Showing papers on "Atmospheric pressure published in 1996"

Numerical modeling of catalytic ignition

Abstract: Catalytic ignition of CH4, CO, and H2 oxidation on platinum and palladium at atmospheric pressure is studied numerically. Two simple configurations are simulated: the stagnation flow field over a catalytically active foil and a chemical reactor with a catalytically active wire inside. The simulation includes detailed reaction mechanisms for the gas phase and for the surface. The gas-phase transport and its coupling to the surface is described using a simplified multicomponent model. The catalyst is characterized by its temperature and its coverage by adsorbed species. The dependence of the ignition temperature on the fuel/oxygen ratio is calculated and compared with experimental results. The ignition temperature of CH4 oxidation decreases with increasing CH4/O2 ratio, whereas the ignition temperature for the oxidation of H2 and CO increases with increasing fuel/oxygen ratio. The kinetic data for adsorption and desorption are found to be critical for the ignition process. They determine the dependence of the ignition temperature on the fuel/oxygen ratio. A sensitivity analysis leads to the rate-determining steps of the surface reaction mechanism. The bistable ignition behavior observed experimentally for lean H2/O2 mixtures on palladium is reproduced numerically. The abrupt transition from a kinetically controlled system before ignition to one controlled by mass transport after ignition is described by the time-dependent codes applied.

Trapped methane volume and potential effects on methane ebullition in a northern peatland

Abstract: A novel way of estimating the gas bubble volume in the floating mat sediment of a peatland was developed at Thoreau's Bog in Concord, Massachusetts. Statistically significant relationships between the buoyancy of the floating Sphagnum mat and atmospheric pressure were observed, and these relationships were used to estimate the gas bubble volume. The mass of CH 4 stored in gas bubbles is estimated to be as much as 3 times the mass of dissolved CH 4 , depending on the time of year. The gas bubble volume is frequently large enough to serve as a significant buffer between microbial production of CH 4 and the release of CH 4 to the atmosphere. Changes in atmospheric pressure, temperature, and water-table elevation may result in modulation of the ebullitive CH 4 flux. Periods of rapidly rising atmospheric pressure or equivalent pressure changes due to water-table elevation are capable of arresting bubble volume growth, thereby halting CH 4 ebullition. Periods of rapid cooling of the bog could also temporarily halt ebullition, as thermally induced contraction of bubbles and dissolution of CH 4 offset bubble volume growth due to methanogenesis.

Absorption of solar energy in the atmosphere : Discrepancy between model and observations

Abstract: An atmospheric general circulation model, which assimilates data from daily observations of temperature, humidity, wind, and sea-level air pressure, was compared with a set of observations that combines satellite and ground-based measurements of solar flux. The comparison reveals that the model underestimates by 25 to 30 watts per square meter the amount of solar energy absorbed by Earth's atmosphere. Contrary to some recent reports, clouds have little or no overall effect on atmospheric absorption, a consistent feature of both the observations and the model. Of several variables considered, water vapor appears to be the dominant influence on atmospheric absorption.

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Abstract: Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

A study of parasitic reactions between NH 3 and TMGa or TMAl

Abstract: The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH3 is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH3. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH3.

Gas mixing apparatus for a ventilator

Abstract: The gas mixing apparatus provides the components of a breathing gas for mixing at approximately ambient atmospheric pressure, and regulates the pressure of a selected gas to approximately ambient atmospheric pressure for mixing with air at ambient atmospheric pressure. The gas mixing apparatus includes a piston disposed within a pump chamber. A flow limiting inlet controls introduction of a first selected gas such as oxygen for mixing with a second selected gas such as air. The pressure of the first selected gas is limited to an acceptable maximum pressure, so that even if a valve for admitting the first selected gas for mixing at ambient pressure fails, breathing gas will not be provided at an excessive pressure. A demand valve is alternatively provided for reducing the pressure of the first selected gas supplied before mixing, but a pressure sensor is also provided downstream of the demand valve for detecting failure of the demand valve, to shut off the supply of the first selected gas to prevent overpressurization.

