Showing papers on "Countercurrent exchange published in 2021"

Experimental Study of Single Taylor Bubble Rising in Stagnant and Downward Flowing Non-Newtonian Fluids in Inclined Pipes

Abstract: An experimental investigation of single Taylor bubbles rising in stagnant and downward flowing non-Newtonian fluids was carried out in an 80 ft long inclined pipe (4°, 15°, 30°, 45° from vertical) of 6 in. inner diameter. Water and four concentrations of bentonite–water mixtures were applied as the liquid phase, with Reynolds numbers in the range 118 < Re < 105,227 in countercurrent flow conditions. The velocity and length of Taylor bubbles were determined by differential pressure measurements. The experimental results indicate that for all fluids tested, the bubble velocity increases as the inclination angle increases, and decreases as liquid viscosity increases. The length of Taylor bubbles decreases as the downward flow liquid velocity and viscosity increase. The bubble velocity was found to be independent of the bubble length. A new drift velocity correlation that incorporates inclination angle and apparent viscosity was developed, which is applicable for non-Newtonian fluids with the Eotvos numbers (E0) ranging from 3212 to 3405 and apparent viscosity (μapp) ranging from 0.001 Pa∙s to 129 Pa∙s. The proposed correlation exhibits good performance for predicting drift velocity from both the present study (mean absolute relative difference is 0.0702) and a database of previous investigator’s results (mean absolute relative difference is 0.09614).

A Semi-Analytical Solution for Countercurrent Spontaneous Imbibition in Water-Wet Fractured Reservoirs

Abstract: Countercurrent spontaneous imbibition (SI) is an important flow mechanism for oil recovery in fractured reservoirs during waterflooding. SI plays a key role in the mobilization of oil in the matrix because it facilitates water infiltration by capillarity even when the matrix permeability is low, which limits fluid transport by advection. However, the modeling of SI in fractured media under dynamic conditions has been insufficiently studied. Most imbibition models assume conventional exponential functions and empirical constants based on experimental results of oil recovery under static conditions. Thus, the modeling of water distribution in the fracture and the matrix has been ignored which may lead to incorrect estimates of the efficiency of countercurrent SI to recover oil. Using the classic fractional flow equation, we present a semi-analytical solution to model countercurrent SI in a water-wet fractured medium by including a transfer function to account for continuous fluid exchange between the fracture and the matrix. The model is numerically solved using finite differences by including an effective imbibition time as a function of water advance in the fracture, which overcomes the difficulty of solving iterative numerical summations as shown in other imbibition models. The novelty of the presented solution is that it enables the modeling of water infiltration in the matrix under dynamic conditions. We verified the semi-analytical model against 2D numerical simulations and validated against experimental data to demonstrate that the model accurately predicts oil recovery and water infiltration in the matrix.

Experimental Investigation on the Hydrodynamic Characteristics of Fluidized Bed Particle Solar Receiver with Gas-Solid Countercurrent Flow Pattern

Abstract: To design a particle solar receiver (PSR), a vital energy conversion system, is still a bottleneck for researchers. This study presents a novel PSR based on countercurrent fluidized bed (CCFB) technology, named CCFB receiver. In this design, downward-moving particles are subjected to the action of an up-flow gas to reduce the falling speed and enhance the radial disturbance, and hence increase the residence time of particles and improve the heat transfer. A cold-mold visual experimental setup is established. The influence factors are investigated experimentally, including the superficial gas velocity, solid flux, aeration gas, particle size and transport tube diameter. The results indicate that the maximum solid holdup can exceed 9% or so with fine particles of diameter dp = 113.5 µm and a tube diameter of 40 mm. It is proved that the CCFB can operate stably and adjust the solid flux rapidly. The results of this study provide a new structure for PSRs in the concentrated solar power field and could fill the research insufficiency in the gas-solid counterflow field.

Nonlinear interaction underlying flow structure transition of inclined oil–water two-phase countercurrent flow

Abstract: Inclined oil–water two-phase flow exhibits random, unstable spatiotemporal structure, associated with the countercurrent caused by gravity and components interacting. In this paper, the nonlinear analysis to give a deeper understanding of inclined oil–water two-phase flow has been conducted. Firstly, we obtained the multivariate measurement of different flow patterns by vertical multi-electrode array system. Then, based on the multivariate refined composite entropy and transfer entropy, a dynamic nonlinear analysis framework is proposed to characterize the inclined oil–water two-phase flow. The results suggest that the proposed framework can reveal the dynamic information hidden in the oil droplet swarms and countercurrent water motion.

