Showing papers on "Total pressure published in 2010"

Regulation of star formation rates in multiphase galactic disks: a thermal/dynamical equilibrium model

Abstract: We develop a model for the regulation of galactic star formation rates ΣSFR in disk galaxies, in which interstellar medium (ISM) heating by stellar UV plays a key role. By requiring that thermal and (vertical) dynamical equilibrium are simultaneously satisfied within the diffuse gas, and that stars form at a rate proportional to the mass of the self-gravitating component, we obtain a prediction for ΣSFR as a function of the total gaseous surface density Σ and the midplane density of stars+dark matter ρsd. The physical basis of this relationship is that the thermal pressure in the diffuse ISM, which is proportional to the UV heating rate and therefore to ΣSFR, must adjust until it matches the midplane pressure value set by the vertical gravitational field. Our model applies to regions where Σ 100 M ☉ pc–2. In low-ΣSFR (outer-galaxy) regions where diffuse gas dominates, the theory predicts that . The decrease of thermal equilibrium pressure when ΣSFR is low implies, consistent with observations, that star formation can extend (with declining efficiency) to large radii in galaxies, rather than having a sharp cutoff at a fixed value of Σ. The main parameters entering our model are the ratio of thermal pressure to total pressure in the diffuse ISM, the fraction of diffuse gas that is in the warm phase, and the star formation timescale in self-gravitating clouds; all of these are (at least in principle) direct observables. At low surface density, our model depends on the ratio of the mean midplane FUV intensity (or thermal pressure in the diffuse gas) to the star formation rate, which we set based on solar-neighborhood values. We compare our results to recent observations, showing good agreement overall for azimuthally averaged data in a set of spiral galaxies. For the large flocculent spiral galaxies NGC 7331 and NGC 5055, the correspondence between theory and observation is remarkably close.

Experiments and Simulations of n-Heptane Spray Auto-Ignition in a Closed Combustion Chamber at Diesel Engine Conditions

Abstract: Auto-igniting n-heptane sprays have been studied experimentally in a high pressure, high temperature constant volume combustion chamber with optical access. Ignition delay and the total pressure increase due to combustion are highly repeatable whereas the ignition location shows substantial fluctuations. Simulations have subsequently been performed by means of a first-order fully elliptic Conditional Moment Closure (CMC) code. Overall, the simulations are in good agreement with the experiment in terms of spray evolution, ignition delay and the pressure development. The sensitivity of the predictions with respect to the measured initial conditions, the spray modelling options as well as the chemical mechanism employed have been analysed. Strong sensitivity on the chemical mechanism and to the initial temperature on the predicted ignition delay is reported. The primary atomisation model did not affect strongly the predicted auto-ignition time, but a strong influence was found on the ignition location prediction.

Kinetics of Char Gasification with CO2 under Regime II Conditions: Effects of Temperature, Reactant, and Total Pressure

Abstract: There are relatively few studies reported in the literature characterizing the kinetics of high-pressure, high-temperature char gasification reactions. Of particular interest to the application of kinetic data to gasification models are studies that provide links between reaction rate data relevant to high-temperature conditions, where some extent of pore diffusion limitation may apply, and lower-temperature, intrinsic gasification reactivity data. This work describes the effects of temperature, reactant partial pressure, and total pressure on the kinetics of the char−CO2 reaction under conditions where reactant diffusion through the pores of the reacting char has been shown to impact the overall conversion rate. A global nth-order rate equation was used to describe these kinetics, in particular, the effect of temperature (via the activation energy) and reactant pressure (via the reaction order). As expected, activation energies at high temperatures (≥∼1300 K) were consistently less than those obtained un...

Internal‐tide energy over topography

Abstract: [1] The method used to separate surface and internal tides ultimately defines properties such as internal-tide generation and the depth structure of internal-tide energy flux. Here, we provide a detailed analysis of several surface-/internal-tide decompositions over arbitrary topography. In all decompositions, surface-tide velocity is expressed as the depth average of total velocity. Analysis indicates that surface-tide pressure is best expressed as the depth average of total pressure plus a new depth-dependent profile of pressure, which is due to isopycnal heaving by movement of the free surface. Internal-tide velocity and pressure are defined as total variables minus the surface-tide components. Corresponding surface- and internal-tide energy equations are derived that contain energy conversion solely through topographic internal-tide generation. The depth structure of internal-tide energy flux produced by the new decomposition is unambiguous and differs from that of past decompositions. Numerical simulations over steep topography reveal that the decomposition is self-consistent and physically relevant. Analysis of observations over Kaena Ridge, Hawaii; and the Oregon continental slope indicate O (50 W m−1) error in depth-integrated energy fluxes when internal-tide pressure is computed as the residual of pressure from its depth average. While these errors are small at major internal-tide generation sites, they may be significant where surface tides are larger and depth-integrated fluxes are weaker (e.g., over continental shelves).

