Mixed Flow Pattern Research Articles

Numerical Simulation of the Micromixing Induced by Ultrasonic Vibration of Sidewall-Trapped Microbubbles

It exist several strategies to mix two fluids in a micro channel. The way micro bubble vibrations influence the mixing flow is still unknown. This paper presents numerical simulations of the mixing within the micro device with and without micro bubble vibrations. A simplified model of the microchannel has been successfully employed, via using moving panels instead of sidewall-trapped bubbles oscillation. The simulation method, which exerted sinusoidal movements on the panels to approximately represent the ultrasonic vibrations of microbubbles, has been used to fully solve the Navier-Stokes equations. The comparison between simulations and previously reported experiments, in terms of flow pattern and the mixing performance within micro channel, exhibits a very good agreement. When ultrasonic vibrations of a frequency of 80 kHz and amplitude of 8 μm were applied, the mixing flow patterns have been reproduced and with a little differences comparing to the experimental results. All of these studies have revealed the mix mechanism under the micrometer scale in a certain way.

2상 유동 해석코드 CUPID를 이용한 CANDU 원자로 감속재 열수력 예비해석

본 연구에서는 기기 스케일 2상 유동(Two-phase flow) 해석 코드 CUPID를 사용하여 CANDU 원자로의 칼란드리아 용기 내부 감속재의 열수력 거동을 분석하기 위한 사전연구를 수행하였다. 먼저, Stern 연구소에서 수행한 단상유동 실험 3종류를 이용하여 CUPID 코드를 검증하였다. 칼란드리아 관다발 영역 격자생성의 복잡성을 피하기 위하여 다공성 매질 모델을 해당 영역에 적용하였고, 다공성 매질 영역의 유동 저항은 실험에서 얻은 관계식을 이용하여 계산하도록 하였다. 계산결과, CUPID 코드는 칼란드리아 용기 내부의 강제 및 자연 대류의 혼합 유동 양식을 성공적으로 예측하였다. 다음으로 2상 유동이 발생하는 경우를 해석하였다. 이들 계산을 통해 CUPID 코드의 CANDU 원자로 감속재 해석 능력을 보였다. 또한, 국부 과냉각 여유도를 예측하는데 사용할 수 있는 유입유량 대비 칼란드리아 용기의 국부 최대 감속재 온도 그래프를 제시하였다. A transient, three-dimensional, two-phase flow analysis code, CUPID, has been developed in KAERI. In this work, we performed a preliminary analysis using the CUPID code to investigate the thermal-hydraulic behavior of the moderator in the Calandria vessel of a CANDU reactor. At first, we validated the CUPID code using the three experiments that were performed at Stern Laboratories Inc. To avoid the complexity to generate computational mesh around the Calandria tube bundles, a porous media approach was applied for the region. The pressure drop in the porous media zone was modeled by an empirical correlation. The results of the calculations showed that the CUPID code can predict the mixed flow pattern of forced and natural convection inside the Calandria vessel very well. Thereafter, the analysis was extended to a two-phase flow condition. Also, the local maximum temperature in the Calandria vessel was plotted as a function of the injection flow rate, which may be utilized to predict the local subcooling margin.

Mixing and atomization characteristics in an internal-mixing twin-fluid atomizer

Twin-fluid atomizers have been successfully used in many industrial applications. This paper presents experimental studies on internal mixing and atomization in a water–air internal-mixing atomizer. Two-phase mixing process and flow patterns in the internal mixing chamber were visually studied through high speed CCD. Observation reveals that internal mixing was dominated by Gas to Liquid mass Ratio (GLR). As GLR increased, the flow patterns changed from slug flow to annular flow. The Oshinowo and Charles’ map can be used to predict the flow patterns for the designed atomizer. Droplet Sauter Mean Diameter (SMD) spatial distributions were measured with Phase Doppler Analyzer (PDA) at different operating conditions. Droplet SMD decreased with the increase of GLR at all operating pressure and locations. In the undeveloped region, a close relationship was observed between flow pattern transformation in internal mixing chamber and droplet SMD distribution, and there was an optimized pressure ranging from 0.2MPa to 0.3MPa for atomization since liquid films became thicker under a higher pressure. In the developed region, pressure promoted to generate finer atomization. Possibility Density Function (PDF) distribution of droplet size at different axial locations was analyzed to quantitatively represent the effect of droplet coalescence and breakup. As axial distance increased, PDF of both fine droplets and large droplets decreased. The particle size with the maximum PDF increased with the axial distance as well. The results imply that best atomization performance was acquired in the undeveloped region.

