Flow Regime Transition Research Articles

Study of wellbore heat transfer considering fluid rheological effects in deep well acidizing

Real-time understanding of the wellbore temperature is a prerequisite for the acidizing treatment design. Fluid flow regime variation caused by fluid rheological property has a great influence on wellbore temperature distribution. According to the physical situation in the acidizing process, the wellbore and formation heat transfer model considering fluid rheological effects is established, the validity of the model is verified by comparing with monitoring data and previous research results. The results indicate that fluid rheology will cause the transition of flow regime, which leads to the variation of heat transfer efficiency. Considering wellbore original water is benefit for accurately predicting bottom-hole temperature. Injection rate has a great influence on fluid flow regime. A critical injection rate with the highest cooling efficiency exists in the low range of pumping rate condition. The findings above can provide theoretical insights for the deep well acidizing, and the model built in this paper is applied in field wellbore temperature simulation.

Improvement of voidage prediction in liquid-solid fluidized beds by inclusion of the Froude number in effective drag relations

A novel effective drag relation for liquid-solid fluidisation is proposed, suitable for application in full-scale installations. This is achieved by presenting new insights related to the influence of the temporal-spatial heterogeneity on the effective hydrodynamic drag for large fluidised systems. While heterogeneous flow behaviour can be predicted increasingly accurately in CFD simulations that explicitly model the heterogeneous solids distribution, for the operation of many large-scale applications it is infeasible to perform such computationally intensive simulations. Therefore, there is a clear need for full-scale drag relations that effectively take into account the heterogeneous behaviour and irregular spatial particle distributions. Our new drag relation is based on a large set of experiments, which shows that the degree of overall expansion is not only dependent on the ratio of laminar-turbulent flow, but also on the amount of homogenous versus heterogeneous flow, which is not included in current full-scale drag relations. To include the effect of heterogeneity, the standard drag relation, based on the Reynolds number, is extended with a specific type of Froude number. Because fully turbulent flow regimes are rare in applications of liquid-solid fluidisation, our focus is not on the turbulent flow regime but instead on laminar and transitional flow regimes. In these regimes, three types of models are investigated. The first type is based on a theoretical similarity with terminal settling, the second is based on the semi-empirical Carman-Kozeny model, and the third is based on empirical equations using symbolic regression techniques. For all three types of models, coefficients are calibrated on experimental data with monodisperse and almost spherical glass beads. The models are validated with a series of calcium carbonate grains applied in drinking water treatment processes as well as data obtained from the literature. Using these models, we show that the voidage prediction average relative error decreases from approximately 5% (according to the best literature equations which use Reynolds number only) to 1–2% (using both Reynolds and Froude number). This implies that our new models are more suitable for operational control in full-scale fluidised bed applications, such as pellet softening in drinking water treatment processes.

Quantifying interfacial parameters of upward and downward annular flow condensation from high-speed visualization

Condensation of R-134a in upward and downward annular flow was experimentally studied in a 5-mm ID transparent test section using a LED-based image processing technique. In addition to the flow orientation, the frequency of the film structures was quantified in terms of the following parameters: total mass flow rate, condensation heat transfer rates (outlet vapor quality) and test section inclination with respect to the horizontal. The image processing technique combined Fourier analysis and the Kernel density estimation technique to calculate the two-dimensional (amplitude–frequency) probability density function of the film flow structure as a function of the independent parameters. Although the tube inclination and the condensation rate showed some influence on the frequency of the interfacial waves, the most significant effect was due to the flow orientation, which resulted in abrupt frequency changes apparently linked to flow regime transitions dictated by shear and gravity forces.

