Volume Fraction Profiles Research Articles

The blending effect on the sooting tendencies of alternative/conventional jet fuel blends in non-premixed flames

In order to study the blending effect on sooting tendencies for practical alternative/conventional jet fuel blends, the present study has investigated soot formation in non-premixed flames of Jet-A blended with three alternative jet fuels (AJFs), i.e., Hydroprocessed Esters and Fatty Acids from camelina (HEFA-Camelina), Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), and Alcohol-to-Jet (ATJ). The smoke points of the AJF/Jet-A blends were measured in a wick lamp per ASTM D1322 standard, and their Threshold Sooting Indices were derived. Soot volume fraction profiles of the AJF/Jet-A fuel blends were also measured using the laser-induced incandescence diagnostics in the counterflow non-premixed flames. The effects of AJF blending ratio, strain rate, and reactant concentration on the soot formation were examined. The experimental results show that the Threshold Sooting Indices of the AJF/Jet-A blends exhibit a linear relationship with AJF mole fraction. While the maximum soot volume fractions of fuel blends show a fairly linear relationship with AJF blending ratio, the nonlinearity becomes prominent at high strain rates and low reactant concentrations. Comparing the three AJF/Jet-A fuel blends, the sooting tendencies are ranked as ATJ/Jet-A > FT-SPK/Jet-A > HEFA-Camelina/Jet-A. Correlations of the maximum soot volume fraction with aromatics content, hydrogen content, smoke point, and Threshold Sooting Index were all also examined. It is found that neither aromatics content nor hydrogen content can correlate with the maximum soot volume fraction for all the fuel blends at a given condition; conversely, smoke point shows a nonlinear correlation and Threshold Sooting Index shows an approximately linear correlation with the maximum soot volume fraction for all the fuel blends. It is suggested that the different sooting tendencies of AJF/Jet-A blends are mainly caused by the different sooting propensities of individual AJFs, which are strongly dependent on their distinct paraffinic compositions.

Intermolecular Voids in Lipid Bilayers in the Presence of Glycyrrhizic Acid.

It is known that glycyrrhizic acid (GA) promotes the enhancement of the activity of several medicines. This is attributed to the fact that GA increases the membrane permeability of small drug molecules. There is an opinion that GA facilitates the formation of additional large voids in the membrane, which enhance the passive diffusion of molecules across the membrane. In this work, we investigate how GA influences the intermolecular voids using the molecular dynamics simulation. We calculate the interstitial spheres (empty spheres inscribed between molecules) in model DPPC and DOPC bilayers, both pure and with the addition of cholesterol. It was observed that the addition of GA does not lead to the formation of new large interstitial spheres; i.e., new large voids do not appear. The distribution of empty volume inside the bilayers is also studied. We calculated the profiles of the empty volume fraction both from the middle plane of the bilayer and from its outer surface (from the lipid-water interface). This analysis has shown that the addition of GA does not cause the increase of the empty volume in the bilayer; moreover, there is a slight decrease in the bilayers with cholesterol. Thus, we have not found a confirmation of the simplest hypothesis that individual GA molecules induce pores in the membrane.

Numerical simulation of vapor volume fraction in a vertical channel under low-pressure conditions

This numerical study involved investigating void behavior under low-pressure subcooled flow boiling by using an Eulerian approach (two-fluid model). In the simulation, a vertical pipe with a length of 0.15 m and diameter of 0.01229 m was considered. Different levels of uniform wall heat flux, mass flux, and inlet subcooling temperature were applied although a constant pressure of 1.65 bar was used for all the simulations. A sensitivity study of the empirical coefficients used to access the predictive capacity of the existing mass transfer models was conducted. Thus, the k-epsilon model was used for the turbulence of the fluid. The axial vapor volume fraction profile, liquid temperature is compared at the operating pressure. Furthermore, the most sensitive flow characteristics of the channel were identified. The results indicated that the predictions of numerical phase evolution relative to the experimental observations were in good qualitative agreement with those obtained in extant studies. Additionally, the changes in drag coefficients were helpful in precisely predicting the void fraction. A commercial CFD solver was used for the implementation of the model.

