Influence of inert and active additives on the initiation and propagation of laminar spherical flames in stoichiometric mixtures of methane,pentane, and hydrogen with air

The propagation of a laminar spherical flame in stoichiometric mixtures of methane, and pentane with air in the presence of argon and carbon dioxide and in hydrogen-air-propylene mixtures at atmospheric pressure in a constant-volume bomb is investigated using high-speed color cinematography. It is shown that, under the experimental conditions employed (at T ₀ = 298 K and a spark energy of E ₀ = 0.91 J), dilution of the combustible mixtures with these additives can cause a more than 10-fold increase in the time of formation of a steady flame front, with the inhibiting effect of carbon dioxide being stronger than that of argon. Small additives of propylene, a chemically active inhibitor, are demonstrated to substantially increase the time it takes to form a steady flame front and reduce the flame propagation velocity.     

Multifrequency Diagnostics of a Vibrationally Equilibrium CO2-Containing Gas Mixture

We present a technique which makes it possible to simultaneously determine the temperature T and the partial pressure of carbon dioxide in a vibrationally equilibrium gas mixture at atmospheric pressure by using the experimentally measured spectral distribution of the absorption factor at the oscillation lines of a tunable CO₂ laser. The technique developed can be employed for monitoring both the energy efficiency and the ecological purity of the processes of combustion of large amounts of hydrocarbon fuels accompanied by release to the atmosphere of combustion products containing carbon dioxide.     

Removal of carbon dioxide from gas mixtures by wollastonite

Wollastonite synthesis and decomposition were analyzed from the viewpoint of thermodynamics (using the TERRA software). It is shown that wollastonite synthesis from limestone and silica takes place at a minimum content of nitrogen (10^?5 N₂) with a release of carbon dioxide. The synthesis temperature is T ≥ 560 K. Wollastonite is decomposed in the presence of flue gas (4N₂) with limestone and silica formation and burial of carbon dioxide in the form of CaCO₃(c). Wollastonite decomposition temperature is T ≤ 420 K. The cyclic reciprocating process for complete removal of carbon dioxide by wollastonite is suggested. Four strokes of the reciprocating system with the fixed temperatures of wollastonite decomposition (T=300 K) and wollastonite synthesis (T=560 K) are presented. Total energy consumption (T = 560 K) is ΔI ≈ 130 kJ/mole, 30 % of energy is spent for heating and 70 % of energy is spent for chemical reaction. This is comparable with the heat of CO₂ solution in ethanolamin.     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Grain Boundary Grooving as an Indicator of Grain Boundary Phase Transformations

The atomic force microscopy (AFM) was used to study the grain boundary (GB) groove profiles far away from the melting temperature T _m. It is shown that AFM allows one to measure the temperature dependence of the GB energy in a rather broad temperature interval (from 0.85 T _m to T _m). The GB energy and GB segregation of Bi were measured at 1123 K in the interval of the Bi bulk concentration x ^v _Bi from 5 to 140 ppm Bi. The transition from monolayer to multilayer adsorption is observed for the 19a GB at 1123 K and x ^v _Bi = 60 at. ppm Bi. At the same point (1123 K and x ^v _Bi = 60 at. ppm Bi) a discontinuity of the first derivative of the GB energy is observed. These features were explained using the model of GB prewetting phase transformation developed previously.     

CARS studies of vibrationally excited nitrogen at low pressures

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational temperature of microwave-excited nitrogen in a N₂–CO–He mixture. CARS spectra, originating from the N₂-vibrational levelsv=0 up tov=3, have been recorded by both narrowband scanning of the resonance region as well as by broadband OSA detection. For the microwave-excited N₂ molecules a vibrational temperatureT _v (N ₂ = (2130±110K) and a lower limit of detection forN ₂(v = 3) = 1.2 x 10¹⁵ cm^–3 was established. The CARS results were independently confirmed by simultaneously recorded and spectrally resolved CO infrared fluorescence studies.     

