remarkable quenching of the major active species, benzyl, phenyl and methyl radicals by acetylene. In spite of quenching, only benzyl contributed products were diminished while phenyl and methyl contributed products were enhanced. Uniquely observed symmetrical PAHs; corrannulene/coronene; formed by the active involvement of PAC, HACA and/or MAC mechanisms, reflects the interdependencies of these mechanisms. Individually, PAC was found highly efficient for endless growth, HACA for filling triple fusing site and MAC for expanding cyclotetra/pentafused into benzenoid structure, respectively.

The main gas phase intermediates, leading the efficient chemical vapor deposition (CVD) growth of single wall carbon nanotubes (SWCNTs), have been explored through systematic experimental and theoretical studies. The growth efficiencies of the basic radical/neutral species (sp2 C2: C2H3/C2H4, sp3 C2: C2H5/C2H6, and sp3 C1: CH3/CH4) have been compared by supplying C2H4, C2H6, and CH4, as carbon sources to grow SWCNTs. The gaseous composition of the exhaust was analyzed by an in situ direct sampling mass spectrometric technique using vacuum ultraviolet (VUV)-single photon ionization (SPI)-time-of-flight mass spectrometry (TOFMS). For verification via theoretical prediction, a CHEMKIN calculation was performed. A kinetic analysis of the experimental and theoretical results was compared with thermal decomposition phenomena of the used hydrocarbons, and hence, it was concluded that the key gas phase intermediates produced from the complex gas phase reactions as a final and efficient species capable of initiati...

Visible absorption spectra of the cyclohexadienyl radical (c-C6H7) assigned to the A ∼ ← X ∼ electronic transition have been measured using laser photolysis coupled with a cavity ring-down spectroscopic detection technique. Spectral simulation of the rotational structure of an origin band at room temperature was performed using molecular constants predicted by quantum chemical calculations. Pressure dependence of the self-reaction of the cyclohexadienyl radical was measured in the pressure range of 10–100 Torr. The reaction rate coefficient of the cyclohexadienyl radical with molecular oxygen at 10 Torr N2 buffer and room temperature was determined to be (5.4 ± 0.7) × 10−14 cm3 molecule−1 s−1, which is in good agreement with previous reported value measured at 760 Torr.

A detailed chemical kinetic model for the mixtures of primary reference fuel (PRF: n -heptane and iso-octane) and toluene has been proposed. This model is divided into three parts; a PRF mechanism [T. Ogura, Y. Sakai, A. Miyoshi, M. Koshi, P. Dagaut, Energy Fuels 21 (2007) 3233–3239], toluene sub-mechanism and cross reactions between PRF and toluene. Toluene sub-mechanism includes the low temperature kinetics relevant to engine conditions. A chemical kinetic mechanism proposed by Pitz et al. [W.J. Pitz, R. Seiser, J.W. Bozzelli, et al., in: Chemical Kinetic Characterization of the Combustion of Toluene, Proceedings of the Second Joint Meeting of the U.S. Sections of the Combustion Institute , 2001] was used as a starting model and modified by updating rate coefficients. Theoretical estimations of rate coefficients were performed for toluene and benzyl radical reactions important at low temperatures. Cross reactions between alkane, alkene, and aromatics were also included in order to account for the acceleration by the addition of toluene into iso-octane recently found in the shock tube study of the ignition delay [Y. Sakai, H. Ozawa, T. Ogura, A. Miyoshi, M. Koshi, W.J. Pitz, Effects of Toluene Addition to Primary Reference Fuel at High Temperature , SAE 2007-01-4104, 2007]. Validations of the model were performed with existing shock tube and flow tube data. The model well predicts the ignition characteristics of PRF/toluene mixtures under the wide range of temperatures (500–1700 K) and pressures (2–50 atm). It is found that reactions of benzyl radical with oxygen molecule determine the reactivity of toluene at low temperature. Although the effect of toluene addition to iso-octane is not fully resolved, the reactions of alkene with benzyl radical have the possibility to account for the kinetic interactions between PRF and toluene.

The gas-phase reaction products of toluene pyrolysis with and without acetylene addition produced in a flow tube reactor at pressures of 8.15-15.11 Torr and temperatures of 1136-1507 K with constant residence time (0.56 s) have been detected in an in situ direct sampling mass spectrometric study by using a vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry technique. Those products range from methyl radical to large polycyclic aromatic hydrocarbons (PAHs) of mass 522 amu (C(42)H(18)) including smaller species, radicals, polyynes, and PAHs, together with ethynyl, methyl, and phenyl PAHs. On the basis of observed mass spectra, the chemical kinetic mechanisms of the formation of products are discussed. Especially, acetylene is mixed with toluene to understand the effect of the hydrogen abstraction and acetylene addition (HACA) mechanism on the formation pathways of products in toluene pyrolysis. The most prominent outputs of this work are the direct detection of large PAHs and new reaction pathways for the formation of PAHs with the major role of cyclopenta-fused radicals. The basis of this new reaction route is the appearance of different sequences of mass spectra that well explain the major role of aromatic radicals mainly cyclopenta fused radicals of PAHs resulting from their corresponding methyl PAHs, with active participation of c-C(5)H(5), C(6)H(5), C(6)H(5)CH(2) ,and C(9)H(7) in the formation of large PAHs. The role of the HACA only seemed important for the formation of stable condensed PAHs from unstable primary PAHs with zigzag structure (having triple fusing sites) in one step by ring growth with two carbon atoms.

Product formation pathways in the photolytically initiated oxidation of CH3OCH3 have been investigated as a function of temperature (298-600 K) and pressure (20-90 Torr) through the detection of HO2 and OH using Near-infrared frequency modulation spectroscopy, as well as the detection of CH3OCH2O2 using UV absorption spectroscopy. The reaction was initiated by pulsed photolysis with a mixture of Cl2, O2, and CH3OCH3. The HO2 and OH yield is obtained by comparison with an established reference mixture, including CH3OH. The CH3OCH2O2 yield is also obtained through the procedure of estimating the CH3OCH2O2/HO2 ratio from their UV absorption. A notable finding is that the OH yield is 1 order of magnitude larger than those known in C2 and C3 alkanes, increasing from 10% to 40% with increasing temperature. The HO2 yield increases gradually until 500 K and sharply up to 40% over 500 K. The CH3OCH2O2 profile has a prompt rise, followed by a gradual decay whose time constant is consistent with slow HO2 formation. To predict species profiles and yields, simple chlorine-initiated oxidation model of DME under low-pressure condition was constructed based on the existing model and the new reaction pathways, which were derived from this study. To model rapid OH formation, OH direct formation from CH3OCH2 + O2 was required. We have also proposed that a new HCO formation pathway via QOOH isomerization to HOQO species and OH + CH3OCH2O2 --HO2 + CH3OCH2O are to be considered, to account for the fast and slow HO2 formations, as well as the total yield. The constructed model including these new pathways has successfully predicted experimental results throughout the entire temperature and pressure ranges investigated. It was revealed that the HO2 formation mechanism changes at 500 K, i.e., HCO + O2 via HCHO + OH and the above proposed direct HCO formation dominates over 500 K, while a series of reactions following CH3OCH2O2 self-reaction and OH + CH3OCH2O2 reaction mainly contribute below 500 K. The pressure dependent rate constant of the CH3OCH2 thermal decomposition reaction has been separately measured since it has large negative sensitivity for HO2 formation and is essential to eliminate the ambiguity in the CH3OCH2 + O2 mechanism at higher temperature.