Oxygen Mass Fraction Research Articles

Variational-principle-optimized porosity distribution in gas diffusion layer of high-temperature PEM fuel cells

Porosity distribution in gas diffusion layer has a significant impact on cell performance by affecting the oxygen transport and the electrical conductivity. This contribution presents a theoretical optimization model for porosity distribution in the cathode gas diffusion layer based on the variational principle. First, the optimization equations for maximizing the total oxygen mass flux diffused through the cathode gas diffusion layer are derived with the constraints of the momentum conservation, the species conservation and the specified average porosity. Then, the application of a typical high-temperature proton exchange membrane fuel cell with twelve parallel flow channels gives the optimized three-dimensional non-uniform porosity distribution that could increase the current density at 0.2 V by 13.7% and the maximum power density by 10.0%. Besides, the optimized porosity distribution also improves the uniformity of oxygen mass fraction field and the current density distribution. Finally, seven design schemes with in-plane non-uniform porosity distribution or thickness-direction linear gradient porosity are proposed and evaluated. The results indicate that the optimized porosity distribution has the best performance, followed by the in-plane non-uniform porosity, and the linear gradient porosity along the thickness direction shows the weakest improvement. This finding further demonstrates the rationality and necessity of the proposed optimization model.

Effect of humidification on distribution and uniformity of reactants and water content in PEMFC

Restricted by experimental conditions, it is difficult to analyze reactants distribution and uniformity through experiment. A simulation model is established by experiment fuel cell. Hydrogen humidification has a great impact on hydrogen distribution and concentrates in inlet and serpentine sections. At anode side membrane water content increases significantly when hydrogen relative humidity is over 50%. Air humidification has little effect on air distribution and water content. The oxygen mass fraction only decreases with relative humidity increase. Hydrogen humidification has greater influence on the distribution of reactants and membrane water content than air humidification, but hydrogen humidification needs to control the relative humidity of hydrogen within a suitable range. According to the simulation results in this article, the relative humidity of hydrogen should be controlled at 25%–50%. This paper proposes mass fraction difference coefficient, as uniformity evaluation index. When hydrogen relative humidity is 50%, uniformity of reactants distribution is the best.

CFD-DEM modelling of raceway dynamics and coke combustion in an ironmaking blast furnace

In this study, a CFD-DEM reacting model featuring thermochemical sub-models is developed for describing gas–solid reacting flow related to raceway dynamics and coke combustion in an ironmaking blast furnace (BF). The simulation results show that a ring-type zone is formed around the cavity where the coke combustion and gasification occur with a large amount of heat released, leading to intense momentum and thermochemical exchanges in this zone. Heat fluxes contributed from radiation and reaction dominate total heat transfer during the coke combustion, while conductive heat transfer is insignificant. Then, the impacts of key operating parameters on raceway evolution are quantified. The simulation results indicate increasing the blast gas velocity from 210 m/s to 230 m/s and oxygen mass fraction from 0.17 to 0.27, the cavity depth is increased by 42.3% and 20.8%, respectively; lower inlet velocity, higher bed temperature, higher oxygen mass fraction leads to more CO production and the resulting better BF performance.

A numerical study on the performance of PEMFC with wedge-shaped fins in the cathode channel

The gas flow field design has a significant influence on the overall performance of a proton exchange membrane fuel cell (PEMFC). A single-channel PEMFC with wedge-shaped fins in the cathode channel was proposed, and the effects of fin parameters such as volume (0.5 mm3, 1.0 mm3, and 1.5 mm3), number (3, 5, and 9), and porosity of the gas diffusion layer (GDL) (0.2, 0.4, 0.6, and 0.8) on the performance of PEMFC were numerically examined based on the growth rate of power density (GRPD) and polarization curve. It was shown that wedge-shaped fins could effectively improve the PEMFC performance. With an increase in fin volume, the distributions of oxygen mass fraction in the outlet area of the cathode channel were lower, the drainage effect of the PEMFC improved, and GRPD also increased accordingly. Similar results were obtained as the number of fins increased. The GDL porosity had a greater effect than the wedge-shaped fins on the improvement in PEMFC performance, but the influence of GDL porosity weakened and the GRPD of porosity decreased as the porosity increased. This study provides an effective guideline for the optimization of the cathode channel in a PEMFC.

