Two-phase Flow Analysis Research Articles

Numerical studies on slope stability in torrential rainfall by using two-phase flow analysis

Numerical studies on slope stability in torrential rainfall by using two-phase flow analysis

Biophysical characterization of organelle-based RNA/protein liquid phases using microfluidics.

Living cells contain numerous membrane-less RNA/protein (RNP) bodies that assemble by intracellular liquid-liquid phase separation. The properties of these condensed phase droplets are increasingly recognized as important in their physiological function within living cells, and also through the link to protein aggregation pathologies. However, techniques such as droplet coalescence analysis or standard microrheology do not always enable robust property measurements of model RNA/protein droplets in vitro. Here, we introduce a microfluidic platform that drives protein droplets into a single large phase, which facilitates viscosity measurements using passive microrheology and/or active two-phase flow analysis. We use this technique to study various phase separating proteins from structures including P granules, nucleoli, and Whi3 droplets. In each case, droplets exhibit simple liquid behavior, with shear rate-independent viscosities, over observed timescales. Interestingly, we find that a reported order of magnitude difference between the timescale of Whi3 and LAF-1 droplet coalescence is driven by large differences in surface tension rather than viscosity, with implications for droplet assembly and function. The ability to simultaneously perform active and passive microrheological measurements enables studying the impact of ATP-dependent biological activity on RNP droplets, which is a key area for future research.

2상 흐름모델에 의한 강우침투를 고려한 불포화 토사 사면의 안정해석

Most previous studies on the rainfall infiltration and its influence on slope stability were based on the single-phase water flow model. The purpose of this study is to study the effect of interaction of air and water flow on the slope stability. In this study, two-phase flow analyses were conducted for weathered residual soil slopes with thin soil overlying impermeable bedrock. Results of the flow analysis were used as input for stability analysis by strength reduction method. Strength reduction method considers mechanical equilibrium to evaluate the effect of rainfall infiltration on the slope stability. The results show that remarkable delaying effects by air flow on the propagation of the wetting front and the deterioration of slope when surface ponding occurs during infiltration. On the other hand, the influence of the pore air flow on the slope stability can be neglected in the non-ponding situation with small rainfall intensity.

CUPID 코드의 유체 물성치 변화를 고려한 자연대류 해석

Without electirc power to cool down the hot reactor core, passive systems utilizing natural circulation are becoming a big specialty of recent neculear systems after the severe accident in Fukusima. When we consider the natural circulation in a pool, thermal mixing phenomena may start from single phase circulation and can continue to two phase condition. Since the CUPID code, which has been developed for two-phase flow analysis, can deal with the phase transition phenomena, the CUPID would be pertinent to natural convection problems in single- and two-phase conditions. Thus, the CUPID should be validated against single- and two-phase natural circulation phenomena. For the first step of the validation process, this study is focused on the validation of single-phase natural circulation. Moreover, the CUPID code solves the fluid properties by the relationship to pressure and temperature from the steam table considering non-condensable gas effects, so that the effects from variable properties are included. Simple square thermal cavity problems are tested for laminar and turbulent conditions against numerical and experimental data. Throughout the investigation, it is found that the variable properties can affect the flow field in laminar condition, but the effect becomes weak in turbulence condition, and the CUPID code implementing steam table is capable of analyzing single phase natural circualtion phenomena.

A study on two phase flows of linear compressors for the prediction of refrigerant leakage

Usage of linear compressors is on the rise due to their high efficiency. In this paper, leakage of a linear compressor has been studied through numerical analysis and experiments. First, nitrogen leakage for a stagnant piston with fixed cylinder pressure as well as for a moving piston with fixed cylinder pressure was analyzed to verify the validity of the two-phase flow analysis model. Next, refrigerant leakage of a linear compressor in operation was finally predicted through 3-dimensional unsteady, two phase flow CFD (Computational fluid dynamics). According to the research results, the numerical analyses for the fixed cylinder pressure models were in good agreement with the experimental results. The refrigerant leakage of the linear compressor in operation mainly occurred through the oil exit and the leakage became negligible after about 0.4s following operation where the leakage became lower than 2.0x10-4 kg/s.