Soret and Dufour effects on doubly diffusive convection of nanofluid over a wedge in the presence of thermal radiation and suction

This paper is devoted to investigate the influences of thermal radiation, Dufour and Soret effects on doubly diffusive convective heat transfer of nanoliquid over a wedge in the presence of wall suction. The governing equations are transformed to nonlinear ordinary differential equations using similarity transformation. The resulting system is solved numerically using fourth-order Runge-Kutta-Gill method with shooting technique and Newton-Raphson method. The solutions are expressed in terms of velocity, temperature, solutal concentration and volume fraction profiles. The effects of pertinent parameters entering into the problem such as wedge angle, thermal radiation, Brownian motion, thermophoresis, Soret and Dufour numbers on the skin friction coefficient, local Nusselt number and local Sherwood number are discussed in detail.

Heat Transfer Analysis of a Williamson Micropolar Nanofluid with Different Flow Controls

ABSTRACTThe present model is devoted for the steady stagnation point flow of a Williamson micropolar nanofluid with magneto-hydrodynamics and thermal radiation effects passed over a horizontal porous stretching sheet. The fluid is considered to be gray, absorbing-emitting but non-scattering medium. The Cogley-Vincent-Gilles formulation is adopted to simulate the radiation component of heat transfer. By applying similarity analysis, the governing partial differential equations are transformed into a set of non-linear ordinary differential equations and they are solved by using the bvp4c package in MATLAB. Numerical computations are carried out for various values of the physical parameters. The effects of momentum, microrotation, temperature and nanoparticle volume fraction profiles together with the reduced skin friction coefficient, reduced Nusselt number and reduced Sherwood number of both active and passive controls on the wall mass flux are graphically presented. The present results are compared with previously obtained solutions and they are in good agreement. Results show that the skin friction is increasing functions of the Williamson parameter in both stretching and shrinking surfaces.

Effect of turbulent dispersion on hydrodynamic characteristics in a liquid jet ejector

Gas induction using a liquid jet is an extensively studied phenomenon. Many studies have been published detailing various hydrodynamic and mass transport aspects of gas induction in ejectors. Conversely, multiple studies have been published detailing jet dynamics, jet break up, nozzle geometry, effects of turbulence etc. In the modelling of the jet ejector systems CFD framework is used as the flow of gas is coupled with the liquid flow and the flow parameters like volume fraction, pressure drop/profile and velocity profiles both local and average are highly interdependent. Hence, it becomes important to capture all the flow physics in the simulations in order to understand the hydrodynamics. In the previous models the effect of turbulent dispersion was not included which led to incorrect calculation of phase profile, and other related parametric values. In the present work, the turbulence effects and its impact on the jet dynamics and gas induction rate in an ejector have been studied. Capturing the correct phase as well as velocity profile is imperative for the extension of the model to mass transfer and reactions. Lastly, other hydrodynamic properties like pressure drop, phase hold up and pressure profile help better the design correlations for jet ejectors.

Interface-resolved simulations of particle suspensions in Newtonian, shear thinning and shear thickening carrier fluids

