Gas Concentration Distribution Research Articles

Gas concentration and temperature reconstruction by genetic simulated annealing algorithm based on multi-wavelengths diode laser absorption spectroscopy

Gas concentration and temperature distribution reconstruction is realized based on tunable diode laser absorption spectroscopy. A measurement system using wavelength multiplexing technology with four absorption transitions is designed to reduce the number of projections and enrich the gas absorption measurement data. According to the nonlinear system established, a hybrid algorithm based on genetic algorithm and simulated annealing algorithm is introduced to obtain the global optimization and improve the search efficiency. A model for H2O concentration and temperature distribution in combustion is assumed, and numerical simulation is utilized to reconstruct the images with the transitions within 1.3—1.5 μm range. Both concentration and temperature distribution reconstruction results are in agreement with the model. The influence of random errors in projections on reconstruction is also analyzed. The increased error in projection would lead to larger mean square error of temperature reconstruction results, but little affect the concentration reconstruction results.

Deposition of solid particles in laminar boundary layer on a flat plate

Results are given of experimental and theoretical investigation of deposition of small solid particles on the surface of a flat plate under conditions of vertical laminar boundary layer. The present investigation is aimed at qualitatively and quantitatively estimating the effect made by the parameters of two-phase flow of the “gas—solid particles” type and by the adhesive properties of particles and surface on the deposition of particles on the plate surface. The flow velocity is 1.5 and 3 m/s. In so doing, the value of Reynolds number along the plate does not exceed 105. Synthetic corundum powders with average sizes of 12, 23, and 32 µm are used as the dispersed phase of two-phase flow. The mass concentration of particles in the flow is 0.01 kg/m3. A flat plate of stainless steel is used as the object of investigation. The distributions of gas velocity and concentration of particles within the boundary layer are measured using laser optical diagnostics. The number of particles deposited along the plate surface is measured by the gravimetric method. The adhesive properties of the “particle-surface” pair are studied using the centrifugal method of detachment of particles from the surface. Logarithmic-normal dependences of the number of adhesion of particles on the force of detachment are obtained. The hydrodynamic parameters of two-phase flow in the vicinity of the plate surface are calculated using the model of two-phase laminar boundary layer.

A Three-Dimensional Navier-Stokes–Based Numerical Model for Squeeze-Film Dampers. Part 1—Effects of Gaseous Cavitation on Pressure Distribution and Damping Coefficients without Consideration of Inertia

Even though most published results detailing damper behavior consider only the liquid phase, the cavitation process in the lubricant film, when it happens, is critical for the damper's performance. A number of modeling approaches, such as the half-Sommerfeld and Elrod models, were proposed in order to account for the effects of cavitation on the pressure generation, without directly simulating the cavitation process. Based on the experimental data, a few other homogeneous cavitation models have also been developed. All these models are based on the classical Reynolds equation. In this article, a three-dimensional numerical model is developed and validated in connection with the operation of a two-phase squeeze-film damper. The full Navier-Stokes equations (NSE), coupled with a homogeneous cavitation model, is solved to simulate the flow of the two-phase lubricant film and the associated pressures. The pressure variation on the journal surface and the gas concentration distribution in the lubricating fluid (cavitated region) will be presented. The damping coefficients predicted by the NSE model are compared to the ones that resulted from the application of the Reynolds equation.

Study on multiphase flow field of gas transportation in drivage face

Based on theories of multiphase fluid mechanics, the gas transportation and concentration distribution in drivage face were studied in this paper. According to gas transportation characteristics in drivage working face, reasonable simplification was made, and two-phase flow transportation model for gas and air was established, then gas transportation characteristics in drivage face were theoretically deduced. In this paper, the gas transportation in drivage working face was also numerically simulated by common CFD software, and the distribution and transportation law of gas in some sections of the laneway were elicited. The results indicate: (1) The gas transportation in mine atmosphere is essentially the quality transmission process of gas in the flow; (2) The main factors having influences on gas transportation in the laneway are convection, molecule diffusion, turbulent flow diffusion and displacement transportation; (3) The main gas accumulation regions are the laneway corners, the laneway roof, the combining part of digging laneway and main laneway.

