颗粒与液相间的湍流涡旋裂变传质模型

湍流在宏观上处于远程混沌无序状态, 而在介微观上处于近程有序状态。从分析湍流场中局 域中涡旋的串级结构出发,通过对N-S方程的涡旋输运形式进行求解, 导出一个具有分形意 义的涡旋群运动表达式,并得到涡旋掠过颗粒表面花费的时间, 然后同Higbie的传质渗透模型相联系,以涡旋的分布函数为权求其数学期望,得到搅拌湍流场中的传质系数表达式,结果与实验数据吻合较好。     

Computational fluid dynamics simulation of hydrodynamics in an uncovered unbaffled tank agitated by pitched blade turbines

Computational fluid dynamics (CFD) simulations were applied for evaluating the hydrodynamics characteristics in an uncovered unbaffled tank agitated by pitched blade turbines. A volume of fluid (VOF) method along with a Reynolds stress model (RSM) was used to capture the gas-liquid interface and the turbulence flow in the tank. The reliability and accuracy of the simulations are verified. The simulation results show that the vortex can be divided into central zone and peripheral zone, and flow field in the tank can be divided into forced vortex flow region and free vortex flow region. With the increase of impeller speed, the vortex becomes deeper, while the critical radius of the two zones keeps almost unchanged. The impeller clearance and the rotational direction have little effect on the vortex shape. The vortex becomes deeper with increasing of the impeller diameter or the blade angles at the same rotational speed. Power number is little influenced by the impeller speed, and decreases by about 30% when impeller diameter varies from 0.25T to 0.5T. When blade angle varies from 30° to 90°, power number increases by about 2.32-times. Power number in uncovered unbaffled tank is much smaller than that in baffled tank, but is very close to that in a covered unbaffled tank. The discrepancy of power number in uncovered unbaffled tank and that in covered unbaffled tank is less than 10%.     

INTERNAL WALL COOLING IN PRESSURE VESSELS EXPOSED TO EXTERNAL FIRE IMPINGEMENT

In many applications there is a requirement to retard accidental heating of pressure vessel shells so that loss of pressure carrying capacity due to increasing shell temperature does not occur. For example, hazardous commodity transport and storage vessels are usually thermally insulated so that in the event of accidental fire the shell is not exposed to the full heating effects of the fire. The present paper deals with a concept device that cools the walls of tanks carrying pressurized liquids by directing 2-phase fluid along the inner surface of the tank when the tank pressure relief valve is in action.

Preliminary experimental results are presented for the case of a short cylindrical tank with the cooling device installed. From the limited tests conducted it was shown that the device cools areas of the tank wall that would normally have been exposed to the full heating effects of the fire.

Based on observations from the tests a simple model was formulated and integrated with an existing tank-in-fire computer model. Simulation results indicate that internal wall cooling could be effective at reducing the risk of thermal ruptures of tanks. The simulation results for internally cooled tanks are compared with simulations and experimental data for an uninsulated tank filled with propane exposed to engulfing fires     

Vortex flow of dilute polymer solutions

It has been observed that very d longchain polymers which are effective in turbulent drag reduction inhibit the formation of a vortex or air core as water drains from a tank. This paper considers the fluid mechanical velocity profile measurements have been performed. There appear to be at least two distinct mechanisms for the vortex inhibition—one involving the viscosity enhancement caused by polymer addition, and the other related to the viscoelastic properties of the polymer solutions. This second mechanism is shown to arise due to the generation of high normal stresses as the air core begins to form. The very close correlation between vortex inhibition and turbulent drag reduction suggests that normal stresses may also play an important role in this latter phenomenon.     

