Mixture Velocity Research Articles

Two-phase co-current flow simulations using periodic boundary conditions in horizontal, 4, 10 and 90° inclined eccentric annulus, flow prediction using a modified interFoam solver and comparison with experimental results

Two-phase oil and gas flow were simulated in an entirely eccentric annulus and compared with experimental data at horizontal, 4, 10, and 90° inclination. The gas-phase was sulphur hexafluoride and the liquid phase a mixture of Exxsol D60 and Marcol 82 for the inclined cases (5–16), and pure Exxsol D60 for the horizontal cases (1–4). The diameter of the outer and inner cylinders was 0.1 and 0.04 m, respectively, for the inclined domains and 0.1 and 0.05 m for the horizontal domain. The cases studied consist of liquid phase fractions between 0.3 and 0.65 and mixture velocities from 1.2 to 4.25 m/s. The mean pressure gradient is within 33% of the expected experimental behavior for all inclined cases. In contrast, the low-velocity horizontal domains exhibit significant deviation, with a drastic over-prediction of the mean pressure gradient by as much as 200–335% for cases 1 and 2. The two remaining horizontal cases (3 and 4) are within 22% of the expected mean pressure gradient. Cases 13–16 are a replication of cases 5–8 at an increased inclination; the mean pressure gradient is within 6.5% of the expected increase due to the increase in hydrostatic pressure. By comparing cases 1–4 to previous published simulations at a lower eccentricity, we found a decrease of the mean pressure gradient by 30–40%, which is in line with existing literature, although for single-phase flows. The simulated and experimental liquid holdup profiles are in good agreement when comparing the fractional data; wave and slug frequencies match to within 0.5 Hz; however, at closer inspection, it is apparent that there is a decrease in the amount of phase-mixing of the simulations compared to the experiments. When increasing the mesh density from 115 k cells/m to 2 million cells/m, the simulations exhibit significantly more phase mixing, but are still unable to produce conventional slugs. In a simplified case, conventional slugs are observed at grid sizing of 1 × 1 × 1 mm, whereas the cells of the 2 million cells/m mesh are roughly 1.5 × 1.5 × 1.5 mm.

Combustion-induced turbulent flow fields in premixed flames

Combustion-induced turbulence is studied in explosions of methane/air in a steel sphere with optical access. Rms turbulent velocities, u', created by four peripheral variable speed fans, were measured by Particle Image Velocimetry, PIV, which indicated near-uniform, isotropic turbulence, with small mean velocities. The gaseous expansion of a near-spherical propagating flame generated a radially outwards velocity of unburned mixture, the intensity of which depended upon the burning velocity and volume increase due to combustion. The study is to measure any extra turbulence created by this outwards velocity pulse. Only wave lengths less than the flame circumference can wrinkle the flame and increase the burning velocity, ut. Consequently the effective rms velocity at the flame front affecting its propagation, uk', is less than u'. Analysis shows, uk', to be a function of flame radius. The velocity pulse generated additional turbulence, enhancing uk'. This enhanced ut, giving further positive feedback. This higher rms value is designated as a spatial rms turbulent velocity, us'. PIV-measured gas velocities are resolved into a mean radial velocity, U-r and, us', with their radial profiles. These are derived from explosions at different equivalence ratios, pressures, and temperatures. At the radii at which ut are measured, us'/uk' is enhanced by the high rates of burning and volumetric expansion. This ratio attained a high value of 2.5, with stoichiometric CH4/air. Values of ut measured in explosions are higher than those measured in steady-state burners, attributable to the induced us', enhancing ut, in explosions, with no such effect in burners.