Multi dimensional analysis of Design Basis Events using MARS-LMR

Abstract KALIMER-600 (Korea Advanced Liquid Metal Reactor), which is a pool type SFR (Sodium-cooled Fast Reactor), was developed by KAERI (Korea Atomic Energy Research Institute). DBE (Design Basis Events) for KALIMER-600 has been analyzed in the one dimension. In this study, the one dimensional analyzed sodium hot pool is modified to a three dimensional node system, because the one dimensional analysis cannot represent the phenomena of the inside pool of a big size pool with many compositions, such as UIS (Upper Internal Structure), IHX (Intermediate Heat eXchanger), DHX (Decay Heat eXchanger), and pump. The results of the multi-dimensional analysis compared with the one dimensional analysis results in normal operation, TOP (Transient of Over Power), LOF (Loss of Flow), and LOHS (Loss of Heat Sink) conditions. First, the results in normal operation condition show the good agreement between the one and multi-dimensional analysis. However, according to the sodium temperatures of the core inlet, outlet, the fuel central line, cladding and PDRC (Passive Decay heat Removal Circuit), the temperatures of the one dimensional analysis are generally higher than the multi-dimensional analysis in conditions except the normal operation state, and the PDRC operation time in the one dimensional analysis is generally longer than the multi-dimensional analysis. In particular, we can see the reverse and mixing flow phenomena in LOHS better than TOP and LOF, which affect the peak temperature in the one dimensional analysis at 330 s but is not observed in the multi-dimensional analysis. In addition, the different temperatures at the central line of fuel and cladding are shown due to different mass flow in the core. The two PDRCs show the almost same results in one and multi-dimensional analysis, respectively. However, in the comparison between the one and multi-dimension, the heat removal capability of PDRC in the multi-dimensional analysis is better than the one dimensional analysis at the beginning of calculation. The aspects change at the ending of calculation due to the different mass flow in PDRC and over flow sodium temperature. In wrap up, the sodium temperatures of the core in the one dimensional analysis are generally higher than the multidimensional analysis. The temperature peak point in the one dimensional analysis under LOHS does not show in the multi-dimensional analysis, because the reverse and mixing flow pattern between the hot pool and the core outlet region. Even thought there are some different results between the two different dimensional analyses, we can say that the KALIMER-600 system has the safety margins.

Large eddy simulation of mechanical mixing in anaerobic digesters

A comprehensive study of anaerobic digestion requires an advanced turbulence model technique to accurately predict mixing flow patterns because the digestion process that involves mass transfer between anaerobes and their substrates is primarily dependent on detailed information about the fine structure of turbulence in the digesters. This study presents a large eddy simulation (LES) of mechanical agitation of non-Newtonian fluids in anaerobic digesters, in which the sliding mesh method is used to characterize the impeller rotation. The three subgrid scale (SGS) models investigated are: (i) Smagorinsky-Lilly model, (ii) wall-adapting local eddy-viscosity model, and (iii) kinetic energy transport (KET) model. The simulation results show that the three SGS models produce very similar flow fields. A comparison of the simulated and measured axial velocities indicates that the LES profile shapes are in general agreement with the experimental data but they differ markedly in velocity magnitudes. A check of impeller power and flow numbers demonstrates that all the SGS models give excellent predictions, with the KET model performing the best. Moreover, the performance of six Reynolds-averaged Navier-Stokes turbulence models are assessed and compared with the LES results.