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

Rotational coherent anti‐Stokes Raman spectroscopy (CARS) in fuel‐rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N₂ H₂ Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self‐broadened nitrogen only, N₂ N₂ [energy‐corrected‐sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N₂ H₂ [Robert–Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self‐broadened nitrogen only, and the case with inclusion of N₂ H₂ Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Spectroscopic diagnostics of R.F. discharges at moderate pressure and chemical applications—I. Pure nitrogen

A vibro-rotational analysis has been performed of the second positive system (SPS) of N₂ and of the first negative system (FNS) of N⁺₂ emitted by 35 MHz discharges in flowing nitrogen operated at pressures of 5–50 torr and power densities of the order of 1–10 W-cm^-3. The distributions of the vibrational and of the rotational levels follow Boltzmann's law in both the SPS and the FNS with T_v = 4000°K and T_R = 2800°K for the N₂(C³Π_u) state and T_v = 5100°K and T_R = 5800°K for the N⁺₂(B²Σ⁺_g) state and independent of pressure. Kinetic gas temperatures are between 1200 and 2000°K and increase with pressure; electron temperatures are in the range 15,000–9,500°K. The reversal of line intensities of the SPS and of the FNS observed with increasing pressure has been tentatively interpreted. Possible chemical implications of these results have been examined.     

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

The rotational (T_R) and vibrational (T_v) temperatures of N₂ molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N₂ and dry air excited by radio‐frequency (rf) power. Both T_R and T_v of the N₂ molecules in the C ³Π_u state were determined from an emission spectroscopic analysis the 2^nd positive system (C ³Π_u → B³Π_g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both T_R and T_v also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N₂ to the Ne in the discharge resulted in higher T_R and T_v values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of an Underexpanded Plasma Jet

Supersonic jets (Mach number M = 1.4 – 2.0) of dense plasma were produced by a plasmatron in a pulsed regime. The shock structure was investigated by fast mirror cameras, by the schlieren technique and by probe and spectroscopic methods. It was found that during jet expansion the bow shock ahead the gas-plasma interface and the shock behind it are superimposed with the quasistationary shock structure which includes the Mach disk. Whereas the plasma in the discharge chamber is in LTE (n_E ≈ 5 × 10²³ m^?3, T ≈ 14000 K), the electron density of the expanding jet plasma falls to a plateau value of n_E ≈ 5 × 10¹⁹ m^?3 below the LTE limit. At this phase, the jet plasma formed a ball-like turbulent plasmoid which moves at a subsonic velocity through the air up to some milliseconds.     

Pressure effect on the anomalous electrical conductivity of magnetite

The temperature dependence of the electrical conductivity of magnetite was measured under hydrostatic pressure up to 18.4 kbar. It is found that the temperature of the conductivity maximum in the high temperature phase is more rapidly reduced by pressure (dT_m/dP = -4.1 K/kbar) than the Verwey temperature (dT_v/dP = -0.27 K/kbar). The discontinuous change of the conductivity at T_v appears to increase with applied pressure as a result of a lowering of T_m.     

Influence of second kind collisions on the electron distribution function in the positive column of low pressure nitrogen discharge

A numerical solution of the Boltzmann equation for the electron gas in the positive column of a DC discharge in nitrogen is presented. The Boltzmann equation was solved with the inclusion of the second kind (superelastic) collisions proceeding from the first six excited vibrational levels of molecular nitrogen. The vibrational level population is supposed to follow a Boltzmann distribution for a given vibrational temperatureT _v, with a possible deviation of the ground level, which can be overpopulated in a given ratio. Apart from the electron distribution functions, which were gained for various values ofE/p ₀,T _v and, the values of some production frequencies and kinetic coefficients are presented in form of tables and plots. It is found that the electron distribution (and also the corresponding production rates) depends above a certain energy limit onT _v and through the normalization constant only.     

DC Townsend Discharge in Nitrogen: Temperature‐Dependent Phenomena

Low‐current Townsend discharge in nitrogen has been studied in the temperature range of T = 100–300 K in a semiconductor‐gas‐discharge structure. It was found that the sustaining voltage U_S increases with time when a current is passed through the structure at low T. This effect was not observed at room temperature. A hypothesis is put forward that a film of a neutral phase of nitrogen is formed on the electrodes under cryogenic discharge conditions. The presence of the condensed thin‐film phase leads to a decrease in the secondary electron emission from the electrode and to a corresponding increase in U_S. A possible mechanism of the phenomenon is associated with the formation of large neutral aggregates in the form of [N⁺₂(N₂)_n]^– in the gas discharge volume. The condensation of these aggregates seems to yield a phase that is comparatively stable at cryogenic temperatures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?