Effect of mixture velocity for given equivalence ratio on flame development in Swiss roll combustor

Small-scale power generation using heat energy from hydrocarbon fuels is a proven technology. In this study, we analyzed 2-D flame development in meso-scale Swiss roll combustor. A mixture of 60% butane and 40% propane was used (0.25?0.55 L per minute). During all the analyses, equivalence ratio (1.1) was kept constant by adjusting air quantity against fuel quantity. The effect of increase in the mixture velocity on the development of flame shapes/patterns was monitored. We found different patterns of flame, e. g., planar, concave, and conical, with the increase in mixture velocity. Increase in combustion chamber temperature was also noted. No flashback was observed and blowout was observed with very high mixture velocity. Combustion chamber temperatures were found to be increasing with the increase in mixture velocity at the same equivalence ratio. Elongation of the flame was observed because of the increased flow velocity. Heat re-circulation to the reactants enhances flame characteristics.

ВЛИЯНИЕ РАЗМЕРНОСТИ МАТЕМАТИЧЕСКОЙ МОДЕЛИ ВНУТРЕННЕЙ БАЛЛИСТИКИ НА РАСЧЕТНЫЕ ПАРАМЕТРЫ ВЫСТРЕЛА ДЛЯ ЗАРЯДОВ ИЗ ЗЕРНЕНОГО ПОРОХА

The problem of the impact of the mathematical model dimension on the calculated intraballistic characteristics of a shot for the charges made of granulated powder is considered. Mathematical models of the shot are studied using the spatial (axisymmetric), one-dimensional, and zero-dimensional (thermodynamic) formulations. The thermodynamic model takes into account the distribution of the pressure and velocity of a gas-powder mixture behind the shot for a channel of variable cross-section. Comparison of simulation results is carried out in a wide range of loading parameters. It is shown that there is a range of the loading parameters for a thermodynamic approach to give satisfactory approximation to the parameters obtained using the gas-dynamic approach, which describes the flow of a heterogeneous reacting mixture with a separate consideration of phases and intergranular interactions between them. Notably that in the entire range of the charging parameters studied in this work, the one-dimensional and twodimensional gas-dynamic models only slightly differ from each other. Therefore, in the main pyrodynamic period, the actuation of the charge, made of granulated powder, can be simulated using a one-dimensional gas-dynamic model or a zero-dimensional thermodynamic model with allowance for spatial distribution of the pressure and velocity of the gas-powder mixture.

Study and Redesign of Aircooler System in a 16 MW Steam Turbine Surface Condenser at Neka Power Plant

According to the study of basic Rankin thermal cycle, the steam exhaust pressure of a typical steam turbine toward the condenser, plays a great role in the efficiency and the net output power of the steam turbine, so most surface condensers that are working in thermal power plants are kept at vacuum condition so that the maximum power of thermal cycle can be achieved. The vacuum pressure at condenser leads to the entering of air and Non-condensable gases from turbine gland seals to condenser so that the special air ejection equipment is being used to take apart air from steam and vent it to out of condenser. In this study, a special steam and air separator mechanism in an evacuating system called “Aircooler” at a 16 MW steam turbine condenser is being studied and the Fluent CFD software is utilized to analyze the behavior of steam plus air in a typical aircooler system of 16 MW steam turbine condenser of Neka power plant to find a way to reduce the risk of cooling tube rupture in aircooler ducts. The critical condition which tube rupture happens is determined and it is demonstrated that in hot seasons of year, by increasing the seawater cooling temperature and increasing in turbine steam exhaust pressure and temperature, the risk of tube rupture due to more mixture velocity at the first row of aircooler cooling tubes increases and also the effect of tube plugged condition on the performance of aircooler shows that the risk of other tubes rupture will increase and thus the efficiency of aircooler decreases due to more aircooler exhaust temperature. Finally, two modified plans at aircooler system design will be studied and simulated via Fluent CFD software which leads to reduce the risk of tube rupture. The results show that by modification of aircooler ducts and holes, the mixture air and steam flow velocity to first aircooler cooling tube row decreases significantly and causes the risk of tube rupture to decrease remarkably and also the exhaust temperature of aircooler decreases and causes the higher ejector performance.

