Liquid Volumetric Fluxes Research Articles

Surface coverage ratio of contaminated Taylor bubbles in a square microchannel

Experiments on contaminated Taylor flows in a microchannel were carried out to investigate the surface coverage ratio for different surfactant properties. Nitrogen and water were used for the gas and liquid phases, respectively. Triton X-100, sodium dodecyl sulfate (SDS) of 90% purity and SDS of 99% purity were used as surfactant. The cross section of the microchannel was square and its width was 200μm. The gas and liquid volumetric fluxes were 0.54 and 0.50 m/s, respectively. Taylor flows were formed by mixing the two phases at the T-junction. Shapes of Taylor bubbles were observed using a microscope and a high-speed video camera. The liquid film thickness changed in the film region, demonstrating that a steep gradient of the interfacial surfactant concentration was formed there. The interface was therefore separated into the clean interface region and the immobilized region by detecting the change in the liquid film thickness. The surface coverage ratio estimated from bubble shapes agreed with that measured from the reduction in the bulk concentration due to surfactant separation by bubbles. The surface coverage ratio increased with increasing the bulk concentration and the increasing rate was smaller than that for the adsorption–desorption equilibrium because of the presence of interfacial advection. The data, however, could be fitted by modifying the functional form for the ad-desorption equilibrium. The bulk concentration normalized by the concentration for the breakdown of the logarithmic relation between the surface tension and the concentration given by the isotherm worked for correlating the surface coverage ratio for the three different surfactants, which can be understood from the order of magnitude of the Marangoni stress.

Effects of liquid viscosity on interfacial and wall friction factors of swirling annular flows in a vertical pipe

Experiments of gas–liquid swirling annular flows in a vertical pipe of diameter D = 30 mm were carried out to obtain interfacial and wall friction factors, fi and fw, based on a three-fluid model. The liquid phase was two kinds of glycerol-water solutions with the kinematic viscosity νL of two or four times larger than that of water. Experimental data were obtained in two axial sections with short and long axial distances from the swirler, which corresponds to swirling and non-swirling annular flow regimes, respectively. Both the azimuthal and axial components of interfacial velocity decayed with increasing νL. The interfacial swirl number si, which is the ratio of the azimuthal component to the axial component of interfacial velocity, was not affected by νL for z*(=z/D) ≤ 3.8, where z is the axial coordinate in the test section. The mean interfacial swirl number si∗, which is the arithmetic mean of si in z* ≤ 3.8, was expressed in terms of dimensionless numbers based on the gas and liquid volumetric fluxes without νL. The interfacial friction factor decreased with increasing νL in swirling flows. The functional form of the Colebrook equation had a prospect of correlating fi in both swirling and non-swirling annular flows. The wall friction factor ratio of fw in the swirling annular flow regime to that in the non-swirling annular flow regime decreased as νL increased for si∗ > 0.33. The wall friction factor ratio was well correlated in terms of si∗ and νL.

Effects of swirl intensity on interfacial and wall friction factors of annular flows in a vertical pipe

Interfacial and wall friction factors, fi and fw, of air-water swirling annular flows in a vertical pipe of diameter D = 30 mm were measured in three sections of L* (=L/D) = 7.3, 13 and 31, where L is the distance from a swirler. The gas and liquid volumetric fluxes, JG and JL, were 12.5 ≤ JG ≤ 20.0 m/s and 0.071 ≤ JL ≤ 0.213 m/s, respectively. The interfacial swirl number, si, defined by Viθ/Viz, where Viθ and Viz are the azimuthal and axial components of interfacial velocity, was introduced as an indicator of swirl intensity. For z* (=z/D) ≤ 7, where z is the axial coordinate in the test section, the si increased with z* at low JG and JL, and slightly decreased with z* at high JG and JL. The si decreased for z* > 7 and reached zero at z* ≈ 30. The fw estimated using the three-fluid model was larger than that estimated using the two-fluid model since the droplet flow rate was not negligible even in swirling annular flows. The three-fluid model was therefore more appropriate for evaluation of fi and fw than the two-fluid model. The ratio fw∗ increased with s¯i for s¯i > 0.25, where fw∗ is the ratio of fw to that of non-swirling flows and s¯i is the averaged value of si in the section. The ratio fi∗ increased with s¯i for s¯i > 0.18, wherefi∗ is the ratio of fi to that of non-swirling flows. Correlations in terms of s¯i have prospect to evaluate the fi and fw in swirling annular flows.