Characteristics of Axial and Radial Development of Solids Holdup in a Countercurrent Fluidized Bed Particle Solar Receiver

Abstract: A novel particle solar receiver (PSR) with gas-solids countercurrent fluidized bed (CCFB) was proposed. The cold-mold prototype was set up to investigate the gas-solids flow structure by using optical fiber probes. The local solids holdup distribution, its evolution with various operating conditions and the fluctuations of the local flow structures were investigated experimentally. The results show that the novel CCFB can achieve much higher solids holdup (∼9%) compared to the traditional downer ones (∼1%). The solid particles are mainly distributed in the near-wall region and the particles are more difficult to get a fully developed state in the near-wall region. The excellent gas-solids mixing and contacting demonstrated by the standard deviation and intermittency index means a better wall-to-bed heat transfer process. The distribution of the solid particles in the CCFB transport tube is revealed, which can provide a significant reference for the structure design of the hot-mold PSR. Moreover, the research can fill in the research gap in the gas-solids counterflow field.

Expression of transport proteins in the rete mirabile of european silver and yellow eel.

Abstract: BACKGROUND In physoclist fishes filling of the swimbladder requires acid secretion of gas gland cells to switch on the Root effect and subsequent countercurrent concentration of the initial gas partial pressure increase by back-diffusion of gas molecules in the rete mirabile. It is generally assumed that the rete mirabile functions as a passive exchanger, but a detailed analysis of lactate and water movements in the rete mirabile of the eel revealed that lactate is diffusing back in the rete. In the present study we therefore test the hypothesis that expression of transport proteins in rete capillaries allows for back-diffusion of ions and metabolites, which would support the countercurrent concentrating capacity of the rete mirabile. It is also assumed that in silver eels, the migratory stage of the eel, the expression of transport proteins would be enhanced. RESULTS Analysis of the transcriptome and of the proteome of rete mirabile tissue of the European eel revealed the expression of a large number of membrane ion and metabolite transport proteins, including monocarboxylate and glucose transport proteins. In addition, ion channel proteins, Ca2+-ATPase, Na+/K+-ATPase and also F1F0-ATP synthase were detected. In contrast to our expectation in silver eels the expression of these transport proteins was not elevated as compared to yellow eels. A remarkable number of enzymes degrading reactive oxygen species (ROS) was detected in rete capillaries. CONCLUSIONS Our results reveal the expression of a large number of transport proteins in rete capillaries, so that the back diffusion of ions and metabolites, in particular lactate, may significantly enhance the countercurrent concentrating ability of the rete. Metabolic pathways allowing for aerobic generation of ATP supporting secondary active transport mechanisms are established. Rete tissue appears to be equipped with a high ROS defense capacity, preventing damage of the tissue due to the high oxygen partial pressures generated in the countercurrent system.

Effects of confinement on absolute and convective instabilities for momentum-driven countercurrent shear layers

Abstract: We present results from the first experimental realization of a two-dimensional confined countercurrent shear layer driven by momentum flows, without the use of suction. Distinct peaks are observed in the frequency spectrum of velocity fluctuations, suggesting the presence of global instabilities. The variation of these frequencies with injected mass flow rate is predicted reasonably well by a spatiotemporal linear stability analysis of local velocity profiles, which checks for the presence of absolute instability, and suggests that confinement destabilizes the shear layer over a broad range of ratio of shear layer thickness to channel width.

Numerical Analysis of Liquid–Liquid Heat Pipe Heat Exchanger Based on a Novel Model

Abstract: Heat pipe heat exchangers (HPHEXs) are widely used in various industries. In this paper, a novel model of a liquid–liquid heat pipe heat exchanger in a countercurrent manner is established by considering the evaporation and condensation thermal resistances inside the heat pipes (HPs). The discrete method is added to the HPHEX model to determine the thermal resistances of the HPs and the temperature change trend of the heat transfer fluid in the HPHEX. The established model is verified by the HPHEX structure and experimental data in the existing literature and demonstrates numerical results that agree with the experimental data to within a 5% error. With the current model, the investigation compares the effectiveness and minimum vapor temperature of the HPHEX with three types of HP diameters, different mass flow rates, and different H* values. For HPs with a diameter of 36 mm, the effectiveness of each is improved by about 0.018 to 0.029 compared to HPs with a diameter of 28 mm. The results show that the current model can predict the temperature change trend of the HPHEX well; in addition, the effects of different structures on the effectiveness and minimum vapor temperature are obtained, which improve the performance of the HPHEX.