Mixing times for process vessels with aspect ratios greater than one

Stirred tank reactors are one of the most important and common pieces of equipment used in speciality, pharmaceutical, and agrichemical processes. It is also typical for these to be operated at large aspect ratios; however, there is very little information in the open literature about mixing times in vessels with aspect ratios greater than one. This paper aims to provide new information in this area that will enable the design of better reactors. Electrical resistance tomography is used to monitor the mixing time of systems with aspect ratios higher than one. The mixing time has been measured on vessels of 0.914 and 0.610 m diameter with Cowles disc, Rushton turbine and mixed flow type impellers to aspect ratios of 2. The current correlation of choice, by Grenville and Nienow (2004), has been compared with the results and found to under predict the mixing time at aspect ratios greater than one. The exponent on the H/ T term has been explored and it has been found that this varies with agitator type, this information has never been shown before. The affect of adding a second impeller on the mixing time and flow pattern is also investigated. Adding a second Rushton turbine creates zoning in the vessel which impedes the mixing; this can be visualised using electrical resistance tomography.

Analysis and interpretation of residence time distribution experimental curves in FM01-LC reactor using axial dispersion and plug dispersion exchange models with closed–closed boundary conditions

The liquid phase mixing flow pattern at low (20 < Re < 120) and intermediate liquid flow rate (120 < Re < 400) was studied by means of residence time distribution (RTD) experimental curve in an up-flow Filter Press electrochemical reactor (FM01-LC) bench scale. For this purpose, a plastic turbulence promoter was used with stainless-steel and platinised titanium structural meshes as electrodes in channel configuration. To visualize and determine the mixing flow pattern in the liquid phase, the stimulus-response technique was employed using dextran blue ( D M = 1.058 × 10 −11 m 2 s −1, 25 °C, in water) as model tracer. A theoretical analysis and approximation RTD experimental curves with axial dispersion model (ADM) and plug dispersion exchange model (PDE), with “closed–closed vessel” boundary conditions were used in order to establish a better approximation of the axial dispersion, stagnant zones, channelling and by-pass (preference flow) effects present at low and intermediate Re. RTD curves show that the liquid flow pattern in the FM01-LC deviates considerably from axial dispersion model at low Re, where the FM01-LC exhibits large channelling, stagnant zones, and dead zone. The PDE model represents fairly this deviation from ideal flow (less dead zone).

Floor heating and cooling combined with displacement ventilation: Possibilities and limitations

Design guidelines envisage that floor heating can be used together with displacement ventilation (DV), provided that the supply air is not overly heated before it can reach heat and contaminant sources. If this is not controlled a mixing flow pattern could occur in the room. The use of floor cooling with DV is also considered possible, although draught risk at ankle level and vertical air temperature differences must be controlled carefully, because they could increase. Few studies on these topics were found in the literature. An indoor environmental chamber was set up to obtain measurements aimed at analysing the possibilities and limitations of combining floor heating/cooling with DV. Air temperature profiles, air velocity profiles, surface temperatures and ventilation effectiveness were measured under different environmental conditions that may occur in practice. These values were compared to equivalent temperature measurements obtained using a thermal manikin. The measurements show that floor heating can be used with DV, obtaining high ventilation effectiveness values. A correlation between the floor heating capacity and the air temperature profile in the room was found. Measurements showed that floor cooling does not increase draught risk at ankle level, although it does increase vertical air temperature differences.

English

Water lubrication technique is one of the most efficient ways to transport viscous oil through pipelines. Flow of two immiscible liquids through conduits gives rise to various flow patterns. In the present study flow patterns of viscous oilwater flow through 5° upward inclined pipeline has been numerically simulated using ANSYS FLUENT TM 6.2. Volume of Fluid (VOF) method has been employed to predict various flow patterns by assuming unsteady flow, immiscible liquid pair (viscous oil and water), constant liquid properties, and co-axial flow. Five flow patterns namely plug flow, slug flow, wavy stratified, stratified mixed and annular flow patterns have been predicted using CFD simulation in upward inclined pipeline. The simulation results have been validated with experimental results. The results gave good proximity with the experimental data.