Pulsed corona and dielectric‐barrier discharge processing of NO in N2

Abstract: Experimental results on pulsed corona and dielectric‐barrier discharge processing of very dilute concentrations of NO in N2 are presented. These NO reduction experiments measure the G value for electron‐impact dissociation of N2 and are used to infer the effective electron mean energy in an N2 discharge plasma at atmospheric pressure. The data have been obtained from three different laboratories using widely differing electrode structures, voltage wave forms, power measurements, and chemical analyses. The NO reduction yields from the discharge reactors tested are all similar, corresponding to an electron mean energy of 4.0±0.5 eV.

Phase behaviors of supercooled water: Reconciling a critical point of amorphous ices with spinodal instability

Abstract: The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphousices(LDA and HDA), and a predicted fragile–strong transition are accounted for in a unified manner by reconciling an idea due to Stanley and co‐workers introducing a second critical point separating LDA and HDA ices with a conjecture proposed by Speedy that LDA is a different phase from a normal water, called water II. The reconciliation is made on the basis of results from extensive molecular dynamics simulations at constant pressure and temperature. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural, and dynamic properties at atmospheric pressure, suggesting liquid–liquid phase transition. This transition is identified with an extension of the experimentally observed LDA–HDA transition in high pressure to atmospheric pressure. Thus, we propose a new phase diagram where the locus of the second critical point is moved into negative pressure region. With this simple modification, it becomes possible to account for the divergence of the thermodynamic response functions at atmospheric pressure in terms of the critical point and the spinodal‐like instability of HDA. The unstable HDA undergoes a transition to LDA phase in lower temperature. The transition is also observed in high pressure region such as 200 MPa while it disappears at negative pressure, −200 MPa. This reinforces our proposed phase diagram in which there is no continuous path from a supercooled state to LDA at atmospheric pressure. It is argued that the HDA–LDA transition is accompanied by a fragile–strong transition. A possible mechanism of avoiding crystallization of aqueous solutions is also discussed in terms of a difference in hydrogen bond number distribution between LDA and HDA.

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Abstract: Pressure probe measurements have been interpreted as showing that xylem pressures below c. -0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below -0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment : (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits ; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms) ; data extended to c. -10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above -0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. -0.5 MPa, cavitation limits its measurement range in situ.

Experimental investigation and characterization of the departure from local thermodynamic equilibrium along a surface‐wave‐sustained discharge at atmospheric pressure

Abstract: Surface‐wave‐sustained discharges (SWDs) form a particular class of high frequency (HF) discharges: their HF sustaining field is provided by a traveling wave that transfers energy as it propagates along the discharge column, yielding a plasma column with an axially decreasing electron density. SWDs have proved to be ideal for investigating experimentally and theoretically both the HF field and discharge aspects of HF plasma sources at reduced gas pressure. In this article, SWDs are utilized at atmospheric pressure to gain insight into the departure from thermodynamic equilibrium (TE) of HF sustained discharges. This departure is found to increase significantly as the electron density decreases along the plasma column whereas the gas temperature and the power absorbed per electron do not vary axially. The two‐temperature plasma model provides an adequate description of this departure from TE.

Signal processing of soil gas radon, atmospheric pressure, moisture, and soil temperature data: A new approach for radon concentration modeling

Abstract: The variability in space and in time of gases (He, 222Rn, CO2) in soils might be used for volcano or seismic fault surveillance or in search of hidden mineral deposits. The gases measured in soils or in the weathered layer of the substratum can, however, be strongly altered by environmental conditions, such as atmospheric pressure, soil temperature, or moisture. An accurate knowledge of the influence of environmental conditions is required to decipher information from deeper phenomena in the earth. Variations in radon concentration, used as a gas flow tracer, are modeled using a new approach based on signal processing techniques in order to express impulse responses from multivariable time series. A general formulation is proposed and the inverse problem is solved by calculating the impulse responses of the parameter to be explained versus the variables responsible for the variability of the parameter. Such problems are generally ill posed and regularization methods must be used to develop the pattern. Due to the block-Toeplitz structure of the correlation matrix, large orders can be used, enabling signal processing of time series controlled by the superposition of fast and slow phenomena. Two examples are processed to illustrate this approach. In the first example (222Rn in soil gas monitoring over a hidden sulphide deposit), radon concentration is controlled only by moisture and temperature, not by atmospheric pressure. The result seems to reveal a diffusive behavior. The impulse response of 222Rn concentration versus rainfall exhibits a great variability, possibly due to the opening of cracks in the surface, the effect being still noticeable 20 days after the rainfall. In the second example (222Rn in a thermal anomaly at summit of Etna volcano), where radon concentration is controlled by soil temperature and atmospheric pressure, the result indicates a convective/advective flow. Radon flux collapsed for 1 day probably due to a break in the rising gas column. In 1993, dramatic increases of signal were recorded for a few hours; such increases are interpreted as gas pulses not related with local seismicity.