Flow pattern and pressure drop for oil\u2013water flows in and around 180\xb0 bends

Pressure drop and flow pattern of oil–water flows were investigated in a 19-mm ID clear polyvinyl chloride pipe consisting of U-bend with radius of curvature of 100 mm. The range for oil and water superficial velocities tested was 0.04 le U_{{{text{so}}}} le 0.950 ;{text{m/s}} and 0.13 le U_{{{text{sw}}}} le 1.10 ;{text{m/s}}, respectively. Measurements were carried out under different flow conditions in a test section that consisted of four different parts: upstream of the bend, at the bend and at two redeveloping flow locations after the bend. The result indicated that the bend had limited influence on downstream flow patterns. However, the shear forces imposed by the bend caused some shift flow pattern transition and bubble characteristics in the redeveloping flow section after the bend relative to develop flow before the bend. Generally, pressure gradient at all the test sections increased with both oil fraction and water superficial velocity and there was a sharp change of pressure gradient profile during phase inversion. The transition point where phase inversion occurred was always within the range of 0.4 le U_{{{text{sw}}}} le 0.54 ;{text{m/s}}. Pressure losses differed at the various test sections, and the difference was strongly linked to the superficial velocity of the phases and the flow pattern. At high mixture velocity, pressure losses at the redeveloping section after the bend were higher than that at the bend and that for fully developed flows. At low mixture velocity, pressure losses at the bend are higher than in the straight sections. Pressure drop generally decreased with level of flow development downstream of the bend.

ОСОБЕННОСТИ РАСПРОСТРАНЕНИЯ ПЛАМЕНИ В УГЛЕ-ПРОПАНО-ВОЗДУШНОЙ ГАЗОВЗВЕСИ

The mathematical model of combustion for a reactive gas suspension of coal dust in a propane-air mixture is developed. The parametric study of the problem is carried out. The observed burning velocity of the propane-air mixture with an admixture of coal particles is determined. Dependences of the observed burning velocity of the propane-air gas suspension on the equivalence ratio and on the radius of the particles are obtained. It is shown that, the observed burning velocity decreases with an increase in the radius of the particles. On the contrary, with an increase in the radius of the particle, the observed burning velocity increases for high-propane mixtures. Moreover, in the case of high-propane mixtures, the observed burning velocity of the gas suspension can be increased by reducing the mass of the particles. The observed burning velocity for a propane-air mixture with particles is significantly less than that for a mixture without particles.

LASER-BASED DIAGNOSTICS OF SLUG FLOW BOILING IN A HORIZONTAL PIPE

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field nonintrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m2 and mass fluxes from 300 to 460 kg/m2 s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapor bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapor bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. This research demonstrates the applicability of laser-based diagnostic techniques to boiling slug flows, to which such experimental methods have not been widely applied in macroscale geometries. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

Study of molecular interaction in a ternary liquid mixture of n-hexane, ethanol and benzene

An extensive theoretical study on the molecular structure of benzene, ethanol, and n-hexane monomer; their dimers and complexes were undertaken using Density Functional Theory (DFT) method. The possibility of the formation of molecular interaction was identified from structural parameter analysis and confirmed with interaction energy calculation. The information about molecular interaction was also assessed by analyzing the excess acoustic parameters. The excess thermo-acoustic parameters have been estimated from the experimentally measured ultrasonic velocity (u) and density (ρ) of the ternary mixture of n-hexane + ethanol + benzene as well as of pure components at (303, 308, 313 & 318) K using standard relation.

RESEARCH INTO KINETIC REGULARITIES OF THE REACTION OF OXIDATIVE DEHYDROGENATION OF METHYLCYCLOHEXANE OVER MODIFIED ZEOLITES

Kinetic regularities of the oxidative dehydrogenation reaction of methylcyclohexane on the -clinoptilolite catalyst were investigated. Absence of internal and external diffusion inhibition was established, the reaction proceeds in the kinetic area, in which all diffusion stages proceed much faster than all chemical stages that make up the mechanism of this reaction. The influence of partial pressures of reagents, the reaction temperature and the space velocity of the reaction mixture on the course of the reaction was studied and optimal conditions for obtaining the intentional reaction product determined.