Geomagnetic dipole moment collapse by convective mixing in the core

Convective mixing in the fluid outer core can induce rapid transient decrease of the geomagnetic dipole. Here we determine rates of dipole moment decrease as a function of magnetic Reynolds number following convective instability in a numerical dynamo and in axisymmetric kinematic flows. Our calculations show that mixing flows induce reversed magnetic flux on the core‐mantle boundary through expulsion of mostly poloidal magnetic field by convective upwellings. The dipole field collapse is accelerated by enhanced radial diffusion and meridional advection of magnetic flux below the core‐mantle boundary. Magnetic energy cascades from the dipole to smaller scales during mixing, producing a filamentary magnetic field structure on the core‐mantle boundary. We find that the maximum rate of dipole moment decrease on century time scales is weakly sensitive to the mixing flow pattern but varies with the velocity of the flow approximately as cRmβ, with Rm the magnetic Reynolds number and (c, β) ≈ (0.2 ± 0.07, 0.78 ± 0.05). According to our calculations, a mixing flow in the outer core with Rm in the range of 200–300 can account for the historically‐measured rate of decrease of the geomagnetic dipole moment, although it is unlikely that a single mixing flow event with this intensity would cause a full dipole collapse or polarity reversal.

Analytical reconsideration of the von Neumann paradox in the reflection of a shock wave over a wedge

The reflection of weak shock waves has been reconsidered analytically using shock polars. Based on the boundary condition across the slipstream, the solutions of the three-shock theory (3ST) were classified as “standard-3ST solutions” and “nonstandard-3ST solutions.” It was shown that there are two situations in the nonstandard case: A situation whereby the 3ST provides solutions of which at least one is physical and a situation when the 3ST provides a solution which is not physical, and hence a reflection having a three-shock confluence is not possible. In addition, it is shown that there are initial conditions for which the 3ST does not provide any solution. In these situations, a four-wave theory, which is also presented in this study, replaces the 3ST. It is shown that four different wave configurations can exist in the weak shock wave reflection domain, a Mach reflection, a von Neumann reflection, a ?R (this reflection is not named yet!), and a modified Guderley reflection (GR). Recall that the wave configuration that was hypothesized by Guderley [“Considerations of the structure of mixed subsonic-supersonic flow patterns,” Air Materiel Command Technical Report No. F-TR-2168-ND, ATI No. 22780, GS-AAF-Wright Field No. 39, U.S. Wright–Patterson Air Force Base, Dayton, OH (October 1947); Theorie Schallnaher Strömungen (Springer-Verlag, Berlin, 1957)] and later termed Guderley reflection did not include a slipstream (see Fig. 7). Our numerical study revealed that the wave structure proposed by Guderley must be complemented by a slipstream (see Fig. 4) in order to be relevant for explaining the von Neumann paradox. Hereafter, for simplicity, this modified GR wave configuration will be also termed Guderley reflection. The domains and transition boundaries between these four types of reflection are elucidated.

Numerical Investigation of Cross Flow Phenomena in a Tight-Lattice Rod Bundle Using Advanced Interface Tracking Method

In relation to the design of an innovative FLexible-fuel-cycle Water Reactor (FLWR), investigation of thermal-hydraulic performance in tight-lattice rod bundles of the FLWR is being carried out at Japan Atomic Energy Agency (JAEA). The FLWR core adopts a tight triangular lattice arrangement with about 1 mm gap clearance between adjacent fuel rods. In view of importance of accurate prediction of cross flow between subchannels in the evaluation of the boiling transition (BT) in the FLWR core, this study presents a statistical evaluation of numerical simulation results obtained by a detailed two-phase flow simulation code, TPFIT, which employs an advanced interface tracking method. In order to clarify mechanisms of cross flow in such tight lattice rod bundles, the TPFIT is applied to simulate water-steam two-phase flow in two modeled subchannels. Attention is focused on instantaneous fluctuation characteristics of cross flow. With the calculation of correlation coefficients between differential pressure and gas/liquid mixing coefficients, time scales of cross flow are evaluated, and effects of mixing section length, flow pattern and gap spacing on correlation coefficients are investigated. Differences in mechanism between gas and liquid cross flows are pointed out.