Laminar flame speeds and oxidation kinetics of benene-air and toluene-air flames

Abstract: Using the counterflow twin-flame technique, the laminar flame speeds of benzene-air and toluene-air mixtures were determined experimentally over an extensive range of equivalence ratios at room temperature and atmospheric pressure. The minimization of stretch effects in the determination of the flame speed was accomplished through both linear and nonlinear extrapolations of the stretched reference flame speed to vanishing stretch, with the nonlinearly extrapolated values being typically 2 cm/s smaller. The laminar flame speeds of toluene were found to be lower than those of benzene, typically by 5–6 cm/s. Numerical simulations of the flame speeds were performed using two reaction mechanisms developed by Emdee, Brezinsky, and Glassman (EBG) and by Lindstedt and Skevis (LS), respectively. Predictions by using both mechanisms were close and were substantially lower than the experimental values. A modified EBG mechanism is then proposed, which adopts a recently measured rate constants of phenyl+O2→ phenoxy+O and takes into account the pressure fall-off for the rate coefficients of several key radical-radical recombination reactions. It is shown that these modifications substantially improve the flame-speed predictions, while minimally affect the available flow-reactor data of benzene and toluene oxidation. The present analysis suggests that the major oxidation pathways of benzene and toluene proposed in the flow-reactor study of Emdee, Brezinsky, and Glassman can account for their oxidation in flames and, therefore, serve as a foundation for a comprehensive model. It further indicates that the development of such a model requires additional studies on the reaction kinetics of phenol and cyclic C5 species.

Influence of gravity and pressure on pool fire-type diffusion flames

Abstract: Experiments are conducted to study the effects of gravity and pressure on the characteristics of diffusion flames of the pool fire type, that is, a diffusion flame stabilized on a burning horizontal fuel surface. In the experiments, the pool fire is simulated by injecting at very low velocity, a gaseous fuel (ethane) through a small-scale, porous, flat, horizontal surface burner and generating a diffusion flame over the burner by the reaction of the gaseous fuel with air. The resulting diffusion flame is characterized by a low Froude number. The diffusion flame characteristics (visual appearance, height, radiant output and temperature, and velocity distribution) are investigated at gravity levels ranging from microgravity (parabolic trajectory of an aircraft) to 12 times normal gravity (centrifuge facility—atmospheric pressure), and at ambient pressures ranging from 0.03 to 0.3 MPa (normal gravity). The results provide information about the effects of these variables on the flame characteristics and data for validation of numerical models of diffusion flames. Furthermore, they also help in understanding some of the limitations of Froude, or pressure, modeling of fires. The experiments indicate that the effect of gravity and pressure on the flame characteristics appears primarily through their effect on the buoyantly induced entrainment of air by the flame plume. Although at elevated pressures the effects are similar on the flame size and shape, important differences are observed on their effect on soot formation. It is found that for pressures above atmospheric, pressure has a major influence in soot formation and, consequently, on the radiant characteristics of the flames, increasing as pressure is increased. It is also found that at pressures below atmospheric pressure and gravity have opposite effects on flame size and soot formation and that consequently their effects on flame radiation also differ.

Diode laser-based air mass flux sensor for subsonic aeropropulsion inlets

Abstract: An optical air mass flux sensor based on a compact, room-temperature diode laser in a fiber-coupled delivery system has been tested on a full-scale gas turbine engine. The sensor is based on simultaneous measurements of O(2) density and Doppler-shifted velocity along a line of sight across the inlet duct. Extensive tests spanning engine power levels from idle to full afterburner demonstrate accuracy and precision of the order of 1-2% of full scale in density, velocity, and mass flux. The precision-limited velocity at atmospheric pressure was as low as 40 cm/s. Multiple data-reduction procedures are quantitatively compared to suggest optimal strategies for flight sensor packages.

