含有不凝气体的蒸汽滴状冷凝传热实验研究

本文对饱和水蒸气和空气的混合物在垂直表面上滴状冷凝传热特性进行了实验研究。考察了不同的压力条件下不凝气对冷凝传热的影响。与膜状冷凝实验结果对比表明,滴状冷凝对含有不凝气的蒸汽冷凝传热有强化作用;在较高的冷凝压力下不凝气体对传热的影响相对较小。     

载气汽油蒸汽在水平圆管内的冷凝

研究了空气质量含量在2%~13%,汽油蒸汽和空气的混合气体在水平管内的冷凝换热。并分析了不凝气体质量分数对管内冷凝换热的影响规律。得到Co随Re变化的冷凝换热系数的实验关联式。     

Influence on fluorocarbons flammability limits in the mixtures of H2-N2O and CH4-N2O

The flammability limits for combustible gas (methane, hydrogen)-nitrous oxide-fluorinated hydrocarbon (trifluoromethane CHF₃, pentafluoroethane C₂HF₅, perfluorobutane C₄F₁₀) mixtures are experimentally determined. The upper and lower flammability limits turned out to be, respectively, significantly higher and lower as compared to those characteristic of the combustion in air. The compositions of the mixtures at the peaks of the flammability curves are characterized by lower values of the fuel equivalence ratio φ (in most cases, φ < 0.1) as compared to the combustion in air and in a 25 vol % oxygen-nitrogen medium (typically φ > 0.5). It was established that the inhibiting concentrations of fluorinated hydrocarbons for the combustion of methane and hydrogen in nitrous oxide are similar, in contrast to the combustion in oxygen- nitrogen oxidative media. A qualitative interpretation of the results is given.     

Preparation, electrical and gas-sensing properties of perovskite-type La1−xMgxFeO3 semiconductor materials

The La_1−xMg_xFeO₃ powder was prepared by sol-gel method using citric acid. The compounds crystallized were perovskite phase with orthorhombic structure. The Mg-doping restrains the growth of the grain size. The conductivity and gas sensing of La_1−xMg_x FeO₃-based sensors were investigated. We found the La_0.92Mg_0.08 FeO₃-based sensors have the best response and selectivity to ethanol gas. Great differences on the conductance-temperature curves of La_0.92Mg_0.08 FeO₃-based sensors between in ethanol gas and air or other gas such as H₂, CO and CH₄ were also found. The conductance in ethanol gas decreased with temperature from 130 to 240 °C. But in air and other gas such as H₂, CO and CH₄ the conductance increases all the time. It indicates that at 240 °C the conductance difference between air and ethanol was the biggest and the response reach the maximum.     

Changes in surface properties of nitrogen-implanted AISI316L stainless steel

The effect of nitrogen ion implantation with an energy of 125 keV and doses of 1 × 10¹⁷–1 × 10¹⁸ at/cm² on such tribological characteristics of AISI316L stainless steel as the friction coefficient, wear resistance, and microhardness was studied. The steel surface layer composition was studied by the methods of RBS, XRD, GXRD, SEM, and EDX. The friction coefficient and abrasion resistance of AISI316L stainless steel were measured in air, oxygen and argon atmospheres, and in vacuum. An increase in the abrasion resistance after implantation was detected, which was different for various media. The largest increase in the wear resistance was observed during testing in air. The largest decrease in the friction coefficient was observed for all implanted samples in argon atmosphere. Tribological tests resulted in an increase in nitrogen, carbon, and oxygen concentrations in worn sample fragments in comparison with their concentrations in surface layers immediately after implantation.     

On the migration of helium bubbles

Abstract

ESR-spectroscopy was used to study free radical formation in solid, polycrystalline pellets of DNA-subunits after heavy charged particle bombardment and after X-ray irradiation. Dose-yield curves were measured at 300 K and analyzed for initial G-values (radicals per 100 eV absorbed energy) and saturation concentrations. Both parameters show a characteristic LET-dependence which will be discussed in terms of the inhomogenous distributions of energy within the particle tracks.     

液氮热管过冷器的设计及分析

过冷器是高温超导电缆(HTS)低温冷却系统的关键设备之一。文中对一种以液氮为工质的新型过冷器-热管过冷器进行了设计计算,并与传统的盘管过冷器作了对比,结果表明热管过冷器能大大减少换热面积,在设计换热量为900W的情况下,热管型过冷换热器所需传热面积仅为盘管型的1/5,热管型冷头换热器面积为盘管型的1/3。并针对热管过冷器分析了不凝性气体对其冷凝段传热效率的影响,结果表明随着不凝性气体浓度逐渐增加,冷凝段换热效率开始时急剧减小,后逐渐趋于平缓。在不凝性气体浓度不变的情况下,提高热管过冷器的工作温度,或减小冷凝段的冷凝温差,都能有效提高冷凝段传热效率;在冷凝段分析的基础上,计算和分析了不凝性气体对整个热管过冷器系统的影响,结果表明随着不凝性气体浓度的升高,热管过冷器的换热量减小,同时工作温度升高,且这两者的变化在不凝性气体浓度较小时较剧烈,较大时较平缓。     

