Countercurrent Flow Conditions Research Articles

Numerical Simulation of Countercurrent Gas-Liquid Flow in a PWR Hot Leg under Reflux Cooling

In reflux cooling, the steam generated in the reactor core and the water condensed in a steam generator form a countercurrent flow in a hot leg. In order to investigate flow patterns in the hot leg under countercurrent flow conditions, countercurrent air-water tests were previously conducted using a 1/15th scale model of a PWR hot leg. Numerical simulation results for the tests using a three-dimensional twofluid model in FLUENT6.3.26, implemented with an appropriate set of correlations for the gas-liquid interfacial friction, were in good agreement with the measured data. In the present study, further numerical simulations were carried out for a full-scale hot leg under PWR plant conditions to investigate the effects of pipe diameter and fluid properties. The predicted countercurrent flow limitation characteristics were well correlated with the Wallis parameters and agreed well with the measured data from the 1/15th scale air-water tests as well as the full-scale steam-water UPTF tests. The results indicate that the set of correlations for the gas-liquid interfacial friction can be utilized to simulate countercurrent flows in the full-scale PWR hot leg.

CFD Simulation of Hydrodynamics, Heat and Mass Transfer Simultaneously in Structured Packing

This paper describes the results of computational fluid dynamic modeling of hydrodynamics, heat and mass transfer simultaneously in Flexipac 1Y operated under a counter-current gas-liquid flow condition. The simulation was performed for a binary mixture of methanol-isopropanol distillation. The pressure drop, the height of equivalent to theoretical plate (HETP) and temperature distribution across the column were calculated and compared with experimental data. The mean absolute relative error (MARE) between CFD predictions and experimental data for the pressure drop, HETP and temperature profile are 20.7%, 12.9% and 2.8%, respectively.

Two-Dimensional Modeling of the Direct ECC Bypass in the APR1400 Downcomer

In a nuclear reactor vessel downcomer incorporating the safety feature of direct vessel injection (DVI), the direct bypass of emergency core coolant (ECC) is activated during the reflood phase of a large-break loss-of-coolant accident due to momentum transfer between the downward liquid film and transverse gas. Direct ECC bypass is reportedly the major bypass mechanism of ECC, and various experiments have been performed to obtain detailed information about the ECC bypass in a DVI downcomer. In the present study, a model of the direct ECC bypass was developed based on two-dimensional two-fluid equations for the adiabatic two-phase flow to predict the ECC bypass flow rate. The direct ECC bypass fractions were calculated with various interfacial friction factor correlations, and the results were compared with the available experimental data. The values predicted by the current model showed reasonably good agreement with the experimental data at bypass fractions >40% when applying the interfacial friction factor model developed in a countercurrent flow condition. However, when the bypass fraction was <40%, models incorporating cocurrent annular flow provided better results than those with countercurrent flow. These results suggest that a transition occurs from a smooth film to a rough film as the gas flow rate increases, and hence, interfacial friction factor models that adequately incorporate this transition are necessary to predict the direct ECC bypass phenomenon.

Poly(lactic‐co‐glycolic acid) hollow fibre membranes for use as a tissue engineering scaffold

Mass transfer limitations of scaffolds are currently hindering the development of 3-dimensional, clinically viable, tissue engineered constructs. We have developed a poly(lactide-co-glycolide) (PLGA) hollow fibre membrane scaffold that will provide support for cell culture, allow psuedovascularisation in vitro and provide channels for angiogenesis in vivo. We produced P(DL)LGA flat sheet membranes using 1, 4-dioxane and 1-methyl-2-pyrrolidinone (NMP) as solvents and water as the nonsolvent, and hollow fibre membranes, using NMP and water, by dry/wet- and wet-spinning. The resulting fibres had an outer diameter of 700 micro m and an inner diameter of 250 micro m with 0.2-1.0 micro m pores on the culture surface. It was shown that varying the air gap and temperature when spinning changed the morphology of the fibres. The introduction of a 50 mm air gap caused a dense skin of 5 micro m thick to form, compared to a skin of 0.5 micro m thick without an air gap. Spinning at 40 degrees C produced fibres with a more open central section in the wall that contained more, larger macrovoids compared to fibres spun at 20 degrees C. Culture of the immortalised osteogenic cell line 560pZIPv.neo (pZIP) was carried out on the P(DL)LGA flat sheets in static culture and in a P(DL)LGA hollow fibre bioreactor under counter-current flow conditions. Attachment and proliferation was statistically similar to tissue culture polystyrene on the flat sheets and was also successful in the hollow fibre bioreactor. The P(DL)LGA hollow fibres are a promising scaffold to address the size limitations currently seen in tissue engineered constructs.