Numerical study on the effects of intake charge on oxy-fuel combustion in a dual-injection spark ignition engine at economical oxygen-fuel ratios

In order to decrease Carbon Dioxide (CO2) emissions, Oxy-Fuel Combustion (OFC) technology with Carbon Capture and Storage (CCS) is being developed in Internal Combustion Engine (ICE). In this article, a numerical study about the effects of intake charge on OFC was conducted in a dual-injection. Spark Ignition (SI) engine, with Gasoline Direct Injection (GDI), Port Fuel Injection (PFI) and P-G (50% PFI and 50% GDI) three injection strategies. The results show that under OFC with fixed Oxygen Mass Fraction (OMF) and intake temperature, the maximum Brake Mean Effective Pressure (BMEP) is each 5.671, 5.649 and 5.646 bar for GDI, P-G and PFI strategy, which leads to a considerable decrease compared to Conventional Air Combustion (CAC). [Formula: see text], [Formula: see text] and [Formula: see text] of PFI are the lowest among three injection strategies. With intake temperature increases from 298 to 378 K, the reduction of BMEP can be up to 12.68%, 12.92% and 12.75% for GDI, P-G and PFI, respectively. Meantime, there is an increase of about 3% in Brake Specific Fuel Consumption (BSFC) and Brake Specific Oxygen Consumption (BSOC). Increasing OMF can improve the performance of BMEP and BSFC, and the trend is more apparent under GDI strategy. Besides, an increasing tendency can be observed for cylinder pressure and in-cylinder temperature under all injection strategies with the increase of OMF.

Modeling the oxygen uptake, transport and consumption in an estivating terrestrial snail, Xeropicta derbentina, by the Colburn analogy.

The present work gives insight into the internal heat management of the respiratory system in the terrestrial snail Xeropicta derbentina, which has to cope with extreme climate conditions in its habitat. A realistic model of the lung´s vein system was constructed and the active diffusive surface of capillaries and main vein was calculated and confirmed by geometrical measurements. We here present a model that is able to validate the measured oxygen consumption by the use of the Colburn analogy between mass and momentum transfer. By combining basic diffusion laws with the momentum transfer, i.e. wall shear stress, at the inner wall of the lung capillaries and the main vein, the progression of the oxygen mass fraction in the hemolymph can be visualized.

Influence of nozzle gas inlets on properties of GTD222 superalloy powders for additive manufacturing

The GTD222 superalloy powders for the additive manufacturing were prepared by the vacuum inert gas atomization (VIGA) with the close-coupled nozzle system in this paper. The effects of nozzle gas inlets on the chemical composition, particle size distribution, sphericility, fluidity, loose packed density, and surface morphology of the GTD222 superalloy powders were studied. The results show that, using the nozzle with an “outer straight inner tangent” structure enhances the suction effect of the nozzle and improves the stability of the closed-coupled gas atomization process. In addition, the tangent inlet system enhances the gas shear effect, therefore improves the yield of the fine powders (15~53 μm). The optimized process parameters are found in the conditions of 200 ℃ superheat and 3 MPa atomization pressure. The oxygen mass fraction of the powders prepared in the optimized conditions is 0.02%, the medium diameter is 57.98 μm, the degree of sphericility is 0.77, the fluidity is 26.15 g/(50 s), and the loose packed density is 4.63 g/cm3, which meet the requirements of the metal additive manufacturing technology.

Optimization of Fuel Cell Performance Using Computational Fluid Dynamics.