蒸汽冷凝过程的分子团聚模型及其对不凝性气体影响传热性能的解释

根据相变过程的微观物理机理和热力学特性,提出了冷凝传热过程中,近壁面蒸汽分子经由团聚阶段进而冷凝成宏观液滴的物理模型.并将团聚体分布与滴状冷凝传热性能相联系,从而研究不凝性气体对滴状冷凝传热过程的影响.在改进的Dillmann和Meier(DM)模型基础上,将分子团聚过程中的临界过饱和度与冷凝过程中的过冷度相联系,以及将团聚体的能量特性与液固界面物理化学特性相联系,将团聚模型与考虑固液界面效应的滴状冷凝传热模型相联系,建立了近壁面条件影响的分子团聚模型.利用模型计算了近壁面蒸汽中团簇体尺寸和分布,以及不凝性气体存在导致的蒸汽冷凝团聚体分布的变化,并结合滴状冷凝传热模型,定量解释了少量不凝性气体的存在,极大影响了冷凝传热性能的现象.模型计算结果与实验结果及文献中含不凝气的蒸汽冷凝传热实验数据进行了比较,两者符合较好,验证了所提出模型的合理性.     

Production of TiN thin films by N2-laser-ablated Ti atoms in nitrogen gas atmospheres

Laser ablation has proven to be an important technique for thin film deposition because of the high velocity heating and quenching of materials. We have used a nitrogen laser, which is capable of producing nanosecond pulses of intense UV (337.1 nm) radiation to generate high-temperature and high-electron-density plasmas when strongly focused on titanium targets. Laser beam pulses with a peak energy of 32 J/cm² and a power density of 1.6 GW/cm² were used in this experiment. As ablated Ti species are allowed to expand further in a nitrogen atmosphere, they cool down and eventually dissipate. In this sequence, they meet a substrate where condensation, nucleation, and growth processes yield TiN thin films because of the reaction with the nitrogen gas. Thin film depositions were made on stainless steel (AISI 1020 and AISI 304) substrates at nitrogen gas pressures ranging from 10^-1 Pa to 200 Pa and for different distances between the substrate and the spot where the laser beam was focused upon the target. For rather short distances (less than 7 mm), optical microscopy shows the effect of a further plasma-thin film interaction, while at larger distances, the depositions exhibit a regular film condensation from the vapor. Microstructure characterization of the thin film deposited was done by several techniques. The typical cubic TiN phase was identified by transmission electron microscopy (TEM) and energy dispersion spectroscopy (EDS).     

Half-Space Problem for the Discrete Boltzmann Equation: Condensing Vapor Flow in the Presence of a Non-condensable Gas

We consider a non-linear half-space problem related to the condensation problem for the discrete Boltzmann equation and extend some known results for a single-component gas to the case when a non-condensable gas is present. The vapor is assumed to tend to an assigned Maxwellian at infinity, as the non-condensable gas tends to zero at infinity. We assume that the vapor is completely absorbed and that the non-condensable gas is diffusively reflected at the condensed phase and that the vapor molecules leaving the condensed phase are distributed according to a given distribution. The conditions, on the given distribution, needed for the existence of a unique solution of the problem are investigated. We also find exact solvability conditions and solutions for a simplified six+four-velocity model, as the given distribution is a Maxwellian at rest, and study a simplified twelve+six-velocity model.     

Investigation of the ozone formation process in a nanosecond microwave discharge in air and oxygen

Results are presented from an investigation of the dynamics of ozone formation in a freely localized discharge generated by a periodic train of 3-cm nanostructured microwave pulses in oxygen and air at a pressure p=3–30 Torr. Conditions for the efficient generation of ozone in air with the minimum production of nitrogen oxides are demonstrated experimentally. It is shown that when the microwave pulse repetition frequency is high, heating of the gas and the buildup of nitrogen oxides in the discharge in air reduce the ozone formation efficiency while under prolonged exposure the ozone formed initially is destroyed. The energy cost of forming ozone in oxygen and air is determined as a function of the microwave pulse length and repetition frequency and the gas pressure. The lowest energy cost of forming a single ozone molecule in these experiments is 16 eV per molecule for a discharge in air and 4 eV per molecule in oxygen. It was observed that circulating the gas through the discharge zone enhances th ozone formation efficiency. It is shown that there are optimal conditions for ozone formation as a function of the reduced electric field in the plasma. Zh. Tekh. Fiz. 67, 9–18 (March 1997)     

Preparation and gas sensing characteristics of p-type semiconducting LnFe0.9Mg0.1O3 (Ln = Nd,Sm, Gd and Dy) materials

Nanoparticulate perovskite-type LnFe_0.9Mg_0.1O₃ (Ln = Nd, Sm, Gd and Dy) oxides were prepared by sol–gel method. X-ray diffraction was used to confirm the phase composition of the compounds. The materials exhibit p-type semiconductor behavior. Their sensitivity and selectivity towards ethanol gas were investigated. It was found that SmFe_0.9Mg_0.1O₃-based sensor had the highest response and selectivity. It is a new potential gas sensing material. The great difference of conductance in air and ethanol gas was found and discussed in detail.     