We present a numerical study of non-colloidal spherical and rigid particles suspended in Newtonian, shear thinning and shear thickening fluids employing an immersed boundary method. We consider a linear Couette configuration to explore a wide range of solid volume fractions ($0.1\leqslant \unicode[STIX]{x1D6F7}\leqslant 0.4$) and particle Reynolds numbers ($0.1\leqslant Re_{p}\leqslant 10$). We report the distribution of solid and fluid phase velocity and solid volume fraction and show that close to the boundaries inertial effects result in a significant slip velocity between the solid and fluid phase. The local solid volume fraction profiles indicate particle layering close to the walls, which increases with the nominal $\unicode[STIX]{x1D6F7}$. This feature is associated with the confinement effects. We calculate the probability density function of local strain rates and compare the latter’s mean value with the values estimated from the homogenisation theory of Chateau et al. (J. Rheol., vol. 52, 2008, pp. 489–506), indicating a reasonable agreement in the Stokesian regime. Both the mean value and standard deviation of the local strain rates increase primarily with the solid volume fraction and secondarily with the $Re_{p}$. The wide spectrum of the local shear rate and its dependency on $\unicode[STIX]{x1D6F7}$ and $Re_{p}$ point to the deficiencies of the mean value of the local shear rates in estimating the rheology of these non-colloidal complex suspensions. Finally, we show that in the presence of inertia, the effective viscosity of these non-colloidal suspensions deviates from that of Stokesian suspensions. We discuss how inertia affects the microstructure and provide a scaling argument to give a closure for the suspension shear stress for both Newtonian and power-law suspending fluids. The stress closure is valid for moderate particle Reynolds numbers, $O(Re_{p})\sim 10$.

Gold nanoparticle distribution in polyelectrolyte brushes loaded at different pH conditions.

Composites made of polymer brushes with inclusions of gold nanoparticles (AuNPs) combine the responsive nature of polymer brushes with the optical properties of the AuNPs, which offers the possibility to be used as colorimetric sensors. To this end, it is crucial to know how AuNPs are distributed inside the brush. Here, this distribution was elucidated by neutron reflectometry with contrast variation and a self-consistent reflectivity analysis based on the analytical parameterization of the volume fraction profiles of all chemical components. In contrast to former studies, this analysis allows the determination of the spatial distribution of components separately from each other: polyelectrolyte, AuNP, and water. Cationic poly-[2-(Methacryloyloxy) ethyl] trimethylammonium chloride (PMETAC) brushes were loaded with 5 nm AuNPs, which were coated with a pH-sensitive capping. The pH was varied during the incubation of the brush in the AuNP suspension. At a lower pH, AuNPs form aggregates in suspension and are attached to the brush periphery. They adsorb into the brush but do not fully penetrate it due to their bulkiness. At a higher pH, AuNP suspensions are electrostatically stabilized and the AuNPs penetrate the brush entirely. However, the AuNP distribution over the brush is not homogeneous but decreases gradually toward the substrate. Penetration of the AuNPs leads to a more extended conformation of the brush. According to the results of the detailed analysis of all components, an increase in water content could be excluded as a reason for brush swelling but replacement of water by the AuNP was observed.

Comparison of two-fluid and discrete particle modeling of gas-particle flows in micro fluidized beds

The formation of a more porous zone in the near wall region has been commonly suggested to be one of the main mechanisms of the delayed onset of fluidization in micro fluidized beds (MFBs). If such geometrical effect still plays a major role once the bed is fluidized, it will challenge the applications of Two-Fluid Model (TFM) to simulate micro fluidized beds. In this work this issue is assessed by taking the simulation results of the Computational Fluid Dynamics coupled with the Discrete Element Method (CFD-DEM) as the benchmark data. Both the delayed onset of fluidization and the advanced onset of turbulent fluidization predicted by CFD-DEM simulations are successfully captured by the TFM simulations. The radial profiles of solids volume fraction and solids axial velocity predicted by TFM and CFD-DEM simulations are also in well agreement. Such results demonstrate that TFM can be appropriately used to model micro fluidized beds systems. Parameter analyses show that TFM simulation results are closely related to the input parameter setting. The minimum bubbling velocity predicted by TFM simulations closely depends on the chosen value of frictional packing limit. And the specularity coefficient has a significant impact on the predicted solids flow behavior.

