Gas Phase Pressure Research Articles

A study of detonation diffraction and failure for a model of compressible two-phase reactive flow

A two-phase model of heterogeneous explosives, with a reaction rate that is proportional to the gas-phase pressure excess above an ignition threshold, is examined computationally. The numerical approach, a variant of Godunov's method designed to accommodate nonconservative terms in the hyperbolic model, extends previous work of the authors to two-dimensional configurations. The focus is on the behavior of an established detonation as it rounds a 90° corner and undergoes diffraction. The dependence of the post-diffraction conduct on the reaction rate is explored by varying the reaction-rate prefactor and the ignition threshold. The aim is to determine whether the model, as postulated, can capture dead zones, which are pockets of unreacted or partially reacted explosive observed in the vicinity of the corner in diffraction experiments. Results of this study are compared with those of a similar investigation on the one-phase ignition-and-growth model.

Peculiarities of electrical transfer and isotopic effects H/D in the proton-conducting oxide BaZr0.9Y0.1O3 − δ

Electrical conduction of the oxide BaZr0.9Y0.1O3 − δ is studied by electrochemical impedance spectroscopy as a function of temperature (300–600°C) and the oxygen partial pressure in gas phase saturated with H2O or D2O vapor. The full electrical conduction is separated into components, in particular, the bulk and grain boundary conduction. It is shown that at PO2> 1 Pa the BaZr0.9Y0.1O3 − δ conduction is contributed significantly by electron holes whose transference number in air atmosphere may be as high as 0.5–0.6. In reductive conditions, the electrical transfer involves proton (deuteron) charge carriers. Isotopic effect H/D in the conduction in the bulk and along the grain boundaries is determined. Isotopic effect H/D in the hole conduction is also revealed. It is shown that this effect comes out of different solubility of deuterons and protons in the oxide (the thermodynamic isotopic effect).

Convective dominated flows in open capillary channels

This paper is concerned with convective dominated liquid flows in open capillary channels. The channels consist of two parallel plates bounded by free liquid surfaces along the open sides. In the case of steady flow the capillary pressure of the free surface balances the differential pressure between the liquid and the surrounding constant pressure gas phase. A maximum flow rate is achieved when the adjusted volumetric flow rate exceeds a certain limit leading to a collapse of the free surfaces. The convective dominated flow regime is a special case of open capillary flow, since the viscous forces are negligibly small compared with the convective forces. Flows of this type are of peculiar interest since the free surfaces possess a quasisymmetry in the flow direction. This quasisymmetry enables the application of a new effective method for evaluation of the flow limit. The flow limit is caused by a choking effect. This effect is indicated by the speed index, S, which is defined by the ratio of the flow velocity and the longitudinal capillary wave speed. The speed index is defined analogously to Mach number and tends toward unity in the case of flow limitation, i.e., when the maximum flow rate is reached. Utilizing the quasisymmetry, a new approach for a very precise determination of the speed index is presented. This approach uses a new approximation for the curvature of the surfaces by means of the empirical surface profiles. On the basis of empirical and theoretical data, the paper discusses the typical features of the stable flow. The experiments were performed under microgravity aboard the sounding rockets TEXUS 41 and TEXUS 42. The experiment setup enables the approach to the flow limit through either increase in flow rate or channel length. The theoretical data have been gained from numerical solutions of a one-dimensional flow model. The empirical and theoretical results are in good agreement and both confirm the choking effect as cause of the flow limitation. A general relation for the speed index as function of the flow rate and the channel length has been found which clarifies the fundamental behavior of the choking effect.

Modeling of phase transition sI−sII in binary gas hydrates of methane and ethane in dependence on composition of gas phase

Calculations of phase transformations ‘structure sI–structure sII’ for binary methane–ethane hydrates were performed in the framework of molecular level model. The conditions of hydrate formation in equilibrium with gas phase and ice have been determined at different gas-phase compositions and pressures. It was shown that even at very low ethane concentration in the gas phase (about 0.5%) hydrate structure sII becomes more stable than sI. Regions of stability in T, P plane of sI and sII structures for mixed hydrates have been predicted. For mixed hydrates with ethane content about 5% in gas phase pressure-induced phase transformation at low temperatures is predicted.

