Gas Phase Mixing Research Articles

Fuel and diluent property effects during wet compression of a fuel aerosol under RCM conditions

Wet compression of fuel aerosols has been proposed as a means of creating gas-phase mixtures of involatile diesel-representative fuels and oxidizer+diluent gases for rapid compression machine (RCM) experiments. The intent of this study is to investigate the effects of fuel and diluent gas properties on the wet compression process, specifically to: (a) explore a range of fuels which could have applicability in aerosol RCM experiments, and illustrate important limitations due to fuel properties, and (b) fundamentally understand how fuel and diluent gas properties affect the wet compression process and assess which ones are most important. Insight gained from this work can be utilized to aid the design and successful operation of aerosol RCMs. A spherically-symmetric, single-droplet wet compression model is used where n-heptane, n-dodecane, 2,2,4,4,6,8,8-heptamethylnonane (isocetane), n-hexadecane (cetane) and n-eicosane are investigated as the diesel-representative fuels, while comparisons are made to water droplets. Nitrogen, neon and argon are selected as the gas-phase diluents while the oxidizer is considered to be oxygen at atmospheric concentrations. Initial droplet diameters of d0=3 and 8μm are used based on results of previous studies where the overall compression time is set to 15.3ms with the maximum volumetric compression ratio 13.4. An overall equivalence ratio of φ=1.0 is used.It is shown that under these conditions, involatile fuels up to ∼n-hexadecane appear to be candidates for aerosol RCM experiments. However, the use of small droplets (d0<5μm) will be necessary in order to ensure complete vaporization and adequate gas-phase mixing in advance of low temperature chemical reactivity. Fuels with higher boiling points might not be useable unless extremely small droplets (d0<1μm) and low pressures (e.g., P0<0.5bar) are employed along with longer compression times. In addition, the boiling curve (i.e., saturation pressure) and Lf are found to be the dominant fuel properties while the density-weighted mass diffusivity, ρgDg, which controls the rate of gas phase mass diffusion, and thus compositional stratification, generally plays a secondary role. The heat capacity and molar mass are the dominant diluent properties that affect the near-droplet and ‘far-field’ conditions. The gas-phase mixture Lewis number (Leg) contributes to either greater compositional (Leg>1) or thermal (Leg<1) stratification. For large hydrocarbons and oxygenated hydrocarbons that are representative of diesel fuels Leg∼3–5, and therefore compositional stratification could be significant; this characteristic has the potential to complicate interpretation of ignition/oxidation data acquired from these machines.

Numerical Investigation of Gas Holdup and Phase Mixing in Bubble Column Reactors

A discrete bubble model (DBM) has been used to study the overall gas holdup and the phase mixing in bubble column reactors. The Eulerian–Lagrangian approach has the advantage that it is possible to study the overall gas holdup and the gas phase mixing in a direct way. Furthermore, by introducing tracer particles, also the liquid phase mixing can be studied in a Lagrangian manner. Comparisons suggest that the overall gas holdups obtained from the discrete bubble model agree very well with the correlations of Kumar et al., Heijnen and Van’t Riet, and Ruzicka et al. within the applied range of the superficial gas velocity. The gas phase dispersion coefficients from the simulation results agree pretty well with Wachi and Nojima’s correlation, which is derived on the basis of the recirculation theory. In addition, the turbulent diffusion coefficient calculated from the tracer particles velocities are very close to those from the literature within the applied range of the superficial gas velocity.

Turbulent mixing in three-dimensional droplet arrays

The atomisation, evaporation and subsequent mixing of fuel from a liquid spray determines the effectiveness of the combustion processes in gas turbines and internal combustion engines. In the present paper, three-dimensional direct numerical simulations (DNS) of the evaporation of methanol droplets in hot environments are presented. The gas phase mixing is assessed by examining the scalar dissipation and the mixture fraction probability density function (PDF). Novel multi-conditional models are proposed that use mixture fraction and structural parameters as the conditioning variables for the scalar dissipation which is found to be well predicted in terms of magnitude and distribution. The β-PDF description of the mixture fraction seems to capture well the global behaviour for a laminar environment and for time-averaged results in turbulent cases. A novel model for the mixture fraction PDF is also proposed based on the multi-conditional model for scalar dissipation and an accurate representation of the PDF is achieved.