Mathematical Analysis of Single and Two-Phase Flow of Blood in Narrow Arteries with Multiple Contrictions

The pulsatile flow of blood in narrow arteries with multiple-stenoses under body acceleration is analyzed mathematically, treating blood as (i) single-phase Herschel-Bulkley fluid model and (ii) two-phase Herschel-Bulkley fluid model. The expressions for various flow quantities obtained by Sankar and Ismail (2010) for single-phase Herschel-Bulkley fluid model and Sankar (2010c) for two-phase Herschel-Bulkley fluid model are used to compute the data for comparing these fluid models in a new flow geometry. It is noted that the plug core radius, wall shear stress and longitudinal impedance to flow are marginally lower for two-phase H-B fluid model than those of the single-phase H-B fluid model. It is found that the velocity decreases significantly with the increase yield stress of the fluid and the reverse behavior is noticed for longitudinal impedance to flow. It is also noticed that the velocity distribution and flow rate are higher for two-phase Herschel-Bulkley fluid model than those of the single-phase Herschel-Bulkley fluid model. It is also recorded that the estimates of the mean velocity increase with the increase of the body acceleration and this behavior is reversed when the stenosis depth increases.

Particle velocimetry analysis of immiscible two-phase flow in micromodels

We perform micro-PIV measurements in micromodels using very simple optical equipment combined with efficient image acquisition and processing. The pore-scale velocity distributions are obtained for single-phase flow in porous media with a typical pore size of 5–40 µm at a resolution of 1.8μm×1.8μm vector grid. Because the application of micro-PIV in micromodels is not standard, extensive effort is invested into validation of the experimental technique. The micro-PIV measurements are in very good agreement with numerical simulations of single-phase flows, for which the modeling is well established once the detailed pore geometry is specified and therefore serves as a reference. The experimental setup is then used with confidence to investigate the dynamics of immiscible two-phase flow in micromodels that represent natural complex porous media (e.g., sandstone). For unstable immiscible two-phase flow experiments, micro-PIV measurements indicate that the flow is highly oscillatory long before the arrival of the invading interface. The dynamics are accompanied with abrupt changes of velocity magnitude and flow direction, and interfacial jumps. Following the passage of the front, dissipative events, such as eddies within the aqueous phase, are observed in the micro-PIV results. These observations of complex interface dynamics at the pore scale motivate further measurement of multiphase fluid movement at the sub-pore scale and requisite modeling.

Experimental and numerical analysis of compressible two-phase flows in a shock tube

The comparative study on seven equation models with two different six equations model for compressible two-phase flow analysis is proposed. The seven equations model is derived for compressible two-phase flow that is in the nonconservation form. In the present work, two different six equations model are derived for two pressures, two velocities and single temperature with the derivation of the equation of state. The closing equation for one of the six equations model is energy conservation equation while another one is closed by entropy balance equation. The partial differential form of governing equations is hyperbolic and written in the conservative form. At this point, the set of governing equations are derived based on the principle of extended thermodynamics. The method of solving single temperature from both six equation models are simple and direct solution can be obtained. Numerical simulation has been tried using one of the six equation models for air–water shock tube problems. Explicit fourth order Runge–Kutta scheme is used with Finite Volume Shock Capturing method for solving the governing equations numerically. The pressure, velocity and volume fraction variations are captured along the shock tube length through flow solver. Experimental work is carried out to magnify the initial stage of liquid injection into a gas. The outcome of six equations model for compressible two-phase flow has revealed the multi-phase flow characteristics that are similar to the actual conditions.

Assessment of Cavitation Models for Flows in Diesel Injectors With Single- and Two-Fluid Approaches

Several recent cavitation models for the analysis of two-phase flows in diesel injectors with single- and two-fluid modeling approaches have been evaluated, including the Saha–Abu-Ramandan–Li (SAL), Schnerr–Sauer (SS), and Zwart–Gerber–Belamri (ZGB) models. The SAL model is a single-fluid model, while the other two models have been implemented with both single- and two-fluid approaches. Numerical predictions are compared with experimental results available in literature, qualitatively with experimental images of two-phase flow in an optically accessible nozzle, and quantitatively with measured mass flow rates and velocity profiles. It is found that at low injection pressure differentials there can be considerable discrepancy in the predictions of the vapor distribution from the three models considered. This discrepancy is reduced as the injection pressure differential is increased. Implementation of the SS and ZGB models with single- and two- fluid approaches yields noticeable differences in the results because of the relative velocity between the two phases, with two-fluid approach providing better agreement with experimental results. The performance of the SS and ZGB models implemented with the two-fluid approach is comparable with the SAL single-fluid model, but with significantly more computational time. Overall, the SAL single-fluid model performs comparatively better with respect to the other two models.