Kinetics and stability of the mixing flow patterns found in champagne glasses as determined by laser tomography techniques: likely impact on champagne tasting

Laser tomography techniques were used in order to make visible the flow patterns induced by ascending bubbles in flutes poured with champagne. The stability of flow patterns was investigated in flutes showing natural (without any specific surface treatment) as well as artificial effervescence (i.e., engraved at their bottom), all along the first 15 min after pouring. Engravement conditions were found to strongly influence the kinetics and the stability with time of the mixing flow phenomena found in champagne glasses.

An experimental study of the fluid flow and heat transfer characteristics in micro-condensers with slug-bubbly flow

Experiments were conducted to study the condensation flow pattern in silicon micro-condensers using water as the medium. Slug-bubbly flow was found to be one of the dominant flows in the micro-condenser and it was a major factor in determining the heat transfer and pressure drop properties of the fluid inside the micro-condenser. The transition from the slug-bubbly flow to a mixed flow pattern was studied. A correlation was obtained to predict when the transition of the flow pattern would occur. Only slug-bubbly flow existed under low steam mass flow rate and high heat transfer rate conditions. As the steam mass flow rate increased or the heat transfer rate dropped, the mixed flow pattern would then appear. In the slug-bubbly flow regime, the heat transfer coefficient and pressure drop in the micro-condensers were investigated in detail. It was found that micro-condensers with smaller channels could exhibit higher heat transfer coefficients with the same Reynolds number. The condensation heat transfer coefficient was higher than that in the tubes with the diameter of centimeter. Pressure drops in the micro-condensers with smaller channels were higher due to the increased transition loss. At the same time, the pressure drop in the micro-condenser was found to be lower than what could be predicted using the macro-scale correlation. Increasing the heat flux would create a longer bubble–film region and fewer unit cells in the micro-condenser resulting in an increased heat transfer coefficient and a decreased pressure drop.

Mixed Travel Behavior in Networks with ATIS and Upper Bound of Efficiency Loss

Abstract In a transportation network supported by the Advanced Traveler Information Systems (ATIS), the drivers equipped with ATIS are directed by a control center to select their routes so as to minimize the total network travel time, while the unequipped ones make the route choice decisions on the base of minimizing their individual travel times. This article presents a variational inequality model to formulate the mixed travel behavior associated with both system optimum and user equilibrium, and derives a general formula for the upper bound of efficiency loss of the mixed behavior. The upper bounds when all link travel cost functions are polynomial are examined against different ATIS market penetrations. It is found that the upper bound is related to the ATIS market penetration and the corresponding mixed equilibrium flow pattern.

Transition between stratified and non-stratified horizontal oil–water flows. Part I: Stability analysis

The characteristics of waves at the interface of oil–water stratified flow and at the onset of entrainment, where drops of one phase appear into the other, were studied. Theoretically a model was developed based on Kelvin–Helmholtz instability to predict the critical wave amplitude at which the waves become unstable for a specific wavelength. According to the model, waves become unstable in stratified flow when at a particular wavelength they exceed a critical amplitude, which decreases with increasing wavelength until it acquires an almost constant value. The model predictions showed that for low-viscosity oils the maximum critical amplitude appears at slip velocity close to zero, while for high-viscosity oils, the maximum amplitude appears for water velocity higher than that of the oil. Also the required entrainment wavelength over the pipe diameter, calculated using literature experimental onset conditions, was found to decrease as the viscosity of the oil increased. Experimentally, wave characteristics before and at the onset of entrainment were investigated by measuring the instantaneous fluctuations of the interface between oil (5.5 mPa s, 828 kg / m 3 ) and water in a 0.038 m ID stainless steel horizontal pipe using a conductivity probe. The formation of drops and the onset of entrainment were identified using a high-speed video camera. At the onset of entrainment, wave characteristics were above the stability lines predicted by the model. Using a semi-empirical characteristic amplitude and wavelength in the model, it was possible to predict the onset of entrainment and transition from stratified to other mixed flow patterns reported in a number of studies.

