Mixing Zone Length Research Articles

Reduction of the harmful NOx pollutants emitted from the ship engines using high-pressure selective catalytic reduction system

Various techniques are used to reduce harmful pollutants such as NOX emissions from ships. Selective catalyst reduction (SCR) systems are the most effective technique used to reduce NOX emissions. In this study, the effects of an SCR reactor on NOX emissions and performance in high-pressure selective catalytic reduction (HP-SCR) systems were investigated numerically. In numerical studies, the effects of SCR system diameter, output form, catalyst activation energy, mixing zone length, and location were investigated as parametric, and the most suitable system geometry was determined. The effects of geometric parameters and catalyst type on emission and performance such as NOX reduction, NH3 slip, velocity, and pressure loss were investigated. It was determined that with increasing system diameter, whereas the NOX reduction performance increased depending on exhaust velocity, the pressure drop decreased, and the most suitable system diameter was determined as 780 mm. Furthermore, the obtained results showed that the performance of NOX reduction decreased after 2 × 106 kJ/kmol activation energy, and the most suitable SCR output form was conical geometry. In terms of the environment, this study will contribute to achieving the UN Sustainable Development Goals such as climate action (SDG 13).

An Environmentally Friendly Compact Microfluidic Hydrodynamic Sequential Injection System Using Curcuma putii Maknoi & Jenjitt. Extract as a Natural Reagent for Colorimetric Determination of Total Iron in Water Samples.

The miniaturization of analytical systems and the utilization of nontoxic natural extract from plants play significant roles for green analytical chemistry methodology. In this work, the microfluidic hydrodynamic sequential injection (HSI) with the LED-phototransistor colorimetric detection system has been proposed to create an ecofriendly and low-cost miniaturized analytical system for online determination of iron in water samples using Curcuma putii Maknoi & Jenjitt. extracts as high stability and good selectivity of a natural reagent. The proposed method was designed for online solution mixing and colorimetric detection on a microfluidic platform. The Curcuma putii Maknoi & Jenjitt. extracts and standard/samples were sequentially aspirated to fill the channel before entering the built-in flow cell. The intensity of iron-Curcuma putii Maknoi & Jenjitt. extract complex was monitored under the optimum conditions of flow rate, sample volume, mixing zone length, and aspiration sequences, by altering the gain control of the colorimetric detector to achieve good sensitivity. The results demonstrated a good performance of the green analytical systems. A linear calibration graph in the range of 0.5-6.0 mg L-1 was obtained with a limit of detection at an adequate level of 0.11 mg L-1 for water samples with a sample throughput of 30 h-1. The precise and accurate measurement results were achieved with relative standard deviations in the range of 1.61-1.72%, and percent recoveries were found in the range of 90.6-113.4. The proposed method offers cost-effective, easy operation over an appropriate analysis time (2 min/injection) with good sensitivity and is environmentally friendly with low consumption of solutions and the use of high stability and good selectivity of nontoxic reagents. The achieved method was demonstrated to be a good choice for routine analysis.

The Influence of Bimodal Heterogeneity on Viscous Fingering of a Miscible Interface in Porous Media

The flow regimes and the dynamics of the front in miscible displacements are controlled by the interactions between the mechanisms of instability involved in such processes. The instabilities may be driven by unfavorable gravity or mobility ratios or by the heterogeneity of the medium providing favorable paths for the more mobile fluid. In this work, the effect of porous medium heterogeneity with two scales of permeability variations on the frontal instability and fluid mixing have been investigated. The base mode of permeability variations has a smaller wavelength and higher frequency while the imposed mode has a larger wavelength. The effect of such a bimodal heterogeneity on the growth of mixing zone length (MZL) has been studied and the development of the previously recognized flow regimes in layered porous media have been examined. Compared to the unimodal medium comprising the base wave, in the bimodal cases with large contrast between the wavelengths of the two periodic profiles the dominance of each wave length at a different time scale predictably enhances the growth of fingers in the early and late stages. Interestingly and less intuitively, even in cases with close wave numbers between the combined modes faster growth of the mixing zone length is observed. In such cases, the coherence of equal layers in a unimodal layered medium is disturbed by the second wave number resulting in fading of the lateral diffusion regime. However, bimodal heterogeneity may attenuate the instability compared to the unimodal system with the imposed wave’s frequency.