Characteristics of Fin Baffle Packing Used in Rotating Packed Bed

For an alcohol/water system and with fin baffle packing, continuous distillation experiments were carried out in a rotating packed bed (RPB) system at atmospheric pressure. The effects of the average high gravity factor (β), liquid reflux ratio ( R) and feedstock flux ( F) on the momentum transfer and mass transfer were investigated. The gas phase pressure drop of RPB increased with the average high gravity factor, liquid reflux ratio and feedstock flux, which was 13.55–64.37 Pa at β of 2.01–51.49, R of 1.0–2.5, and F of 8–24 L·h −1 for a theoretical tray in the RPB with fin baffle packing. The investigation on the mass transfer in the RPB with different packings showed that the number of transfer units of RPB with a packing also increased with the average high gravity factor, reflux ratio and feedstock flux. It is found that the fin baffle packing (packing III) presents the best mass transfer performance and lowest pressure drop for the height equivalent to a theoretical plate (HETP), which is 6.59–9.84 mm.

Time Dependent Analysis of a Metal Hydride Bed

In this paper we present a study of the time dependent analysis of a metal hydride bed (MHB) which provides constant flow to a fuel cell at required power loading and pressure. The hydrogen gas phase pressure, the hydrogen concentration in the metal hydride and the hydrogen desorption rate are consider as key variables in this study. Both the space scale and time scale analysis are performed. Parameters investigations are also conducted. All the simulations are treated at isothermal conditions and applied using COMSOL software.

Theory of absolute rates of gas/solid interface reactions

The discussed results yielded from development of the absolute reaction rate theory for surface processes, which was initiated by works of M. I. Temkin on uniform and nonuniform solid surfaces. His works also studied the interaction effect of adsorbed particles to the kinetics of surface reactions and discussed several adsorption sites blocked by adsorbed particles, kinetic complications caused by low mobility of adsorbed particles, and effect of high pressures in gas phase. At the present time, the theory of surface processes provides simultaneous account of all the major physical factors: nonuniform surface, interaction of adsorbed particles, size of particles, limited mobility of reagents, and rearrangement of surface. This review analyzes the role of self-consistent description of elementary reaction rates and equilibrium reaction system, which was emphasized by M.I. Temkin. The self-consistency requirement for dense surface phases, which realize cooperative behavior of reagents, indicates to the restrictions of the earlier derived kinetic equations.

Flow Rate Limitation of Steady Convective Dominated Open Capillary Channel Flows Through a Groove

An open capillary channel is a structure that establishes a liquid flow path when the capillary pressure caused by surface tension forces dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. To maintain a steady flow through the channel the capillary pressure of the free surface has to balance the pressure difference between the liquid and the surrounding constant pressure gas phase. Due to convective and viscous momentum transport the pressure along the flow path of the liquid decreases and causes the free surface to bend inwards. The maximum flow rate through the channel is reached when the free surface collapses and gas ingestion occurs near the outlet. This stability limit depends on the geometry of the channel and the properties of the liquid. In this paper we present an experimental setup which is used in the low-gravity environment of the Bremen Drop Tower. Experiments with convective dominated systems have been performed where the flow rate was increased up to the maximum value. In comparison to this we present a one-dimensional theoretical model to determine important characteristics of the flow, such as the free surface shape and the limiting flow rate. Furthermore we present an explanation for the mechanism of flow rate limitation for these flow conditions which is similar to the choking problem for compressible gas flows.