Transport of uranium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor

Summary Membrane based non-dispersive solvent extraction (NDSX) of U(VI) from aqueous nitric acid medium was carried out using a commercial hollow fibre module containing microporous hydrophobic polypropylene capillaries. The NDSX operation was carried out by pumping U(VI) solutions at varying concentrations of nitric acid (1-5 M) through the tube side and organic solvent (di-n-hexyl octanamide (DHOA) in normal paraffin hydrocarbon (NPH)) through the shell side of the hollow fibre capillaries. Extraction studies were performed under different hydrodynamic conditions and the overall mass transfer was evaluated under counter-current flow conditions. Counter-current mode was found to be better as compared to the co-current mode. The U(VI) mass transfer rates were positively influenced by the nitric acid as well as DHOA concentrations.

Application of Hollow Fiber Contactor in Nondispersive Solvent Extraction of Pu(IV) by TBP

Data on dispersion‐free solvent extraction (DFSX) of Pu(IV) from acidic nitrate media using microporous hydrophobic polypropylene hollow fiber membrane contactor with tri‐n‐butylphosphate (TBP) employing as an extractant are being presented. The DFSX operation was carried out with various concentration of TBP in n‐dodecane by passing acidic feed containing Pu (IV) through the tube side at the flow rate of 5.83 cm3 s−1 and organic extractant through the shell side at the flow rate of 1.53 cm3s−1. Extraction studies were performed under different hydrodynamic conditions and the overall mass‐transfer coefficient was evaluated with countercurrent flow condition. It was possible to extract and concentrate Pu(IV) from aqueous phase by employing this technique. For back extracting the Pu, uranous nitrate and hydroxylamine hydrochloride solution as strippants were examined, which flowed through the tube side (flow rate: 6.11 cm3 s−1) and the loaded organic was passed through the shell side with the flow rate of 1.66 cm3s−1. Results revealed that ∼80% of Pu(IV) from oxalate supernatant waste could be extracted by utilizing this technique.

Thermal model validation of plate heat exchangers with generalized configurations

Thermal models of plate heat exchangers rely on correlations for the evaluation of the convective heat transfer coefficients inside the channels. It is usual to configure the exchanger with one countercurrent single-pass arrangement for acquiring heat transfer experimental data. This type of configuration approaches the ideal case of pure countercurrent flow conditions, and therefore a simplified mathematical model can be used for parameter estimation. However, it is known that the results of parameter estimation depend on the selected exchanger configuration because the effects of flow maldistribution inside its channels are incorporated into the heat transfer coefficients. This work presents a parameter estimation procedure for plate heat exchangers that handles experimental data from multiple configurations. The procedure is tested with an Armfield FT-43 heat exchanger with flat plates and the parameter estimation results are compared to those obtained from the usual method of single-pass arrangements. It can be observed that the heat transfer correlations obtained for plate heat exchangers are intimately associated with the configuration(s) experimentally tested and the corresponding flow distribution pattern(s).

Dialyzer performance in the HEMO Study: in vivo K0A and true blood flow determined from a model of cross-dialyzer urea extraction.

Inlet and outlet blood urea concentrations (Cin and Cout) can be used to directly measure dialyzer performance if simultaneous blood flow measurements (Qb) are available. Dialyzer clearance, for example, is the product of the urea extraction ratio [ER = (Cin - Cout)/Cin] and Qb. Urea concentrations are measured routinely in all hemodialysis clinics, but Qb is usually reported as the product of the pump rotational speed and pump segment stroke volume, which can be inaccurate at high flow rates. Dialyzer urea extraction is also a function of Qb, dialysate flow (Qd), and the membrane permeability-area coefficient (K0A) for urea. To determine true in vivo values for Qb and K0A in the absence of direct flow measurements, we developed a model based on an existing mathematical equation for hemodialyzer ER under conditions of countercurrent flow. Qb, K0A, and other variables were adjusted to fit the modeled ER to ER measured in 1,285 patients treated with Qb that ranged from 200 to 450 ml/min during the HEMO Study. Fitting was performed by least squares nonlinear regression using parametric and nonparametric methods for estimating true flow. As Qb rose above 250 ml/min, both methods for estimating actual Qb showed increasing deviations from the flow reported by the blood pump meter. Modeled values for K0A differed significantly among dialyzer models, ranging from 71% to 96% of the in vitro values. The previously described 14% increase in K0A, as Qd increased in vitro from 500 to 800 ml/min, was much less in vivo, averaging only 5.5 +/- 1.5% higher. Dialyzer reprocessing was associated with a 6.3 +/- 1.0% reduction in K0A and an approximate 2% fall in urea clearance per 10 reuses (p < 0.001). Multiple regression analysis showed a small but significant dialysis center effect on ER but no independent effects of other variables, including the ultrafiltration rate, diabetic status, race, ethnicity, sex, method of reuse, treatment time, access recirculation, and use of central venous accesses. The new algorithm allowed a more accurate determination of true Qb and in vivo K0A in the absence of direct flow measurements in a large population treated with a wide range of blood flow rates. Application of this technique for more than 1000 patients in the HEMO Study confirmed that in vitro measurements using simple crystalloid solutions cannot readily substitute for in vivo measurements of dialyzer function, and permitted a more accurate calculation of each patient's prescribed dialysis dose and urea volume.