Simulation and testing of pollutant dispersion during preventive maintenance in a cleanroom

Sulfur hexafluoride (SF 6) of >99.9% purity was artificially released to simulate the emission sources in the etching-thin film area of a working cleanroom in a semiconductor fab at the rate of 492 g/h. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF 6 concentrations at different locations of the cleanroom. A three-dimensional numerical model was also used to predict the unsteady gas concentration distribution and the results were compared with the experimental data. Due to high dilution of the pollutant in the cleanroom, it is found that the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and to ensure good product yield. After comparison with the validated numerical results, the well-mixed model is found to predict the peak pollutant concentrations within a reasonable range which is 0.34–1.33 times the experimental values except when the monitored distance is very close to the release point. The well-mixed model is shown to be capable of predicting a reasonable attainable maximum concentration once a pollutant leaks in the cleanroom.

Concentration profile of jet gas in the feed injection zone of a FCC riser

The concentration profiles of jet gas are investigated in the feed injection zone of a cold-model FCC riser by using a hydrogen trace technique. Experimental results demonstrate that four types of jet gas concentration profiles can be used to describe the mixing between the jet gas and pre-lift gas in the feed injection zone. The four types are a distinct M-shaped profile for weak mixing, an indistinct M-shaped profile for medium mixing, a sharp profile for strong mixing, and a parabolic profile for full mixing. The heights for regions of initial and full mixing reduce when decreasing the jet gas velocity or increasing the pre-lift gas velocity. Furthermore, the momentum ratio, M j/ M r, (where M j is the momentum of jet gas, M r is the mixture momentum of pre-lift gas and solid particles) is introduced to describe the effects of gas–solid physical properties, operating conditions, and equipment configuration on the jet gas concentration distribution. The heights for regions of initial and full mixing between jet gas and pre-lift gas are found to be 0–0.375 and 0.375–0.675 m when M j/ M r < 0.29, 0.375–0.675 and 1.075–1.375 m when M j/ M r > 0.54, and 0–0.375 and 0.675–1.075 m when M j/ M r between 0.29 and 0.54, respectively.

Experimental measurement of gas concentration distribution in an impinging entrained-flow gasifier

On a laboratory-scale testing platform of impinging entrained-flow gasifier with two opposed burners, the detailed measurements of gas concentration distribution have been performed for carbonaceous compound (diesel oil) at atmospheric pressure. Under the condition of 1.48–2.36 O/C ratios (kg/kg), radial gas samples are collected at three axial positions and the syngas exit position with stainless steel water-cooled probes, the concentration distribution of the major gases (H 2, CO, CO 2, CH 4 and O 2) under stable operating state was determined with a mass spectrometry. These data are used to clarify mixing and reaction characteristics within the reactor, to give insight into the combustion process and provide a database for evaluating predictive mathematical models.

Numerical analysis of interphase heat and mass transfer of cluster in a circulating fluidized bed

Interphase heat and mass transfer characteristics of a naphthalene particle cluster in a circulating fluidized riser are numerically analyzed using a three-dimensional CFD model. Heat and mass transfer characteristics of gas over an in-line array of three naphthalene particles and an isolated naphthalene particle are analyzed. Distributions of gas concentration, temperature and velocity are obtained. The heat and mass transfer rates of gas-to-cluster increase with the increase of the cluster porosity and Reynolds number. Present simulations indicate that the small cluster gives higher heat and mass transfer coefficients than those of the large cluster. The heat and mass transfer rates of individual particles in the cluster are lower than that of an isolated particle and particles in an in-line array under a given cluster porosity.