Study on gas separation by supported liquid membranes applying novel ionic liquids

In the present study, the permeability of H 2, N 2 and CO 2 was investigated through supported liquid membranes prepared by using four types of novel ionic liquids (VACEM 42, VACEM 44, VACEM 47, VACEM 58) under various gas phase pressures (2.2 bar, 1.8 bar, 1.4 bar) and temperatures (30°C, 40°C, 50°C). VACEM type ionic liquids, which are built up of a common hexafluorophosphate anion and different cations, were tailored in order to dissolve CO 2. Since CO 2 can cause unfavorable changes in the mechanical and chemical properties of the PVDF membranes used as the supporting phase, the stability of the membranes was studied by measuring decrease in H 2 permeability after the permeation of CO 2. For determining selectivity the membranes were used to separate binary and ternary gas mixtures. It was found that the permeability of CO 2 was much higher than the permeability of the other two gases through the membranes prepared with VACEM type ionic liquids. Gas permeability exhibited an increase with increasing temperature and a slight decrease with increasing gas phase pressure. Experiments concerning stability showed that after several CO 2 permeations through membranes containing VACEM type ionic liquids the permeability of H 2 slightly decreased while the CO 2/H 2 selectivity improved. It was found that supported ionic liquid membranes prepared with VACEM type ionic liquids could be successfully used in separating CO 2 from other gases.

A simplified model for natural-gas vehicle catalysts with honeycomb and foam substrates

A mathematical model is developed for the simulation of the chemical and transport phenomena in three-way catalysts for natural-gas vehicles, with honeycomb and foam substrates. The chemical phenomena are modelled on the basis of a simplified approach, which assumes that pollutant conversion is governed by oxygen storage and release dynamics. The transport phenomena, i.e. heat transfer, gas-phase mass transfer, washcoat diffusion, and pressure drop, are modelled on the basis of correlations adapted from the literature. The model is validated using lambda scan tests and a cold-start driving cycle, carried out with a honeycomb catalyst and a ceramic foam catalyst. The experimental data show a decline in methane conversion for lean mixtures, which is modelled by an inhibition term of methane oxidation from nitric oxide. The proposed model may capture the performance of both substrates with sufficient accuracy, both under cold-start and under hot-mode conditions. The model-based comparison of the two substrates shows that the ceramic foam presents inferior carbon monoxide conversion in the extra-urban part of the cycle. Although the gas-phase mass transfer in the ceramic foam is faster than with the honeycomb monolith, diffusion in the washcoat pores is much slower. As a result, the mass-transfer-limited conversion efficiency of the foam is lower. This is primarily attributed to the higher washcoat thickness of the foam sample tested.

Influence of the gas phase resistance on hydrogen flux through thin palladium–silver membranes

Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773 K and at pressure differences up to 6 bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd–Ag layer (20 wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts’ law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance.

Characterization and CFD-modeling of the hydrodynamics of a prismatic spouted bed apparatus

Recently the importance of spouted bed technology has significantly increased in the context of drying processes as well as granulation, agglomeration or coating processes. However, the understanding of the complex interactions within and between the single phases is still low and needs further improvement. Several research groups apply both continuum as well as discrete element simulations to understand the hydrodynamics of the spouting process. This work focuses on the simulation of the hydrodynamic behavior of a prismatic spouted bed apparatus by applying the Euler/Euler continuum approach in the commercial computational fluid dynamics (CFD) software package FLUENT 6.2. The simulations are validated by experiments. Calculated and experimental (by PIV-measurements) obtained velocity vector maps, as well as measured and calculated gas phase pressure fluctuations over the entire bed are compared. The aim of this work is to improve the understanding of the hydrodynamics of the spouting process.