Unsteady free convective boundary layer flow of a nanofluid past a stretching surface using a spectral relaxation method

ABSTRACTAn analysis is presented to study the unsteady, two-dimensional nanofluid flow of heat and mass transfer with vanishing nanoparticles flux at the wall in the presence of heat generation or absorption with viscous dissipation. In this analysis, it is assumed as nanoparticle flux is zero on the boundaries. The highly nonlinear differential equations governing the boundary layer flow, heat and mass transfer are numerically solved by spectral relaxation method and validated with MATLAB solver bvp4c. The results are obtained by some values of the physical parameters, namely, the Brownian motion parameter, the thermophoresis parameter, unsteadiness parameter, Eckert number, heat generation or absorption parameter, Prandtl number and Lewis number. Physical features for all relevant parameters on the dimensionless velocity, temperature, nanoparticles volume fraction and heat and mass transfer rates are analysed and discussed. The thermal boundary layer thickness increases with an increase in the thermophoretic parameter. The nanoparticle volume fraction profile significantly descends from the surface with an increase in Brownian motion parameter.

Reconstruction and simulation of temperature and CO2 concentration in an axisymmetric flame based on TDLAS

Two-dimensional (2D) distributions of temperature and species concentration are crucial information to understand PAH inception and soot formation in diffusion flames. We report a tunable diode laser absorption spectroscopy (TDLAS) measurement to reconstruct the temperature and carbon dioxide concentration profiles in axisymmetric methane co-flow diffusion flame. The TDLAS sensor selected the v3 bandhead region of CO2 molecule at 4.2 um to ensure high temperature sensitivity and avoid ambient background absorption. A Fourier analysis Abel inversion transform algorithm was used to reconstruct 2D distribution of attenuation coefficient and showed higher inversion accuracy compared with other commonly used inversion methods. Evaluation of the reconstruction accuracy of the present Fourier analysis and traditional Onion Peeling algorithm was also performed using the simulated temperature and CO2 concentration profiles from CFD calculation. The experimental reconstructed temperature and CO2 volume fraction profiles were obtained and compared with thermocouple measurement and CFD simulation. The results show that the temperature from TDLAS measurement agrees well with CFD simulation. Flame temperature measured by thermocouple is lower compared with TDLAS and CFD because of the perturbation introduced by insertion of the thermocouple, especially at the edge of the flame.

Visualization of flow structures inside a conical spouted bed by electrical capacitance volume tomography

Electrical capacitance volume tomography (ECVT) was conducted to measure instantaneous gas–solid flow structures in a conical spouted bed. The sensor has 24 electrodes/channels and was used to make three-dimensional measurements of the solid volume fraction. Flow structures in the conical spouted bed were analyzed using quantitative ECVT images. Three-dimensional images of gas–solid flow structures and time-averaged vertical and radial solid volume fraction profiles are presented. Different fluidization regimes in spouted beds were characterized by analyzing pressure fluctuation signals and ECVT images. ECVT was shown to be robust and powerful for the three-dimensional visualization and quantitative measurement of a gas–solid fluidization system.

Comparison of hollow fiber and flat mixed-matrix membranes: Theory and simulation

We extend effective medium theory (EMT) to composite hollow fiber mixed matrix membranes, considering the asymmetric filler volume fraction profile arising from finite system size. This volume fraction profile leads to strong variation of the driving force (i.e. pseudo-bulk concentration gradient) in the regions adjacent to the composite ends, and to sensitivity of the effective permeability of the composite to the geometrical configuration. The new theory is validated against rigorous simulations of the transport in mixed-matrix membranes (MMMs) using both concentration-independent and concentration-dependent diffusivities in the MMM constituent phases. Both theory and simulations show that flat mixed-matrix membranes (F-MMMs) have higher effective overall permeability than hollow fiber mixed-matrix membranes (HF-MMMs), upon comparison of systems having identical operating conditions, filler phase loading and particle size. Furthermore, we show here that the sensitivity to the geometry vanishes with increase of inner radius of the hollow fiber membrane at fixed thickness, and the effective permeability of a HF-MMM is found to asymptotically approach that of a F-MMM.