Hyporheic transverse mixing zones and dispersivity: Laboratory and numerical experiments of hydraulic controls

Mixing of surface water and groundwater in shallow sediments is important to biogeochemical cycling and contaminant migration, and is often used to define the hyporheic zone. Yet knowledge of mixing processes in hyporheic zones is supported by surprisingly few rigorous lab or field observations, and differ from those in deeper groundwater by presence of enhanced head gradients, sediment heterogeneity, and temporal fluctuations. In a laboratory sediment (sand) tank we photographed a conservative dye to analyze transverse mixing zones between upwelling groundwater and bidirectional hyporheic exchange flows. We then conducted numerical modeling to investigate processes behind observed phenomena and estimate dispersivities. We found that transverse mixing zones were thin (i.e. mixing thickness measured in direction of steepest concentration gradient, δ, less than 5 cm), consistent with a small calibrated transverse dispersivity (~0.1 mm) and prior lab studies conducted at similar scales. In steady-state experiments and simulations, δ and estimated dispersion coefficients increased with the surface water head drop driving exchange flows. Given relatively constant deeper groundwater heads, increased Δh led to increased mixing zone length for both steady-state and transient conditions, indicating larger bedforms or weaker gaining conditions enhance subsurface mixing. However, Peclet number and flux-related dilution index simultaneously increased and decreased, respectively, indicating that enhancement of subsurface advection outpaced that of dispersion. In transient experiments and simulations, δ was greater than for steady-state, probably from temporary addition of longitudinal dispersion. During transient experiments, δ exhibited temporal noise, perhaps due to the mixing zone moving past varying patterns of sediment packing. Our results provide basic knowledge of mixing zone behavior in hyporheic zones with implications for hyporheic zone definitions, solute transport, mixing-dependent reaction, and water quality.

Study of Near Field Performance of Multi-ports Diffuser in Relatively Deep Rivers.(Dept.C)

In this paper an experimental study is presented for the problem of the thermal power stations, which is discharging the hot water into relatively deep rivers. The receiving water of the river is said to be deep when the jet is free to mix due to entrainment and buoyancy and is not constrained by neither the river bed nor the water surface (6), which increases the temperature dilution. The water depth (H) is constant during the experiments. The main objective of this paper is to develop a new general equation which can be used in designing the multi-ports diffuser of the thermal power plant discharging hot water into relatively deep rivers. Also, the rapid dilution and minimizing of the mixing zone length of the hot water 10 reduce the environmental effects are considered. In this paper, the effects of the different parameters related to both the out-fall structure and the river characteristics on the rapid dilution of the hot water concentration (Cm/Co) are considered. These parameters are, the effluent/ambient velocity ratio (V/U), the number of ports (n), the spacing between the ports with respect to its diameter (s/D). the vertical angle of ports in combination with the ratio of its height to the flow depth (θh/H), and the horizontal angle between the diffuser line and the flow direction (β). The analysis of the experimental results shows that the relative thermal concentration ( Cm/Co ) is directly proportional to V/U, n, θh/Hand β. while it is inversely proportional to SD. Also, it is found that the maximum value of Cm/Co occurs when the diffuser is aligned parallel to the direction of flow. A general equation between the relative thermal concentration (Cm/Co) and the aforementioned parameters is developed. The comparative study between the experimental results, which is carried out by Mowed [7], and the corresponding results by applying the developed equation, illustrates that there is a good compatibility between the two results.

Stochastic radiative transfer in random media: Pure absorbing cases.

We study stochastic radiation transport through random media in one dimension, in particular for pure absorbing cases. The statistical model to calculate the ensemble-averaged transmission for a binary random mixture is derived based on the cumulative probability density function (PDF) of optical depth, which is numerically simulated for both Markovian and non-Markovian mixtures by Monte Carlo calculations. We present systematic results about the influence of mixtures' stochasticity on the radiation transport. It is found that mixing statistics affects the ensemble-averaged intensities mainly due to the distribution of cumulative PDF at small optical depths, which explains well why the ensemble-averaged transmission is observed to be sensitive to chord length distribution and its variances. The effect of the particle size is substantial when the mixtures' correlation length is comparable to the mean free path of photons, which imprints a moderately broad transition region into the cumulative PDF. With the mixing probability increasing, the intensity decreases nearly exponentially, from which the mixing zone length can be approximately estimated. The impact of mixed configuration is also discussed, which is in line with previous results.