CFD modeling of a prismatic spouted bed with two adjustable gas inlets

AbstractSince the invention of the spouted bed technology by Mathur and Gishler (1955), different kinds of apparatus design were developed and a huge number of applications in nearly all branches of industry have emerged. Modeling of spouted beds by means of modern simulation tools, like discrete particle models or continuum models, has become a precious opportunity to understand the complexity of multiphase flows in these apparatus. In this work, two‐dimensional continuum simulations of a novel kind of spouted bed apparatus with adjustable gas inlets are presented. Different gas–particle drag models are used to obtain a realistic flow structure. Simulated solid phase concentrations are compared with images taken during experiments. Furthermore, simulated and measured gas phase pressure fluctuations over the entire bed are contrasted allowing more accurate comparisons between the simulated and the real hydrodynamic behaviour of the spouted bed.

Water Evaporation Versus Condensation in a Hygroscopic Soil

The liquid/vapour phase change of water in soil is involved in many environmental geotechnical processes. In the case of hygroscopic soils, the liquid water is strongly adsorbed on the solid phase and this particular thermodynamic state can highly influence the phase change kinetics. Based on the linear Thermodynamic of Irreversible Processes ideas, the non-equilibrium phase change rate is written as a linear function of the water chemical potential difference between the liquid and vapour state. In this relation, the system is characterized by a phenomenological coefficient that depends on the state variables. Using an original experimental set-up able to analyze the response of a porous medium subjected to non-equilibrium conditions, the phase change coefficient is determined in various configurations. This paper focuses on the influence of the gas phase pressure and underlines that a low gas pressure decreases the phase change kinetics. Then, evaporation and condensation processes are compared showing an asymmetric behaviour. These experimental results are interpreted from a microscopic point of view by relying on recent works dealing with molecular dynamics numerical simulation of the liquid/gas interface.

Dynamic Thermodynamics with Internal Energy, Volume, and Amount of Moles as States: Application to Liquefied Gas Tank

Dynamic models for process design, optimization, and control usually solve a set of heat and/or mass balances as a function of time and/or position in the process. To obtain more robust dynamic models and to minimize the amount of assumptions, internal energy, volume, and amount of moles are chosen as states for the conservation laws of the dynamic model. Temperature, pressure, and the amount and composition of the phases are calculated on the basis of these states at every time step. The Redlich−Kwong and Peng−Robinson (RK-PR) cubic equation of state is used as the thermodynamic model. This study describes the aspects of this approach and additionally gives a wide view over the whole internal energy and volume surface in specific phase diagrams. A complete separation between the dynamic balance model and the thermodynamic model is achieved. Several examples show the application of this approach for a liquefied gas tank and demonstrate that the method is applicable to one and two phases in a wide temperature and pressure range, from liquid and/or gas phase to supercritical conditions.

Water management studies in PEM fuel cells, Part II: Ex situ investigation of flow maldistribution, pressure drop and two-phase flow pattern in gas channels

Two-phase flow of water and reactant gases in the gas distribution channels of proton exchange membrane fuel cells (PEMFCs) plays a critical role in proper water management. In this work, the two-phase flow in PEMFC cathode parallel channels is studied over a wide range of superficial air velocity (air stoichiometry) and superficial water velocity in a specially designed ex situ experimental setup, which enables the measurement of instantaneous flow rates in individual gas channels and simultaneous visualization of the water flow structure. It is found that the two-phase flow at low superficial air velocities (air stoichiometry below 5) is dominated by slugs or semi-slugs, leading to severe flow maldistribution and large fluctuations in the pressure drop. Slug residence time, measured from the video observation and the instantaneous flow rate data, is found to be a new parameter to describe the slug flow. At higher air velocities, a water film is formed on the channel walls if they are hydrophilic. The pressure drop for the film flow is characterized by smaller but frequent fluctuations, which are found to result from the water buildup at the channel-exit manifold interface. As the superficial air velocity increases further, mist flow is obtained where little water buildup is observed. The water buildup in the gas channels at the two-phase flow is well described by the two-phase friction multiplier, defined as the ratio of the two-phase pressure drop to the single gas phase pressure drop. It is found that the two-phase friction multiplier increases with increasing water flow rate. A flow pattern map is developed using superficial water and air velocities with clearly defined transition regions.