Liquid film thickness of upward air–water annular flow after passing through 90° bend

Axial development of the base film thickness of an upward air–water annular flow after passing through a 90° bend was investigated. The pipe diameter D of the channel was 5.0 mm. The ratios, R*, of the radius of curvature of a bend to D were 6.0 and 10. The dimensionless axial distances, z* (=z/D), of measurement positions were 4, 12 and 20, where z is the axial coordinate in the vertical section. The circumferential measurement positions, θ, from the outer side to the inner side of the bend at each z* were 0, 45, 90, 135 and 180°. The liquid film thicknesses on the top and bottom sides in the horizontal section were also measured. At z* = 4, the base film at θ = 0° was thicker than that at the other θ in most of the present experimental conditions. In contrast, the circumferential distribution of the base film thickness was symmetric even at z* = 4 in the cases of high gas volumetric flux and low liquid volumetric flux. The circumferential distributions of the base film thicknesses at z* = 12 and 20 were symmetric in all experimental conditions. At z* = 12 and 20, the circumferential averaged base film thickness after passing through the bend with R* = 10 was larger than that after passing through the bend with R* = 6. The axial velocity in the liquid film was also measured. The circumferential distribution of the axial velocity was nearly symmetric at z* = 12 and 20.

Euler-Euler numerical simulations of upward turbulent bubbly flows in vertical pipes with low-Reynolds-number model

In this work, numerical simulations of upward turbulent bubbly flows in vertical pipes are conducted in the Euler-Euler framework with a low-Reynolds-number (LRN) model for liquid. It is found that the existing correlation for the drag coefficient of a single bubble in a shear flow, which has been successfully used along with the high-Reynolds-number (HRN) models, cannot be used with the LRN model. The reason is that drag forces of bubbles calculated using such correlation are enormous in near-wall cells (order of [Formula: see text]), which causes a divergence of numerical simulations with LRN. Therefore, a modified correlation for the drag coefficient of a single bubble in shear flow, that can be used successfully with the LRN model, has been proposed. The results of numerical simulations, performed with a new correlation for the drag coefficient, are analysed and compared to selected experimental measurements for different pipe diameters and different flow conditions of gas and liquid. It is shown that the largest effect of the application of the new correlation for the drag coefficient of a single bubble in shear flow in numerical simulations can be achieved on the reduction of the relative velocity between gas and liquid. The degree of this reduction depends on the pipe diameter and liquid volumetric flux.

Circumferential incoherent distribution of film thickness for multi-dimensional two-phase annular flow

Annular flow in rod bundle geometry is characterized by multi-dimensional properties in terms of two distinct length scales, rendering no consensus yet on an available model for predicting the circumferential incoherent distribution of film thickness. Current paper presents a semi-empirical but original correlation based on pertinent underlying physics. Gas phase Reynolds number is used to illustrate the gas–liquid interfacial shear-off mechanism. Liquid source term is scaled by the combination of void fraction and liquid volumetric flux. Circumferential shear stress produced by secondary flow is employed to denote mass transport in subchannel. Four available datasets including 189 data points are collected to develop and validate the proposed correlation from both cross-sectional and subchannel local regions perspectives. The majority of data can be correlated with the maximum relative error of ± 20.0%. Based upon the heuristic correlation, the circumferential incoherent distribution of film thickness is analyzed. The average film thickness on center rod increases slightly as the liquid velocity increased, since most of liquid phase migrates to corner region leading to a dramatically increase in film thickness on rod nearby duct wall. Two dependent factors are mainly responsible for the rod peripheral distribution of liquid thickness, in which the hydrodynamic effect dominates over the secondary flow factor, but the secondary flow makes the peripheral distribution of film thickness more uniform. Among the hydrodynamic parameters, both void fraction and gas velocity contribute to the thinning of the liquid film, but the increasing of liquid thickness along with rod periphery is attributed to the variation of subchannel characteristic length.