Effect of Surface Water Stage Fluctuation on Mixing‐Dependent Hyporheic Denitrification in Riverbed Dunes

AbstractThe hyporheic zone, where surface water (SW) and groundwater (GW) interact in shallow sediments beneath rivers, is uniquely reactive and attenuates pollutants. Mixing of reactants from SW and GW enables mixing‐dependent (MD) reactions, which may be the last opportunity for GW contaminants to react before entering SW. Yet little is known about hyporheic MD reactions, particularly how they respond to daily or seasonal SW fluctuations or sediment heterogeneity. We used MODFLOW and SEAM3D to simulate non‐mixing‐dependent (NMD) aerobic respiration and MD denitrification in a riverbed dune with nitrate from SW and dissolved organic carbon from GW. We varied SW heads and heterogeneity of sediment hydraulic conductivity. For longer‐term fluctuations (i.e., seasons), increasing SW depth from 0.1 to 1.0 m increased NMD aerobic respiration by 270% and MD denitrification by 78% in homogeneous sediment. MD reactions thus were controlled by mixing zone length or size and would be stronger when SW stage is elevated, for example, during wintertime. Adding sediment heterogeneity to the long‐term scenarios, particularly by increasing hydraulic conductivity correlation length, increased flow focusing and consequently increased MD denitrification by 20–30%. By contrast, the net effect of daily SW fluctuations on MD denitrification in homogeneous sediment was minor. In sum, SW fluctuations are an important control on hyporheic MD reactions, primarily by controlling mixing zone length. The hyporheic zone may attenuate nitrate in upwelling GW plumes, but temporal fluctuations may be considerable as quantified above.

Experimental analysis of density fingering instability modified by precipitation.

We analyze the effect of precipitate formation on the development of density induced hydrodynamic instabilities. In this case, the precipitate is BaCO3, obtained by reaction of CO2 with aqueous BaCl2. CO2(g) dissolution increases the local density of the aqueous phase, triggering Rayleigh-Taylor instabilities and BaCO3 formation. It was observed that at first the precipitate was formed at the finger front. As the particles became bigger, they began to fall down from the front. These particles were used as tracers using PIV technique to visualize the particle streamlines and to obtain the velocity of that movement. This falling produced a downward flow that might increase the mixing zone. Contrary to expectations, it was observed that the finger length decreased, indicating that for the mixing zone development, the consumption of CO2 to form the precipitate is more important than the downward flow. The mixing zone length was recovered by increasing the availability of the reactant (higher CO2 partial pressure), compensating the CO2 used for BaCO3 formation. Mixing zone development rates reached constant values at shorter times when the precipitate is absent than when it is present. An analysis of the nonlinear regime with and without the precipitate is performed.

EFFECT OF ATOMIZER INTERNAL GEOMETRY ON THE INTERNAL AND EXTERNAL TWO-PHASE FLOW IN EFFERVESCENT ATOMIZATION

An experimental study was conducted to study the internal and external two-phase flow in an effervescent atomizer using high-speed imaging. The impact of the aerator tube configuration and mixing zone length on the size of bubbles inside the mixing zone and the spray droplet characteristics at different gas-to-liquid flow rates ratios were studied. An aerator tube with the conical end base was manufactured and tested. It is observed that the separation bubble at the trailing edge is suppressed by this configuration, which resulted in more uniform and smaller bubbles compared to the standard aerator tube with a flat base. The length of the mixing zone was found to have an impact on the bubble size distribution inside the mixing zone; more uniform and smaller bubbles are generated in the shorter mixing zone. The mixing zone length, however, does not show a distinct impact on the droplet velocity and size. It is concluded that a conical base aerator tube and a shorter mixing zone could significantly improve the spray steadiness and the atomization process.

