Combustion characteristics of Mg vapor jet flames in CO2 atmospheres

An experimental study was performed on the combustion characteristics of a jet diffusion flame of Mg vapor injected through a small nozzle into CO₂ atmospheres at low pressures from 8 to 48 kPa with a view to using Mg as fuel for a CO₂-breathing turbojet engine in the Mars atmosphere. The Mg vapor jet produced three types of the flame. At lower pressures and higher injection velocities, a red-heated jet flame formed, in which the injected Mg vapor was heated by spontaneous reactions, turning red. At medium pressures and injection velocities, a stable luminous lifted-like flame developed above the rim of the chimney, a tube-like combustion product for the Mg vapor passage that grew on the nozzle during combustion. The flame had similar flame length properties to laminar jet diffusion flames of gaseous fuels. At higher pressures and lower injection velocities, a stable luminous attached flame developed at the rim of the chimney. The same reactions, producing MgO(g), CO and MgO(c), proceeded preferentially for all flames and chimneys. Carbon was only subordinately generated. Burning behavior of Mg vapor jets in a CO₂ atmosphere has been represented, including the homogeneous reaction of Mg vapor with CO₂, the diffusion of CO₂, and the condensation and deposit of MgO. The injection velocity of Mg vapor at the rim of the chimney and the exothermic reactions with diffused CO₂ that occur there play a crucial role in the attachment and development of the flames. The flame structure may be explained in terms of the relatively low gas-phase reaction rate of Mg with CO₂.     

零质量射流激励下诱发液体相变及其格子Boltzmann方法模拟

使用格子Boltzmann方法对零质量射流激励下液体的相变演化过程进行了数值模拟和分析.首先,提出了此特定零质量射流进出口边界的处理格式.然后,结合Shan和Doolen提出的单组分多相模型,模拟了方腔内液体受到此零质量射流激励而诱发产生空化的过程,着重分析了三个重要射流参数ε/T,T和v out/v in对方腔内液体相变的影响.分析表明:演化过程中方腔内气相节点数量在初始阶段急剧增长,然后经振荡趋于一个稳定值.由于ε/T和v out/v in可以反映射流在出入方腔两个过程间相互转换时的急剧变化,所以能够影响方腔中的液体相变的演化;而改变参数T并不影响射流速度的变化程度,所以T对液体相变的影响较弱.对于本文给定的参数取值,ε/T较小时,方腔内液体相变生成的孤立气泡脱离壁面;较大的ε/T下产生附着于方腔壁面的气泡,并且能够加速液体的相变进程;v out/v in的增加使方腔内相应的孤立气泡所覆盖的范围略有减小.研究结果揭示了零质量射流激励诱发的液体相变过程,为进一步探索液体空化的控制途径奠定了基础.     

Controlled mixing enhancement in turbulent rectangular jets responding to periodically forced inflow conditions

We present numerical studies of active flow control applied to jet flow. We focus on rectangular jets, which are more unstable than their circular counterparts. The higher level of instability is expressed mainly by an increased intensity of mixing of the main flow with its surroundings. We analyse jets with aspect ratio A_r = 1, A_r = 2 and A_r = 3 at Re = 10,000. It is shown that the application of control with a suitable excitation (forcing) at the jet nozzle can amplify the mixing and qualitatively alter the character of the flow. This can result in an increased spreading rate of the jet or even splitting into nearly separate streams. The excitations studied are obtained from a superposition of axial and flapping forcing terms. We consider the effect of varying parameters such as the frequency of the excitations and phase shift between forcing components. The amplitude of the forcing is 10% of the inlet centreline jet velocity and the forcing frequencies correspond to Strouhal numbers in a range St = 0.3–0.7. It is shown that qualitatively different flow regimes and a rich variety of possible flow behaviours can be achieved simply by changing aspect ratio and forcing parameters. The numerical results are obtained applying large eddy simulation in combination with a high-order compact difference code for incompressible flows. The solutions are validated based on experimental data from literature for non-excited jets for A_r = 1 at Re = 1.84 × 10⁵ and A_r = 2 at Re = 1.28 × 10⁵. Both the mean velocities as well as their fluctuations are predicted with good accuracy.     