A global sensitivity analysis of two-phase flow between fractured crystalline rock and bentonite with application to spent nuclear fuel disposal

Geological disposal of spent nuclear fuel in deep crystalline rock is investigated as a possible long term solution in Sweden and Finland. The fuel rods would be cased in copper canisters and deposited in vertical holes in the floor of deep underground tunnels, embedded within an engineered bentonite buffer. Recent experiments at the Äspö Hard Rock Laboratory (Sweden) showed that the high suction of unsaturated bentonite causes a de-saturation of the adjacent rock at the time of installation, which was also independently predicted in model experiments. Remaining air can affect the flow patterns and alter bio-geochemical conditions, influencing for instance the transport of radionuclides in the case of canister failure. However, thus far, observations and model realizations are limited in number and do not capture the conceivable range and combination of parameter values and boundary conditions that are relevant for the thousands of deposition holes envisioned in an operational final repository.In order to decrease this knowledge gap, we introduce here a formalized, systematic and fully integrated approach to study the combined impact of multiple factors on air saturation and dissolution predictions, investigating the impact of variability in parameter values, geometry and boundary conditions on bentonite buffer saturation times and on occurrences of rock de-saturation. Results showed that four parameters consistently appear in the top six influential factors for all considered output (target) variables: the position of the fracture intersecting the deposition hole, the background rock permeability, the suction representing the relative humidity in the open tunnel and the far field pressure value. The combined influence of these compared to the other parameters increases as one targets a larger fraction of the buffer reaching near-saturation. Strong interaction effects were found, which means that some parameter combinations yielded results (e.g., time to saturation) far outside the range of results obtained by the rest of the scenarios. This study also addresses potential air trapping by dissolution of part of the initial air content of the bentonite, showing that neglecting gas flow effects and trapping could lead to significant underestimation of the remaining air content and the duration of the initial aerobic phase of the repository.

Numerical simulation and analysis of solid-liquid two-phase three-dimensional unsteady flow in centrifugal slurry pump

Based on RNG k-e turbulence model and sliding grid technique, solid-liquid two-phase three-dimensional (3-D) unsteady turbulence of full passage in slurry pump was simulated by means of Fluent software. The effects of unsteady flow characteristics on solid-liquid two-phase flow and pump performance were researched under design condition. The results show that clocking effect has a significant influence on the flow in pump, and the fluctuation of flow velocity and pressure is obvious, particularly near the volute tongue, at the position of small sections of volute and within diffuser. Clocking effect has a more influence on liquid-phase than on solid-phase, and the wake-jet structure of relative velocity of solid-phase is less obvious than liquid-phase near the volute tongue and the impeller passage outlet. The fluctuation of relative velocity of solid-phase flow is 7.6% smaller than liquid-phase flow at the impeller outlet on circular path. Head and radial forces of the impeller are 8.1% and 85.7% of fluctuation, respectively. The results provide a theoretical basis for further research for turbulence, improving efficient, reducing the hydraulic losses and wear. Finally, field tests were carried out to verify the operation and wear of slurry pump.

Nonorthogonality Correction Method for a Two-Phase Semiimplicit Algorithm on a Polyhedral Unstructured Mesh

Nonorthogonality is often encountered in structured or unstructured mesh generation for computational fluid dynamics (CFD). A flow solution from CFD can degenerate because of mesh nonorthogonality. Sometimes, it is difficult to obtain a converged solution because of a numerical instability originating from a nonorthogonal mesh, especially in the case of a two-phase flow analysis. In this study, a method for nonorthogonality correction of a pressure-correction equation is proposed for application to a two-phase implicit continuous Eulerian (ICE) algorithm. In the case of a two-phase flow with buoyant force, a pressure gradient must be carefully reconstructed. A density-weighted interpolation of the pressure can be used for the pressure gradient, but it may generate an erroneous solution. Effort was devoted to improve the method and implement it for a general polyhedral mesh. The methods are implemented in the CUPID code and tested by solving conceptual two-phase flow problems.