Magnetic resonance imaging analysis on the in-situ mixing zone of CO2 miscible displacement flows in porous media

The in-situ mixing zone represents dynamic characteristics of CO2 miscible displacement flows, which is important for carbon dioxide enhanced oil recovery (CO2-EOR) projects. However, the migration characteristics of the in-situ mixing zone under reservoir conditions has been neither well studied nor fully understood. The in-situ mixing zone with the flowing mixture of supercritical CO2 and n-decane (nC10) was investigated by using a magnetic resonance imaging apparatus at a reservoir condition of 8.5 MPa and 37.8 °C in porous media. The experimental results showed that the CO2-frontal velocity was larger than the mixing-frontal velocity. The mixing zone length was linearly declined in the miscible displacement process. And the declining rate of the mixing zone length was increased with injection rate. It indicates that the mixing zone length is not constant in a vertically stable CO2 misible displacement and a volume contraction due to phase behavior effects may occur. Then, an error function based on the convection-dispersion equation was fitted with CO2 miscible displacement experiments. The error function was well fitted both at a series of fixed core positions and a series of fixed displacement times. Furthermore, the longitudinal dispersion coefficients (Klx and Klt) and the longitudinal Peclet numbers (Ped and PeL) were quantified from the fitting results. The evolutions of the longitudinal dispersion coefficient were reduced along the displacement time. And the declining rate was increased with injection rate. And with proceeding, the longitudinal dispersion coefficient was tending towards stability and constant. But the evolutions of the longitudinal Peclet numbers were increased along the displacement time. And the increasing rate was increased with injection rate.

Scaling and unified characterization of flow instabilities in layered heterogeneous porous media

The physics of miscible flow displacements with unfavorable mobility ratios through horizontal layered heterogeneous media is investigated. The flow model is solved numerically, and the effects of various physical parameters such as the injection velocity, diffusion, viscosity, and the heterogeneity length scale and variance are examined. The flow instability is characterized qualitatively through concentration contours as well as quantitatively through the mixing zone length and the breakthrough time. This characterization allowed us to identify four distinct regimes that govern the flow displacement. Furthermore, a scaling of the model resulted in generalized curves of the mixing zone length for any flow scenario in which the first three regimes of diffusion, channeling, and lateral dispersion superpose into a single unifying curve and allowed us to clearly identify the onset of the fourth regime. A critical effective Péclet number w_{c} based on the layers' width is proposed to identify flows where heterogeneity effects are expected to be important and those where the flow can be safely treated as homogeneous. A similar scaling of the breakthrough time was obtained and allowed us to identify two optimal effective Péclet numbers w_{opt} that result in the longest and shortest breakthrough times for any flow displacement.

Mixing of surface and groundwater induced by riverbed dunes: Implications for hyporheic zone definitions and pollutant reactions

[1] The hyporheic zone is often defined as where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers. This mixing is credited with creating unique biogeochemical conditions that can attenuate contaminants from either upstream surface water or groundwater under gaining conditions. However, reactions of contaminants upwelling from groundwater may be more dependent on such mixing than contaminants from surface water. We numerically modeled mixing between hyporheic flow paths induced by riverbed dunes and flow paths of adjacent upwelling of deeper groundwater. Results show that only 12.7% or less tracer mass upwelling from deeper groundwater dispersed across into hyporheic flow paths originating in surface water. The spatial extent of a mixing-defined hyporheic zone was smaller than a hyporheic zone defined as hydrologic flow paths leaving and returning to surface water. Mixing-dependent reactions will therefore be localized within a thin mixing zone yet vary considerably with site conditions. For example, mixing in homogeneous sediments was controlled most by variation in hydraulic conductivity and upwelling flow rate which primarily affected mixing zone length. By contrast, introduction of heterogeneity increased mixing primarily by increasing mixing zone thickness, consistent with studies of flow focusing in aquifers. Finally, dispersivity is a critical parameter for which data are needed for shallow sediments. Our results help clarify hyporheic zone definitions and potential for mixing-dependent reactions. In particular, the biogeochemically reactive portion of riverbed sediments from the perspective of upwelling contaminants does not necessarily spatially coincide with more traditional hydrologic conceptions of the hyporheic zone.