Sintering mechanism of CaO during carbonation reaction in the presence of water vapor

In this paper, CaO sintering in the presence of water vapor for CO₂ capture were carried out by ReaxFF(Reactive Force Field) molecular dynamics. The CaO sintering model was simulated at different temperatures (873 K-1273 K) and atmospheres (CO₂, H₂O), respectively. The results showed that water vapor could significantly promote the sintering process of CO₂ capture by CaO. The Mean-square displacement (MSD) and Boltzmann–Arrhenius dependency were used to study the diffusion properties of CaO particles. The decreased diffusion activation Ea and increased pre-exponential factor D₀ indicate that CaO particles have a stronger initial diffusivity and a lower diffusion barrier in the presence of CO₂ and H₂O. The inner and outer regions of CaO atoms were analyzed and it was found that the activation energy is the main factor to enhance the diffusion in the presence of CO₂ for CaO sintering process, whereas the pre-exponential factor dominates with both CO₂ and H₂O. Water vapor enhanced the sintering pf CaO carbonation reaction is mainly achieved by promoting atoms in the inner layers of CaO particles. The types and numbers of sintering atoms during the sintering process were counted, and the distances between Ca and O atoms were calculated, which found that water vapor first dissociates into hydroxyl and H protons on the CaO surface, and the hydroxyl group will stay on the surface of CaO and combine with CO₂, while the H proton will combine with O inside CaO to promote the sintering of CaO further.     

The quenching of Pb(6p 2 1 D 2) atoms in collisions with molecules

The collisions of metastable Pb(6p ^{2 1} D ₂) atoms with various molecules were studied by the diagnostics of radiation from a hollow cathode lamp and a laser on lead vapor. Experiments were performed for a gas flow of lead atoms with argon. The Pb(6p ^{2 1} D ₂) states were excited in a gas discharge in the presence of reagent gas molecules. The absolute rate constants for the quenching and chemical reactions of lead atoms in the ground and excited states were determined. The quantum efficiency of chemical reactions was close to one for the N₂O, CH₂Cl₂, SF₆, and CuBr molecules. Long-lived chemical compounds were formed in these reactions.     

Space resolved density measurements of argon and helium metastable atoms in radio-frequency generated He-Ar micro-plasmas

Space resolved concentrations of helium He* (³S₁) and argon Ar* (³P₂) metastable atoms in an atmospheric pressure radio frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Even small absorptions down to 10^-4 could be measured using lock-in technique. The absolute density of metastable atoms densities at different rf-power, flow rate and gas mixture was deduced from measured absorption rates. Metastable concentrations range from 10⁹ to 10¹¹ cm^-3. Analysis of spectral profiles provided the pressure broadening coefficients of both metastable atoms by helium. The spatial distribution of metastable atoms in the plasma volume was obtained for various discharge conditions. Density profiles between the electrodes reveal the sheath structure and reflect the plasma excitation distributions in the discharge volume. It reveals the dominance of the α-mode discharge.     

On the 1920 Å continuous spectrum of the supersonic argon jet excited by an electron beam

The T₀/?(T₀) dependences (T₀ is the intensity of the 1920 Å continuum at its maximum, ? is the concentration of argon atoms at the intersection of an electron beam and a jet, T₀ is the temperature of argon at the nozzle) for the 1920 Å continuum radiated by a supersonic argon jet excited by an electron beam were investigated at various pressures of argon at the entry to a nozzle. The experiments performed suggested a possible generation mechanism of the 1920 Å continuum emitted by an argon jet.     

Improvement of transport properties for polycrystalline silicon prepared by plasma-enhanced chemical vapor deposition