Improvement of CUPID code for simulating filmwise steam condensation in the presence of noncondensable gases

Abstract In a nuclear reactor containment, wall condensation forms with noncondensable gases and their accumulation near the condensate film leads to a significant reduction in heat transfer. In the framework of nuclear reactor safety, the film condensation in the presence of noncondensable gases is of high relevance with regards to safety concerns as it is closely associated with peak pressure predictions for containment integrity and the performance of components installed for containment cooling in accident conditions. In the present study, CUPID code, which has been developed by KAERI for the analysis of transient two-phase flows in nuclear reactor components, is improved for simulating film condensation in the presence of noncondensable gases. In order to evaluate the condensate heat transfer accurately in a large system using the two-fluid model, a mass diffusion model, a liquid film model, and a wall film condensation model were implemented into CUPID. For the condensation simulation, a wall function approach with a heat/mass transfer analogy was applied in order to save computational time without considerable refinement for the boundary layer. This paper presents the implemented wall film condensation model, and then introduces the simulation result using the improved CUPID for a conceptual condensation problem in a large system.

Analysis of Two-Phase Cavitating Flow with Two-Fluid Model Using Integrated Boltzmann Equations

In the present work, both computational and experimental methods are employed to study the two-phase flow occurring in a model pump sump. The two-fluid model of the two-phase flow has been applied to the simulation of the three-dimensional cavitating flow. The governing equations of the two-phase cavitating flow are derived from the kinetic theory based on the Boltzmann equation. The isotropic RNG k — e — kca turbulence model of two-phase flows in the form of cavity number instead of the form of cavity phase volume fraction is developed. The RNG k—e—kca turbulence model, that is the RNG k — e turbulence model for the liquid phase combined with the kca model for the cavity phase, is employed to close the governing turbulent equations of the two-phase flow. The computation of the cavitating flow through a model pump sump has been carried out with this model in three-dimensional spaces. The calculated results have been compared with the data of the PIV experiment. Good qualitative agreement has been achieved which exhibits the reliability of the numerical simulation model.

Slag Prediction in Submerged Rocket Nozzle Through Two-Phase CFD Simulations

A computational procedure has been established to predict the slag in a practical solid rocket motor with submerged nozzle. Both single-phase and two-phase flow analyses have been performed in the rocket motor port. Three-dimensional Navier-Stokes equations along with SST turbulence model have been solved for gas-phase calculations. The effect of ejected alumina particles from the propellant geometry on the flow field has been simulated through Lagrangian tracking method. The computational methodology is firstly validated by comparing against other numerical results of rocket motors available in the literature before applying the same to predict the slag accumulation of a submerged rocket motor for strategic applications. Burn-back geometries at different instants have been simulated and parametric studies were performed to find out the effect of Al 2 O 3 particle size. It was observed that the slag capture rate increases uniformly with A1 2 O 3 particle size. The predicted slag accumulation data match closely with the ground test data for the range of conditions simulated in the present work. Defence Science Journal, Vol. 65, No. 2, March 2015, pp.99-106, DOI:http://dx.doi.org/10.14429/dsj.65.7147 Normal 0 false false false EN-US X-NONE X-NONE

Моделювання процесу кристалізації води в ультразвуковому полі

The article is headlined to prospects of low-temperature water treatment technologies. It describes the available scientific and technical contradictions and the mathematical formulation of the problem. The principle of the block freezing eliminates cold losses that are characteristic of traditional cryoconcentration systems. Further studies are aimed at intensification of mass transfer processes during the formation of ice. For two-phase systems ice - possibility of a general mathematical description of crystallization from solution are currently questionable. Therefore, in modeling of this problem it is advisable to use the maximum approaches which are known in the analysis of two-phase flow in the absence of external influences, but also the research on the heat transfer intensification by means of various fields. It is shown that the time taken to process water treatment, decreases when exposed to water-crystal system of acoustic fields and discussed mathematical models of such influence. The problem was analyzed by using the theory of generalized variables.