Diffusive and Convective Mechanisms during CO2 Sequestration in Aquifers

CO2 emissions originated from industrial sources can be captured, transported, and stored in depleted gas/oil fields and deep saline aquifers. The transport mechanisms, occurred during CO2 sequestration in deep saline aquifers, are examined in this study. After injecting CO2 until the tolerable pressure for the aquifer is reached, the wells are closed and CO2 is deposited as free gas and soluble gas in water under the sealing rock. During injection and waiting periods, the concentration profile of CO2 within the aquifer is formed by diffusion and convection mechanisms. The Rayleigh number and mixing zone length concepts are used for investigating the effect of reservoir properties, such as dispersivity, permeability, porosity, and others on the aforementioned mechanisms. The results of convective dominant mechanism in aquifers with 1 md and 10 md permeability values are so near in that diffusion-dominated system. After 10 md, the convection mechanism begins to dominate gradually and it becomes totally convection dominated for 50 md and higher permeability values. These results are also verified by the Rayleigh number and mixing zone lengths.

Pattern formation driven by an acid-base neutralization reaction in aqueous media in a gravitational field

We report the hydrodynamic instabilities found in a simple exothermic neutralization reaction. Although the heavier aqueous NaOH solution was put below the lighter layer of aqueous HCl solution, fingering at the interface in a Hele-Shaw cell was observed. The reaction front, which propagates downward, becomes buoyantly unstable in the gravity field. The mixing zone length and wave number depend on the reactant concentrations. The mixing zone length increases and the wave number decreases when the reactant concentrations decrease.

Liquid Holdup in Large-Diameter Horizontal Multiphase Pipelines

This paper studies the liquid holdup within the mixing zone of the slug. The results of the study show that the liquid holdup begins at the liquid holdup of the liquid film before the slug and then increases until the end of the mixing zone is reached. Once past the mixing zone of the slug, the average liquid holdup becomes constant. As the height of the liquid film and/or viscosity increases, so does the liquid holdup at any given film Froude number. Since the liquid holdup becomes constant once past the mixing zone of the slug, the mixing zone length was determined for the film Froude numbers studied. The results show that the mixing zone length increases linearly with film Froude number and is independent of the viscosity of the liquid in the slug for a viscosity range of 1 to 16.6 cP.

Quantitative Auger electron spectroscopy in thin film depth profiling

Auger Electron Spectroscopy (AES) analyzes the very first atomic layers with a characteristic intensity which exponentially decreases with depth from the surface. Therefore, any quantification strongly depends on the in depth distribution of composition within these first few monolayers. During depth profiling by ion sputtering, the surface composition is changed due to many phenomena such as ion beam induced atomic mixing and surface roughening, radiation enhanced diffusion, preferential sputtering and segregation. Even if the last ones can be neglected, the most fundamental effects of atomic mixing and ion beam induced roughening lead to a significant deviation between the measured profile and the AES depth profile that is predicted by the usual quantification method assuming an ideal layer by layer sputtering behavior. The difference depends mainly on the ratio of the Auger electron information depth and the mixing zone length and on the elemental distribution within these regions. Taking into account these dependencies in an appropriate model of the three parameters mixing length, roughness and information depth (M,R,I), the measured AES depth profiles can be calculated with high accuracy in the atomic monolayer regime. As demonstrated for GaAs AIAs multilayers, this model enables an accurate description of the depth resolution function as well as the conversion of the sputtering time in a depth scale which are the key issues for reconstruction of the original depth distribution profile.

Towards the numerical simulation of the horizontal slug front

AbstractSteps towards the numerical simulation of the flow behind the slug front in horizontal slug flow performed with a streamfunction‐vorticity representation of the mean flow and an energy dissipation model for the turbulence are discussed. The flow field consists of two vortices, one saddle point and four stagnation regions. Attention is focused on the following boundary conditions: moving wall jet, moving wall, free jet velocity discontinuity and vertical liquid‐gas open surface. A dissipation flux boundary condition is suggested to simulate the interaction of the turbulent eddies with the open surface. A method to assess the necessity to use a transport model equation for the dissipation rather than a geometric specification of a length is suggested. Three different ways to characterize the mixing zone length are proposed.