Exploring Two-Phase Flow Regime Transition Mechanisms Using High-Resolution Virtual Experiments

Recent advancements in computing power allow utilization of state-of-the-art direct numerical simulations (DNSs), coupled with interface tracking techniques, to perform fully resolved simulations of complex two-phase flows, such as flow regime transitions. Studying the highly resolved temporal and spatial information produced from these virtual experiments can advance our understanding of the phenomenon and inform coarser models. With these improved models, better predictions of flow regime behavior and location in boiling water reactors can be made. The presented research uses the PHASTA code, which employs the level set method for interface tracking, to examine the mechanisms of flow regime transition, specifically the slug-to-bubbly and slug-to–churn-turbulent regime transitions. The DNS was validated using theoretical and experimental work found in open literature. Different geometries, including pipes and minichannels, were explored in order to improve the fundamental understanding of the complex flow phenomenon. Using advanced analysis techniques, the transient flow properties were analyzed at resolutions not available to other methods. The numerical data analysis allows for calculation of both time and spatially averaged properties as well as local instantaneous properties. Possible mechanisms for the transition are discussed. Examples include liquid kinetic energy/surface tension energy balance and interfacial shear forces in the liquid film. It is also noted that the transition out of slug flow can take at least two pathways: interfacial wave-induced instability development in the Taylor bubble, leading to its disintegration, or strong bubble shearing at the tail of the bubble.

Numerical investigation of Non-Darcy flow regime transitions in shale gas production

Shale gas reservoirs are organic rich formations with often ultra-low permeability. Gas is stored in free and adsorbed form. Conventional Darcy flow cannot fully describe the gas transport in such porous media. It is thus crucial to study the shale gas production considering different flow regimes and time dependent permeability, which can improve well-induced fracture design and ultimate gas recovery. In particular, this paper will focus on the transition in non-Darcy flow regimes near fracture-matrix interfaces using mathematical modelling. Especially, we investigate conditions at which these effects vanish, and Darcy flow assumptions become reasonable.The model describes a representative well-induced high permeability fracture surrounded by shale matrix. Investigated Non-Darcy mechanisms include apparent permeability, Knudsen diffusion, gas desorption and Forchheimer flow. Pressure depletion is the main driving force for single phase gas flow from the matrix to the fracture and from the fracture to the well. Pressure dependent gas desorption is defined by Langmuir isotherm and is a key production mechanism. This model is implemented in Matlab using Marcellus shale data.Scaling the model shows that recovery of gas depends on two dimensionless number that incorporates geometry relations, time scales of flow, intrinsic parameters of the porous media, non-Darcy constants, adsorption and boundary conditions. The dimensionless numbers define respectively if 1) the fracture or matrix limit the gas production rate 2) if non-Darcy flow is significant in the fracture or matrix. When one of the media limit production, the non-Darcy flow in the other medium has reduced importance and can be excluded from the model. Non-Darcy flow is important if it limits flow in the medium limiting the production. By checking the magnitude of the selected dimensionless numbers, the modelling approach can be determined in advance and significant computational time can be saved.The proposed model provides a tool for interpretation of complex shale gas production systems. It can be used for screening of flow regimes at different operational configurations and hence appropriate modelling approaches. The model can be used to optimise fracture network design and potentially in identifying stimulation operations that may significantly improve production rates and ultimate recovery from unconventional gas reservoirs.

Integrated hydrodynamic model for simulation of river-lake-sluice interactions

In this paper, an integrated model for simulating the hydrodynamic process of river-lake-sluice (RLS) systems is presented. It includes a novel one-dimensional (1D) and two-dimensional (2D) coupling method called the coupling-zone iteration-correction (CZIC) method, and an improved numerical algorithm for the sluice problem. The 1D river-network model and the 2D lake model are coupled by establishing a coupling region, and iterative correction is carried out to ensure the accurate transfer of hydraulic parameters. The convergence conditions of the CZIC method are discussed theoretically, and the proper spatial step of the coupling zone is adopted according to different inflow conditions to ensure stable computation. In order to deal with the transition of flow regimes during the gate operation, a method for calculating the discharge capacity is presented. In addition, a general difference coefficient of the river reach is deduced for hydrodynamic calculation with sluices included. Simulations on open channels demonstrate that (1) simulated values of the CZIC method are consistent with the results of the full 2D model; (2) the sluice solving algorithm can stably handle the flow transition between the orifice flow and weir flow. Furthermore, the developed integrated model is applied to the middle and lower reaches of the Huaihe River, including the Hongze Lake and fifteen sluices. Numerical simulation results reproduced the hydrodynamic process during the flood season of 2007 accurately and efficiently. The errors of the present model are also compared with that of the MIKE model, and the results show that the proposed methods perform better than MIKE, especially in rising and flood periods. Therefore, it seems likely that the developed integrated model will work well in hydrodynamic modelling of large-scale complex RLS systems.