LNG储罐分层涡旋事故的热对流特性

不同密度的LNG液体在储槽内会形成稳定分层。本文根据相似性定律,分析了盐水模型与LNG分层的相似性条件,建立了LNG储罐分层模拟实验台,以模拟LNG储罐中分层时的密度场、外热源场。通过分层形成条件实验和不同漏热条件下的分层破环实验,描述了分层涡旋事故的演化过程,得到了分层形成的条件、破坏机理,涡旋产生的原因及其时间预测,并讨论了初始密度差、分层高度和漏热等因素对分层涡旋事故的影响。     

Integrating mechanical mixing,headspace, and rheology in a computational model for a fermentation process

Although computational fluid dynamics (CFD) modelling has been applied to study bioprocesses where mechanical mixing and aerations are key, the effect of having a free surface has usually been ignored. This work studies a mixing tank for bioprocess applications to analyze the impact that the assumption of a flat liquid level has upon the modelling results, with and without aeration. The methodology takes an experimental and modelling approach. A dual impeller mixer with a speed of 400 rpm is used for batch systems with both non-Newtonian and Newtonian fluids to characterize the axis torque and surface vortex formation. Several multi-phase CFD models are applied to study the modelling and numerical effects of the headspace on the results. Regarding modelling accuracy, the CFD models are shown to be able to capture the effect of the free surface on the fluid dynamics of the stirred tank for different fluid rheologies. Therefore, the simplification of the liquid level as a flat and fixed surface should not be applied, especially for a process with aeration or when unaerated conditions can lead to a surface vortex. Regarding the numerical accuracy, it is concluded that the mixture model does not predict the interface shape as well as the Eulerian model. However, only the mixture model shows to be numerically stable. Overall, this work provides validated CFD configurations able to predict the effect of a free surface on the mixing mechanisms in stirred reactors.     

Time-dependent flow structures and Lagrangian mixing in Rushton-impeller baffled-tank reactor

The object of this work is to investigate the role of large-scale convective structures in promoting mixing in a stirred tank. We focus on a standard geometry (flat bottom, four-baffle reactor stirred by a six-blade Rusthon impeller) and we use an Eulerian-Lagrangian approach to investigate numerically the dispersion of fluid particles. The three-dimensional, time-dependent, fully developed flow field is calculated with a computationally efficient procedure using a RANS solver with k-ε turbulence modeling and the flow field is assessed precisely against experimental data. Then, fluid parcels are tracked in the calculated flow field. Analyzing the trajectory of fluid parcels, the segregated regions within the flow are identified and mixing indicators are calculated (mixing time, circulation length and sojour time distribution). A physical explanation is thus proposed to establish a link between large-scale mixing and complex fluid dynamics generated by the interactions of radial-discharge jet, ring vortices, and upper counter rotating vortex.     

Trapping region of impinging jets in a cross-shaped channel

This study follows our previous report (Zhang et al., Phys. Fluids, vol. 31, 2019, 034105) by describing the formation and evolution of the engulfment flow in the cross-shaped channel. First, the flow regimes were studied by planar laser induced fluorescence (PLIF). Results show the formation of a spiral vortex in the center of the chamber and the appearance of a well-mixed zone inside the spiral vortex. Second, we proposed a novel experimental method to analyze the residence time of the fluid in the chamber, and discover an unexpected trapping region inside the well-mixed zone. There is almost no fluid transport into or out of this region. Furthermore, three-dimensional numerical simulation is used to reveal the origination of this trapping region. Simulation results reveal that the fluid recirculates in the trapping region and the flow feature is caused by the bubble-type vortex breakdown.     

Intensification of a vortex during free draining

Three regimes of air-core vortex formation are demonstrated to exist for the free draining of liquids containing angular momentum. These comprise of a rapidly intensifying air-core vortex, a quasisteady vortex and a decaying vortex. The regimes are shown to be strongly influenced by the size of the drain-port used in relation to the tank diameter. The influence of Froude number on air-core vortex is discussed.     

基于涡街压电信号的质量流量计

通过对涡街流量传感器的信号分析,发现涡街信号的幅值和流体的密度及其流速的平方成正比,而涡街信号的频率与流体的速度成正比,所以,涡街信号中包含了流体的流速和密度的信息,提出用涡街信号的幅值进行密度补偿的方法,并通过实验加以验证。     

A Three‐Dimensional Two‐Phase Model for a Membraneless Fuel Cell Using Decomposition of Hydrogen Peroxide with Y‐Shaped Microchannel