Behavior of shock trains in a diverging duct

Abstract: A shock train inside a diverging duct is analyzed at different pressure levels and Mach numbers. Nonreactive pressurized cold gas is used as fluid. The structure and pressure recovery inside the shock train is analyzed by means of wall pressure measurements, Schlieren images and total pressure probes. During the course of the experiments, the total pressure of the flow, the back pressure level and the Mach number upstream of the compression region have been varied. It is shown that the Reynolds number has some small effect on the shock position and length of the shock train. However, more dominant is the effect of the confinement level and Mach number. The results are compared with analytical and empirical models from the literature. It was found that the empirical pseudo-shock model from Billig and the analytical mass averaging model from Matsuo are suitable to compute the pressure gradient along the shock train and total pressure loss, respectively.

Instability of shock-induced nozzle flow separation

Abstract: We investigate experimentally the causes of jet plume instability and enhanced mixing observed in the exhaust of shock-containing convergent-divergent nozzles. Key features of the internal flow are the separation shock, separation shear layers, and pattern of alternating expansion and compression waves downstream of the shock. We focus on two possible reasons for this instability—the motion of the separation shock and the wave pattern downstream of the shock. The nozzle flow was generated in a planar facility with variable area ratio and pressure ratio, and the motion of the shock was tracked using time-resolved wall pressure measurements. The isolated effect of the wave pattern was investigated in a separate facility wherein a sonic shear layer, simulating the nozzle separation shear layer, was disturbed with compression and expansion waves emanating from a wavy wall. In both instances, the instability of the shear layer was characterized by time-resolved measurements of the total pressure. In the nozzle flow, the amplitude of shock motion increases with shock strength. Correlation of shock motion with shear layer total pressure is virtually absent for weak shocks but becomes significant for strong shocks. However, impingement of stationary waves on the shear layer had no impact on its growth rate. We conclude that the enhanced shear layer instability is strongly coupled to shock motion, and that the wave pattern by itself is not a cause of enhanced mixing. The occurrence of asymmetric separation at large shock strengths is a further contributor to the enhancement of instability.

Experiment and analysis for an improved design of the inlet and nozzle in Tesla disc turbines

Abstract: In this article, the performance of the inlet to a Tesla disc turbine has been studied. The losses in the inlet and nozzle are known to be a major reason why the overall efficiency of disc turbines is not high. A new nozzle utilizing a plenum chamber inlet has been designed and tested here. Experiments have demonstrated less than 1 per cent loss in total pressure in the new design compared to losses in the range 13–34 per cent for the original nozzle and inlet. Other than the dramatic improvement in loss reduction, the new plenum-integrated nozzle achieves a considerable enhancement in the uniformity of the jet. This has been demonstrated here both by experimental traverses of Pitot tubes as well as computational fluid dynamics studies. The greater uniformity of the jet means that a single Pitot measurement approximately positioned at the centre of the jet would record a value close to the true centre-line total pressure, and that calculations based on centre-line values of total pressure would gi...

Measurements of Losses and Reynolds Stresses in the Secondary Flow Downstream of a Low-Speed Linear Turbine Cascade

Abstract: Experimental measurements of the mean and turbulent flow field were preformed downstream of a low-speed linear turbine cascade. The influence of turbulence on the production of secondary losses is examined. Steady pressure measurements were collected using a seven-hole pressure probe and the turbulent flow quantities were measured using a rotatable x-type hotwire probe. Each probe was traversed downstream of the cascade along planes positioned at three axial locations: 100%, 120% and 140% of the axial chord (Cx ) downstream of the leading edge. The seven-hole pressure probe was used to determine the local total and static pressure as well as the three mean velocity components. The rotatable x-type hotwire probe, in addition to the mean velocity components, provided the local Reynolds stresses and the turbulent kinetic energy. The axial development of the secondary losses is examined in relation to the rate at which mean kinetic energy is transferred to turbulent kinetic energy. In general, losses are generated as a result of the mean flow dissipating kinetic energy through the action of viscosity. The production of turbulence can be considered a preliminary step in this process. The measured total pressure contours from the three axial locations (1.00, 1.20 and 1.40Cx ) demonstrate the development of the secondary losses. The peak loss core in each plane consists mainly of low momentum fluid that originates from the inlet endwall boundary layer. There are, however, additional losses generated as the flow mixes with downstream distance. These losses have been found to relate to the turbulent Reynolds stresses. An examination of the turbulent deformation work term demonstrates a mechanism of loss generation in the secondary flow region. The importance of the Reynolds shear stress to this process is explored in detail.Copyright © 2010 by ASME