Numerical Analysis of the Effect of Gas Sparging on Bubble Column Hydrodynamics

A discrete bubble model (DBM) has been used to study the effect of gas sparger properties on the hydrodynamics in a bubble column. As a first step the performance of the model was evaluated by comparison with experimental data. Subsequently, four different perforated plates with different sparged areas were used as a gas sparger. Distributions of liquid velocity, turbulent kinetic energy, and void fraction in the central plane were compared for the four different systems. Furthermore, the effect of the sparger location was also investigated. It was found that the liquid-phase circulation becomes more pronounced as the sparged area location is more distant from the center of the bottom plate. Finally, gas-phase residence time distributions (RTD) were obtained from the simulations. By employing standard axial dispersion model, the gas-phase mixing in the bubble column was characterized. Results show that the extent of mixing increased when the sparged area decreased. The axial dispersion coefficient increased as the sparged area was shifted to the edge of the bottom plate.

A mixture fraction framework for the theory and modeling of droplets and sprays

A mixture fraction is carefully defined for evaporation and combustion of droplets and sprays. The definition is valid at points in either the liquid or gas phases and care is taken to distinguish between definitions based on conserved scalars appropriate for heat transfer and those for mass transfer. Results are presented for Spalding B numbers and values of the mixture fraction at the droplet surface for the fast chemistry case and for the case where the droplet cannot sustain an envelope flame. The classical theory for an isolated droplet with spherical symmetry yields simple formulae when expressed in mixture fraction terms. New results are then readily obtained for several quantities of interest in spray modeling. The formulation provides a seamless unification of droplet evaporation processes with gas-phase mixing and reaction. Mixing in a turbulent spray jet is identified as a model problem that clarifies the role of large scale structures in the overall mixing process. Important constraints on the parameter space for sprays are shown to be greatly clarified when expressed in the mixture fraction framework. It is shown how the classical approach for segregated flow with Eulerian/Lagrangian modeling of dispersion and transfer processes in turbulent sprays can be upgraded to include fluctuations in the temperature and composition surrounding the droplets on top of those coming from the turbulent velocity fluctuations. Such preliminary calculations that assume a simple chemically reacting system can readily be upgraded using flamelet functions derived from counterflow experiments or computations: these can then form the starting point for full chemistry calculations using such approaches as conditional moment closure.

All solid-state battery with sulfur electrode and thio-LISICON electrolyte

A high-capacity type of all solid-state battery was developed using sulfur electrode and the thio-LISICON electrolyte. New nano-composite of sulfur and acetylene black (AB) with an average particle size of 1–10 nm was fabricated by gas-phase mixing and showed a reversible capacity of 900 mAh g −1 at a current density of 0.013 mA cm −2.

Recovery of Sulfur Dioxide Using Non-Dispersive Absorption

Removal of SO2 from gas emissions by selective absorption into a liquid is a common method to reduce air pollution and environmental risks. The absorption efficiency is determined by the interaction between the gases and the liquid. A great number of gas desulphurization methods have been developed where aqueous or organic solvents are used as sorbents.N,N-dimethylaniline (DMA) is an organic solvent used in the industry because its affinity with SO2. This absorption is neither too strong nor too weak, thus absorption and desorption can occur leading to a regenerative process where SO2 can be recovered. However, a direct contact between SO2 and DMA leads to several environmental problems caused by solvent evaporation and drops dragging into the gas stream.In order to increase the process efficiency and reduce environmental risks, a non-dispersive absorption process using hollow fibre membrane modules is developed in this work for a solvent zero emission process. The mass transfer into a fibre may be described by three main assumptions: gas-phase laminar flow, gas-phase plug-flow and gas-phase mixing. A numerical calculation was carried out to establish the performance of a hollow fibre membrane contactor for the removal of SO2 when water and N,N-dimethylaniline are used as sorbents in order to compare both, wetted and non-wetted operating modes.