Determination of the atomic mixing layer in sputter profiling of Ta/Si multilayers by TEM and AES

AbstractSputter‐deposited multilayers of Si and Ta with a nominal period length of a double layer (Ta + Si) of 20 nm were studied with AES depth profiling and with transmission electron microscopy (TEM). The asymmetric shape of the measured Si layer profiles was fitted to model calculations based on preferential sputtering of Si and an atomic mixing zone length of 4 nm for 3 keV Ar+ ions at a 36° incidence angle. TEM images of cross‐sections of the original sample show sharp Si/Ta and Ta/Si interfaces, with a width of ∼ 0.5 nm. The atomic mixing zone length of the sputter‐profiled sample was observed directly by TEM and was determined to be 4.0 nm, in accordance with the AES profile evaluation. It is concluded that the observed asymmetric broadening of the shape of the measured AES sputtering profiles is due mainly to atomic mixing and is enhanced by preferential sputtering of Si.

The Effects of Gravity Segregation in Laboratory Studies of Miscible Displacement in Vertical Unconsolidated Porous Media

Abstract This study of the effect of gravity segregation was carried out in a vertical core of unconsolidated sand, 4 ft long and 2 in. in diameter. Fluids of varying viscosity and density were injected into the top of the core and moved at constant rates of 25, 50 or 100 ft/day by use of a positive displacement pump. The composition of the efflux was analyzed by measurement of the refractive index. The main variables in this study were in viscosity ratio, the density difference between in-place and displacing phases, and the rate of flow. The experiments fell into four logical groups:favorable viscosity ratio and favorable density difference;favorable viscosity ratio and unfavorable density difference;unfavorable viscosity ratio and favorable density difference; andunfavorable viscosity ratio and unfavorable density difference. The behavior of these four systems at various rates of flow was determined by measuring the length of the mixing or transition zone which developed between the displaced and displacing phases. The data indicated that gravity segregation could act to shorten this mixing zone when the displacing material was the less dense phase, and lengthen the zone for unfavorable density differences. The magnitude of the effect was most marked at slow rates when sufficient residence time existed to allow significant flow to occur. The change in the length of the mixing zone with density difference, rate and viscosity ratio was plotted, and from these graphs it became evident that the length of the mixing zone was really dependent upon the ratio of the viscous forces to the gravity forces. If the ratio of these quantities is used as a parameter, we find that the plots of mixing zone length vs the dimensionless quantity V/Vc, yields a greatly simplified presentation of the data. Such plots confirm the importance of the ratio of the viscous forces to the gravity forces in analyzing the flow behavior in vertical systems. Introduction The idea of using miscible phase displacements to increase recovery of crude oil has been under active study for more than 15 years. Most of this work has been carried out in the research laboratories of the industry and in universities. In such studies a great deal of attention has been directed toward determining the amount of miscible material required to recover all of the oil from a porous medium. The nature and extent of the mixing zone which develops between the two miscible materials has also been observed for many situations, including those where the fluids used have different densities. In many of the cases where density differences exist, however, the effect of gravity segregation on the mixing zone has not been explicitly studied. It seems quite likely that serious errors might be present in such experimental work. That is, the literature presents certain data which suggest changes in efficiency of the process as a result of some explicit change in a variable, when it may well be that the change in the efficiency results from gravity effects. The objective of this study, therefore is to determine the effect of density differences of miscible fluids on the observed efficiency of the displacement process. Since such density differences cause selective movement of fluids within the porous medium, the study becomes vitally concerned with gravity segregation. Moreover, since the amount of gravity segregation which can occur for a given set of fluids depends upon residence time, the rate of flow in these experiments also becomes an important variable. Finally, since the actual distribution of the fluids within the system (fingers, etc.) also affects the results, the viscosity ratio is another pertinent variable. Hence, this study may be characterized as a determination of the nature of the mixing zone between miscible phases as a function of density differences, viscosity ratio, and rate of flow. Fortunately these variables are inter-related. SPEJ P. 1ˆ