Application of a novel model for bubble-induced turbulence to bubbly flows in containers and vertical pipes

The baseline model for the CFD-simulation of turbulent bubbly flows in the Euler-Euler framework was proposed several years ago. It defines a fixed set of closures including all constants to avoid case by case tuning and to have a basis for sustainable model development. The present paper extends the baseline model by improving the modelling of bubble-induced turbulence. The closure terms in the transport equations of the k-ω SST model are revisited and replaced with the model recently proposed by Ma et al. (Ma et al., 2017. Physical Review Fluids 2, 034301) which is based on an analysis of the turbulent kinetic energy budget obtained from direct numerical simulation data. Detailed validation results for various flow configurations with a wide range of gas and liquid volumetric fluxes are presented. In case of vertical pipe flow significant improvements in the predicted gas volume fraction and velocity profiles are obtained, especially in high gas volume fraction cases where bubble-induced turbulence is dominant. Simulations of other configurations, such as uniform and non-uniform bubble columns, show that the model of Ma et al. (2017) results in an also for these cases overall improvement. Therefore, the baseline model is now updated to include the new model for bubble-induced turbulence.

Prediction of rod film thickness of vertical upward co-current adiabatic flow in rod bundle

Prediction of rod film thickness and its distribution in annular flow in rod bundle is of essential importance to nuclear power industry. This is attributed to the possible film dryout on fuel rods under certain conditions, which will result in a drastically increasing cladding temperature and possible fuel damage. Though there exist a lot of models or correlations to predict film thickness for annular flow in pipes, there are few models to predict rod film thickness inside rod bundle. The one-dimensional models or correlations for pipe flow cannot be used for rod bundle directly due to different geometry and flow dynamics. In relation to this, a new approach to predict the rod film thickness inside rod bundle has been developed. This approach first divides the rod bundle cross-section into three regions according to their characteristic length scales. Then, the gas and liquid flow distribution inside rod bundle has been given with reasonable assumptions. Based on the flow distribution profile, the sub-channel area-averaged gas and liquid volumetric fluxes have been given. Finally, the correlation developed for pipe flow is adopted for each sub-channel based on the local two-phase parameters in the rod bundle. Compared to experimental data obtained in-house and found in the literature, the newly proposed approach can predict rod film thickness with satisfactory accuracy.

Analysis of LES-based combustion models applied to an acetone turbulent spray flame

ABSTRACTTwo different combustion models are analyzed for the prediction of an acetone turbulent diluted spray flame. Simulations are conducted in the Large Eddy Simulation (LES) framework, coupled with the Flamelet Generated Manifold (FGM) chemistry reduction method. To represent the polydispersed spray the Eulerian-Lagrangian specification is applied. Both combustion models consist of the Artificially Thickened Flame (ATF) and the presumed PDF approach. Effects of the evaporative cooling and the presence of droplets into the combustion modeling are accounted for. Results achieved with both models are validated against experimental data. These consist in statistical data of droplets velocities, liquid volumetric flux, a characteristic diameter, and temperature. A general good agreement with experimental data is observed. Analysis of simulations results allow deeper interpretation of additional flame features, for instance the double flame structure. As an outcome, the concept of the burning potential is introduced in this paper to assist the interpretation of the underlying mechanisms to the occurrence of different flame modes.

Countercurrent flow limitation in a pressurizer surge line

In order to clarify the characteristics of countercurrent flow limitation (CCFL) in a pressurizer surge line, CCFL in a vertical pipe with a large diameter was evaluated using previous experimental data and a one-region model with an existing correlation of interfacial shear stress. The CCFL is defined by the relationship between the gas and liquid volumetric fluxes under a quasi-steady state after onset of flooding. A correlation for CCFL at the sharp-edged upper end of vertical pipes was constructed and it was compared to CCFL in the slightly inclined pipe. The comparison indicated that the liquid volumetric flux at the sharp-edged upper end was lower than that in a slightly inclined pipe when they have a large diameter such for an actual-scale pressurizer surge line. However, it was found that the liquid volumetric flux in integral experiments (including a slightly inclined pipe, a vertical elbow, and a vertical pipe) was lower than that in slightly inclined pipes and vertical pipes. Considering system characteristics from experimental data for the pressurizer surge line, it was concluded that the Kutateladze parameter should be applied with the slope of m = 0.94 and the constant of CK = 1.24 ± 0.1 for the Wallis type CCFL correlation.