OS10-9 統計解析手法による稠密炉心内流体混合現象の解明(OS10 混相流の計測技術と解析,循環型社会における動力エネルギー技術)

As a candidate for next generation reactor, the innovative Flexible-fuel-cycle Water Reactor (FLWR) adopts a remarkably tight triangular lattice arrangement with about 1mm gap spacing between adjacent fuel rods. In relation to its design, this study presents a statistical evaluation of numerical simulation results of a detailed two-phase flow simulation code (named TPFIT). In order to make clear mechanisms of cross flow in such tight lattice rod bundles, the TPFIT is used to simulate cross flow between two modeled subchannels. Attention was focused on instantaneous fluctuation characteristics of differential pressure between two subchannels and gas/liquid mixing coefficients. With the calculation of correlation coefficients between the differential pressure and gas/liquid mixing coefficients, the time scales of cross flow, e.g. lag times were evaluated, and the effects of mixing section length, flow pattern and gap spacing on correlation coefficients were extensively investigated. The difference in mechanism between gas and liquid cross flows was pointed out.

Theoretical analysis of conversion enhancement in isothermal polymeric catalytic membrane reactors

A theoretical study of polymeric catalytic membrane reactors (PCMR) is performed when a reversible A ↔ B reaction is taking place inside the catalytic membrane. The PCMR is assumed to be isothermal, and with a perfectly mixed flow pattern for both retentate and permeate chambers. An analytical expression is derived to calculate the reactor conversion and the reactor conversion enhancement for the case in which the membrane permeability is the same for the reactant and product. When the membrane has different permeabilities for the reactant and product, a semi-analytical solution is presented. The results show that, at least theoretically, conversion above thermodynamic equilibrium can be achieved with the PCMR when the membrane has higher permeability for the product than for the reactant. It is also pointed out that, in the PCMR, the environment of the catalyst particle should be quite different from that existing in conventional reactors. This should change the properties of the catalyst particle and it must be carefully considered.

Experimental and numerical study of mixed convection and flow pattern in a lid-driven arc-shape cavity

Experimental and numerical study has been performed to investigate the combined effects of lid movement and buoyancy on flow and heat transfer characteristics for the mixed convective flow inside a lid-driven arc-shape cavity. The numerical methodology is based on a numerical grid generation scheme that maps the complex cross section onto a rectangular computation domain. The discretization procedure for the governing equations is based on the finite-volume method. In experiments, steady-state temperature data are measured by T-type thermocouples, and the flow field is visualized by using kerosene smoke. Reynolds number and Grashof number are two major independent parameters representing the effects of lid movement and buoyancy, respectively. Flow pattern, friction factor, and Nusselt numbers are investigated in wide ranges of these independent parameters. Close agreement in the comparison between the predicted and the visualized flow patterns shows the validity of the numerical methods.

Simulating catalytic membrane reactors using orthogonal collocation with spatial coordinates transformation

It is presented in this study a new numerical scheme using orthogonal collocation together with an independent variable (spatial coordinate) transformation, useful for solving the model equations associated to membrane reactors with catalytic membranes. This new scheme takes advantage of a noticeable feature of the concentration profile inside a catalytic membrane: close to the membrane surfaces, this profile becomes steeper and steeper with the Thiele modulus. Using traditional numerical methods for solving the model equations of a catalytic membrane reactor, namely finite differences with equispaced intervals or orthogonal collocation, for example, may lead to imprecise results. In order to illustrate the ability of this new numerical scheme for solving such equations, it is applied to the resolution of a case where an analytical solution is available (a generic A ⇌ B reaction carried out in a catalytic membrane with perfectly mixed flow pattern in both retentate and permeate sides). Then, the same numerical scheme is used for solving a model describing the cyclohexane dehydrogenation, A ⇌ B + 3 C , carried out in a porous membrane with the same flow pattern as above—and the results are compared with the ones obtained using an adaptive wavelet-based method. For these two models, solutions were also obtained using straight orthogonal collocation and finite differences with homogeneously distributed grid points for the sake of the comparison. The results show that the new numerical approach is useful in dealing with this kind of problems, especially for high Thiele modulus values, showing high accuracy and demanding low computation time. Finally, this new scheme is applied to the resolution of a more complex model: a generic reaction 2 A ⇌ B , carried out in a dense catalytic cylindrical membrane with plug–flow pattern for both retentate and permeate sides.