Estimation of gas phase mixing in packed beds

An improved model is presented for estimation of the mixing of gaseous species in a packed bed for fuel conversion. In particular, this work clarifies the main characteristics of mixing of volatiles and oxidizers in a burning bed of high-volatile solid fuel. Expressions are introduced to represent the active role of degradation of the solid particles in the mixing within the gas phase. During drying and devolatilization the solids modify the behavior of the gas flow: the volatiles released from the surface of the particles increase the turbulence in the system, and hence the rates of the homogeneous reactions under mixing-limited conditions. Numerical experiments are carried out to test the validity of this conclusion regarding mixing in different geometries. The flow of volatiles leaving the fuel particles is shown to contribute significantly to mixing, especially at low air flows through a bed. However, the fraction of the particle surface where volatiles are released and its orientation in the bed should be better determined in order to increase the accuracy of the estimates of turbulent mixing.

Growth and properties of Si–N–C–O nanocones and graphitic nanofibers synthesized using three-nanometer diameter iron/platinum nanoparticle-catalyst

Cone-shaped nanostructures of mixed composition (nanocones) and largely graphitic nanofibers were synthesized on silicon substrates using iron/platinum alloy nanoparticles as the catalyst in a direct-current plasma enhanced chemical vapor deposition reactor. The catalyst nanoparticles were monodisperse in size with an average diameter of 3 (±1) nm. The nanocones were produced on laterally widely dispersed catalyst particles and were oriented perpendicular to the substrate surface with an amorphous internal structure. The nanocones were produced by gas phase mixing and deposition of plasma-sputtered silicon, nitrogen, carbon, and oxygen species on a central backbone nucleated by the Fe–Pt catalyst particle. Field emission measurements showed that a very high turn-on electric field was required for electron emission from the nanocones. In contrast, the graphitic nanofibers that were produced when silicon sputtering and redeposition were minimized had the “stacked-cup” structure, and well-defined voids could be observed within nanofibers nucleated from larger catalyst particles.

Mechanism of mass transfer from bubbles in dispersions: Part II: Mass transfer coefficients in stirred gas–liquid reactor and bubble column

Experimental data on the average mass transfer liquid film coefficient ( k L) in an aerated tank stirred by Rushton turbine and in bubble column are presented. Liquid media were used as 0.8 M sodium sulphite solution, pure or with the addition of Sokrat 44 (copolymer of acrylonitrile and acrylic acid) or short-fiber carboxymethylcellulose (CMC) for the Newtonian and long-fiber CMC for the non-Newtonian viscosity enhancement and ocenol ( cis-9-octadecen-1-ol) or polyethylenglycol (PEG) 1000 for surface tension change. Volumetric mass transfer coefficient ( k L a) and specific interfacial area ( a) were measured by the Danckwerts’ plot method. Coefficients k L measured by pure oxygen absorption in pure sulphite solution and Newtonian viscous liquids are well fitted by the “eddy” model in the form of k L = 0.448 ( e v/ ρ) 0.25 ( D/ v) 0.5 with a mean deviation of 20%. Surface-active agents (ocenol and PEG) and non-Newtonian additive (long-fiber CMC) reduced k L value significantly but their effect was not described satisfactorily neither by surface tension nor by surface pressure. It is shown that the decisive quantity to correlate k L in the stirred tank and bubble column is power dissipated in the liquid phase rather than the bubble diameter and the slip velocity. Absorption of air did not yield correct k L data, which did not depend on or slightly decreased with increasing power. This is due to the application of an improper gas phase mixing model for absorption data evaluation.

Gas-Phase Mixing and Dispersion in a Diffusion Furnace

Gas-phase mixing and dispersion in a typical commercial-scale diffusion furnace was investigated using computational fluid dynamic simulations and experimentation. Five regions were defined within the furnace: entrance volume, annular volume around the wafer-source stack, radial volumes between wafer-source pairs, intermediate volumes between successive wafer carriers, and exit volume. The mixing characteristics within each of these volumes was quantified and appropriate mixing models were defined based on the relative amount of backmixing or gas exchange within and between these volumes. A dispersion-based approach was then used to develop a simplified model describing the transient transport of water vapor during a typical hydrogen injection cycle. Model parameters were calibrated using experimental data from tracer impulse studies. © 2003 The Electrochemical Society. All rights reserved.