Thermoacoustic Properties of Binary Liquid Mixtures of Sulfolane with Aniline, N,N-Dimethylaniline and N,N-Diethylaniline at Different Temperatures and Atmospheric Pressure

Densities and sound velocities of the binary liquid mixtures of sulfolane + aniline, sulfolane + N,N-dimethylaniline, sulfolane + N,N-diethylaniline over the whole range of composition and their pure component were measured at temperatures (T = 303.15, 308.15, 313.15 K) and atmospheric pressure. A high precision vibrating-tube densitometer was used for the measurements. From the measured values, excess adiabatic compressibility (βs E), excess sound velocity (uE), excess internal pressure (Pi E) and deviation of surface tension (Δγ) were calculated for each of the systems. The excess properties and surface tension deviation were fitted to the Redlich-Kister equation. All these properties have been discussed in terms of molecular interactions.

Oily Water Treatment by Ceramic Membrane: Modeling and Simulation

The separation process of oily water using membranes has attracted the attention of researchers and engineers. The greater problem in the use of membrane separation process is the reduction in permeate flux due to clogged pores by oil deposition inside the membrane or by the effect of the concentration polarization. For this purpose, a theoretical study of a water/oil separation module was performed. This device consists of a tubular ceramic membrane provided with a rectangular inlet section. Numerical simulations were performed using Ansys CFX software to solve the mass and momentum conservation equations in the fluid and porous domains. Here was adopted the RNG k-ε turbulence model. The effect of the membrane porosity and the inlet velocity of the fluid mixture on the two-phase flow behavior inside the separation module were evaluated. Results of the volumetric fraction, velocity and pressure fields of the oil and water phases are presented and analyzed. The results indicate a higher oil concentration within the membrane for the cases of higher porosity, and that the inlet fluid mixture velocity does not substantially affect the velocity profile within the separation module. It is found that the maximum separation efficiency of the module was obtained with feed velocity of 40 m/s and membrane porosity of 0.44.

Numerical Prediction of Two-Phase Flow in Tube Bundles with a CFD Porous Media Approach Based on Mixture Model and a Void Fraction Correlation

Being able to predict the void fraction is essential for a numerical prediction of the thermohydraulic behaviour in steam generators. Indeed, it determines two-phase mixture density and affects two-phase mixture velocity which enable to evaluate the pressure drop of heat exchanger, the mass transfer and heat transfer coefficients. In this study, the flow is modelled by coupling Ansys Fluent with an in-house code library where a CFD porous media approach is implemented. In this code, the two-phase flow has been modelled so far using the Eulerian model. However, this two-phase model requires interaction laws between phases which are not known and/or reliable for a flow within a tube bundle. The aim of this paper is to use the mixture model, for which it is easier to implement suitable correlations for tube bundles. By expressing the relative velocity, as a function of slip, the void fraction model of Feenstra et al. developed for upward cross-flow through horizontal tube bundles is introduced. With this method, physical phenomena that occur in tube bundles are taken into consideration in the mixture model. The developed approach is validated based on the experimental results obtained by Dowlati et al.

A comprehensive study on laminar burning velocity and flame stability of oxy-producer gas mixtures. Part-2: Laminar burning velocity and Markstein length analysis

Part-1 reported the extent of variation in each species in producer gas (PG) mixture(s) after a thorough literature study. Forty-five PG variants were identified to analyse the effect of each species on the combustion characteristics of PG-O2 mixtures. Spherical flame method was used for measuring laminar burning velocities and burned gas Markstein lengths of PG at 1 bar, 300 K and ϕ = 0.8 in a 4.5 L spherical chamber. Extrapolation (linear and non-linear) and non-extrapolation based schemes were used to estimate unstretched flame speed. LBV extracted from all three schemes were in good agreement within ±4.92%. Only H2/CO-rich-PG-O2 mixtures having mole fraction of combustible components (CC) less than 20% displayed negative Lb values and their effective Lewis numbers were less than the respective critical values and were corroborated with experimental results. CH4-rich-PG-O2 mixtures were unconditionally stable. Their LBV was the highest at most conditions due to high flame temperatures, with exception to LBV of H2-N2-rich-PG-O2 mixtures at 45%CC owing to large thermal diffusivities. LBV varied non-linearly as CO2 in non-combustible components (NCC) increased beyond 50% and its suppressing effect was the highest in CH4-rich-PG. Simulations were performed using 1D expanding spherical flame model in COSILAB with GRI-Mech3.0. Simulated LBV showed a deviation of ±13.2% with experimental values at 45%CC. Unstretched LBV obtained from non-extrapolation-mapping method which involved simulation results showed good agreement with experiments. Combustion of PG with higher H2, CO and N2 content and relatively lesser amounts of CH4 and CO2 were stable to preferential diffusion effects and predictable to fluctuations in composition.