Thermoelectric Model of a Tubular SOFC for Dynamic Simulation

A one-dimensional transient model of a tubular solid oxide fuel cell stack is proposed in this paper. The model developed in the virtual test bed (VTB) computational environment is capable of dynamic system simulation. This model is based on the electrochemical and thermal modeling, accounting for the voltage losses and temperature dynamics. The single cell is discretized using a finite volume method where all the governing equations are solved for each finite volume. The temperature, the current density, and the gas concentration distribution along the axial direction of the cell are presented. The dynamic behavior of electrical characteristics and temperature under the variable load is simulated and analyzed. For easy implementation in the VTB platform, the nonlinear governing equations are discretized in resistive companion form. The developed model is validated with experimental results and can be used for dynamic performance evaluation and design optimization of the cell under variable operating conditions and geometric condition.

Consequence analysis and safety verification of hydrogen fueling stations using CFD simulation

Now that environmental awareness is enhanced on a global basis, great hopes are placed on the expanded use of hydrogen stations and fuel-cell vehicles (FCVs) that economize hydrogen energy. Hydrogen stations must be safe and secure because they store large quantities of hydrogen under higher pressure than the hydrogen actually consumed by FCVs. Thus, multiple safety measures are taken to ensure that hydrogen does not leak from the stations. Furthermore, in the unlikely event of leakage, the damage needs to be kept on an allowable level. For this reason, it is necessary to understand the behavior of hydrogen gas leaking from the stations.It is well known that hydrogen is a highly explosive gas, being combustible over a wide range of concentrations. Due to this property of hydrogen, tests for verifying its behavior must be done outdoors. In the open air, however, tests are affected by weather conditions and hydrogen sensors’ responses are delayed, which makes it difficult to observe an accurate distribution of hydrogen concentrations. Therefore, with respect to assumed cases of hydrogen leakage, a concentration distribution of hydrogen gas was obtained by means of computational fluid dynamics (CFD). The analysis results were used for verifying the safety of hydrogen stations.

Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence

The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbersReTranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that asReTincreases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocityKLwere obtained and their normalized values (KLSc0.5/uHT) depend onReTwith exponent −0.25. The spectra of the covariance termc′w′ indicate that the contribution ofc′w′ is larger in the lower-frequency region for cases with smallReT, whereas for the other cases with higherReT, the contribution ofc′w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux$\overline{c^\prime w^\prime}$profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δeis the thickness of the gas boundary layer. The measured turbulent flux (c′w′) is of the same order as the total flux (j), which shows that the contribution ofc′w′ to the total flux is significant.

Evaluation of downer reactor performance by catalytic ozone decomposition

Radial distribution profiles of ozone concentrations were measured along an 8.50 m high and 0.09 m inside diameter gas/solid co-current down-flow circulating fluidized bed (downer) to characterize the reactor performance. Tests were conducted under a series of operating conditions at room temperature and near atmospheric pressure, with FCC particles as the bed material. Results show that the concentration distribution of the ozone tracer gas correlates well with the flow structure of the downer. There is quite a uniform radial distribution of ozone concentrations in the core region of all tested axial sections in the fully developed region of the downer, except for the near-wall region where there is a sharp decrease in ozone concentration. And there exists a relatively significant non-uniform distribution in the entrance acceleration region of the downer.

A mathematical model of a tubular solid oxide fuel cell with specified combustion zone

A numerical model has been developed to simulate the effect of combustion zone geometry on the steady state and transient performance of a tubular solid oxide fuel cell (SOFC). The model consists of an electrochemical submodel and a thermal submodel. In the electrochemical model, a network circuit of a tubular SOFC was adopted to model the dynamics of Nernst potential, ohmic polarization, activation polarization, and concentration polarization. The thermal submodel simulated heat transfers by conduction, convention, and radiation between the cell and the air feed tube. The developed model was applied to simulate the performance of a tubular solid oxide fuel cell at various operating parameters, including distributions of circuits, temperature, and gas concentrations inside the fuel cell. The simulations predicted that increasing the length of the combustion zone would lead to an increase of the overall cell tube temperature and a shorter response time for transient performance. Enlarging the combustion zone, however, makes only a negligible contribution to electricity output properties, such as output voltage and power. These numerical results show that the developed model can reasonably simulate the performance properties of a tubular SOFC and is applicable to cell stack design.