Emulsion Combustion and Flame Spray Synthesis of Zinc Oxide/Silica Particles

Two flame spray methods, emulsion combustion method (ECM) and flame spray pyrolysis (FSP), were compared for synthesis of pure and mixed SiO2 and ZnO nanoparticles. The effect of silicon precursor was investigated using liquid hexamethyldisiloxane (HMDSO) or SiO2 sol, while for ZnO zinc acetate (ZA) was used. Gas phase reaction took place when using HMDSO as Si precursor, forming nanoparticles, whereas the SiO2 sol used as Si source was not evaporated in the flame, creating large aggregates of the sol particles (e.g. 1 μm). The FSP of ZA produced ZnO homogeneous nanoparticles. Lower flame temperatures in ECM than in FSP resulted in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes. The FSP of HMDSO and ZA led to intimate gas-phase mixing of Zn and Si, suppressing each other's particle growth, forming nanoparticles of 19 nm in BET-equivalent average primary particle diameter. Nucleation of ZnO and SiO2 occurred independently by ECM of HMDSO and ZA as well as by FSP of the SiO2 sol and ZA, creating a ZnO and SiO2 mixture. The reaction of ZnO with SiO2 was likely to be enhanced by ECM of the SiO2 sol and ZA where both Zn and Si species were not evaporated completely, resulting in ZnO, β-willemite and Zn1.7SiO4 mixed phase.

Performance of a Titan Rocket Engine Using Laser-Induced Fluorescence of OH

Rocket engine performance can be modeled by considering separately the propellant delivery system distribution, combustion efe ciency including propellant vaporization and gas-phase mixing, and nozzle expansion efe ciency. Although these quantities can be modeled, experimental verie cation is extremely helpful for separately understanding these processes and for design improvements. Laser-induced e uorescence of OH, excited by a KrF excimer laser operating at 248 nm, is used to measure the concentration of the OH radical across the exit plane of a e ring Titan IV, stage I, liquid rocket engine, from which the combusting mixture ratio proe le could be inferred. Thesemeasurementsallowassessmentofthedegreeofmixing and potential e owstratie cationbetween theinjector core, combustion bafe es, and combustion chamber fuel-e lm cooling and can help to provide the basis for future performance optimization. Nomenclature A = Einstein A coefecient Aa = area of the laser beam (height times thickness ) B12 = Einstein second coefe cient for stimulated absorption c = speed of light E = laser energy per pulse fB.T/ = temperature-dependent Boltzmann fraction of the absorbing state g.o/ = spectral overlap function ho = energy of a scattered photon K = non-noise-free gain factor M = magnie cation of the imaging system Nc = number of counts recorded by the camera per pixel Np = number of laser pulses integrated NT = total number density of the gas Pc = rocket engine combustion chamber pressure Qpre = predissociation rate S = e uorescence signal T = gas temperature uradial = radial component of the velocity V = collection volume ´ = collection efe ciency ÂOH = mole fraction of OH A = collection solid angle per pixel ! = wave number of the laser Subscript pp = per pixel

Experimental observations of spot radiative ignition and subsequent three-dimensional flame spread over thin cellulose fuels

Non-piloted radiative ignition and transition to flame spread over thin cellulose fuel samples was studied aboard the USMP-3 STS-75 Space Shuttle mission, and in three test series in the 10 second Japan Microgravity Center (JAMIC). A focused beam from a tungsten/halogen lamp was used to ignite the center of the fuel sample while an external air flow was varied from 0 to 10 cm/s. Non-piloted radiative ignition of the paper was found to occur more easily in microgravity than in normal gravity. Ignition of the sample was achieved under all conditions studied (shuttle cabin air, 21%–50% O 2 in JAMIC), with transition to flame spread occurring for all but the lowest oxygen and flow conditions. Although radiative ignition in a quiescent atmosphere was achieved, the flame quickly extinguished in air. The ignition delay time was proportional to the gas-phase mixing time, which is estimated by using the inverse flow rate. The ignition delay was a much stronger function of flow at lower oxygen concentrations. After ignition, the flame initially spread only upstream, in a fan-shaped pattern. The fan angle increased with increasing external flow and oxygen concentration from zero angle (tunneling flame spread) at the limiting 0.5 cm/s external air flow, to 90 degrees (semicircular flame spread) for external flows at and above 5 cm/s, and higher oxygen concentrations. The fan angle was shown to be directly related to the limiting air flow velocity. A surface energy balance reveals that the heat feedback rate from the upstream flame to the surface decreases with decreasing oxygen mass transport via either imposed flow velocity or ambient oxygen concentration. Quenching extinction occurs when the heat feedback rate from the flame is no longer sufficient to offset the ongoing surface radiative heat losses. Despite the convective heating from the upstream flame, the downstream flame was inhibited due to the ‘oxygen shadow’ of the upstream flame for the air flow conditions studied. Downstream flame spread rates in air, measured after upstream flame spread was complete and extinguished, were slower than upstream flame spread rates at the same flow. The quench regime for the transition to flame spread was skewed toward the downstream, because of the augmenting role of diffusion for opposed flow flame spread, versus the canceling effect of diffusion at very low cocurrent flows.