鉛直管内における気液対向流での蒸気・空気混合気体のリフラックス凝縮伝熱（層流条件）

鉛直管内における気液対向流での蒸気・空気混合気体のリフラックス凝縮伝熱（層流条件）

Experimental Study of the Impact of Boundary Conditions on Oil Recovery by Co-Current and Counter-Current Spontaneous Imbibition

Spontaneous imbibition (SI) is a very important oil recovery mechanism from low-permeability fractured reservoirs. The impact of different sample shapes and boundary conditions on oil recovery by counter-current and co-current SI into low-permeability (2 mD) strongly water-wet outcrop chalk have been investigated. The experimental program was divided into two parts. In part 1, counter-current SI was studied by submerging cubic, cylindrical, and irregular rock samples totally in the aqueous phase, whereas co-current SI was investigated in part 2 by exposing different portions of a cubic rock sample to an oil-phase. The scaling law of Ma et al. -1 was used to correlate counter-current SI data with all parameters than the characteristic length, L C , held essentially constant. The results confirm the ability for L C to account for cylindrical (diameters in the range from 2 to 10 cm) and cubic sample shapes and different no-flow surface boundary conditions (2, 4 and 5 faces were coated with polyester on the cubic rock samples; top and bottom faces on the cylindrical cores). Rock samples having irregular shapes (increasing and decreasing cross-sectional areas were investigated) could not be scaled according to L C because the overall shape of the imbibition curves deviated from curves generated by rock samples having regular shape. No scaling law correlating imbibition data for different boundary conditions exists for tests performed under co-current flow conditions; hence the results in part 2 of the experimental program are interpreted qualitatively. When using the SI rate for a cube with all faces open to imbibition as the reference (counter-current flow conditions), the results show that depending on the area fraction of the cube covered by oil, co-current SI can either be faster (water-oil area-ratio (WOAR) > 1), equal (WOAR = 1) or slower (WOAR < 1)) than the reference system. In all cases, ultimate recovery is significantly higher for tests performed under co-current flow conditions than under counter-current (70% of initially oil in place (IOIP) vs 58% IOIP for counter-current). The results indicate that when evaluating oil recovery potential on reservoir rock samples due to SI experimentally, tests should be performed both under co-current and counter-current flow conditions because the latter one might under certain circumstances predict too low rates and ultimate recoveries.

Wind and Stream Flow Induced Reaeration

Reaeration in combined wind/stream driven flows was experimentally studied in a laboratory wind-water tunnel. Wind velocities, stream flow properties, and oxygen transfer rates were measured under a variety of cocurrent and countercurrent flow conditions. For pure open channel flows and pure wind-driven flows, respectively, the results of oxygen transfer measurement can be scaled by the simple forcing parameters and show good agreement with other laboratory studies. For wind/stream combined flows, the transfer velocity depends on the stream flow condition and wind shear velocity. Based on the magnitude of the bottom-shear induced transfer and the wind-shear induced transfer velocity, a criterion was proposed to determine the dominant mechanism for gas transfer in combined wind/stream driven flows. A simple additive model is developed for the reaeration coefficient in combined wind and stream flow-influenced environments. Also, the effects of tidal flow on reaeration in tidal estuaries are briefly discussed.