Computer-based studies of diffusion through stomata of different architecture.

The influence of stomatal architecture on stomatal conductance and on the developing concentration gradient was explored quantitatively by comparing diffusion rates of water vapour and CO(2) occurring in a set of three-dimensional stoma models. The influence on diffusion of an internal cuticle, a sunken stoma, a partially closed stoma and of substomatal chambers of two different sizes was considered. The study was performed by using a commercial computer program based on the Finite Element Method which allows for the simulation of diffusion in three dimensions. By using this method, diffusion was generated by prescribed gas concentrations at the boundaries of the substomatal chamber and outside of the leaf. The program calculates the distribution of gas concentrations over the entire model space. Locating the stomatal pore at the bottom of a stomatal antechamber with a depth of 20 microm decreased the conductance significantly (at roughly about 30 %). The humidity directly above the stomatal pore is significantly higher with the stomatal antechamber present. Lining the walls of the substomatal chamber with an internal cuticle which suppresses evaporation had an even stronger effect by reducing the conductance to 60 % of the original value. The study corroborates therefore the results of former studies that water will evaporate preferentially at sites in the immediate vicinity to the stomatal pore if no internal cuticle is present. The conductance decrease affects only water vapour and not CO(2). Increasing the substomatal chamber increases CO(2) uptake, whereas transpiration increases if an internal cuticle is present. Variation of stomatal structure may, with unchanged pore size and depth, profoundly affect gas exchange and the pathways of liquid water inside the leaf. Equations for calculation of stomatal conductance which are solely based on stomatal density and pore depth and size can significantly overestimate stomatal conductance.

Chemical perturbations in the planetary boundary layer and their relevance for chemistry transport modelling

The role of perturbations of reactive trace gas concentration distributions in turbulent flows in the planetary boundary layer (PBL) is discussed. The paper focuses on disturbances with larger spatial scales. Sequential nesting of a chemical transport model is applied to assess the effect of neglecting subgrid chemical perturbations on the formation and loss of ozone, NO x , peroxyacetyl nitrate (PAN) and HNO3 calculated with a highly complex chemical mechanism. The results point to characteristic differences regarding the process of mixing of chemically reactive species in the PBL and lower troposphere.

A Transient Model of Vadose Zone Reaction Rates Using Oxygen Isotopes and Carbon Dioxide

The importance of identifying and quantifying subsurface geochemical reaction rates and processes by monitoring and modeling CO2 and O2 concentrations is well established. These parameters, however, are typically studied independently under presumed steady‐state conditions. Here we present models of seasonally variable vadose zone CO2 and O2 concentrations that use δ18O of O2 as a constraint to create a dynamic link between these three parameters under transient conditions. The gas transport modeling was used to quantify the controls of biogeochemical processes and parameters (i.e., temperature and moisture content) on vadose zone distributions of CO2 and O2 gas concentrations. The investigation was conducted on a 3‐m‐thick, unvegetated, fine‐sand vadose zone located in northern Alberta, Canada (56°40′N, 111°07′W). Using the modeled molar ratio of surface fluxes for O2 and CO2, the change in reaction rate for a temperature change of 10°C (Q10), moisture content at maximum reaction rates, and biogeochemical discrimination against consumption of 18O16O (αk), we determined that organic C oxidation by microbial respiration was the predominant mechanism consuming O2 and producing CO2 The mean αk was determined to be 0.973, suggesting that subsurface respiration was via the alternative oxidase pathway, which may be common in cold climates. Modeling revealed that the moisture content of a moist, surficial clayey sand layer (0.1–0.3 m thick) had a dramatic effect on pore‐gas CO2 and O2 concentrations and on δ18OO2 The vadose zone in this study was at an unvegetated site to simplify the model application; however, it can be modified to include root respiration and applied to natural vadose zones to help quantify the role of subsurface respiration in global O2 and C budgets.