Analysis of tradeoffs between purification factor and yield for high-performance countercurrent membrane purification for protein separations.

Abstract: High-performance countercurrent membrane purification (HPCMP) has recently been presented as a new approach for protein separations, exploiting differences in diffusive transport across a semipermeable membrane to achieve high selectivity for protein separations. This study presents a set of design equations and diagrams that describe the tradeoff between the yield and purification factor in HPCMP processes in terms of two parametric variables: the diffusive membrane selectivity and the ratio of the draw to bulk solution flow rates. Conditions are identified that provide the high yields and purification factors of interest in bioprocessing. In addition, hydrodynamic models for solute transport were used to evaluate the selectivity as a function of the membrane pore size distribution for purely size-based separations. Model calculations demonstrate that diffusive transport provides significantly greater selectivity than traditional pressure-driven membrane separations for the same pore size distribution due to differences in hindered transport rates for diffusion and convection. These results provide a framework that can be used for the development of HPCMP processes for highly selective protein separations.

Mathematical Model of Maximum Commutation Half Cycle for Thermal Countercurrent Oxidation of Low-Concentration Gas in Coal Mine Ventilation

Abstract: The Fluent computational fluid dynamics software was used to study the relevant factors affecting the maximum commutation half cycle for thermal countercurrent oxidation of low-concentration gas in coal mine ventilation. Based on orthogonal experiments, the maximum commutation half cycle for thermal countercurrent oxidation of the exhaust gas in the coal mine ventilation under 25 working conditions with the combination of different methane concentrations, inlet speeds, porosities, and oxidation bed filling lengths is investigated. SPSS data processing software was used to perform regression analysis on the numerical simulation data, and a mathematical model for predicting the maximum commutation half cycle under the influence of four factors was obtained. Through experiments, the mathematical model of the maximum commutation half cycle by the numerical simulation was verified. After introducing the wall heat loss correction coefficient, the complete prediction model of the maximum commutation half cycle was obtained. Comparing the experimental test value with the calculated value using the corrected model, the relative error was not more than 3%. The complete mathematical model corrected can be applied to the design calculation of the maximum commutation half cycle for thermal countercurrent oxidation of low-concentration gas in actual coal mine ventilation.

Mass transfer analysis and kinetic modeling for process design of countercurrent membrane supported reactive extraction of carboxylic acids

Abstract: Countercurrent membrane supported reactive extraction (MSRE) was studied for removal of carboxylic acids from aqueous streams with a PTFE capillary membrane. Analysis of the mass transfer rates was performed to support modeling of the process. Total mass transfer coefficients ranging from 2.0·10-7 to 4.0·10-7 m/s were obtained when extracting lactic acid with 20 wt% tri-N-octyl amine in 1-decanol with membrane thicknesses of 260 µm and 80 µm. The limiting mass transfer resistance in all experiments was in the membrane phase. The developed model based on mass transfer and reaction in parallel allows to predict countercurrent extraction. Experimental validation with 5, 7 and 12 m long membrane modules showed excellent accordance for two acids, validating the model simulations. Simulated membrane contactor lengths required for single, two and three countercurrent stages varied between 10 and 39 m/stage for lactic, mandelic, succinic, itaconic and citric acid, depending on acid, membrane, and diluent.

Modeling and Simulation of Either Co-Current or Countercurrent Operated Reverse-Osmosis-Based Air Water Generator

Abstract: Technologies for obtaining drinkable water are becoming more important as global water consumption steadily increases and climate change progresses. One possibility for obtaining water is the extraction of water vapor from ambient air by means of air water generators (AWG). Previous studies in the field of AWG have mainly dealt with the condensation of humidity on cold surfaces with a cooling system or with absorption and thermal desorption. In this paper, another possibility for AWG is investigated, specifically AWG using absorption and reverse osmosis. For this purpose, models have been set up for an absorber operated in countercurrent and reverse osmosis membrane modules operated in co-current and countercurrent. With these models, simulations with different boundary conditions were then carried out using the programming language Python. The simulations have shown that the reverse osmosis membrane modules operated in countercurrent generally have a lower energy demand and require fewer reverse osmosis stages than those operated in co-current.