A low cost bipolar plate materials with a high fuel cell performance is important for the establishment of Proton Exchange Membrane (PEM ) fuel cells into the competitive world market. In this research, the effect of different bipolar plates material such as Aluminum (Al), Copper (Cu), and Stainless Steel (SS) of a single stack of proton exchange membrane (PEM) fuel cells was investigated both numerically and experimentally. Firstly, a three dimensional (3D) PEM fuel cell model was developed, and simulations were conducted using commercial computational fluid dynamics (CFD) ANSYS FLUENT to examine the effect of each bipolar plate materials on cell performance. Along with cell performance, significant parameters distributions like temperature, pressure, a mass fraction of hydrogen, oxygen, and water is presented. Then, an experimental study of a single cell of Al, Cu, and SS bipolar plate material was used in the verification of the numerical investigation. Finally, polarization curves of numerical and experimental results was compared for validation, and the result shows that Al serpentine bipolar plate material performed better than Cu and SS materials. The outcome of the investigation was in tandem to the fact that due to adsorption on metal surfaces, hydrogen molecules is more stable on Al surface than Cu and SS surfaces.

Mathematical modeling of mass, heat and fluid flow in a reverberatory furnace for melting nickel-containing raw materials

The article presents analytical results on the use and distribution of energy in reverberatory furnaces with medium pressure gas burners to melt nickel-containing raw materials, by means of a numerical simulation of the furnace’s combustion space. The combustion flow and radiation of a reverberatory furnace powered by natural gas have been calculated and validated. The chemical composition of natural gas makes it the cleanest of fossil fuels, for this reason its use increases worldwide. The higher hydrogen/carbon ratio in its composition, compared to other fossil fuels, causes combustion to emit less CO2 per unit of energy produced. The flue gas outlet duct and medium pressure burners are represented and located as in a typical design to determine the effect on the efficiency of the reverberatory furnace. The results show the temperature field, the mass fraction of oxygen and fuel within the furnace’s combustion space and the comparison between the theoretical and practical efficiency of a furnace.

Hydrodynamic Simulation of Two-Phase Flow in an Industrial Electrowinning Cell with New Scheme

Electrowinning is the process of depositing copper of the electrolyte solution inside the cell to the cathode. In the present study, the hydrodynamic simulation of the electrowinning cell of Miduk Copper Complex is studied using computational fluid dynamics. The software used is Ansys CFX. The Navier-Stokes and continuity equations are considered in the form of two phases of fluid and gas, turbulent, incompressible and steady state, and the equation for copper concentration in the electrolyte is solved with consideration of its specific boundary condition. The flow turbulence is modeled using K-ω relationships. Due to large variations in the properties near cathode and anode, and also the large size of the electrowinning cell, to create a good grid, and increase the speed and accuracy of the results, global and local simulations are used together. First, in global simulation, the entire geometry of the cell is modeled by creating an appropriate grid, then, in the local simulation, the volume between two cathodes of the cell only is considered and modeled with a much smaller mesh. Data on boundary conditions of the common border plates in the local simulation are obtained from global simulation data, which increases the accuracy of modeling. The results of this simulation are the velocity vectors, the concentrations of acid and copper, turbulence Intensity, the amount of pressure, and the volume ratio of the oxygen phase in the entire electrowinning cell domain. Finally, for model validation, the model is compared with experiments conducted on actual cells in the industry. Results show high accuracy with less than 2.5% deviation of this modeling technique. Then, the mass transfer coefficient values for the different electrode intervals are obtained by this modeling and the results are validated using the results of the experimental relations, indicating a deviation of only 0.5%. In the next step, the electrolyte mixture containing different mass fractions of oxygen is sprayed into the electrowinning cell from the inlet of the simulated cell. The results demonstrate that spraying 16 liters per second of oxygen at the inlet can increase the overall mass transfer coefficient of the electrode plates up to 7%. The effect of changing inlet temperature and flow rate of the electrolyte on the mass transfer coefficient is also investigated by the obtained model.