Downward Annular Flow of Air–Oil–Water Mixture in a Vertical Pipe

The paper presents the results of a study concerned with the hydrodynamics of an annular downward multiphase flow of gas and two mutually non-mixing liquids through a vertical pipe with a diameter of 12.5 mm. The air, oil and water were used as working media in this study with changes in superficial velocities in the ranges of jg = 0.34–52.5 m/s for air, jo = 0.000165–0.75 m/s for oil, and jw = 0.02–2.5 m/s for water, respectively. The oil density and viscosity were varied within the ranges of ρo = 859–881 kg/m3 and ηo = 29–2190 mPas, respectively. The research involved the identification of multiphase flow patterns and determination of the void fraction of the individual phases. New flow patterns have been identified and described for the gravitational flow conditions of a two-phase water–oil liquid and a three-phase air–water–oil flow. New flow regime maps and equations for the calculation of air, oil and water void fractions have been developed. A good conformity between the calculated and measured values of void fraction were obtained. The map for the oil–water–air three-phase flow is valid for the following conditions: j3P = 0.35–53.4 m/s (velocity of three-phase mixture) and oil in liquid concentration βo* = 0.48–94% (oil in liquid concentration). In the case of a downward annular oil–water two-phase flow, this map is valid for liquid mixture velocity jl = 0.052–2.14 m/s and βo* = 0.48–94%.

Effect analysis on energy conversion enhancement and NOx emission reduction of ammonia/hydrogen fuelled wavy micro-combustor for micro-thermophotovoltaic application

Ammonia (NH3) is regarded as an alternative fuel not only as a carbon-free fuel but also as a renewable hydrogen-carrier. It is possible that the safety in micro-combustor can be improved through partial NH3 substitution for hydrogen. However, knowledge of the thermal performance and nitrogen oxides (NOx) emission of ammonia/hydrogen combustion, especially in the micro-combustor, has been insufficient. In order to enhance thermal performance, reduce NOx emission and improve flame stabilization of ammonia/hydrogen fuelled micro-combustors for thermophotovoltaic (TPV) application, three types of micro-combustors with a wavy profile are designed and evaluated. For this, a three-dimensional (3D) numerical model with a detailed chemical reaction mechanism has been verified and applied to assess the thermal performance of the modified micro combustors in terms of the outer wall temperature distributions. The average temperature of these wavy combustors is found to be much higher than that of the conventional smooth combustor, regardless of the hydrogen/ammonia mixture flow velocity. Moreover, the wavy is a more effective measure to improve temperature uniformity when the mixture velocity is greater than 12 m/s. Comparing the flame stability behaviours of hydrogen/ammonia/air blended combustion in both the conventional and the proposed wavy combustors reveals that the blowout limit is effectively broadened. Finally, the effects of 1) hydrogen/ammonia blended ratio and 2) fuel-air equivalence ratio on NOxemissions are examined in detail. It is found that approximately 21.2% of NOx emission reduction could be achieved in the ARC wavy micro-combustor. NOx emission reduction can be gradually improved, as the nitrogen fuel mass ratio is increased. This present research sheds lights on an effective design of a micro-combustor with enhanced thermal performance and reduced NOx emission.