Flow Resistance Modeling for Coolant Distribution within Canned Motor Cooling Loops

Taylor–Couette–Poiseuille (TCP) flow dominates the inner water-cooling circulation of canned motor reactor coolant pumps. Current research on TCP flow focuses on torque behaviors and flow regime transitions through experiments and simulations. However, research on axial flow resistance in a large Reynolds number turbulent state is not sufficient, especially for the various flow patterns. This study is devoted to investigating the influence of annular flow on the axial flow resistance of liquid in the coaxial cylinders of the stator and rotor in canned motor reactor coolant pumps, and predicting the coolant flow distribution between the upper coil cooling loop and lower bearing lubricating loop for safe operation. The axial flow resistance, coupled with the annular rotation, is experimentally investigated at a flow rate with an axial Reynolds number, Rea, from 2.6 × 103 to 6.0 × 103 and rotational Reynolds number, Ret, from 1.6 × 104 to 4.0 × 104. It is revealed that the axial flow frictional coefficient varies against the axial flow rate in linear relation sets with logarithmic coordinates, which shift up when the flow has a higher Ret. Further examination of the axial flow resistance, with the Rea extending to 3.5 × 105 and Ret up to 1.6 × 105, by simulation shows gentle variation rates in the axial flow frictional coefficients against the Rea. The relation curves with different Ret values converge when the Rea exceeds 3.5 × 105. A prediction model for TCP flow consisting of a polygonal approximation with logarithmic coordinates is developed to estimate the axial flow resistance against different axial and rotational Reynolds numbers for the evaluation of heat and mass transfer during transition states and the engineering design of the canned motor chamber structure.

Comparison of methods for measuring infiltration rate of pervious concrete

The infiltration rate of both a pervious concrete material itself and a pervious pavement system are important values for understanding how a pervious concrete pavement will drain water. While there is currently a standard test method to measure the infiltration rate of a pervious concrete pavement system (ASTM C1701), there is no standard test procedure for measuring the infiltration rate of only the pervious concrete material. The goal of this research was to compare various methods for measuring the infiltration rate of pervious concrete in isolation. The constant head, falling head, and a modified ASTM C1701 test were compared along with a horizontal version of the constant head test for multiple pervious concrete cylinders from the same mix. It was found that the sample-to-sample variation in infiltration rate is typically much larger than the experimental uncertainty present in a given test method. The infiltration rate depended non-linearly on head level, demonstrating that flows were in the transition flow regime between laminar and turbulent.Therefore, test results are presented in terms of both infiltration rate and modified Darcy’s law parameters.

Effect of Surface Tension on Hydrodynamics and Mass Transfer Coefficient in Airlift Reactors

AbstractThe hydrodynamics and volumetric mass transfer coefficient play important roles in the design and scale‐up of airlift reactors. The effect of surface tension on hydrodynamics and volumetric mass transfer coefficient in internal loop airlift reactors was investigated. With reduction of the surface tension of the fluid, the hydrodynamic parameters raised, namely, gas phase holdup, flow regime transition point, and interfacial area, whereas the bubble diameter as well as the liquid velocity decreased and the volumetric mass transfer coefficient increased. Empirical correlations are proposed for gas‐phase holdup and volumetric mass transfer coefficient in terms of dimensionless numbers and can be applied in the design of airlift reactors.

Validation of numerically simulated ventricular flow patterns during left ventricular assist device support.