Performance of a Titan rocket engine using laser-induced fluorescence of OH

Rocket engine performance can be modeled by considering separately the propellant delivery system distribution, combustion efficiency including propellant vaporization and gas-phase mixing, and nozzle expansion efficiency. Although these quantities can be modeled, experimental verification is extremely helpful for separately understanding these processes and for design improvements. Laser-induced fluorescence of OH, excited by a KrF excimer laser operating at 248 nm, is used to measure the concentration of the OH radical across the exit plane of a firing Titan IV, stage I, liquid rocket engine, from which the combusting mixture ratio profile could be inferred

A Composite Modeling Study of Civil Aircraft Impacts on Ozone and Sulfate over the Taiwan Area

Aircraft induced ozone and sulfate perturbations calculated from a Lagrangian plume model were composed into a regional distribution to quantify the impacts of aircraft emissions at the international and domestic flight altitudes of about 11.5 and 6.5 km, respectively, over Taiwan. The evolution of pollutant concentrations within a Lagrangian aircraft plume was first calculated with consideration of gas-phase chemistry and mixing of ambient air. Due to reaction with high concentration of aircraft emitted NO, the O 3 concentration in fresh aircraft plumes rapidly decreased by more than 15 ppb during the first few hours after emissions. Then, becauseof mixing with the ambient air that contained higher O 3 and lower NO x concentrations, as well as the conversion of NO into NO 2 , the O 3 concentrations in aged plumes increased and became somewhat (up to 2 ppb) higher than in the ambient air as the plumes were transported further downwind. The sulfate concentration within the aircraft plumes, on the other hand, was enhanced due to chemical conversion of the emitted SO 2 . At both the domestic and international flight altitudes, enhancements of up to 10 and 5 ppt greater than in the background were achieved a couple of days after emissions. In the aged and diluted plumes, however, the production of sulfate can be lower than in the ambient because of the lower OH concentration. The plume results were converted into regional-scale distributions by including all flight frequencies at each international and domestic flight corridor, as well as the probability distribution of the wind speed at the cruising altitudes to account for the advection effect. These composite results indicated that aircraft emissions led to negative perturbation of O 3 concentration at and just downwind of the flight corridors, with maximum O 3 reduction of about 1 and 4% at the international and domestic flight altitudes, respectively. The total column (vertically integrated through troposphere) O 3 , however, was enhanced because the emitted aircraft plumes were transported by various wind speeds and those plumes that stay longer

A networks-of-zones analysis of mixing and mass transfer in three industrial bioreactors

The original version of the networks-of-zones (N-o-Z) model developed for the description of gas–liquid flow in stirred-vessel reactors (R. Mann, Gas–liquid stirred vessel mixers: towards a unified theory based on network of zones, Transactions of the Institution of Chemical Engineers 64 (1986) 23–34) has been extended and enhanced to cover distributed bubble sizes, gas–liquid mass transfer, bioreaction kinetics and multiple-impeller operation. In addition, a modified version of the N-o-Z model for bubble columns has been simply derived from the impeller version, assuming the existence of a two-loop axisymmetrical circulation pattern induced by the non-uniform distribution of gas holdup in bubble columns. The liquid circulation velocity has been expressed as a function of gas flow rate and the density difference between the gas and liquid phases, based on Zehner's circulation model (P. Zehner, G. Schuh, A concept for the description of gas phase mixing in bubble columns, German Chemical Engineering 5 (1985) 282–289). These two variants of the N-o-Z model have been used for modelling three different industrial fermenters: 3 and 31 m 3 triple-impeller stirred reactors, and a 236 m 3 bubble column reactor. The performance of these three reactors, typical of the fine chemicals, bioprocessing and pharmaceutical process industries was evaluated and compared in terms of geometry/size, gas flows, power inputs, pressure, liquid mixing, oxygen mass transfer, reaction speed and spatial variability of behaviour. This provides potentially valuable insights into the relative factors influencing the selection of an appropriate reactor type.