The carrier transport property of polycrystalline silicon (poly-Si:H:F) thin films was studied in relation to film microstructure, impurity, in situ or post-annealing treatments to obtain better carrier transport properties. Poly-Si:H:F films were prepared from SiF₄ and H₂ gas mixtures at temperatures <300 °C. Dark conductivity of the films prepared at high SiF₄/H₂ gas flow ratio (e.g., 60/3 sccm) exhibits a high value for intrinsic silicon and its Fermi level is located near the conduction band edge. The carrier incorporation is suppressed well, either by in situ hydrogen plasma treatment or by post-annealing with high-pressure hot-H₂O vapor. It is confirmed that weak-bonded hydrogen atoms are removed by the hot-H₂O vapor annealing. In addition, evident correlation between impurity concentrations and dark conductivity is not found for these films. It is thought that the carrier incorporation in the films prepared at high SiF₄/H₂ gas flow ratios is related to grain-boundary defects such as weak-bonded hydrogen. By applying hot-H₂O vapor annealing at 310 °C to a 1-μm-thick p-doped (400)-oriented poly-Si:H:F film, Hall mobility was improved from 10 cm²/Vs to 17 cm²/Vs. Received: 7 August 2000 / Accepted: 2 March 2001 / Published online: 20 June 2001     

Study on concurrent measurements of velocity and density distributions for high-speed CO2 flow

Velocity and density distributions of a high-speed and initial CO₂ jet flow have been analyzed concurrently by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Two high-speed cameras have been used for the tomography and one for the PIV since a shape of a nozzle for the jet flow is bilaterally symmetric and the starting flow is fast and unsteady. The speckle movements between no flow and CO₂ jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO₂ jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method concurrently and instantaneously.     

Large-scale flow structures in a planar offset attaching jet with a co-flowing wall jet

Conditional averaging techniques were used to examine the periodic motions that were observed in flows consisting of an offset planar jet and a co-flowing planar wall jet. The offset jet is one jet height (H_j) away from the wall and has a Reynolds number of approximately 40, 000, based on H_j and flow-rate averaged velocity U_o; for the co-flowing jets, different heights (i.e., 0.18H_j and 0.5H_j) and velocities (i.e., 0.56U_o and 0.36U_o) were considered. The flows had periodic motions with frequencies fH_j/U_o = 0.28 and 0.49 or fH_c/U_o = 0.23 and 0.25, where H_c is the distance between the jets. The periodic motions were present in both the inner shear layer of the offset jet above the re-circulation region and the outer shear layer of the wall jet below the re-circulation region. The motions from the inner shear layer of the offset jet persisted in the shear layer that formed downstream of the re-circulating region. There were periodic motions in the outer shear layer of the offset jet particularly in the flow with the smaller wall jet. The relative contribution of the motions to the total fluctuations increased as the flow evolved downstream reaching a maximum approximately 4H_c downstream of the flow exit. The relative contribution of the periodic motions to the turbulent fluctuations was similar in the two flows but the periodic motions had a much larger impact on the near-wall velocity and pressure fluctuations in the flow with the smaller wall jet due to the trajectory of the periodic structures, the distance of the periodic structures to the wall and the size of these structures.     

Atmospheric pressure plasma treatment on graphene grown by chemical vapor deposition

We demonstrate the surface treatment of graphene using an atmospheric pressure plasma jet (APPJ) system. The graphene was synthesized by a thermal chemical vapor deposition with methane gas. A Mo foil and a SiO₂ wafer covered by Ni films were employed to synthesize monolayer and mixed-layered graphene, respectively. The home-built APPJ system was ignited using nitrogen gas to functionalize the graphene surface, and we studied the effect of different treatment times and interdistance between the plasma jet and the graphene surface. After the APPJ treatment, the hydrophobic character of graphene surface changed to hydrophilic. We found that the change is due to the formation of functionalities such as hydroxyl and carboxyl groups. Furthermore, it is worth noting that the nitrogen plasma treatment induced charge doping on graphene, and the pyridinic nitrogen component in the X-ray photoelectron spectroscopy spectrum was significantly enhanced. We conclude that the atmospheric pressure plasma treatment enables controlling the graphene properties without introducing surface defects.     

On the light amplification in the reactions of atoms with condensing gases

Results of the experimental investigations of the reactions of oxygen or hydrogen atoms with gaseous NO, which is rapidly cooling in the supersonic jet, are discussed. It is shown that the intense chemiluminescence arises in the reaction of atoms with cluster droplets (NO)_n. The possibility is shown of light amplification in photorecombination reactions of atoms with condensing gases. Such heterophase reactions have a variety of advantages in laser applications compared with reactions in the homogeneous gaseous phase.     