Gas–liquid mass transfer in a 15 cm diameter reciprocating plate column

AbstractData are reported on the absorption of oxygen by water under countercurrent flow conditions, in a 15 cm diameter reciprocating plate bubble column. Amplitude (half stroke) is set at 12.7 mm and reciprocation frequencies up to 5 Hz have been employed. Five different types of plate have been compared; two of the types are multiperforated and three of the types are ‘donut’ plates with a single central hole. When the comparison is made in terms of the volumetric mass transfer coefficient (kla) and power input per unit mass, the best of the five types of plate is a multiperforated plate with small holes of diameter 6.35 mm.© 2003 Society of Chemical Industry

Effects of novel manufacturing technology on blood and dialysate flow distribution in a new low flux "alpha Polysulfone" hemodialyzer.

The main target for low flux hemodialyzers is an efficient low molecular weight solutes clearance. Such efficiency is largely dependent on the optimization of diffusion between blood and dialysis solution. The diffusion process can be impaired if there is a mismatch between blood and dialysate flow distribution in the dialyzer. Thus optimized flow distribution both in the blood and dialysate compartment becomes quintessential for the maximal efficiency of the diffusion process within the hemodialyzer. The present paper describes the distribution of the blood and dialysate flows in a new low flux polysulfone hollow fiber hemodialyzer characterized by a specific undulation of the fibers and a new cutting technology of the fibers for an improved micro-flow condition in the blood compartment headers. Twelve Diacap alpha Polysulfone LO PS 15 (1.5 sqm) (B. Braun Medizintechnologie, Melsungen Germany) were employed for the study. Six were analyzed in vitro and six were studied in vivo. Blood flow distribution was studied in vitro by dye injection in the blood compartment during experimental extracorporeal circulation utilizing human blood with hematocrit adjusted at 33%. Sequential images were obtained with a helical scanner in a fixed longitudinal section of the dialyzer 1 cm thick. Average and regional blood flow velocities were measured utilizing the reconstructed imaging sequence. The method allowed the calculation of single fiber blood flow (SF Qb) and the mass transfer zone (MTR) definition in digitally subtracted images. The patterns 20-10 and 40-30 were utilized. The same technology was used to evaluate flow distribution in the dialysate compartment after dye injection in the Hansen's connector. Regional dialysate flow was calculated in central and peripheral sample areas of 1 cm2. Six in vivo hemodialysis treatments on patients with end stage renal disease were performed at three different blood flow rates (250-350 and 450 ml/min) in order to measure urea, creatinine and phosphate clearance. Macroscopic and densitometrical analysis revealed that flow distribution was homogeneous in the blood compartment while in the dialysate compartment a slight difference between the peripheral and central regions in terms of flow velocity was observed. This however was not generating channeling phenomena. Urea creatinine and phosphate clearances were remarkably high and so were the Kt/V observed in all sessions, especially in relation to the studied blood flows. In conclusion, a significant blood to dialysate flow match with optimized countercurrent flow condition was observed in the studied hollow fiber hemodialyzers. Such optimization might be due both to the improved dialyzer design at the level of the blood header and to the specific fiber undulation that prevents dialysate channeling.

Capillary magnetophoresis of human blood cells and their magnetophoretic trapping in a flow system

The performance of a capillary magnetophoretic device was improved by enhancing the magnetic field gradient using a pair of small iron tips attached to the Nd–Fe–B magnets. The magnetophoretic intensity, B(d B/d x), was determined as a function of distance along the gap between the tips from the magnetophoretic velocity of a 3 μm polystyrene microparticle in 0.6 M manganese(II) chloride solution. The maximum intensity was increased 4.5 times by the attached iron pieces. The magnetophoresis of a single human blood cell in 0.1 M manganese(II) solution was studied by this method and its magnetic susceptibility was estimated. Magnetophoretic trapping of red blood cells was demonstrated under counter-current flow conditions in the capillary.

Dispersion-Free Solvent Extraction and Stripping of Gold Cyanide with LIX79 Using Hollow Fiber Contactors: Optimization and Modeling