Plasma-assisted deposition at atmospheric pressure

Abstract: Most plasma-assisted deposition methods currently available use gas discharges at pressures below 1 hPa. In many cases, the process temperature can be kept low due to the fact that the energy necessary for the initiation of chemical reactions is transferred via charged particles. However, a low pressure requires a large amount of vacuum equipment. Processes at atmospheric pressure are more favourable if results similar to those of existing methods can be achieved. Barrier discharges provide the basis for a new plasma-assisted deposition method at atmospheric pressure. These discharges consist of a large number of transient microdischarges in parallel which are distributed statistically on the surface to be coated. Starting with some basic considerations on the properties of microdischarges, the deposition of thin polymeric films on glass surfaces is described using barrier discharges at atmospheric pressure and acetylene. Uniform polymeric films up to 1 μm are obtained if trains of voltage pulses are used. The parameters influencing the deposition rate and the film quality are discussed. In addition, it is estimated whether further improvements of the deposition process are possible.

Precision high pressure control assembly

Abstract: A precision high-pressure control assembly for supercritical fluids comprises a continuous flow system having a pressure control loop which includes a source of fluid communicating with a piston driven pump. A pressure sensor monitors the pressure of the supercritical fluid in the outlet line of the pump. A pressure controller has an input for receiving a signal relating to the pressure sent by the pressure sensor and the pressure controller yields an electronic output signal to an electropneumatic regulator. A source of air communicates with the electropneumatic regulator to provide pressurized regulated driver air directed to the pump. The electropneumatic regulator controls the regulated driver air pressure of the pump in accordance with the signal received from the pressure controller. The piston head of the pump is in a cryogenic chamber to minimize flash and cavitation.

Method for the ionization of heavy molecules at atmospheric pressure

Abstract: The invention relates to a device and method for the ionization of non-vaporizable substance molecules at atmospheric pressure by chemical ionization (APCI=atmospheric pressure chemical ionization). The invention consists of desorbing the analyte substances which are mixed with decomposable substances (matrix substances) in solid form on a solid support, by laser irradiation at atmospheric pressure into a gas stream, and to add sufficient ions for proton transfer reactions to the gas stream. Explosives like cellulosis trinitrate or trinitro toluene (TNT) form a preferred class of decomposable matrix substances.

Influence of High Hydrostatic Pressure and pH on the Rate of Maillard Browning in a Glucose−Lysine System

Abstract: Glucose−lysine solutions, initial pH 5.1, 6.5, 8.0, and 10.1, were incubated at temperatures in the range 40−60 °C under atmospheric pressure and 600 MPa. The rate of Maillard browning at 50 °C was shown to be retarded by pressure in solutions of initial pH 5.1 and 6.5 but significantly enhanced in solutions of initial pH 8.0 and 10.1, while the effect of pressure was negligible at pH 7.0−7.5. At pH 10.1 the activation energies for the two systems (high pressure and atmospheric pressure) were not significantly different. The rates of reaction of these systems carried out in phosphate and bicarbonate buffer, at 600 MPa and alkaline pH, were slower than expected. It is proposed that pressure-induced ionization of the carboxylate group on the lysine at low pH, or of the phosphate and bicarbonate groups at high pH, causes a decrease in pH and subsequent reduction in the rate of reaction. In systems in which the pH is unaffected by pressure, i.e., those buffered by the amino groups of lysine, pressure accelera...

Homogeneous ignition of hydrogen-air mixtures over platinum

Abstract: The homogeneous ignition of hydrogen-air mixtures over platinum is studied at atmospheric pressure using a stagnation-point flow model with detailed gas-phase kinetics, surface kinetics, and transport phenomena. The momentum, energy, and mass balances are discretized using a second-order finite difference scheme. The obtained set of algebraic equations is then solved using Newton's method. An are-length continuation is employed to determine ignitions and to perform parametric studies. Inhibition of the homogeneous ignition caused by the catalyst is observed in agreement with published experiments. A local maximum in the homogeneous ignition temperature is found at the surface stoichiometric point at ∼15% H 2 in air, a composition determined by surface reactions and multicomponent transport effects. The gasphase inhibition is induced mainly by product formation through the termination reaction H+O 2 +M→HO 2 +M and by depletion of the fuel below ∼15% H 2 in air or of the oxidant above ∼15% H 2 in air. Sensitivity analysis shows that desorption of radicals has a minor effect on gas-phase ignition of H 2 in air. On the other hand, the dissociative adsorption of O 2 through O 2 +2°→2O°, (° denotes adsorbed species or an adsorption site) has a strong influence on the homogeneous ignition temperature. Reaction path analysis reveals a change in the major surface reaction path for formation of H 2 O° from 2OH°→H 2 O°+O° when H 2 in air is less than ∼15% to OH°+H°→H 2 O°+° when this composition is greater than ∼15%. Implications for homogeneous ignition of hydrocarbons near catalysts are also discussed.