Diffusion-controlled premixed flames

Steady laminar flow structures with mixing, chemical reaction, and normal strain qualitatively representative of turbulent combustion at the small scales are analysed. A contrived counterflow configuration is examined to focus more easily on the premixed flames that are known to occur in multiple-flame structures; the premixed flame can be driven by a controlled heat source which is a surrogate for the diffusion flame in the practical multi-flame structure. Reduction to a one-dimensional similar form is obtained with density and properties variations. It is shown that premixed flames at fuel-rich or fuel-lean conditions sufficiently far from the stoichiometric condition are diffusion-controlled with a propagation velocity very weakly dependent on chemical kinetic details. In particular, heat diffused from a downstream source (i.e. the surrogate for a diffusion flame) is needed to sustain the flame. Effects of normal-strain rate, pressure level, and transport properties, combustible mixture composition, and heat source strength are determined through a parameter study with variations in Damköhler number, Prandtl number, upstream mass fractions of oxygen and propane fuel, and wall temperature. The premixed flame becomes less dependent on the downstream source as strain rate decreases and pressure increases. Small changes in heat and mass diffusivities have stronger effects than an order-of-magnitude change in Damköhler number.

Experimental and computational investigation of extinction and autoignition of propane and n-heptane in nonpremixed flows

An experimental and computational investigation is carried out to characterize the influence of reactants on critical conditions for extinction and for autoignition of propane and n-heptane in nonpremixed counterflow configurations. Propane or vaporized n-heptane mixed with nitrogen is transported in one stream while the other stream is made up of air mixed with nitrogen. Measurements of the oxidizer stream temperature needed for autoignition are made at fixed values of the strain rate, either with the fuel mass fraction varied at a fixed oxygen mass fraction or with the oxygen mass fraction varied at a fixed fuel mass fraction. Extinction strain rates for propane are measured as a function of the oxygen mass fraction with room-temperature feed streams and the fuel mass fraction fixed and for n-heptane as a function of the fuel mass fraction with the oxygen mass fraction and feed-stream temperatures fixed. Predictions of critical conditions for extinction and autoignition are made employing detailed kinetic mechanisms. Predictions of critical conditions for extinction are in reasonable agreement with measurements, but there are significant discrepancies for autoignition. Measurements show that increasing the mass fraction of either fuel or oxygen increases the overall reactivity thereby reducing the autoignition temperature. The kinetic models predict the increase in reactivity of the mixing layer with increasing mass fraction of fuel but predict very little change in reactivity of the mixing layer with increasing mass fraction of oxygen, thus failing to predict the influence of oxygen on autoignition. It is concluded that there may exist kinetic pathways responsible for this disagreement that are yet to be discovered, and paths that fail to explain the results are identified.

Combustion mode and wave multiplicity in rotating detonative combustion with separate reactant injection

Abstract Numerical simulations with detailed chemistry are conducted for two-dimensional rotating detonative combustion with separate injection of fuel and oxidant. The influences of the fuel and oxidant compositions on combustion mode and detonation wave multiplicity are studied. It is found that in the parts of the flow beyond the fuel refill zone, there are two distinct, highly inhomogeneous zones with fuel-rich and fuel-lean compositions, respectively. Both detonative and deflagrative regimes are observed and proceed mostly under a premixed combustion mode with the deflagration confined mostly to the fuel-lean zone. The results from our simulations show that limited H2 is detonated or deflagrated close to stoichiometric conditions and more than 70% of H2 is detonated or deflagrated under fuel-lean conditions. Over 70% of the detonated H2 is consumed in a premixed combustion mode. Our analysis also suggests that the detonation fraction increases with increased inlet pressure, decreased inlet temperature or increased injection orifice number. Additionally, the range of mixture fraction over which the composition is detonable is narrower than the range for deflagration. The number of detonation waves increases with increased oxygen mass fraction in the oxidant stream, with the additional waves being formed by mutual enhancement of an explosive hot spot and a travelling shock wave. Stabilization of the multiple waves follows a chaotic period involving both co-rotating and counter-rotating waves. Furthermore, the deficit of the detonation speed relative to the ideal Chapman − Jouguet value increases with the number of waves but also decreases monotonically with the level of reactant mixing. The reactant mixing effects along the detonation wave height are further discussed through quantifying the statistics of height-wise mixture fraction, heat release rate and OH mass fraction.