This paper reports experimental data on hollow fiber dispersion-free membrane extraction (DFSX) of gold(I) from alkaline cyanide media using microporous hydrophobic polypropylene hollow fiber contactor. A recently developed new solvent extraction reagent, namely, LIX79 (Henkel Corporation), was employed as an extractant. The DFSX operation was carried out with 12% LIX79 in n-heptane by passing alkaline feed containing gold through the tube side and organic extractant through the shell side. Extraction studies were performed under different hydrodynamic conditions and mass correlations for the tube and shell side were presented for cocurrent and countercurrent flow conditions. A model is presented that identifies the different rate-controlling steps under various chemical and hydrodynamic experimental conditions in feed, organic, and membrane phases. The mass-transfer resistance resulting from a chemical extraction reaction at the surface was observed to be dominant during the entire extraction process. The validity of this model was evaluated with experimental data and found to tie in well with theoretical values. It was possible to separate and concentrate Au(I) (up to 400 mg/L) in the presence of other metal cyanide salts such as Fe(II), Cu(I), Ni(II), Ag(I), and Zn(II) (NaCN = 1000 mg/L) by employing this technique. The stripping of gold from LIX79 was accomplished efficiently by using NaOH, which flowed through the tube side (11.11 cm3/s) and the organic was passed through the shell side at (6.94 cm3/s).

Two-phase absorption complicated by a secondorder chemical reaction in the liquid phase in irrigated channels: The solution for the third quadrant of the hydrodynamic plane

Two-phase chemisorption in the liquid in vertical channels irrigated by a thin film of absorbent solution under the countercurrent flow conditions is investigated for the case that the process is complicated by an irreversible second-order chemical reaction. The problem is solved for the third quadrant of the plane of dimensionless hydrodynamic variables. A method for designing a contact device under arbitrary conditions is presented.

Pressure drops in gas—liquid flow in a 15 cm reciprocating platecolumn

AbstractTime‐averaged pressure drops have been measured in a 15 cm internal diameter reciprocating plate column, using the water/air system under conditions of cocurrent, countercurrent and single phase (water) flow. The reciprocation frequency was varied from 1 to 4 Hz, while the stroke (peak to trough amplitude) was set at 2.54 cm. Two types of perforated plate were used, having perforation diameters 14.3 and 6.35 mm.Under single phase flow conditions, the pressure drops were in agreement with an earlier model due to Noh and Baird (1984). Under cocurrent conditions the gas flow had the effect of increasing the pressure drop, particularly at low reciprocation rates; this was interpreted in terms of enhanced circulation velocities in the liquid phase. In countercurrent flow the pressure drop was also affected by bubbles clustering at the plates.

Limit conditions of steady-state countercurrent annular flow and the onset of flooding, with reference to the CHF of boiling in a bottom-closed vertical tube

An analytical study on the limit conditions of steady-state countercurrent annular flow in a vertical tube has been performed, aiming at the prediction of the onset of flooding in tubes where it is dilfficult for unstable wave motion to occur. A new expedient method is developed to determine the limit conditions under the involved flow conditions. It is revealed that the limit conditions in this flow system appear only when the interfacial shear stress increases with the liquid film thickness. A prediction model that takes the full effects of the void fraction ϵ into account, and a simplified model that considers only the effect of the liquid film thickness on the interfacial shear stress under a fixed condition of ϵ = 1 are dealt with, showing that they predict the experimental data fairly well at low liquid flow rates, and that the latter model performs better than the former one. Finally, there is a brief discussion as to the possibility of the application of tbe present analysis to the prediction of the critical heat flux of boiling in a bottom-closed vertical tube.

Limit Conditions of Steady-State Countercurrent Annular Flow and Onset of Flooding, with Referenceto CHF of Boiling in a Bottom-Closed Vertical Tube.

Limit conditions for the existence of steady-state countercurrent annular flow have been studied analytically, including laminar, transition, and turbulent states of both the core gas and liquid film flow. A new method is developed to determine the limit conditions effectively by computer, and it reveals that the major cause of the limit conditions is interfacial shear stress between the two fluids and its characteristics. Then, based on this finding, a non-α model, assuming no effects of the void fraction α, is introduced in addition to an a model taking the effect of a into consideration. The limit conditions thus predicted compare fairly well with experimental data regarding the CHF of boiling in a bottom-closed vertical tube as well as the onset of flooding.

Counterflow double-pipe heat exchanger analysis using a mixed lumped-differential formulation

An analysis is made of double-pipe heat exchangers under a thermally developing countercurrent flow condition. A lumped-differential mixed formulation is employed, by radially lumping the temperature field in the outer channel, which results in a more involved boundary condition for the inner differential system, involving the axially varying outer channel bulk temperature. The generalized integral transform technique is utilized to provide a reliable and straightforward analytical solution to this class of problems. Numerical results for heat transfer quantities are presented in terms of the dimensionless governing parameters along the thermal entry region, allowing for critical comparisons against the concurrent flow situation, limiting solutions and engineering-type correlations.