A consistent Artificially Thickened Flame approach for spray combustion using LES and the FGM chemistry reduction method: Validation in Lean Partially Pre-vaporized flames

A consistent modeling approach in the context of the Artificially Thickened Flame (ATF) method is proposed for the prediction of turbulent Lean Partially Pre-vaporized (LPP) spray flames. Large Eddy Simulations (LES) are carried out in an Eulerian–Lagrangian framework to represent the turbulent two-phase flow. The chemical reactions are described with the Flamelet Generated Manifold (FGM) chemistry reduction method. Various aspects of the thickening of the scalar fields in the frame of reacting two-phase flows are systematically investigated relying on simplified numerical test cases. A correction for the solution of the dispersed phase equations for droplets interacting with the flame is successfully validated for different scenarios. The influences of the presence of evaporating droplets on the reaction development are carefully analyzed along with the adopted FGM database. At this stage a new theoretical expression to predict the laminar spray flame propagation speed is derived. Enhancements on the thickening procedure could be achieved with the inclusion of mixture quality aspects. Finally, the complete designed model is applied to a turbulent LPP burner for three different operating conditions. Results of the droplets velocity, characteristic diameters, liquid volumetric fluxes and reaction progress variable are compared with experimental data. Further findings regarding the mixture stratification, the impact of the SGS wrinkling modeling and the thickening onto the flame structure as well as onto the dispersed phase behavior are also addressed.

鉛直管内における気液対向流制限

Characteristics of counter-current flow limitation (CCFL) inside vertical pipes with sharp-edged upper end (CCFL-P/SE) and with round-edged upper and lower ends (CCFL-P) were evaluated to clarify the dependence of the limiting locations. The CCFL is defined by the relationship between the gas and liquid volumetric fluxes (JG and JL) under a quasi-steady state after onset of flooding. CCFL correlations were constructed for CCFL-P/SE and CCFL-P based on the previous experimental data. The correlations were compared to the characteristics of CCFL at the vertical pipes of the sharp-edged upper end (CCFL-U) and the sharp-edged lower end (CCFL-L). CCFL-P has the highest liquid volumetric flux when CCFL occurs, and the lowest is CCFL-L. For the Wallis correlation, selection of the characteristic length of w = D(1-β)Lβ (where D and L respectively denote the diameter and the Laplace capillary length) is quite important. Our results have indicated that CCFL-P/SE and CCFL-P are also expressed by β ≈ 0.5, CCFL-U is expressed by β = 1, and CCFL-L is expressed by β = 0.

加圧溶解法により生成した微細気泡の気泡径・数密度

Effects of liquid volumetric flux JL and flow pattern in a decompression nozzle on the mean diameter and number density of micro-bubbles generated by using a pressurized dissolution method are experimentally investigated. The bubble diameter and number density are measured by using a phase Doppler anemometry at the exit of the decompression nozzle. The flow pattern in the decompression nozzle was observed by using a high speed camera. The experimental results indicate that the diameter and number density strongly depends on the flow pattern in the decompression nozzle, and their characteristics are classified into three regimes: (1) few bubbles are generated when no cavitation occurs, (2) when bubble cavitation takes place, the mean bubble diameter keeps low values which do not depend on JL so much and the bubble number density increases with JL, and (3) the mean bubble diameter increases with JL whereas the number density does not depend on JL when flow pattern in the nozzle is sheet cavitation.

Air-lift pumps characteristics under two-phase flow conditions

Air-lift pumps are finding increasing use where pump reliability and low maintenance are required, where corrosive, abrasive, or radioactive fluids in nuclear applications must be handled and when a compressed air is readily available as a source of a renewable energy for water pumping applications. The objective of the present study is to evaluate the performance of a pump under predetermined operating conditions and to optimize the related parameters. For this purpose, an air-lift pump was designed and tested. Experiments were performed for nine submergence ratios, and three risers of different lengths with different air injection pressures. Moreover, the pump was tested under different two-phase flow patterns. A theoretical model is proposed in this study taking into account the flow patterns at the best efficiency range where the pump is operated. The present results showed that the pump capacity and efficiency are functions of the air mass flow rate, submergence ratio, and riser pipe length. The best efficiency range of the air-lift pumps operation was found to be in the slug and slug-churn flow regimes. The proposed model has been compared with experimental data and the most cited models available. The proposed model is in good agreement with experimental results and found to predict the liquid volumetric flux for different flow patterns including bubbly, slug and churn flow patterns.