Chemical Reaction Effects on Nano Carreau Liquid Flow Past a Cone and a Wedge with Cattaneo-Christov Heat Flux Model

AbstractChemical reaction aspect is utilized for heat mass transfer analysis of nano non-Newtonian liquid flow past a cone and a wedge. Flow is steady, laminar and two dimensional created due to a cone and a wedge. The Carreau liquid and Cattaneo-Christov heat flux models are utilized. The magneto-nano Carreau liquid material occupies the porous space. The relevant PDEs are rendered into coupled non-linear ODEs via appropriate transformations before treated them numerically through Runge-Kutta and Newton’s method. The computed results are plotted for employing the various values of physical constraints on the profiles of velocity, temperature and nanoparticle volume fraction. Moreover, vitiation of the friction factor, Nusselt number and Sherwood number against physical parameters are presented numerically. It is figured out that convective heating and Brownian motion effects are constructive for thermal boundary layer growth. Aspect of chemical reaction is significant to control the solute layer growth and mass transfer rate.

Conformation of Single and Interacting Lipopolysaccharide Surfaces Bearing O-Side Chains

The outer surfaces of Gram-negative bacteria are composed of lipopolysaccharide (LPS) molecules exposing oligo- and polysaccharides to the aqueous environment. This unique, structurally complex biological interface is of great scientific interest as it mediates the interaction of bacteria with antimicrobial agents as well as with neighboring bacteria in colonies and biofilms. Structural studies on LPS surfaces, however, have so far dealt almost exclusively with rough mutant LPS of reduced molecular complexity and limited biological relevance. Here, by using neutron reflectometry, we structurally characterize planar monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific O-side chains in the presence and absence of divalent cations and under controlled interaction conditions. The model used for the reflectivity analysis is self-consistent and based on the volume fraction profiles of all chemical components. The saccharide profiles are found to be bimodal, with dense inner oligosaccharides and more dilute, extended O-side chains. For interacting LPS monolayers, we establish the pressure-distance curve and determine the distance-dependent saccharide conformation.

Two-dimensional temperature and carbon dioxide concentration profiles in atmospheric laminar diffusion flames measured by mid-infrared direct absorption spectroscopy at 4.2 μm

We present a multi-line flame thermometry technique based on mid-infrared direct absorption spectroscopy of carbon dioxide at its $$v_3$$ fundamental around 4.2 μm that is particularly suitable for sooting flames. Temperature and concentration profiles of gas phase molecules in a flame are important characteristics to understand its flame structure and combustion chemistry. One of the standard laboratory flames to analyze polycyclic aromatic hydrocarbons (PAH) and soot formation is laminar non-premixed co-flow flame, but PAH and soot introduce artifact to most non-contact optical measurements. Here we report an accurate diagnostic method of the temperature and concentration profiles of CO2 in ethylene diffusion flames by measuring its $$v_3$$ vibrational fundamental. An interband cascade laser was used to probe the R-branch bandhead at 4.2 μm, which is highly sensitive to temperature change, free from soot interference and ambient background. Calibration measurement was carried out both in a low-pressure Herriott cell and an atmospheric pressure tube furnace up to 1550 K to obtain spectroscopic parameters for high-temperature spectra. In our co-flow flame measurement, two-dimensional line-of-sight optical depth of an ethylene/N2 laminar sooting flame was recorded by dual-beam absorption scheme. The axially symmetrical attenuation coefficient profile of CO2 in the co-flow flame was reconstructed from the optical depth by Abel inversion. Spatially resolved flame temperature and in situ CO2 volume fraction profiles were derived from the calibrated CO2 spectroscopic parameters and compared with temperature profiles measured by two-line atomic fluorescence.