Diode Laser Detection of Greenhouse Gases in the Near-Infrared Region by Wavelength Modulation Spectroscopy: Pressure Dependence of the Detection Sensitivity

Abstract: We have investigated the pressure dependence of the detection sensitivity of CO2, N2O and CH4 using wavelength modulation spectroscopy (WMS) with distributed feed-back diode lasers in the near infrared region. The spectral line shapes and the background noise of the second harmonics (2f) detection of the WMS were analyzed theoretically. We determined the optimum pressure conditions in the detection of CO2, N2O and CH4, by taking into consideration the background noise in the WMS. At the optimum total pressure for the detection of CO2, N2O and CH4, the limits of detection in the present system were determined.

The Impact of Assimilating Surface Pressure Observations on Severe Weather Events in a WRF Mesoscale Ensemble System

Abstract: Surface pressure observations are assimilated into a Weather Research and Forecast ensemble using an ensemble Kalman filter (EnKF) approach and the results are compared with observations for two severe weather events Several EnKF experiments are performed to evaluate the relative impacts of two very different pressure observations: altimeter setting (a total pressure field) and 1-h surface pressure tendency The primary objective of this study is to determine the surface pressure observation that is most successful in producing realistic mesoscale features, such as convectively driven cold pools, which often play an important role in future convective development Results show that ensemble-mean pressure analyses produced from the assimilation of surface temperature, moisture, and winds possess significant errors in regard to mesohigh strength and location The addition of surface pressure tendency observations within the assimilation yields limited ability to constrain such errors, while the as

On the Efficiency of Secondary Flow Suction in a Compressor Cascade

Abstract: An experimemtal investigation in a high speed compressor cascade has been carried out to show the effect of different types of secondary flow suction. In order to get deeper insight into the separated three dimensional flow topology and to determine appropriate suction positions, numerical simulations are performed additionally for the baseline cascade. To obtain the flow solution, an implicit, pressure based solver, elaN3D (by ISTA TU Berlin), is employed in steady RANS mode, whereby the Menter SST-k model is used for turbulence treatment. Both investigations are conducted at Mach number Ma = 0.67 and Reynolds number Re = 560.000. The aerodynamic design condition is used. The examined cascade consists of NACA65-K48 type vanes. The experiments include measurements with four different types of suction geometries plus reference measurements. Total pressure and flow angle measurements in the wake show the flow deflection, total pressure loss and the rise of the static pressure of the cascade. The best suction geometry follows the design of R.E. Peacock, designed for low Mach number cascades, with small changes. Using a maximum suction rate of 2% of the main flow the total loss coefficient was reduced by 23%. In this case the stage efficiency — calculated with a reference rotor — is increased by almost 1%. The vacuum pump energy consumption has been taken into account for this calculation. In another case the suction geometry has been chosen in a way that the suction slot is placed along the sidewall from suction side to pressure side following the wall streamlines. With an increased suction rate of 5% of the main flow, the vortex system in the passage is eliminated and the total loss coefficient is decreased to 0.055, which equals to a decrease of 37%. Taking into account that compressors in aero-engines provide bleed air for the plane’s air system, enormous efficiency increase is possible. For this the air bleed valves need to be redesigned.Copyright © 2010 by ASME

In Situ XRD Detection of Reversible Dawsonite Formation on Alkali Promoted Alumina: A Cheap Sorbent for CO2 Capture

Abstract: Alkali-promoted aluminas are inexpensive and robust materials with significant basicity that allow CO 2 uptake at relatively high temperature and pressure. In situ XRD experiments show that bulk crystalline carbonate K-Dawsonite [KAlCO 3 (OH) 2 ] phase is formed on such materials under relatively high pressure of an equimolar mixture of CO 2 and steam (total pressure of 10 bar) and at temperatures up to 300 °C. In parallel, typical needle-shaped Dawsonite crystallites are observed by SEM after exposure to similar conditions. Furthermore, the in-situ experiments show that the carbonate crystalline phase disappears between 300-400 °C, and that K-Dawsonite crystalline phase can be reformed by lowering the temperature in the range 200-300 °C and contacting the material with both steam and CO 2 at sufficiently high partial pressure. In a fixed-bed reactor a high breakthrough capacity of 1.5-1.7 mmol g -1 has been measured. The experimental results reported herein highlight the high potential of alkali-promoted alumina for cyclic CO 2 removal in industrial systems.

Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

Abstract: The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing Experimental results are obtained for various O2/Ar gas mixtures (0?100%), applied powers (50?600?W) and total discharge pressures (2?25?mTorr) The measurements reveal that the O? ion dominates over with the latter less than 2% of the total observed Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200?W) and oxygen partial pressures (10% O2) The negative ion density was found to increase with the chamber pressure and the main loss reaction for O? was determined to be ion?ion recombination with O+, and Ar+In this study, the maximum negative ion density obtained was found to be 77 ? 1015?m?3 at 200?W applied power, 25?mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, ? = 14, indicating that the plasma is moderately electronegative These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O2 gas mixtures during dc sputtering The influence of these ions on thin film growth is briefly discussed

Ab initio quantum-mechanical calculation of CaCO3 aragonite at high pressure: thermodynamic properties and comparison with experimental data

Abstract: Structure and vibrational frequencies (at the Γ point) of CaCO 3 aragonite have been calculated from first principles, by using the hybrid Hartree-Fock/DFT B3LYP Hamiltonian, at different unit-cell volumes in the 185–242 A 3 range. By using the frequencies evaluated at such different volumes, the mode- γ Gruneisen’s parameters were estimated for each vibrational mode, and the zero point and thermal pressure contributions to the total pressure, at each volume and temperature, have then been determined by means of standard thermodynamic formulas, within the limits of the quasi-harmonic approximation. This allowed for the determination of ( i ) the equation of state at different temperatures; ( ii ) the thermal expansion as a function of pressure and temperature, and ( iii ) the evaluation of some thermodynamic properties (entropy and specific heat) together with their temperature dependences. Results were directly compared with relevant experimental data. The agreement of the calculated frequencies with the experimental data, at variable pressure, shows that the ab initio simulation can reproduce, at a relatively low computational cost, the full vibrational spectra of crystalline compounds of mineralogical interest. Moreover, the elastic properties (bulk modulus in particular), thermal expansion and thermodynamic properties, which play an important role in the characterization and in the understanding of the stability relations between carbonate phases, at high-pressure and high-temperature conditions, can be satisfactorily estimated. Precisely, at room temperature and pressure conditions, the calculated bulk modulus was 64.7 GPa, to be compared with an experimental value of 65(1) GPa (mean of three different experimental determinations); the estimated thermal expansion was about 6.1 · 10 −5 K −1 , which is only slightly underestimated with respect to the experimental datum [6.3(2) · 10 −5 K −1 ]; the calculated entropy ( S ) and the constant-pressure specific heat ( C P ) were 87.5 and 83.1 J mol −1 K −1 respectively, which are in close agreement with the experimental data [84(6) and 82.6 J mol −1 K −1 , for S and C P respectively].

Growth rate control and solid–gas modeling of TFA-YBa2Cu3O7 thin film processing

Abstract: The trifluoroacetate metal-organic decomposition route to YBa2Cu3O7 film growth was investigated in order to bring new insights in the growth mechanism and its dependence on processing conditions and critical current density. Precursor films were processed on LaAlO3 substrates at different total pressure, oxygen partial pressure, water vapor partial pressure, and volume gas flow rate keeping the growth temperature at 740??C. The influence of these various experimental parameters on the film growth rate, which was evaluated by in?situ electrical resistance measurements, was studied thoroughly. It was found that the growth rate is nearly independent of the oxygen pressure and proportional to the square root of the water pressure. Additionally, the growth rate increases with a decrease of the total pressure or an increase of the gas flow rate. An empirical multi-exponential model simulates the experimental data, however, a better understanding was achieved using a theoretical solid?gas reaction?diffusion model, including both the diffusion of the product HF gas from the surface and the chemical reaction kinetics at the YBCO/precursor interface. A complex cross-linked relationship between the film growth rate and the processing parameters has been properly verified from the experimental data. Finally we showed that the growth rate is an important parameter influencing the critical current densities of YBCO films because it controls the nucleation of non-c-axis oriented grains.