A simplified model of gas–liquid two‐phase flow pattern transition

AbstractAn experiment of upward gas–liquid two‐phase flow was conducted in an air–water isothermal system under atmospheric pressure. The differential pressure was measured at the fully developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high‐speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one‐dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas‐component advection based on the spatiotemporal void fraction behaviors by considering the gas compressibility, the wake, and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple‐like, rectangular, distorted rectangular, and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns corresponded well to the experimentally observed flow patterns. The transitions among void wave patterns agree well with the Mishima–Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 445–461, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20029

Gas–liquid two-phase flow patterns in a miniature square channel with a gas permeable sidewall

Characteristics of air–water two-phase flow patterns in a miniature square channel having a gas permeable sidewall were investigated experimentally. Water was fed into the channel from its entrance, while air was injected uniformly into the channel along the permeable sidewall. This configured two-phase flow problem is encountered in direct feed methanol fuel cells. Flow patterns in both vertical upward and horizontal flows were identified using a high-speed motion analyzer. The visualization shows that the typical flow pattern encountered in the conventional co-current gas–liquid two-phase flow, such as bubbly flow, plug flow, slug flow and annular flow were also observed in the present work. However, unlike the conventional co-current gas–liquid two-phase flow in a channel with gas and liquid uniformly entering from one of its ends, for the flow configuration considered in this work, the stratified flow and wavy flow were not found in horizontal flow. And a so-called “single layer bubbly flow” was found in vertical upward flow, which is characterized by a mono small-gas-bubble layer existing adjacent to the surface of the permeable sidewall with the reminding space occupied by the liquid phase. Four transitional flow patterns such as bubbly-plug flow, bubbly-slug flow, plug–slug flow, and slug-annular flow, were found to exist between the distinct flow patterns. Finally, the flow regime maps for various liquid volumetric fluxes are presented in terms of mass quality versus the volumetric flux of gas phase.

A Simplified Model of Gas-Liquid Two-Phase Flow Pattern Transition.

An experiment of upward gas–liquid two-phase flow was conducted in an air–water isothermal system under atmospheric pressure. The differential pressure was measured at the fully developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high-speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one-dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas-component advection based on the spatiotemporal void fraction behaviors by considering the gas compressibility, the wake, and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple-like, rectangular, distorted rectangular, and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns corresponded well to the experimentally observed flow patterns. The transitions among void wave patterns agree well with the Mishima–Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 445–461, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20029

전자기유량계를 이용한 기포 및 슬러그 유동 측정방법 연구

In order to investigate the characteristics of electromagnetic flowmeter in two-phase flow, an AC electromagnetic tlowmeter was designed and manufactured. In various tlow conditions, the signals and noises from the tlowmeter were obtained and analyzed by comparison with the observed tlow patterns with a high speed CCO camera. The experiment with the void simulators in which rod shaped non-conducting materialbr/ was used was carried out to investigate the efTect of the bubble position and the void fraction on the tlowmeter. Based on the results from the void simulator, two-phase tlow experiments encompassed from bubbly to slug tlow regime were conducted. The simple relation △UTP＝ △UTP(１ -a) was verified with measurements of the potential difference and the void fraction. Due to the lack of homogeneity in a real two-phase tlow, the discrepancy between the relation and the present measurement was sl ightly increased with void fraction and also liquid volumetric flux )

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.

The use of MRI to quantify multi-phase flow patterns and transitions: an application to horizontal slug flow

In this paper we discuss the current benefits and limitations of using magnetic resonance imaging (MRI) to examine multi-phase fluid flow patterns and transitions. The advantages and disadvantages are highlighted in the context of an ongoing collaborative research effort between the University of California, San Francisco (UCSF) School of Medicine MRI Center and the Department of Nuclear Engineering at Oregon State University (OSU). Of particular interest, are the MRI measurements of the liquid volumetric flux distribution, the void fraction and the interfacial area concentration for slug flow in a horizontal air–water system. The data presented herein is unique relative to the slug flow conditions examined and the method implemented for data acquisition. The special scanning sequences designed by UCSF were capable of imaging at repetition intervals as fast as 7 ms.