Simulation of effect of catalyst particle cluster on dry methane reforming in circulating fluidized beds

Abstract A study was undertaken to determine whether the performance of dry methane reforming could be affected by catalyst particle clusters in circulating fluidized bed reformers. Flow behavior and chemical reactions through a quiescent catalyst cluster were predicted. The distributions of gas temperature and concentrations of gas species were obtained using a simple two-dimensional model with Rh/Al2O3 catalyst particles. Numerical results show that the reforming reaction rate on catalyst particles in the cluster is much smaller than the rate on isolated catalyst particle in the stream. The molar ratio of H2/CO is increased due to catalyst particle clustering. The effects of gas temperature, inlet gas velocity and reactor pressure on reforming reactions are also investigated.

Concentration distribution of the exhaust gas near a roadway with a stochastic traffic flow

The mechanical process of transfer and dispersion of the exhaust gas near a motor road has been studied based on the hypothesis of a stochastic traffic flow on the considered roadway section, which is described by a Poisson process with a constant intensity. Using the three-dimensional diffusion equation, solutions have been obtained for problems on the concentration distribution of pollutants near an extended roadway and a road crossing. Basic stochastic characteristics of the concentration distribution of the exhaust gas from a stochastic traffic flow have been determined. Calculated results have been compared with experimental data.

Dynamic Interfacial Tension Method for Measuring Gas Diffusion Coefficient and Interface Mass Transfer Coefficient in a Liquid

This paper presents a new dynamic interfacial tension method for measuring the gas diffusion coefficient and the interface mass transfer coefficient in a liquid at a high pressure and a constant temperature. In the experiment, a see-through windowed high-pressure cell is filled with a test gas at a prespecified pressure and a constant temperature. Then a liquid sample is introduced by using a syringe delivery system to form a pendant liquid drop inside the pressure cell. With the dissolution of the gas into the pendant liquid drop, the dynamic interfacial tension between the test gas and the liquid keeps reducing and eventually reaches its equilibrium value when the saturation state is achieved. The sequential digital images of the pendant liquid drop are acquired and analyzed by applying computer-aided digital image acquisition and processing techniques to measure the dynamic interfacial tensions. Theoretically, a mass transfer model is developed to study the diffusion process of the gas inside the pendant liquid drop. This model is solved numerically by applying the semidiscrete Galerkin finite element method to obtain the transient gas concentration distribution inside the pendant liquid drop at any time. With a predetermined calibration curve of the equilibrium interfacial tension versus the equilibrium gas concentration for the gas−liquid system, the corresponding dynamic interfacial tension is calculated. The gas diffusion coefficient and the interface mass transfer coefficient are, thus, determined by finding the best fit of the theoretically calculated dynamic interfacial tensions to the experimentally measured data. This newly developed dynamic interfacial tension method is applied to measure the diffusion coefficient and the interface mass transfer coefficient of CO2 in a reservoir brine sample at P = 0.1−6.0 MPa and T = 27 °C.

Internal air current patterns depend on the ventilation method in a scaled-up culture vessel for micropropagation

Air current patterns were visualized inside a scaled-up culture vessel under natural or forced ventilation. Metaldehyde particles were used as tracers, and their patterns were recorded as video images by a high-resolution-and-contrast camera. Under natural conditions, the air currents were mainly influenced by natural convection that developed due to the lighting scheme, which caused differences in temperature among various articles in the chamber, including a sweet potato plantlet, supporting material, a multi-cell tray, and the culture vessel. Under forced ventilation, the air current pattern and air speed were affected by ventilation rates and by air-supply methods that were either parallel downward or circular upward. Uniformity of air movement could be achieved with air distribution pipes inside a modified vessel. Under forced ventilation, growth, photosynthetic rate, and transpiration of the micropropagated plantlets were enhanced around the air outlet as well as the inlet in the large-scale vessel. Those plant responses were probably induced by uniform spatial distribution of air current and gas concentrations.