Homogeneous Gas Mixture Research Articles

The virial expansion of the grand potential at spherical and planar walls

The graphs of the virial expansion of the grand potential of molecules in a spherical cavity are related to the graphs of the virial coefficients of a homogeneous gas mixture. The second virial coefficient is obtained for hard spheres in a spherical cavity with a square-well wall, and for square-well molecules in a cavity with a hard wall. The planar limit of these results is used to analyse several different proposals for separating the grand potential into a bulk and surface part for arbitrary wall potentials. It is shown that none of these is entirely satisfactory and that, for systems of arbitrary geometry, no unique division is possible.

Efficiency of resonance laser-collisional conversion of hydrogen fluoride laser radiation

High-power hydrogen fluoride lasers with good energy parameters are now available.1 However, there are several factors why the radiation emitted from these lasers is not of satisfactory quality. In the case of cw hydrogen fluoride la- sers one of the main problems is the loss of optical homo- geneity by the active media as a result of mixing of the rea- gents. In the case of pulsed hydrogen fluoride lasers the optical homogeneity is disturbed by inhomogeneous initia- tion of the reaction. Suppression of these inhomogeneities is sometimes a very difficult task. Successful solution of many scientific and applied problems by hydrogen fluoride lasers requires that the divergence of the radiation from these la- sers should be close to the diffraction limit. A method for reducing the divergence of hydrogen flu- oride laser radiation based on resonance laser-collisional conversion (see Ref. 2-5 and the bibliography cited there) is currently being discussed. In this method the molecules of HF in an optically homogeneous gas mixture are pumped by HF laser radiation. This results in stimulated emission from HF molecules involving high rotational sublevels when the populations are inverted for transitions in the P branch. Wang et al? realized such resonance laser-collisional conversion emitted by a cw hydrogen fluoride laser. The maximum efficiency of the conversion process was 5.3% rel- ative to the pump power. In predicting the capabilities of the method it is important to know also the efficiency of conver- sion relative to the absorbed power. This efficiency was not determined in Ref. 3. The small-signal gain of a converter of radiation from a cw hydrogen fluoride laser was calculated in Ref. 4. The results obtained indicated that, in principle, the conversion process could have a high efficiency. We shall report the results of calculations of the effi- ciency of resonance laser-collisional conversion of radiation from a cw hydrogen fluoride laser. We shall assume that the laser radiation is absorbed and reemitted by a homogeneous gas stream of constant density and velocity. This allows us to ignore the equations of gasdynamics and to assume that χ = ut, where χ is the downstream coordinate, u is the stream velocity, and t is the time. Our calculations allowed for laser pumping, for VT, VV, and/?7relaxation processes, and for stimulated emission. The absorption cross sections were calculated using formulas from Ref. 6 on the assump- tion of an exact resonance. The FT relaxation rate of the first vibrational level of the HF molecule was taken from Ref. 7. It was assumed that the dependence of the FT relaxation rate on the serial number of the vibrational level was cubic: K υ. υ-ι — y3AT1>0.Theendothermal KFrelaxation rates were taken from Ref. 8 and their temperature dependences from Ref. 7. The R T relaxation processes were described by the model of strong collisions.910 The rotational relaxation times were taken from Ref. 11. The power of stimulated radi- ation was calculated in the constant-gain approximation. The following assumptions were made about the spectral composition of the pump radiation: 1) 45% of the total pow- er corresponded to the 0-1 band and 55% to the 1-2 band (such a distribution is typical of cw hydrogen lasers—see Ref. 12); 2) in each vibrational band the pump laser emits as a result of one transition in the P branch with / = 5 (this re- striction is unimportant for the model in question, but it makes it possible to reduce greatly the calculation time). We also assumed that reemission in the converter occurs due to one transition in the P branch for each vibrational band of the HF molecule. The rotational quantum number/, of this transition was treated as a parameter. Calculations showed that Jx = 9 was optimal from the point of view of the conver- sion efficiency in the case of long durations of interaction between the pump radiation and the gas stream.