Stability and emissions control using air injection and H2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H₂ pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO_x emission benefits from the air jet, while, with H₂ addition, NO_x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO_x. The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO_x reduction.     

Exact expressions for the potential functions of a molecule in terms of the probability amplitudes of electron transitions and their utilization in the inverse spectroscopic problem

The frequencies of 5s ³ S ₁-np ³ P ₁ Rydberg transitions, quantum defects for n=15–50, and energies of high-lying P states of the Zn atom were measured by three-step laser excitation with subsequent ionization by a pulsed electric field. Free Zn atoms were produced through effusion of Zn vapor from an atomizer during the thermal dissociation of ZnS molecules. The evaporation kinetics was studied, and the ZnS vapor pressure was measured by the detection of photoionic signals of Zn.     

Fifth harmonic generation in sodium vapor

We report experimental investigation of direct fifth harmonic of Nd³⁺: glass laser radiation (1060 nm) in metal vapor. Using various mixtures of sodium vapor and xenon in order to achieve a phase-matching, we observed direct as well as cascade nonlinear processes. The first measurement of effective fifth-order nonlinear susceptibility χ⁽⁵⁾_eff in atoms, is performed.     

平面射流场中纳米颗粒的成核与凝并

采用大涡模拟和直接积分矩方法,数值模拟了在Reynolds数为8300的平面射流中,水蒸气（相对湿度φ=70%）和硫酸蒸气（质量分数为5×10^-6）二元体系中纳米颗粒的成核与凝并,详细分析了颗粒数密度、体积密度和平均粒径的分布.计算结果表明.射流场混合动量厚度的增长和实验结果一致;射流场的拟序结构导致了涡核中心处硫酸蒸气浓度的明显减小,而纳米颗粒数密度则明显增加;拟序结构的出现导致颗粒碰撞概率增大,提高了颗粒凝并效率;在颗粒数密度较大的涡核中心,颗粒成核作用增强,从而加 关键词： 纳米颗粒 成核 凝并 平面射流     

Supersonic Jets with Periodic Structure due to Arc plasma Expansion into a Low pressure ambient

A supersonic plasma jet was produced by a d.c. arc plasma generator operated at normal pressure and connected to a low-pressure (p∞ = 0.2-50 kPa) chamber via cylindrical nozzle with diameter of 2.5 mm. The argon gas flow rate was G = 0.025 to 0.35 g.s^?1. In some experiments current I_E ≦ 30 A passed coaxially through the initial part of the jet. Photographic records of the jet and pressure measurements are in agreement with theoretical predictions by a simple one-dimensional, gasdynamical model capable of self-consistent calculations throughout the plasma source/jet system. Periodic jet structure is observed over a wide range of experimental conditions, incl. in highly under-expanded flow. The jet expansion angle and Mach disc position vary with p∞, G and I_E, but are nearly constant at different arcing currents.     

Visualization of dilute hydrogen jet flame in air flow

The structure of hydrogen jet flame diluted by CO₂ in air flow is studied by various visualization techniques, such as schlieren, direct photograph, tracer injection and reactive Mie scattering method, which allow understanding of the influence of CO₂ on the characteristics of the hydrogen jet flame. The experimental result indicates that the flame structure consists of laminar fuel jet and surrounding reaction zone near the nozzle exit. When the CO2 fraction is increased, the width of the fuel jet grows and the reaction zone is reduced in size. These observations are further confirmed by quantitative measurements of temperature and velocity fields in the flame, which are evaluated by thermocouple and particle image velocimetry (PIV), respectively. These results indicate that the flame temperature is decreased and the flow rate of the fuel jet is increased by the influence of diluents, which are due to the reduced calorific value and larger density of fuel, respectively.     

The violet emissions produced by laser excitation of Na vapor in the 570–595 nm region

We have observed atomic Na and molecular Na₂ emissions in the violet region when Na vapor in a heatpipe (～10³—10¹⁶ atoms/cm³) is irradiated with a pulsed dye laser with output wavelength in the 570–597 nm region. The Na atomic emissions probably result from recombination of Na⁺ + e^- and energy-pooling involving highly excited atoms and molecules, while the diffuse violet emission bands are probably produced through collisions among excited Na atoms and Na₂ molecules.     

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.