Decomposition of methylene chloride by electron beam and pulsed corona processing

Experiments on the plasma-assisted decomposition of dilute concentrations of methylene chloride in atmospheric-pressure N₂ and dry air streams by electron beam and pulsed corona processing are presented. This paper presents the first experimental evidence showing that the decomposition of methylene chloride in a non-thermal plasma at ambient gas temperature proceeds via reaction with nitrogen atoms. The decompositions is more efficient with energy deposition in electron beam generated plasmas because of the higher rate for electron-impact dissociation of N₂. In dry air mixture, the decomposition of methylene chloride is degraded substantially because the nitrogen atoms are consumed in the production of nitrogen oxides. At higher gas temperatures (300°C), the decomposition of methylene chloride in dry air is shown to proceed via reaction with oxygen atoms. The main products of methylene chloride decomposition in dry air mixtures are CO, CO₂, HCl, and probably Cl₂.     

Characterization and gas-sensing properties of NiO nanowires prepared through hydrothermal method

NiO nanowires with high aspect ratio and dispersive distribution have been synthesized by a hydrothermal reaction of NiCl₂ with Na₂C₂O₄ and H₂O in the simultaneous presence of ethylene glycol (EG) and polyethylene glycol (PEG). Then the products were obtained by the subsequent annealing at 400 °C in air. The effect of –OH from EG and –O– from PEG in the formation of nanowires was discussed. And the gas sensing properties of the as-prepared NiO nanowires toward ethanol were investigated. A novel formation mechanism of nanowires was presented and the NiO nanowires were proved to have an excellent gas sensing performance.     

Bose–Einstein Condensation: A Mathematically Unsolved Problem

The interacting Bose gas with repulsive potential is considered in the polymer representation and some of the yet unsolved mathematical questions for establishing the existence of Bose condensation in this setting are discussed. PACS: 05.30.-d. Talk delivered in honor of Professor Stanley Gudder at the IQSA-Quantum Structures 2004 conference (Denver)     

Back discharge in multipoint-plane geometry in flue gases

The paper presents investigations of back discharge occurring in air and flue gases produced by the process of burning of liquefied petroleum gas or charcoal. The discharge was generated between a multineedle electrode and plate covered with fly ash layer. The aim of this work was to determine the effects of back discharge in multineedle-to-plate electrode configuration on the fly ash layer covering the plate electrode. Level of NO_x and CO emission was also measured. It was found that the chemical composition of flue gas can be changed in the domains where the back discharge occurs, for example, additional amounts of nitrogen oxides (NO and NO₂) are produced and also carbon oxide (CO) was produced at higher discharge current.     

Implementation of the Effective Combustion of Gas Mixtures with a Low Emission of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and CO

The method for burning stoichiometric or near-stoichiometric gas mixtures in a burner device with a volumetric matrix is proposed, which provides low concentrations of harmful substances in the combustion products. It is shown that the concentrations of nitrogen oxide and carbon monoxide in the combustion products can be reduced to 10 ppm at an air excess ratio of ~1 and an output firing rate of ~300 W/cm².     

低不凝性气体含量下水平螺旋扁管管束中的沸腾换热实验研究

以汽油和空气为工质,在低质量含气率的条件下,对导程分别为100 mm和150 mm的螺旋扁管管束外的沸腾换热进行了实验研究。得到了沸腾换热系数随质量含气率变化的基本规律。在不凝性气体含量较低的情况下沸腾换热系数随质量含气率的增加而增加;当继续增加不凝性气体时,沸腾换热系数受到了一定的抑制。采用渐进逼近模型给出了低含气率条件下的沸腾换热系数准则方程。将用准则方程计算的三种流量下的两相沸腾换热系数与实验处理值进行比较,误差均在6%以内。     

Gas sensing behavior of SnO1.8:Ag films composed of size-selected nanoparticles

Size-selected SnO_1.8:Ag mixed nanoparticle films have been prepared using a gas phase condensation method. Transmission electron microscopy showed that the applied size-selection technique yields well-defined, monodisperse and spherical SnO_1.8 and Ag nanoparticles, both with a fixed diameter of 20 nm. The technique allows an independent variation of the particle size of both materials as well as the concentration of Ag. It allows to assess the influence of these parameters on the gas-sensing properties of the films, here for ethanol vapor in synthetic air. SnO_1.8:Ag nanoparticle films show optimal values of the sensor signal and response time at a Ag nanoparticle concentration of 5%. Due to the fact that the Ag nanoparticles are clearly distinct from the SnO_1.8 nanoparticles in the film, the most probable mechanism leading to improved sensor properties is chemical sensitization via a spill-over effect.