The synthesis of a specific isotopomer, C₆D₄H(ortho)-H(ortho)D₄C₆ of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ²), of these twists. (φ²) is found to grow linearly withT for 50<T<200 K. From the slope of (φ²) vs.T the frequency (the wave number[(v)\tilde]\tilde v) is derived. The result is[(v)\tilde] = 20\tilde v = 20 cm⁻¹. ForT<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The temperature dependence of the magnitude of the order parameter of the IC phase is obtained.     

High temperature multiple transitions in bismuth-based superconductors

We report observation of possible superconducting transitions at 133 K, 117K and 105 K in Bi-Sr-Ca-Cu-O bulk samples which attain zero resistance state above the liquid nitrogen temperature (T_c(0)) = 80K). The transition at 105 K is prominent and shows a significant drop in the sample resistance. X-ray data depict characteristic superconducting peak at 20 = 5–6°. All the specimens show Meissner effect when cooled in liquid nitrogen.     

Phase transitions in [NH(CH3)3]2MeCl4 (Me = Cd,Zn,Cu) crystals

Dielectric properties of the new [NH(CH₃)₃]₂ZnCl₄ and [NH(CH₃)₃]₂CdCl₄ crystals from the [(CH₃) _n NH_4-n ]₂MeCl₄ group have been investigated in a wide temperature range (4.2–320 K). A series of phase transitions has been discovered at T₃ = 325 K,T₄ = 251 K,T₅ = 193 K, for [NH(CH₃)₃]₂CdCl₄ and at T₃ = 309 K, T₄ = 282 K, T₅ = 269 K for [NH(CH₃)₃]₂ZnCl₄. A ferroelectric phase has been discovered in the temperature interval T₄—T₅ from the temperature and frequency dependence of the dielectric permittivity ε(T, v). According to optical investigations the existence of ferroelastic phases in the temperature interval T₁ = 349 K–T₂ = 391 K and below T₅ for [NH(CH₃)₃]₂CdCl₄ and both above T₃ and below T₅ for [NH(CH₃)₃]₂ZnCl₄ has been ascertained.     

Step-like structure and assisted tunneling in two coupled modulated waveguides

The propagation of an initially-localized optical beam along two coupled modulated waveguides is investigated. A universal step-like structure emerges in the propagation process once |d|\delta v/v₀|v_0| ~ 1 and T = 2 π/m v₀m v_0, both for the linear and nonlinear optical waveguides. The reason is that the coupling between the two waveguides can be completely suppressed at z = m T or (2 m+1)T/2. When the nonlinearity is strong, assisted tunneling can be found for the initially-localized optical beam by slightly increasing the strength of the modulation.     

Behavior of the Lorenz number in the light heavy-fermion system YbInCu4

The electrical resistivity and thermal conductivity of two polycrystalline YbInCu₄ samples prepared by different techniques at the Ioffe Physicotechnical Institute, RAS (St. Petersburg, Russia), and the Goethe University (Frankfurt-am-Main, Germany) are studied within the temperature range 4.2–300 K. At T _v～75–78 K, these samples exhibited an isostructural phase transition from a state with an integer valence (at T>T _v) to a state with an intermediate valence (at T<T _v) of the Yb ions. It is shown that at T<T _v; i.e., in the temperature range where YbInCu₄ is assumed to be a light heavy-fermion compound, the Lorenz number behaves as it should in a classical heavy-fermion system. At T>T _v, where YbInCu₄ is a semimetal, the Lorenz number has a value characteristic of standard metals.     

二维颗粒气体在堆积过程中的能量耗散

通过高速摄像的方法观测了玻璃颗粒组成的准二维气态颗粒流的冷凝耗散过程,并和理想情况下的均匀耗散的颗粒流体理论作了比较,实验发现气态颗粒部分在耗散堆积过程中近似地满足高斯分布;从动能的结果来看,实际耗散过程和流体理论所预测的不同.实验发现冷凝分为两个阶段：当动能的贡献以气体颗粒为主时,发现颗粒以恒定的速度堆积,动能耗散主要由其中以气态分布的颗粒的沉积速率α,颗粒温度T和气态部分的平动速度ν_g决定;当气态颗粒数目趋向于0,能量耗散主要来自于密堆颗粒的表面层部分 关键词： 离散体系 耗散性