Influence of sample thickness and measurement set-up on the experimental evaluation of permeability of metallic foams

Abstract: The pressure drop measured when a fluid flow through a porous medium is presumed a linear function of the medium thickness along the flow direction Recent works have, however, reported that such linear dependence is not valid for highly porous foams The deviations observed were attributed to the relative contribution of entrance and exit effects on the total pressure drop In this work, the effect of the measurement set-up and of the specimen thickness on the pressure drop was investigated Permeability measurements were carried out with nickel-chromium foams Two different set-ups for holding the samples were tested Pressure drop vs velocity curves were obtained with water at room temperature The results obtained showed that the normalized pressure drop was affected by the specimen thickness and the set-ups used The size of the annular section supporting the specimens has a clear influence on the magnitude of the deviation from the expected linear behavior between the pressure drop and the medium thickness This effect has been attributed to the fluid diffusion and flow expansion into the annular region covered by the specimen support The magnitude of this effect is more important as the thickness of the specimens increased No significant entrance or exit effect was observed when the supporting annular section was minimized The dispersion of permeability constants caused by this effect was determined and discussed

Continuous spin detonation of a hydrogen-air mixture with addition of air into the products and the mixing region

Abstract: Results of an experimental study in a flow-type annular cylindrical combustor with an outer diameter of 30.6 cm are described. The influence of air addition to the products of continuous spin detonation of a hydrogen-air mixture and to the mixing region on parameters of detonation waves, pressure in the combustor, and specific impulse is studied. The range of continuous spin detonation of the hydrogen-air mixture is extended to specific flow rates of the mixture equal to 560 kg/(sec · m2) and fuel-to-air equivalence ratios equal to 0.5–4.4. It is demonstrated that addition of air decreases the detonation velocity, increases the pressure in the combustor and thrust, and decreases the specific flow rate of the fuel. The total pressure loss due to the mixing process and heat transfer to a colder gas increases. The minimum specific flow rate of hydrogen reached in the combustor of the examined geometry is 0.04 kg/(h · N).

Polystyrene Immobilized Ir(III) Complex as a new Material for Optical Oxygen Sensing

Abstract: The luminescence spectra of tris(phenylpyridine)iridium embedded in Amberlite (polystyrene adsorbent) have been measured under various O2/N2 mixtures (total pressure = 1 atm) It is shown that the Stern-Volmer plots are downward curved and that the curvature can be explained either by a two-site model or by a negative deviation from Henry's law A curve fitting procedure cannot distinguish between the two models Using a frequency modulated blue LED and a fiber optic set-up, O2 partial pressure of 04 mbar could be detected Oxygen can be determined at 200 mbar with a 15% accuracy

Experimental and theoretical determination of the saturation vapor pressure of silicon in a wide range of temperatures

Abstract: Reference books and original studies devoted to the determination of the saturation vapor pressure of silicon in a wide range of temperatures have been analyzed. It has been established that no reliable experimental data in the range of high temperatures (above 2000 K) are available in the literature. It has been demonstrated that there is a need to perform additional theoretical and experimental investigations with the use of different methods. The total pressure and partial pressures of Si n (n = 1−6) molecules over liquid silicon are calculated in the temperature range 1700–3400 K. The calculation of the composition of the gas phase over the “Si(l)-container” systems is performed. Materials of the crucibles intended for the use in experimental investigations of the temperature dependence of the saturation vapor pressure of silicon over the liquid phase are recommended.

The influence of hydrogen pressure on the heterogeneous hydrogenation of β-myrcene in a CO2-expanded liquid

Abstract: This paper presents the second part of work on the effect of hydrogen partial pressure on the hydrogenation of a terpene in a CO 2 -expanded liquid. The effect of hydrogen partial pressure on the hydrogenation of β-myrcene possessing three C C bonds catalysed by alumina-supported ruthenium and rhodium was studied. Experiments were performed at various hydrogen pressures in the range from 2.0 up to 4.5 MPa at a fixed total pressure of 12.5 MPa. In all the conditions the reaction proceeded in two phases (liquid + gas), that is, the total pressure was below the critical pressure of the CO 2 + β-myrcene + H 2 system. The liquid phase volume is expanded in relation to the initial volume of β-myrcene in a fashion that is strongly dependent on the hydrogen and carbon dioxide pressures. An increase of H 2 pressure concomitantly diminishes carbon dioxide pressure, which leads to the enhancement of the liquid phase in hydrogen and a terpene. It does not direct to straightforward higher reaction rate, but surprisingly the effect of higher concentrations either hydrogen or β-myrcene is opposite. It is attributed to the fact that the hydrogenation of β-myrcene rate-controlling factor turns out to be the hydrogen to β-myrcene ratio which decreases as the hydrogen pressure increases. These unexpected appealing results present that lower pressures of hydrogen guide to higher hydrogen/β-myrcene ratios in the liquid phase, but on the other hand they also amplify the initial reaction rate constant. The obtained results are opposite to the results achieved for effect of hydrogen pressure on the Pd-catalysed hydrogenation of limonene consisting of two C C double bonds.