Studies on kinetics of ammonia synthesis over ruthenium catalyst supported on active carbon

Abstract: The rate of ammonia synthesis was measured for ruthenium and iron catalysts under high (100 bar) and under atmospheric pressures. The effect of ammonia concentration in the gas phase on reaction rate was measured at 400, 430, and 470°C for p = 100bar at 370 and 400°C for p = 1bar . It was found that ruthenium is less sensitive than iron to the increase of ammonia pressure in the gas phase. It is also much less sensitive to changes in the total pressure (for p H 2 :p N 2 =const. ). At relatively high conversion degrees the ruthenium catalyst used was several times more active than the fused iron catalyst at 100 bar and 25–40 times more active under atmospheric pressure.

Spectral and Electrical Diagnostics of Gliding Arc

Abstract: Using spectroscopic and electric measurements, vibrational and rotational molecular gas temperatures as well as free electron temperature and concentration were determined in different regions of a time-periodical like, atmospheric pressure non-equilibrium lοw current gliding arc. It was shown that this discharge includes an initial quasi-equilibrium zone, with the quasi-equilibrium temperature of 4 to 6 kK, and the non-equilibrium zone with the electron temperature about 10 kK, the vibrational temperature about 3 kK, rotational and translational temperatures from 1 to 1.5 kK. The transition between two mentioned zones coincides with the phenomenon of the arc "length explosion" already observed in moderate-current gliding arc. PACS numbers: 33.10.Jz, 52.70.Kz, 52.25.Kn

Sensor and method for detecting trace underground energetic materials

Abstract: A subsurface soil contaminant identification system employs a cone penetrometer unit for continuously measuring the concentration of energetic materials in potentially contaminated soils. The sensor is rugged, reliable, and has a fast response time. The invention utilizes two pulsed, time-delayed miniature lasers. An infrared laser is used for decomposing the energetic material into NO and other products, while a visible laser operating near 452 nm is used for NO detection by (2+2) resonance-enhanced multiphoton ionization (REMPI). The system employs a fiber optic to transmit the output radiation at distances of approximately 30-50 meters, a lens assembly to focus one or both laser beams, a pair of miniature electrodes to collect the ions, a penetrometer, and data acquisition/processing equipment. A REMPI spectrum of 0.1% NO at atmospheric pressure reveals that the spectral resolution is sufficient such that characteristic spectral features of NO can be identified unequivocally.

Room-temerature oxidation of Ni(110) at low and atmospheric oxygen pressures

Abstract: Oxidation of atomically clean (110) nickel single crystals has been studied at room temperature and in pure oxygen or air at pressures from 1×10−9 torr to atmospheric X-ray photoelectron (XPS) and Auger electron spectroscopic (AES) data indicate that the standard regimes of dissociative chemisorption, oxide nucleation, and oxide lateral growth to coalescence were observed at low pressures After the NiO layer coalesced at low pressures, exposure of the sample to atmospheric oxygen or air did not cause further growth of the oxide thickness at room temperature Instead the growth of a high-energy shoulder on the O 1s XPS peak indicated the formation of Ni(OH)2 on the surface The presence of the hydroxide is consistent with high-resolution, electronenergy-loss spectroscopy (HREELS) data and chemical shifts in the Ni 2p spectra While the oxide thickness is constant, the hydroxide thickness increased with exposure and time at high pressure Surface analysis and lowpressure techniques are appropriate for the study of room-temperature, ambient-oxide formation and allow a determination of the kinetics and reaction products critical to the passivation of Ni