Intraventricular flow patterns during left ventricular assist device support have been investigated via computational fluid dynamics by several groups. Based on such simulations, specific parameters for thrombus formation risk analysis have been developed. However, computational fluid dynamic simulations of complex flow configurations require proper validation by experiments. To meet this need, a ventricular model with a well-defined inflow section was analyzed by particle image velocimetry and replicated by transient computational fluid dynamic simulations. To cover the laminar, transitional, and turbulent flow regime, four numerical methods including the laminar, standard k-omega, shear-stress transport, and renormalized group k-epsilon were applied and compared to the particle image velocimetry results in 46 different planes in the whole left ventricle. The simulated flow patterns for all methods, except renormalized group k-epsilon, were comparable to the flow patterns measured using particle image velocimetry (absolute error over whole left ventricle: laminar: 10.5, standard k-omega: 11.3, shear–stress transport: 11.3, and renormalized group k-epsilon: 17.8 mm/s). Intraventricular flow fields were simulated using four numerical methods and validated with experimental particle image velocimetry results. In the given setting and for the chosen boundary conditions, the laminar, standard K-omega, and shear–stress transport methods showed acceptable similarity to experimental particle image velocimetry data, with the laminar model showing the best transient behavior.

Preswirl and Mixed-Flow (Mainly Liquid) Effects on Rotordynamic Performance of a Long (L/D = 0.75) Smooth Seal

Abstract Measured results are presented for rotordynamic coefficients and mass leakage rates of a long smooth annular seal (length-to-diameter ratio L/D = 0.75, diameter D = 114.686 mm, and radial clearance Cr = 0.200 mm) tested with a mixture of silicone oil (PSF-5cSt) and air. The test seal is centered, the seal exit pressure is maintained at 6.9 bars-g while the fluid inlet temperature is controlled within 37.8–40.6 °C. It is tested with three inlet-preswirl inserts, namely, zero, medium, and high (the preswirl ratios (PSRs), i.e., the ratio between the fluid's circumferential velocity and the shaft surface's velocity, are in ranges of 0.10–0.18, 0.30–0.65, and 0.65–1.40 for zero, medium, and high preswirls, respectively), six inlet gas-volume fractions GVFi (0%, 2%, 4%, 6%, 8%, and 10%), four pressure drops PDs (20.7, 27.6, 34.5, and 41.4 bars), and three speeds ω (3, 4, and 5 krpm). The targeted test matrix could not be achieved for the medium- and high-preswirl inserts at PD ≥ 27.6 bars due to the test-rig stator's dynamic instability issues. Spargers were used to inject air into the oil, and GVFi values higher than 0.10 could not be consistently achieved because of unsteady surging flow downstream from the sparger mixing section. Leakage mass flow rate m˙ and rotordynamic coefficients are measured, and the effect of changing inlet preswirl and GVFi is studied. The test results are then compared with predictions from a two-phase, homogeneous-mixture, bulk-flow model developed in 2011. Generally, both measurements and predictions show little change in m˙ as inlet preswirl changes. Measured m˙ remains unchanged or slightly increases with increasing GVFi, but predicted m˙ decreases. Measured m˙ is comparable to predicted values but consistently lower. Dynamic-stiffness coefficients are measured using an ensemble of excitation frequencies and curve-fitted well by frequency-independent stiffness Kij, damping Cij, and virtual mass Mij coefficients. Planned tests with the medium- and high-preswirl inserts could not be accomplished at PD = 34.5 and 41.4 bars because the seal stator became unstable with any finite injection of air. The test results show that the instability arose because the seal's direct stiffness K became negative and increased in magnitude with increasing GVFi. The model predicts a drop in K as GVFi increases, but the test results dropped substantially more rapidly than predicted. Also, the model does not predict the observed strong tendency for K to drop with an increase in preswirl in moving from the zero-to-medium and medium-to-high preswirl inserts. The authors believe that the observed drop in K due to increasing GVFi is not explained by either (a) a reverse Lomakin effect from operating in the transition flow regime or (b) the predicted drop in K at higher GVFi values from the model. A separate and as yet unidentified two-phase flow phenomenon probably causes the observed results. The negative K results due to increasing GVFi and moving from the zero to medium, and medium to high preswirl observed here could explain the instability issue (sudden subsynchronous vibration) on a high-differential-pressure helico-axial multiphase pump (MPP), reported in 2013. Effective damping Ceff combines the stabilizing effect of direct damping C, the destabilizing effect of cross-coupled stiffness k, and the influence of cross-coupled mass mq. As predicted and measured, increasing inlet preswirl significantly increases k and decreases Ceff, which decreases the seal's stabilizing properties. Ceff increases with increasing GVFi—becomes more stable.