Spark ignition energies for dust-air mixtures: Temperature and concentration dependences

Energy requirements for the spark ignition in air of polyethylene powder, lycopodium spores, and Pittsburgh seam bituminous coal dust were measured in a 1.2-L furnace and a 20-L chamber. Dust concentrations and initial temperatures were carefully controlled (at ambient pressure) so that the mixtures were above their lean limits of flammability and below their spontaneous autoignition temperatures. Electrical ignition energy requirements are given in terms of both effective spark gap energies, ε eff , and stored capacitor energies, 1/2 CE 2 . At ambient temperature and pressure, the average minimum ε eff -values for the three dusts were: 40 mJ for lycopodium, 50 mJ for polyethylene, and 160 mJ for the coal dust. The concentrations at which those minima were observed were 300, 500, and 750 g/m 3 , respectively. The measured electrical ignitability ranking is consistent with the data of other researchers but the measured values are systematically higher, probably because of the higher flow and turbulence levels used here. For lycopodium, its ease of ignition allowed its lean limit to be approached readily with spark sources, and measurements of its temperature dependence confirm the validity of the modified Burgess-Wheeler law for a dust. While the concept of a minimum electrical ignition energy for homogeneous gas mixtures is well established, the data reported here, and their analysis, suggest that the concept may not be particularly useful for heterogeneous dust-air mixtures. So many contradictory requirements are involved in experimental conditions, and such extraordinary difficulties and complexities are encountered for dusts, that a reliable determination of a minimum ignition energy that reflects intrinsic flammability behavior is dubious.

Spatially resolved methane band photometry of Saturn: II. Cloud structure models at four latitudes

Spatially resolved measurements of Saturn's reflectivity in the 6190-, 7250-, and 8996-Å methane bands are analyzed to determine cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near latitudes ±30°. Radiative transfer models are computed for a simple two-parameter structure. The parameters are A 0, the methane column abundance in an aerosol-free layer at the top of the atmosphere, and A 1, the specific abundance of methane in a semi-infinite homogeneous gas and cloud mixture deep in the atmosphere. For the Equatorial Zone, a model with A 0 = 37 ± 3 m-am and A 1 = 26 ± 2 m-am fits all three bands. For the North Temperate Region, a model with A 0 = 39 m-am and A 1 = 47 m-am comes close to fitting all three bands. For the South Equatorial Belt and South Temperate Region, a single A 0 and A 1 do not fit all three bands. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. Some suggestions are proposed to explain differences between model parameters derived using different absorption bands.

Homogeneous Gas Mixtures in the Bain Circuit

Homogeneous Gas Mixtures in the Bain Circuit

Energy loss of charged particles to molecular gas targets

The energy-loss spectrum of fast charged particles penetrating a dilute molecular gas target has been analyzed theoretically, with a homogeneous gas mixture in the state of complete dissociation as a reference standard. It is shown that the geometrical structure of molecules causes the energy-loss straggling and higher moments over the energy-loss spectrum to be greater than the corresponding quantities for a completely dissociated gas of equal composition. Such deviations from additivity are shown to be most pronounced at energies around the stopping-power maximum. Supporting evidence is found in the experimental literature. (AIP)

Spontaneous Ignition Delay of a Fuel Droplet in High Pressure and High Temperature Gaseous Environments

Experimental and theoretical studies on the ignition delay of a single droplet in high pressure gaseous environments were carried out. In the experiments, a movable furnace was dropped over a droplet to cause the ignition and the measurement of ignition delay was made for normal paraffin hydrocarbon droplets at 0 atg40 atg of ambient gas pressure and 220°C700°C of ambient gas temperature. Experimental data were correlated with the following expression. [numerical formula] where P, T, and φd are pressure, and temperature and oxygen concentration of ambient gas of which increase makes the ignition delay short. Ignition delay was independent of droplet size and increased with an increase in the number of carbon atom in a molecule of fuel. In the theory, a model was developed for droplet ignition, which consisted of the evaporation of a droplet and ignition of homogeneous gas mixture, and ignition delay was calculated. Both the experimental and the theoretical results showed reasonable agreements.