Numerical investigation on implications of four strut injectors on combustion characteristics of a Doubly-Dual cavity-based scramjet combustor

The present study numerically investigates the implications of four strut injector on combustion characteristics of a hydrogen fueled doubly-dual cavity based scramjet combustor. Large Eddy Simulation (LES) is adopted to reveal the combustion characteristics of the doubly-dual cavity based scramjet combustor. Firstly, the fluid flow characteristics of a three strut injector are compared with four strut injector to highlight the influence of 4-strut injector. Thereafter, a separate study is carried out to investigate the influence of Mach number on the performance of the combustor. Our study reveals that the four strut injector arrangement greatly affects the shock wave interaction like, shock-shock and the shock-shear layer which leads to enhance the formation of vortices and recirculation region as compared to the three strut injector arrangement. In addition, introduction of 4-strut injectors found to result in better mixing and complete combustion as evidenced by the higher static temperature and the lesser amount of oxygen mass fraction at the outlet of the combustor.Also, the flame blow out condition is seen at the trailingedge of a strut and found to be minimum when 4 strut arrangement is use dowing to maximum value of OH mass fraction. Further, the higher air-fuel mixing rate is observed at Mach 2.5 of the free stream of their inlet which aids to stabilize the flame leading to complete combustion.

Critical condition relevant to the extinction of a carbon particle in the course of combustion

In order to examine extinction of carbon particle in the course of combustion, an attempt has been made to obtain its critical condition. Main concern has been put on the particle extinction in the lower limit that can occur at the end of the particle combustion although not only the lower limit but also the upper limit of the critical conditions has been obtained. By conducting asymptotics, with focusing on the temporal variation of the particle temperature, an analytical expression has been obtained for the limit of the particle diameter, as functions of the pressure ratio, oxygen mass-fraction, ambient temperature, and/or radiative heat flux. An approximate expression is also obtained from the analytical solution. It is found that the approximate expression can fairly represent the limits. In addition, use has been made of the Arrhenius plot of a comprehensive parameter, consisting of the particle diameter and pressure ratio, in order to identify regions for the particle combustion sustained, which are next to those for the particle extinction. Comparisons have also been conducted by use of experimental data in the literature, with presenting a fair degree of agreement, as far as the trend and approximate magnitude are concerned. It has been confirmed that the formulation has captured the essential feature, that the reduction in particle size does not necessarily favor the particle combustion, and that the particle extinction can occur when the particle diameter is reduced to the critical value.

Systematic study of the impact of chemistry and radiation on extinction and carbon monoxide formation at low scalar dissipation rates

In fire flames scalar dissipation rates in the reaction zone are low and radiation becomes important for extinction. The chemistry mechanism also strongly affects extinction and CO formation. The present study applies the unsteady laminar flamelet approach (or, equivalently, the conditional moment closure (CMC) method in mixture fraction domain, without transport in physical space). Six chemistry mechanisms are analyzed. In addition to adiabatic conditions, three radiation models are considered: RADCAL-based gray gas; weighted sum of gray gases (WSGG); and prescribed radiative fraction model. The oxygen mass fraction values for extinction are discussed, diluting the oxidizer by adding nitrogen. Carbon monoxide profiles are reported as well. The radiative fraction model is not capable of predicting the strong radiative effects at low scalar dissipation rates, while the RADCAL-based and the WSGG model are able to do so. The analysis of extinction and CO reveals strong sensitivity to the mesh in mixture fraction space, as well as to the shape of the conditional scalar dissipation rate profile. Results with the ARM2 mechanism and GRI3 are in very close agreement, so that significant time savings are possible, using ARM2.