Hydrodynamic review on liquid–solid magnetized fluidized bed

Abstract The magnetic field has been successfully used to intensify the liquid–solid contact performance in the fluidized bed, creating the magnetized fluidized bed (MFB). The MFBs with purely magnetizable particles and with the binary admixture of magnetizable and nonmagnetizable particles could be simply termed the pure MFB and admixture MFB, respectively. Their potential application in the chemical and biochemical industries has been thoroughly explored in the literature. However, a fundamental investigation on the hydrodynamics therein is far from sufficient, severely hindering the commercial application. For this reason, this review summarized the relevant findings, including (1) flow regime transition, (2) boundaries between two adjacent flow regimes, (3) unique features of the magnetically stabilized bed, (4) hysteresis phenomenon and bed voidage, (5) minimum fluidization velocity and terminal velocity, (6) numerical simulation and segregation of the admixture MFB, and (7) some explored applications. More importantly, the existing controversies and unsolved issues in this area were identified. Among others, the flow regime transition and unique hydrodynamic characteristics of each flow regime should be first clarified, only after which could the terminology describing all the flow regimes be unified and the results from different scholars be compared.

Void fractions in a rod bundle geometry at high pressure–Part Ⅱ: Drift-flux model assessment and development

The present paper is Part Ⅱ of an investigation into void fraction measurement and modeling in a triangular-array rod bundle geometry under high-pressure conditions (P = 5–9 MPa and G = 100–350 kg m−2 s−1). In Part I of the study, the experimental facility and measurement techniques were described, and the measured local, chordal, and area-averaged void fractions were presented and analyzed. In this paper, six existing drift-flux models for rod bundles are assessed using the present data. Results show that most of the models are inconsistent with the measured void fraction under low void fraction conditions. This is because the effects of pressure and mass flux on drift-flux parameters are not considered. Hence, a new drift-flux model is developed with flow parameters and flow regime transition being considered. Additionally, the general applicability of this new model is verified by both the present data and existing data in the literatures. Results show that the new drift-flux model agrees well with the experimental data, with the relative deviations of void fraction less than 20%, indicating that the new model is applicable under a wide range of mass-flux conditions.

Preparation of microdispersed droplets by phase inversion in gas/liquid/liquid microdispersion system

A simple and effective method was conducted for preparing microdispersed droplets with decreased size and increased uniformity, in which phase inversion in high-phase-ratio liquid/liquid system was combined with addition of microbubbles. Five typical flow patterns were observed, including W/O, O/W/O, transition regime, O/W and G/W/O regimes. Flow regime transition was observed, according to which physical properties of systems and operating conditions for preparing single-oil-droplet-in-water emulsion were determined in different liquid/liquid and gas/liquid/liquid modes. Effects of liquid viscosities as well as interfacial tension on droplet size (DO) and its standard deviation (SD) were studied systematically and the most effective gas/liquid/liquid mode was recommended. Mathematical models were established for correlating DO in five different modes, which show relatively good agreement with experimental results. DO decreases by 42% via introducing phase inversion and further reduces to 25% by addition of microbubbles, both under the same operating conditions.