Phase Change of Water in a Hygroscopic Porous Medium. Phenomenological Relation and Experimental Analysis for Water in Soil

The phenomenological relation of non-equilibrium liquid-gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hy- groscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilib- rium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.

H 2S Abatement in a biotrickling filter using iron(III) foam media

Airstreams polluted with H 2S at inlet loads ranging from 2.4 to 40.9 g H 2S m −3 h −1 were treated in a biotrickling reactor packed with hematite bearing, open pore foam units, at Empty Bed Residence Times (EBRT) ranging from 20 to 60 s over a period of 80 d, with almost complete removal of the pollutant from the startup of the system. The media had been seeded with sludge from a local water works facility, and removal efficiencies in excess of 80% were consistently observed along the operation of the reactor, with an average of 98%. Based on section performance, being a section one third of the bed length, observed elimination capacities (EC) reached up to 88.7 g H 2S m −3 h − 1 and 72.0 g H 2S m −3 h −1 at section EBRT of 10 and 7 s, respectively. The observed EC values compared much better than data reported on other packed bed reactors using biological iron oxidization to treat H 2S airstreams indirectly, and so did it when comparing the EC per unit of specific area in a similar study using polyurethane (PU) foams. Further, and unlike PU packed biofilters, no compaction occurred due to the iron foam rigidity, which translated in much better observed gas phase pressure drop as opposed to other conventional biofilters. Denaturing gel gradient electrophoresis was performed on the biomass collected in the packing after the biofilter service, and it was found that though a multi bacterial colony was seeded in the system via the sludge, the only surviving genus was the iron oxidizing Alicyclobacillus spp.

Light gas adsorption of all-silica DDR- and MFI-type zeolite: computational and experimental investigation

Grand canonical Monte Carlo simulations (GCMC) of adsorption of CO2 and N2 were carried out using the software Materials Studio 4.1 on all-silica DDR- and MFI-type zeolite. The simulated values of sorption capacity, isosteric heat of adsorption and Henry's constant for all-silica DDR- and MFI-type zeolite showed good agreement with experimental data. Accurate pure and binary adsorption data were measured for all-silica DDR-type zeolite crystals. Furthermore, the GCMC simulation was used to assess the adsorption properties and selectivity of CO2/N2 mixture gas as a function of gas phase composition and pressure. Thermodynamic properties and binary mixtures of adsorption showed good agreement with our simulation and experiments.

Properties of disturbance waves in vertical annular two-phase flow

Disturbance waves play an important role in interfacial transfer of mass, momentum and energy in annular two-phase flow. In spite of their importance, majority of the experimental data available in literature on disturbance wave properties such as velocity, frequency, wavelength and amplitude are limited to near atmospheric conditions (Azzopardi, B.J., 1997. Drops in annular two-phase flow. International Journal of Multiphase Flow, 23, 1–53). In view of this, air–water annular flow experiments have been conducted at three pressure conditions (1.2, 4.0 and 5.8 bar) in a tubular test section having an inside diameter 9.4 mm. At each pressure condition liquid and gas phase flow rates are varied over a large range so that the effects of density ratio, liquid flow rate and gas flow rate on disturbance wave properties can be studied systematically. A liquid film thickness is measured by two flush mounted ring shaped conductance probes located 38.1 mm apart. Disturbance wave velocity, frequency, amplitude and wavelength are estimated from the liquid film thickness measurements by following the statistical analysis methods. Parametric trends in variations of disturbance wave properties are analyzed using the non-dimensional numbers; liquid phase Reynolds number ( Re f), gas phase Reynolds number ( Re g), Weber number ( We) and Strouhal number ( Sr). Finally, the existing correlations available for the prediction of disturbance wave velocity and frequency are analyzed and a new, improved correlation is proposed for the prediction of disturbance wave frequency. The new correlation satisfactorily predicted the current data and the data available in literature.