Hydrodynamics of Bubble Columns: Turbulence and Population Balance Model

This paper presents an in-depth numerical analysis on the hydrodynamics of a bubble column. As in previous works on the subject, the focus here is on three important parameters characterizing the flow: interfacial forces, turbulence and inlet superficial Gas Velocity (UG). The bubble size distribution is taken into account by the use of the Quadrature Method of Moments (QMOM) model in a two-phase Euler-Euler approach using the open-source Computational Fluid Dynamics (CFD) code OpenFOAM (Open Field Operation and Manipulation). The interfacial forces accounted for in all the simulations presented here are drag, lift and virtual mass. For the turbulence analysis in the water phase, three versions of the Reynolds Averaged Navier-Stokes (RANS) k-ε turbulence model are examined: namely, the standard, modified and mixture variants. The lift force proves to be of major importance for a trustworthy prediction of the gas volume fraction profiles for all the (superficial) gas velocities tested. Concerning the turbulence, the mixture k-ε model is seen to provide higher values of the turbulent kinetic energy dissipation rate in comparison to the other models, and this clearly affects the prediction of the gas volume fraction in the bulk region, and the bubble-size distribution. In general, the modified k-ε model proves to be a good compromise between modeling simplicity and accuracy in the study of bubble columns of the kind undertaken here.

Thermal-mechanical-electrical buckling behavior of functionally graded micro-beams based on modified couple stress theory

This paper presents thermal-mechanical-electrical buckling analysis of micro-beams that are made of functionally graded materials(FGMs) with temperature-dependent thermo-elastic properties. The material properties of the micro-beam are assumed to be graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. The governing equations are based on the principle of the minimum total potential energy, von Kármán geometric nonlinearity theory, and modified couple stress theory is employed to consider the size effect. A differential quadrature (DQ) method combined with an iteration process is used to predict the critical buckling external axial force and buckling temperature increment. The effects of the volume fraction profile parameter, dimensionless length scale parameter, initial gap ratio, ground electrode shape parameter, the applied voltage, slenderness ratio and the axial residual stress on the thermal-mechanical-electrical buckling behavior are evaluated in detail through parametric studies.

Shock waves induced by a planar obstacle in a vibrated granular gas

The low value of the speed of sound in dilute granular media permits the study of the properties of supersonic flows for a wide range of Mach numbers. In this paper, we report the experimental observation of a subsonic–supersonic transition in a vibrated granular gas. The shock fronts studied are obtained by simply pushing a rectangular obstacle into the granular gas for different obstacle velocities. The supersonic regime is characterized by the formation of normal shock waves whose width increases when the Mach number decreases to values close to 1. The bimodal model proposed by Mott-Smith in the 1950s provides a good description for the velocity distributions as well as the macroscopic quantities for shock waves in molecular gases but remains inadequate for dissipative media like granular gases and plasmas. Here by examining the shock front structure for a wide range of Mach numbers, we adapt the Mott-Smith bimodal description to a dissipative medium. By using balance equations from granular kinetic theory and taking into account different dissipation sources, the proposed model allows us to understand how this dissipation modifies temperature, mean velocity and volume fraction profiles through the shock front.

CFD modeling of dilute phase pneumatic conveying in a horizontal pipe using Euler–Euler approach

ABSTRACTComputational fluid dynamics simulations were performed to investigate the behavior of dilute phase pneumatic conveying of plastic pellets in a horizontal circular pipe. The pellets are 200 µm in diameter and 1000 kg/m3 in density. A parametric study was performed to investigate the effects of turbulence model and model collision parameters on pressure drop, solid’s volume fraction and velocity profiles. Among model collision parameters, specularity coefficient has considerable effect on the pressure drop. Moreover, the results from simulations carried out for different solid loadings and velocities were compared with experimental data found in the literature. The air velocities range from 6 to 15 m/s and solids to air mass flow ratios range from 1 to 3. At higher air velocities, the pressure drops predicted by the standard k-omega turbulence model are higher than the pressure drops predicted by the standard k-epsilon model. In contrast, at lower gas velocities, the standard k-epsilon model predicts higher pressure drops compared to the standard k-omega turbulence model. However, no significant difference in solids and air velocity profiles is observed for the two different turbulence models.