The flow regime and hydrodynamic performance for a pitching hydrofoil

The objective of this paper is to study the flow regime and hydrodynamic performance for a pitching Clark-Y hydrofoil. The aims are to (1) improve the understanding of the interplay between the hydrodynamic performance, unsteady flow structures and dynamic motion of the hydrofoil, (2) study the influence of the pitching rate on the transition of different flow regimes. The experimental investigations were conducted in the looped cavitation tunnel, and the dynamic moment measurement system was applied to obtain the hydrodynamic forces. The pitching hydrofoil is controlled to rotate from α+ = 10° to α+ = 15° firstly, then goes from α+ = 15° to α− = 5°, finally goes back to α+ = 10° from α− = 5°. The pitching rate is varying with the Reynolds number Re = 4.4 × 105. The numerical investigations were performed by solving the incompressible URANS equations using the coupled k-ω SST turbulence model and γ-Reθ transition model. The numerical results agree well with the experimental measurements. The pitching motion affects the turbulence kinetic energy distribution around the hydrofoil, leading to the delay or acceleration of the transition between different flow patterns. During the pitching process, higher level of turbulence kinetic energy distribution causes earlier transition from laminar to turbulence. Moreover, hysteresis effect of the hydrodynamic force is observed. For the upstroke stage, the higher pitching rate promotes the laminar separation slightly and intensifies the delay of turbulence separation. For the downstroke stage, the higher pitching rate promotes the turbulence separation extensively. The first leading edge vortex (LEV) and anticlockwise trailing edge vortex (TEV) are delayed with the increase of pitching rate, which is responsible to the delay of dynamic stall. The lower pitching rate shrinks the hysteresis loops and intensifies the fluctuation of the dynamic force.

Hydrodynamics of narrow-tube fixed bed reactors filled with Raschig rings

The local flow structure and pressure drop in random packings of Raschig rings are analyzed using sequential Rigid Body Dynamics (RBD) method and Computational Fluid Dynamics (CFD) simulation. Tube-to-pellet diameter ratios, N, between 3 and 6 are investigated for laminar, transitional and turbulent flow regimes (5 ≤ Rep ≤ 3,000). The computed pressure drops are in good agreement with the empirical correlation of Nemec and Levec (2005), while the Ergun equation exhibited high deviations of more than 60%, even when it is modified to explicitly account for non-sphericity of pellets. This deviation is ascribed to additional sources for eddy formation offered by Rashig rings, compared to spheres and cylinders, which cannot be counterbalanced by the usage of a higher specific surface area. The 3D results of flow structure demonstrate a large influence of packing topology on the velocity distribution: rings oriented parallel to the flow accelerate the local velocity through their axial holes, while rings oriented perpendicular to the flow provide additional space for vortex formation. The flow fields are substantially different from that found in packings of spheres and cylinders, both in terms of volume of backflow regions and velocity hotspots. This implies a higher order of local flow inhomogeneity in azimuthal and axial directions compared to spherical and cylindrical packings. Furthermore, it is found that azimuthal averaging of the 3D velocity field over the bed volume, which has been used to improve classical plug-flow pseudo-homogenous models to account for the role of tortuous velocity fields, cannot reflect the appearance of vortex regions and thereby leads to underestimation of the local axial velocity values by over 500% of the inlet velocity.