Patterns due to quintic kinetics in a diffusion-reaction system with global interaction

We study the process of pattern selection in a catalytic ribbon or disk subject to global interaction. The diffusion-reaction system, xt−Δx=f(x,y)−〈f(x,y)〉; yt=ε(−αx−y), with a quintic source function f(x,y)=−x(x2−1)(x2−a2)+y, qualitatively describes the behavior of catalytic or electrochemical oscillations subject to control or gas-phase mixing and the kinetics describes a system with two simultaneous or consecutive reactions. This model shows a richer class of solutions than the extensively studied one with a cubic source function (f=−x3+x+y) since f(x)=0 is tristable and for a wide separation of time scales the system admits, without global interaction, coexistence of a stable and oscillatory states. Also the reaction-diffusion equation with a quintic source may admit one large and two small fronts and their domains of existence and stability are mapped. Under global interaction the system exhibits all the patterns unveiled with the “cubic kinetics,” along with multifront patterns and new patterns at the border of instability of the large front.

Mathematical modeling of fluidized bed combustion — 2: combustion of gases

Combustion of volatiles within a fluidized bed combustor is expected to influence the pollutant formation and emission characteristics as well as the combustion behavior of carbon. The difficulties involved in developing system models which include details of the release of volatiles from coal and their subsequent combustion are well recognized. In this paper, the behavior of combustible gas mixtures is considered as a sub-model of the more complex problem. The hydrodynamic description of the bubbling fluidized bed is based on the three-phase dispersion model, which accounts for the variation of the visible bubble flow with height above the distributor (the ability of this model to explain experimental data on gas-phase mixing and reaction in isothermal bubbling beds has already been established). The non-isothermality of the bed resulting from combustion reactions is taken into account through inclusion of an energy balance for the bubble phase. Finally, the effect of the variation in superficial gas velocity, on bubble properties and cross-flow, is included through an overall mass balance. Species (intermediates and products) and energy balances are solved numerically. Transient calculations show that the ignition first occurs close to the top of the bed; with time, the ignition front moves closer to the distributor. Steady-state parametric analyses show that the ignition front moves closer to the distributor with increase in bed temperature and particle size, and decrease in the excess gas velocity. Model calculations were also compared with experimental data reported in the literature on the axial variation in mole fractions of various reactant and product species. These comparisons show that under typical fluidized bed combustion conditions, methane and propane burn in a region of parametric sensitivity; thus, significant variations in calculated mole fractions result from comparatively minor variations in reaction rate parameters. Nevertheless, with activation energies well within acceptable limits established in independent kinetic studies, good agreement is obtained with experimental data for combustion of gases in fluidized beds.

A hot-flow model analysis of the MSW incinerator

A hot-flow model incinerator was constructed to simulate the combustion chamber of mass-burn municipal solid waste (MSW) incinerator. Experiments using the reduced scale simplified model were designed to reproduce the physical processes. The Current study focuses on the gas phase mixing and the subsequent oxidative destruction of incomplete combustion products. Excessive carbon monoxide is generated by operating the gas burner under fuel-rich conditions, and is supplied into the furnace chamber as a hot gas stream, where fresh air is provided by the secondary air injection. Mixing is quantified by measuring the local gas concentrations (CO and O2) and temperatures since the destruction rate of CO is controlled by oxygen availability and chemical kinetic rates. The degree of mixing is monitored while the design alternatives of the secondary air injection pattern are systematically adjusted. This effect of the secondary air injection schemes on the degree of mixing are observed, and the measurements of temperature fluctuation with fine-wire thermocouples are performed for the quantitative evaluation of mixing. The results agree well with the plausible scenario of mixing and provide a better understanding of the mixing process. The hot flow accompanying chemical reactions in the model incinerator is also analysed numerically using the computational fluid dynamics codes to bolster the understanding of the experimental results. The modelling results compare reasonably well with experimental data. This comparison helps to cross-check the results. © 1997 John Wiley & Sons, Ltd.