Numerical and Experimental Investigations on Combustion Characteristics of Premixed Lean Methane–Air in a Staggered Arrangement Burner with Discrete Cylinders

Premixed combustion of lean methane–air in an artificial porous media burner with staggered alumina cylinders was experimentally and numerically performed. Numerical simulations were conducted at gas mixture velocities of 0.43–0.86 m/s and equivalence ratios of 0.162 and 0.243, respectively. Through comparison with experimental results, temperature distribution, peak temperature and flame propagation velocity are analyzed and discussed in detail. The numerical calculated temperature profile over the axis of the combustor coincided well with test data in the post-flame zone, however a certain deviation was found in the preheated zone. A two-dimensional flame shape was observed and the flame thickness was the size of cylinder diameter. The peak temperature increased with the gas mixture inlet velocity at the certain equivalence ratio, and its peak value was about 1.8–2.16 times higher than the adiabatic combustion temperature under the desired equivalence ratio, which indicates that super-adiabatic combustion was the case for all the numerical simulations. The flame propagating velocity had a positive correlation with the gas mixture inlet velocity.

Models with phase slip in the hydrodynamics of granular media

Two-phase flows in granular media are widely used in the chemical and energy industries as they provide a high intensity of transfer processes. One of the key characteristics of such flows is the relationship between the pressure drop cross the granular layer and the mass velocity of the two-phase mixture. To date, no single approach has been developed to calculate the characteristics of such flows. In this paper, we compare three models constructed given the difference in the velocities of the liquid and vapor phases. All models are used to solve both the direct problem of calculating the pressure drop for a given mass velocity and the inverse problem of calculating the mass velocity for a known pressure drop. The accuracy of the models was compared using two sets of experimental data with a combined pressure variation range of 0.6–15 MPa, a flow quality of 0.002–0.3, a mass velocity of 120 – 1100 kg/m2s, ball particle diameters of 2–4 mm, and a packed bed height of 50-355 mm. The calculation with the new gas-dynamic model demonstrated the best result in the entire range of experimental data.

Mathematical modeling of CO2 separation using different diameter hollow fiber membranes

This research presents a 2D mass-transfer simulation model using computational fluid dynamics (CFD) for separation of CO2 from a binary gas mixture of CO2/CH4 by means of polytetrafluoroethylene (PTFE) hollow fiber membrane contactor (HFMC). Governing equations with their corresponding boundary conditions are solved using COMSOL Multiphysics and the results are validated against reported experimental data. Convection and diffusion flux vectors and concentration gradient of CO2 species in the radial and axial directions of the HFMC are investigated. This study provides an opportunity to investigate the effects of gas and liquid cross flow velocities on the overall performance of membrane contacting system. The results demonstrate that increasing the liquid phase velocity improves the CO2 absorption performance of the membrane system, while increasing the gas mixture velocity deteriorates the CO2 separation of the system. The impact of hollow fiber geometry on the removal of CO2 is investigated and the results indicate that hollow fibers with smaller inner diameter provides higher effective mass-transfer area and therefore superior CO2 removal performance. Furthermore, the modeling predictions for the CO2 removal are in good agreement with the experimental data under various ratios of gas to liquid velocity. The PTFE hollow fiber membrane contacting system showed a great potential for separation of CO2 from gas mixtures.

A Divergence‐free Mixture Model for Multiphase Fluids

AbstractWe present a novel divergence free mixture model for multiphase flows and the related fluid‐solid coupling. The new mixture model is built upon a volume‐weighted mixture velocity so that the divergence free condition is satisfied for miscible and immiscible multiphase fluids. The proposed mixture velocity can be solved efficiently by adapted single phase incompressible solvers, allowing for larger time steps and smaller volume deviations. Besides, the drift velocity formulation is corrected to ensure mass conservation during the simulation. The new approach increases the accuracy of multiphase fluid simulation by several orders. The capability of the new divergence‐free mixture model is demonstrated by simulating different multiphase flow phenomena including mixing and unmixing of multiple fluids, fluid‐solid coupling involving deformable solids and granular materials.