Expanding the concept of end effect in vortex chambers

Background. Mechanisms of vortex structures generation in geometric inhomogeneities of streamlined surfaces as the basis for the development of low-cost control methods for hydromechanical and thermophysical characteristics of technical devices, aircraft and water vehicles.Objective. Studying the features of the formation and interaction of vortex formations inside and near a cross streamlined semi-cylindrical groove on a flat surface, as well as the structure, kinematic and dynamic characteristics of the boundary layer using statistical analysis.Methods. Experimental investiganions in a wind tunnel of the fields of averaged and fluctuation velocity components in semi-cylindrical groove and its near wake on a hydraulically smooth flat surface by hot-wire measurements in the range of Reynolds numbers calculated along the plate length to the front edge of the recess Rex=U0x/n from 3,8×104 up to 6,9×105.Results. According to measurements in the mid-section of the groove of the fields of actual velocities, the probability density functions of the longitudinal velocity fluctuations, their coefficients of asymmetry and excess are obtained. The profiles of the averaged longitudinal velocity and the isotaches of the averaged and fluctuation velocity components inside the groove are constructed. The regions of formation of the reverse flow in the groove, the velocity field in the shear layer, and the features of its interaction with the stern wall of the groove were found. In the transition from the laminar flow regime of a groove with a low vortex formation intensity to a turbulent one, at first an intense flow is formed in the near-bottom region of the cavity, directed towards the direction of the main flow. That is the reason of generation of the large-scale vortex structures that are periodically ejected out from the groove. As a result of the transition, the intensities of the vortex motion and the circulation flow in the groove volume are significantly increased, and in the near wake there is a separation region with intense velocity fluctuations. It has been established that in the boundary layer above the groove, the law of the probability density distribution of the longitudinal velocity fluctuations is close to normal one, but in the recess to the Maxwellian one. With the deep of semi-cylindrical groove the asymmetry and excess coefficients increase.Conclusions. A complex vortex flow that forms inside a half-cylindrical cavity leads to a change in the structure of the boundary layer above the plate. In the transition and turbulent flow regimes in the boundary layer on the plate, the features and regularities of both the generation of large-scale vortex structures inside the groove and their ejection out from the groove into the near wake are revealed. Thus, the conditions are established under which the transversely streamlined semi-cylindrical groove can serve as an effective generator of vortices, which can be used to control hydroaerodynamic drag and heat and mass transfer processes in structural elements of vehicles, power machines and apparatuses.

Numerical Modelling of Microchannel Gas Flows in the Transition Flow Regime Using the Cascaded Lattice Boltzmann Method.

In this article, a lattice Boltzmann (LB) method for studying microchannel gas flows is developed in the framework of the cascaded collision operator. In the cascaded lattice Boltzmann (CLB) method, the Bosanquet-type effective viscosity is employed to capture the rarefaction effects, and the combined bounce-back/specular-reflection scheme together with the modified second-order slip boundary condition is adopted so as to match the Bosanquet-type effective viscosity. Numerical simulations of microchannel gas flow with periodic and pressure boundary conditions in the transition flow regime are carried out to validate the CLB method. The predicted results agree well with the analytical, numerical, and experimental data reported in the literature.

Lattice Boltzmann simulation of diluted gas flow inside irregular shape microchannel by two relaxation times on the basis of wall function approach

The rarefied gas flow through a long microchannel with multi throats is studied in both slip and transition flow regimes by the lattice Boltzmann method with Two Relaxation Times (TRT). The power-law analytical function is applied to improve the slippage velocity on the microchannel surfaces. The slip velocity correction at the corner point of microchannel is used. Therefore, a slip coefficient basis on the kinetic theory is considered. The Zou-He and Bounce back-Specular Reflection boundary conditions are used at the inlet-outlet of the geometry and on the microchannel wall surfaces, respectively. The present model results coincide with the Direct Simulation Monte Carlo and the Information Preservation values. The present model predicts the slippage velocity with higher accuracy as compared with the single relaxation time one. Moreover, this model has low cost and running time in comparison with the mention models.

Mechanism of loop seal clearance occurring at U-shaped pipe with air-water two-phase flow

Main purposes of the LOCAL (LOop seal Clearance separate effect test of ATLAS) facility are to investigate the effect of cross-over leg (COL) diameter at the loop seal clearance (LSC), to evaluate the two-phase flow regime through high speed visualization, and to suggest the mechanistic models for the LSC prediction at Small Break-Loss of Coolant Accident (SB-LOCA) in Light Water Reactors (LWRs). In this paper, we introduce a key mechanism of the onset of the LSC occurring at U-shaped COL with air-water two-phase flow condition. The onset of LSC is coupled with the flow regime transition at vertical upward and horizontal section of COL. Slug flow with single Taylor bubble results in decrease of pressure difference at vertical upward section and causes rapid flow regime transition at horizontal section from the stratified to the slug. The critical velocity of gas leading to flow regime transition at horizontal section can explain the onset of LSC and high speed visualization can provide the clear understanding of LSC phenomena at the local point of view.