A three‐dimensional, two‐phase, multi‐component mixture model in conjunction with a finite‐volume‐based computational fluid dynamics (CFD) technique is applied to simulate the operation of membraneless fuel cell with Y‐shape channel. Hydrogen peroxide is employed both as fuel and oxidant, which are dissolved in diluted sodium hydroxide and sulfuric acid solutions, respectively. Almost all transport phenomena occurring in the fuel cell such as fluid flows, mass transport, electrochemical kinetics, and charge transport are accounted in this model. The oxygen O₂ gas, which is a product on the anode electrode, is assumed to be insoluble. The presence of gas phase acts to prevent the processes of reactant supply and product removal. Thus, the cell performance is hindered, while it is operated at the normal current density situation. On the other hand, the capillary action is found to enhance the electrolyte transport in the anode porous electrode, which may slightly improve the cell performance at the high‐current density situation. Besides, a secondary vortex flow is induced due to the transportation of the gas phase, which drifts from the bottom to the top of the channel. The mixing zone is then inclined, which may result in serious fuel crossing phenomenon.     

Gas absorption at a liquid surface agitated by vortex rings

Underwater vortex rings have been formed at orifices of between 25.4 and 63.5 mm diameter and projected upwards at the surface of the water. The conditions of ring formation at which significant surface deformation occurs have been correlated. The absorption of pure carbon dioxide by the water surface with impinging vortex rings has been measured and related to the following variables: time of fluid impulse through the orifice, orifice diameter, volume of displaced liquid, and depth of submergence. A simplified mass transfer model based on the penetration theory approximately correlates the data.     

Particle suspension in top-covered unbaffled tanks

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels. However, they may be expected to provide significant advantages in a wide range of applications (e.g. crystallization, food and pharmaceutical processes, etc.), where the presence of baffles is often undesirable. In the present work solid–liquid suspension in an unbaffled stirred tank is investigated. The tank was equipped with a top-cover in order to avoid vortex formation. A novel experimental method (the “steady cone radius method”, SCRM) is proposed to determine experimentally the minimum impeller speed at which solids are completely suspended. Experimental N_js and power consumption data are provided over fairly wide ranges for particle size, density and concentration. Dependence of N_js on particle density and concentration is similar to that observed in baffled tank. Conversely, a negligible dependence of particle diameter on N_js is observed in the unbaffled tank, a difference from baffled vessels with important practical implications.Finally, the mechanical power required to achieve complete suspension in unbaffled tanks is shown to be much smaller than in baffled vessels. This, in conjunction with the previously ascertained excellent particle-fluid mass-transfer promotion, could make unbaffled tanks a best choice for many solid–liquid operations, where mass transfer is the main limiting factor.     

Operation of a glass-melting furnace with a heat-insulated melting tank

The operating advantages of a glass-melting furnace with a thermally insulated melting tank are considered. A scheme of thermal insulation is presented. The efficiency of the heat insulation of the melting tank in the production of glass with decreased light transmission is noted. Translated from Steklo i Keramika, No. 2, pp. 8–9, February, 2000.     

Numerical simulation of fluid dynamics in the stirred tank by the SSG Reynolds Stress Model

The Speziale, Sarkar and Gatski Reynolds Stress Model (SSG RSM) is utilized to simulate the fluid dynamics in a full baffled stirred tank with a Rushton turbine impeller. Four levels of grid resolutions are chosen to determine an optimised number of grids for further simulations. CFD model data in terms of the flow field, trailing vortex, and the power number are compared with published experimental results. The comparison shows that the global fluid dynamics throughout the stirred tank and the local characteristics of trailing vortices near the blade tips can be captured by the SSG RSM. The predicted mean velocity components in axial, radial and tangential direction are also in good agreement with experiment data. The power number predicted is quite close to the designed value, which demonstrates that this model can accurately calculate the power number in the stirred tank. Therefore, the simulation by using a combination of SSG RSM and MRF impeller rotational model can accurately model turbulent fluid flow in the stirred tank, and it offers an alternative method for design and optimisation of stirred tanks.     