Numerical Simulation Study on the Air/Water Counter- current Flow Limitation in Nuclear Reactors

Abstract: After a loss-of-coolant accident (LOCA) in a Pressurized Water Reactor (PWR), the temperature of the fuel elements cladding increases dramatically due to the heat produced by the fission products decay, which is not adequately removed by the vapor contained in the core. In order to avoid this sharp rise in temperature and consequent melting of the core, the Emergency Core Cooling System is activated. This system initially injects borated water from accumulator tanks of the reactor through the inlet pipe (cold leg) and the outlet pipe (hot leg), or through the cold leg only, depending on the plant manufacturer. Some manufacturers add to this, direct injection into the upper plenum of the reactor. The penetration of water into the reactor core is a complex thermofluidodynamic process because it involves the mixing of water with the vapor contained in the reactor, added to that generated in the contact of the water with the still hot surfaces in various geometries. In some critical locations, the vapor flowing in the opposite direction of the water can control the penetration of this into the core. This phenomenon is known as Countercurrent Flow Limitation (CCFL) or Flooding, and it are characterized by the control that a gas exerts in the liquid flow in the opposite direction. This work presents a proposal to use a CFD to simulate the CCFL phenomenon. Numerical computing can provide important information and data that is difficult or expensive to measure or test experimentally. Given the importance of computational science today, it can be considered a third and independent branch of science on an equal footing with the theoretical and experimental sciences.

Modified methodology for determining the temperature profiles of inverted u-tubes steam generators used in pwr power plants

Abstract: The most common and reliable methodology for determining temperature profiles of Inverted U-tubes Steam Generators is using Computational Fluid Dynamics (CFD) programs. In this work, we developed a modified methodology, using the Wolfram Mathematica software, in order to determine, with good approximation, the temperature profiles of these kind of equipment. The first step was to determine expressions for the physical properties of the water in the operational conditions, like density, thermal conductivity, specific heat and dynamic viscosity. Geometrical parameters like tubes diameter and sub-channel flowing area, as well as the flow parameters like flow mass of primary and secondary fluid, were also considered for determining the numbers of Reynolds, Prandtl, Nusselt and, consequently, the variation of convective coefficients and the global heat transfer coefficient. With subroutines that use the method of the lines we were able to solve the partial differential equations applied to parallel and countercurrent heat exchangers with no phase change. The U-tubes SG were divided in two regions which the first one was calculated considering a parallel heat exchanger and the second one was calculated considering a countercurrent heat exchanger, depending on the flow direction of the primary and secondary circuit. During the phase change, a constant variation of the enthalpy was considered, making the primary fluid temperature decrease following a linear behavior. Using the developed methodology called “Enthalpy Ruler”, the encountered results were considered adequate, since the defined lengths are compatible with the constant variation of the enthalpy from the compressed liquid to saturated steam.

Influence of counterdiffusion effects on mass transfer coefficients in stirred tank reactors

Abstract: Accurate knowledge of volumetric oxygen mass transfer coefficients in multiphase systems is important for successfully designing and efficient operation of process plants. As a result, a large number of papers on the determination of volumetric mass transfer coefficients k L a have been published in the literature. The dynamic degassing method is probably the most common method for determining the volumetric mass transfer coefficient. However, little work is known on the influence of countercurrent diffusion effects, arising from further dissolved components, on the volumetric mass transfer coefficient. For this reason, the effect of countercurrent diffusion is investigated in the present work by using different stripping gases to determine the volumetric oxygen mass transfer coefficient. Furthermore, the effect of counterdiffusion is investigated for both conventional aeration and aeration with microbubbles. The present work shows that the choice of stripping gas has a decisive influence on the determination of the volumetric oxygen mass transfer coefficient. Moreover, it can be shown that this effect is directly proportional to the solubility of the stripping gas in the aqueous phase. Simultaneous measurements of bubble size distributions allow determining mass transfer coefficients k L . A model is developed describing the decrease in the mass transfer coefficient as a function of the solubility of the stripping gas. Furthermore, it can be shown that for the experimental determination of volumetric oxygen mass transfer coefficients, the choice of stripping gas should be adapted to secondary gas types occurring within real processes achieving a better comparability.