Three-dimensional magnetohydrodynamic two-phase flow and heat transfer analysis in electroslag remelting process

A transient three-dimensional (3D) coupled mathematical model is developed in this paper in order to explore the electromagnetic, flow, and temperature fields, as well as the solidification in the electroslag remelting (ESR) process. Maxwell's equations are solved by the Finite Volume Method. The Joule heating and electromagnetic force (EMF), the source terms in the energy and momentum equations, are recalculated during each iteration as function of the phase distribution. The movement of the metal droplets is described by the Volume of Fluid (VOF) approach. Additionally, the solidification of the metal is modeled by an enthalpy-based technique, where the mushy zone is treated as a porous medium with a porosity equal to the liquid fraction. The results show that the electric current tends to go through the metal droplet first. The EMF varies with the falling metal droplet, always blocking the motion of the metal droplet. The zone with the highest temperature appears under the outer radius of the inlet. A larger Joule heating density, as well as a higher average temperature of the molten slag, is obtained when the current increases. Bigger metal droplets form with more heat and momentum, resulting in a deeper liquid metal pool.

Ligament and Droplet Generation by Oil Film on a Rotating Disk

The lubrication and heat transfer designs of bearing chamber depend on an understanding of oil/air two-phase flow. As initial and boundary conditions, the characteristics of ligament and droplet generation by oil film on rotating parts have significant influence on the feasibility of oil/air two-phase flow analysis. An integrated model to predict the oil film flow, ligament number, and droplet Sauter mean diameter (SMD) of a rotating disk, which is an abstraction of the droplet generation sources in a bearing chamber, is developed based on the oil film force balance analysis and wave theory. The oil film thickness and velocity, ligaments number, and droplet SMD are calculated as functions of the rotating disk radius, rotational speed and oil volume flow rate and oil properties. The theoretical results show that the oil film thickness and SMD are decreased with an increasing rotational speed, while the radial, transverse velocities, and ligament number are increased. The oil film thickness, radial velocity, and SMD are increased with an increasing oil flow rate, but the transverse velocity and ligament number are decreased. A test facility is built for the investigation into the ligament number of a rotating disk, and the measurement of ligament number is carried out by means of a high speed photography.

F232 二相流解析コードTPFITの垂直矩形管における空気と水二相流に対する検証(OS9 熱・流動(6))

In Japan Atomic Energy Agency, the detailed two-phase flow analysis code TPFIT has been developed to simulate and evaluate two-phase flow characteristics in nuclear systems. In this study, a numerical simulation of bubbly flow in a vertical square channel was performed to validate the applicability of TPFIT code on bubbly flow simulation. By checking bubble distribution development in the flow direction, the calculation of the forces acting on bubbles was validated through comparing simulation results and experimental results from Matos et al. (2004). Comparisons between the experimental and numerical data revealed, in general, good agreement except serious bubble coalescence appeared in numerical simulation.

CUPID 코드를 이용한 CANDU 원자로 칼란드리아 탱크 내부유동 열수력 예비 해석

The CUPID code has been developed for a transient, three-dimensional, two-phase flow analysis at a component scale. It has been validated against a wide range of two-phase flow experiments. Especially, to assess its applicability to single- and two-phase flow analyses in the Calandria vessel of a CANDU nuclear reactor, it was validated using the experimental data of the 1/4-scaled facility of a Calandria vessel at the STERN laboratory. In this study, a preliminary thermal-hydraulic analysis of the CANDU reactor moderator tank using the CUPID code is carried out, which is based on the results of the previous studies. The complicated internal structure of the Calandria vessel and the inlet nozzle was modeled in a simplified manner by using a porous media approach. One of the most important factors in the analysis was found to be the modeling of the tank inlet nozzle. A calculation with a simple inlet nozzle modeling resulted in thermal stratification by buoyance, leading to a boiling from the top of the Calandria tank. This is not realistic at all and may occur due to the lack of inlet flow momentum. To improve this, a new nozzle modeling was used, which can preserve both mass flow and momentum flow at the inlet nozzle. This resulted in a realistic temperature distribution in the tank. In conclusion, it was shown that the CUPID code is applicable to thermal-hydraulic analysis of the CANDU reactor moderator tank using the cost-effective porous media approach and that the inlet nozzle modeling is very important for the flow analysis in the tank.