Oxidation of Petroleum Asphaltenes Coupled with Iodination

Oxidation of petroleum asphaltenes by potassium iodide (KI) is described. Oxidation under mild conditions is shown to be accompanied by iodination of the asphaltene aromatic core. The inducing reaction is the oxidation of sulfide sulfur atoms in the asphaltenes with reduction of iodate anion to molecular iodine; the coupled reaction, electrophilic substitution. X-ray photoelectron spectroscopy detected an increase in the mass fraction of oxygen as compared with the initial sample and formation of C–I bonds. IR spectroscopy found that oxidation of the asphaltenes is accompanied by the formation of carboxyl and sulfoxide groups. Raman spectroscopy revealed a decrease in the size of the oxidized asphaltene molecules as compared to the native asphaltenes. X-ray powder diffraction analysis showed a decrease in the interplanar spacing of the asphaltene oxidation product that was due to ordering and a denser arrangement of the aliphatic fragments.

The effect of baffle shape on the performance of a polymer electrolyte membrane fuel cell with a biometric flow field

Flow field design on the cathode side, inspired by leaf shapes, leads to a high performance, as it achieves a good distribution of reactants. Furthermore, the addition of baffles to the cathode channel also increases the supply of reactants in the cathode catalyst. However, research on the addition of baffles to the cathode channel has still been limited to straight channels and conventional flow fields. Therefore, in this work, a numerical study was conducted to investigate the effect of baffles on the leaf flow field on the performance of a polymer electrolyte membrane fuel cell. The generated 3D model is composed of nine layers with a 25-cm2 active area. The beam and chevron shapes of the baffles, which were inserted into the mother channel, were compared. The simulation results revealed that the addition of beam-shaped baffles that are close to each other can increase the current and power densities by up to 18% due to the more uniform distribution of the oxygen mass fraction.

Study on the Oxygen Enrichment Effect of Individual Oxygen-Supply Device in a Tunnel of Plateau Mine.

Complex characteristics of the plateau environment such as low oxygen content seriously restrict the exploitation of abundant mineral resources in plateau areas. To regulate the hypoxia environment and improve the comfort of workers engaged in intense physical labor like tunnel excavation operations in plateau mines, an individual oxygen-supply device for tunnel of plateau mine was proposed to create local oxygen enrichment in the area around the human nose. The Computational Fluid Dynamics (CFD) method was used to judge the application’s effect of the individual oxygen-supply device in plateau mine, revealing the oxygen diffusion law under the influence of different oxygen enrichment factors. The orthogonal design and range analysis were used to measure the degree of influence of major factors such as oxygen-supply velocity, oxygen-supply concentration, and tunnel airflow velocity. The results demonstrate that the oxygen mass fraction of the air inhaled by the human had a positive correlation exponential function, a positive correlation linear function, and a negative correlation exponential function, respectively, concerning oxygen-supply velocity, oxygen-supply concentration, and tunnel airflow velocity. The range analysis revealed that the major influencing factors of oxygen enrichment in the tunnel of the plateau mine were, in a descending sequence, as follows: oxygen-supply concentration, tunnel airflow velocity, and oxygen-supply velocity, and the corresponding ranges were 2.86, 2.63, and 1.83, respectively. The individual oxygen-supply device achieved the best oxygen enrichment effect when the oxygen-supply velocity was 5 m/s, the oxygen-supply concentration was 60%, and the tunnel airflow velocity was 0.2 m/s, which increased the oxygen mass fraction of air inhaled by the human to 30.42%. This study has a positive guiding significance for the improvement of the respiration environment in the tunnel of plateau mine.

Limit conditions of smoldering spread in counterflow configuration: Extinction and smoldering-to-flaming transition

This paper presents an experimental and theoretical study on limit conditions for smoldering combustion of biomass fuel. A fuel rod was burned in an oxidizer flow in a Tsuji-Yamaoka type, counterflow configuration, and the spread rate and temperature distribution were measured. The spread rate and maximum temperature increased with an increase in oxygen mass fraction in the oxidizer flow. Smoldering spread was not possible when the oxygen mass fraction was below a lower critical value, whereas a smoldering-to-flaming transition (SFT) occurred when the mass fraction was above an upper critical value. A one-dimensional model was then developed based on experimental observations to describe steady smoldering spread. The limit conditions were predicted by assessing the existence of solution. It was found that extinction occurs due to finiteness of char-oxidation rate, while the SFT limit can be predicted by considering gas-phase combustion of pyrolysis products.