搅拌釜内固-液悬浮体系的CFD-DEM模拟(英文)

Computational fluid dynamics-discrete element method （CFD-DEM） coupled approach was employed to simulate the solid suspension behavior in a Rushton stirred tank with consideration of transitional and rotational motions of millions of particles with complex interactions with liquid and the rotating impeller. The simulations were satisfactorily validated with experimental data in literature in terms of measured particle velocities in the tank. Influences of operating conditions and physical properties of particles （i.e., particle diameter and density） on the two-phase flow field in the stirred tank involving particle distribution, particle velocity and vortex were studied. The wide distribution of particle angular velocity ranging from 0 to 105 r·min-1 is revealed. The Magnus force is comparable to the drag force during the particle movement in the tank. The strong particle rotation will generate extra shear force on the particles so that the particle morphology may be affected, especially in the bio-/polymer-product related processes. It can be concluded that the CFD-DEM coupled approach provides a theoretical way to under-stand the physics of particle movement in micro-to macro-scales in the solid suspension of a stirred tank.     

Mixing effects and hydrodynamics of vortex rings

Vortex rings have been formed in water by impulsive flow through an orifice, and mixing effects due to the lings have been measured. Laboratory tests on the mixing of stratified layers of salt solution by vortex rings formed at a 6.35 cm diameter orifice have indicated that Froude number is an important factor in determining the energy efficiency of mixing. Field tests using a 25.4 cm orifice in Hamilton Harbour (18 m depth of water) have shown a small mixing effect. Further experiments with a 25.4 cm orifice in a test tank have shown that the vortex rings have a range of at least 30 m.     

SIMULATION OF SPRAY DRYING OF A SOLUTION ATOMIZED IN A PULSATING FLOW

Spray drying of NaCl solution was carried out under an intense oscillating flow field generated by a pulse combustor. A pulse combustion spray drying system was constructed. An optical analyzer was used to measure the particle diameter distribution of droplets atomized by a pulsating flow. The momentum, heat and mass transfer in both gaseous and particulate phases during spray drying inside the drying chamber were simulated using the computational fluid dynamics method. The simulated profiles of flow field, temperature and humidity of the gaseous phase, as well as the particulate phase, in the drying chamber were presented. The simulation showed changes of the flow field and particle trajectories in the drying chamber during one pulsating period. A large-scale vortex was observed in the upper part of the drying chamber because of the unstable state of flow field and particle trajectories. Short drying time and large evaporation rate are characteristics of pulsating spray drying. The influence of gas stream pulsation frequency on the drying process is also analyzed.     

Modelling of heavy and buoyant particle dispersion in a two-dimensional turbulent mixing layer

Heavy and buoyant particle dispersion in the turbulent mixing layer was investigated numerically using a two-phase flow discrete vortex modelling. It was revealed from the modelling that inclusion of two-way momentum coupling is essential for properly modelling heavy particle dispersive transport in turbulent free shear flows. For heavy particles with small Stokes numbers, the dispersion is predominated by the large-scale vortex structures and they exert small influence on the carrier fluid flow. Heavy particles with large St directionally align along the braid region between the neighbouring vortices. However, the lateral dispersion of particles of large St is smaller than that of particles of small St.For buoyant particles with the density being slightly greater than that of the carrier fluid, numerical simulation revealed that the buoyant particles scatter over the whole vortex core rather than collect along the fringes of the vortex. The Lagrangian statistics calculation of buoyant particle dispersion showed that both the inertial and crossing-trajectory effects affect the particle dispersion behaviour and particle eddy diffusivity. The dispersion behaviour of buoyant particles is highly associated with the particle Stokes number. Large St buoyant particles exhibit a larger dispersion. It was also indicated from the numerical simulation that buoyant particles might disperse larger than the fluid tracers. The correlation between the buoyant particle and fluid tracer velocities was affected by including the coupling effect.     

Calculation of Temperature Distribution in Glass Tanks by Use of a Special-Purpose Electric Analog

A low-cost special-purpose electric analog simulating heat flow in a transverse half section of a glass tank is described. It is easily constructed and is useful for studying steady-state temperature distributions and heat-flux densities in regions where convection can be ignored. Results are presented for physically realistic boundary conditions at the outer refractory surface, for an assumed symmetry about the transverse center line, and for three classes of boundary conditions at the free surface of the melt, namely the temperature distribution specified, heat-flux density distribution specified, or a given relation between temperature and heat-flux density. Heat transport by radiation is treated as radiation conductivity, and it is shown that even near the boundaries of the glass the error from this approximation is small provided the boundaries can be treated as blackbodies.