Estimation of solution and asymptotic behavior of the problem of flame propagation in a homogeneous gas mixture

Estimation of solution and asymptotic behavior of the problem of flame propagation in a homogeneous gas mixture

Effect of heats of chemical reactions on turbulence in homogeneous gas mixtures

The effect of the heats of chemical reactions on the turbulence in a turbulent gas flow is considered. An example of the application of the obtained results to a dissociating polyatomic gas is given.

Turbulent flame propagation in a homogeneous gas mixture

This paper predicts turbulent flame velocities theoretically and compares the results with experimental data. The turbulent flame model is a one-dimensional inviscid flame propagating in a tube at constant velocity and pressure through a premixed combustible gas. The chemical kinetics is taken to be a direct one-step, first-order irreversible exothermic reaction subject to an Arrhenius thermal dependence. The time-mean average concept is used to describe the turbulent flow. Isotropic turbulence of low intensity of turbulence and small scale of turbulence is also adopted. The conservation of species and energy equations are solved by two different numerical integration schemes; both methods are shown to yield similar results. The following information can be obtained from the equations: the laminar or turbulent flame velocity, and the temperature and species profile. Three specific cases were computed: (1) laminar flame propagation, (2) turbulent flame of five per cent intensity, and (3) turbulent flame of eight per cent intensity. The dimensionless integral scale of turbulence was assumed to be 0·85 and 0·64 respectively, for cases (2) and (3). In all cases a stoichiometric methane-air mixture was assumed. The results of the study can be summarized by the following equation: U T / U L = c + d( u′/ U L )( L y / b) where U T and U L are the turbulent and laminar flame propagation, respectively: c and d are constants depending upon the fuel, fuel air ratio and fluid flow conditions: u′ is the root-mean-square value of the fluctuating velocity; L y is the integral scale of turbulence; and b is a characteristic length. It can be shown that turbulent flame propagation increases with an increase in intensity of turbulence u′ if the scale of turbulence, L y laminar flame propagation, and fuel/air ratio are held constant. Also, turbulent flame propagation increases with a decrease in scale of turbulence L y if the intensity of turbulence u′, laminar flame propagation, and fuel/air ratio are held constant. The theoretical and experimental data are in close agreement.

Electrical modeling of the combustion process by means of a low-temperature plasma

The authors discuss the possibility of modeling the process of combustion of a homogeneous gas mixture on the basis of identical temperature dependences of the reaction rate and the electrical conductivity of a low-temperature plasma.

Vacuum-Type Gas-Flow Calibrator

A vacuum-type gas-flow calibrator was designed and constructed for making accurate calibrations of flowmeters needed for the investigation of high-pressure flames. This calibrator operates by automatically measuring the time required to pressurize a known volume from a near vacuum to 1 atmosphere pressure. The gas volume flow rate is obtained by dividing the calibration volume by the pressurization time and applying corrections from PVT data. This apparatus has been successfully used to calibrate flowmeters for use with hydrogen, oxygen, nitrogen, air, methane, nitric oxide, carbon monoxide, and various homogeneous nonexplosive gas mixtures.

Boundary Layer Flame Stabilization

An initial experimental investigation of boundary layer flame interaction has been carried out . Homogeneous propane-air gas mixtures with laminar free stream flow were studied. The flames were essentially two-d imensional and intersected a boundary layer flowing over a flat plate or airfoil shaped plate. Two distinct types of flame stabilization were found, depending upon the nature of the boundary layer. Experimental nonsteady phenomena of blowoff, flashback, and boundary layer flame oscillation are reported and discussed.

Some properties of rod-stabilized flames C homogeneous gas mixtures

Some properties of rod-stabilized flames C homogeneous gas mixtures