Gas-liquid Flow Ratio Research Articles

Insights into the bubble formation dynamics in converging shape microchannels using CLSVOF method

Abstract Bubble formation in a square microchannel having a converging shape merging junction has been studied using the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method. The influence of variations in merging junction angles, fluid properties, and operating conditions on the bubble length and pressure drop has been analyzed. The results show a direct relationship between surface tension, gas-liquid flow ratio, and the inverse relation of continuous phase viscosity with the bubble length. Moreover, opposite variations of these parameters are observed for pressure drop. This work reveals a discerning influence of the angle variations of merging junction on the interplay between inertial, viscous, and surface tension forces in the bubble formation mechanism. We envisage that this numerical work will be of significant interest for the process intensification in various industries that deal with gas-liquid microfluidic systems.

Control of pressurized microbubble generation by multi-channel membranes: Experiments and modeling

Microbubbles have been attracting a lot of attention in the fields of chemical and petrochemical engineering, environmental protection, medicine, and aquaculture etc. Herein, a controlled pressurized microbubble generation system is developed. Numerous microbubbles are generated under certain of pressure by multi-channel ceramic membranes in a surfactant-free system consisting of dispersed gaseous and continuous aqueous phases. The average diameter and size distribution of the microbubbles are significantly affected by the membrane pore size, membrane area, liquid viscosity, pressure and gas-liquid flow ratio. A mathematical model about the average diameter of microbubbles is established for the first time by directly introducing the membrane parameters including the membrane area and membrane pore size, and the error of the as-established model is less than 10%. More importantly, the model can be used to predict the optimal membrane pore size and membrane area under different operating conditions under the premise of achieving the expected bubble size. The work will aid the understanding and development of microbubble generation with high performance.

N‐shape pressure drop curve of gas–liquid microchannel reactor and modeling

AbstractAccurate prediction of gas–liquid pressure drop is essential to microreactors design; however, the understanding of general rules of pressure drop in a wide gas–liquid flow ratio, especially smaller than 1.0, remains insufficient, Accordingly, this work systematically studies the pressure drop rule within the gas–liquid flow ratio of 0.2–2.0. The results show that, under a given gas velocity, the pressure drop first increases and then decreases, and finally back increases with the liquid flow velocity, and the named N‐shape pressure drop curve could be clearly observed. Besides, the rules of gas–liquid unit length and gas‐phase holdup are explored to reveal the mechanism behind the N‐shape pressure drop curve. Finally, three semi‐empirical correlations based on the gas–liquid unit length and separated flow model are successfully proposed for the N‐shape pressure drop. This work provides some important fundamental information for the reliable design of gas–liquid microreactors.

Study on effect of gas-liquid two phase physical feature on slug flow in microchannels

At present, there are relatively few studies on the slug flow generation mode obtained by exchanging gas-liquid two-phase inlets. In this study, an experimental system combining microfluidic devices and high-speed cameras was used to study the effects of gas-liquid two-phase flow rate, liquid physical parameters, etc., On the characteristic length, generation period and other generation characteristics of slug flow, and dimensionless analysis was conducted to investigate the main factors affecting the characteristic length of gas slug. Results show that 1) when the gas flow rate affects the aeroelastic generation characteristics, the aeroelastic characteristic length increases from 443 μm when the gas flow rate increases changes to 657 μm. The generation period decreases rapidly at first and then the change amplitude slows down. The maximum value of aeroelastic generation frequency is 217 s-1; 2) when studying the effect of different liquid flow rates, increasing the liquid flow rate, the characteristic length of the gas bomb gradually decreases, and the generation period of the gas bomb gradually increases. Aeroelastic characteristic length from 770 μm changes to 378 μm. The range of aeroelastic generation cycle is 4–13.4 ms, and the maximum value of aeroelastic generation frequency is 250 s-1; 3) there is a functional relationship between the ratio of aeroelastic characteristic length to channel size L/d and dimensionless gas-liquid flow ratio Q*, Reynolds number Re, Weber number We: L/d=3.677Q*0.58/Re0.11.

Hydrodynamics and mass transfer performance of gas–liquid microflow in viscous liquids

Mass transfer performance is markedly improved in microchannels compared with traditional gas–liquid chemical devices. The residence time is a crucial parameter for determining the mass transfer performance. However, investigation of the residence time in gas–liquid absorption systems remains insufficient, especially in viscous gas–liquid systems with obvious flow non-uniformity between the two phases. Herein, the microscale gas–liquid flow and mass transfer performance in viscous liquids (9.0–55.4 mPa·s) based on the true residence time are investigated. Owing to the broad operating conditions of the gas–liquid flow ratio (0.2–6.7), both Taylor flow and bubbly flow patterns were observed. The results show that the bubble velocity gradually decreases to the minimum value with increasing flow distance in the Taylor flow pattern owing to gas absorption. Subsequently, bubble velocity gradually increases under the bubbly flow pattern, and the dominant factor of bubble velocity is pointed out. The true bubble residence time initially increases and then gradually decreases compared to the superficial time as the gas holdup decreases, based on the superficial velocity using the initial flow rates. The transition point occurs at a gas holdup of 0.2. In addition, the turning point of the different rules of the pressure drop occurs at a gas holdup of 0.5. Finally, a new dimensionless equation was proposed to predict the mass transfer coefficient and reveal the equation differences between low- and high-viscosity liquids. This work enriches the understanding of microflow characteristics and mass transfer performance in viscous solutions and provides a reliable guide for microreactor design.

Study of multiphase flow inside straight and spiral microchannel and effect of two phase flow on Dean’s vortices

A combined experimental and numerical simulation study of gas–liquid two-phase flow inside straight and spiral microchannels is presented. Dynamics of Taylor bubble formation was investigated in both types of channels and it was canvassed that presence of centrifugal force in spiral channel has consequential influence on the Taylor bubble dynamics. Effect of gas–liquid flow ratio and hydrophobicity of channel wall on the Taylor bubble formation was also investigated through numerical simulations. Formation of Dean's vortices inside spiral microchannel was shown via 3D CFD based simulations. Quantification of the strength of such vortices was done by calculating vorticity (ω) which is the curl of the velocity vector field. The effect of curvature on the formation of Dean's vortices was shown using Dean Number (De) and Dean velocity (VDe). The effect of two phase flow on the vorticity was also emphasized by presenting a comparative study with single phase flow. Presence of Dean's vortices inside such spiral micro-channel can be utilized in many futuristic micro-contrivances.

In silico screening of venturi designs and operational conditions for gas-liquid mass transfer applications

Venturi ejectors are commonly used to facilitate gas-liquid mass transfer, but their mass transfer characteristics depend on various operational and ejector design parameters, which make experimental identification of optimal conditions cumbersome. A CFD model using a two-phase Eulerian approach with a free surface model to describe local venturi hydrodynamics was developed and used to characterize the influence of gas-liquid flow ratio and venturi design on venturi-mediated gas-liquid mass transfer in bulk liquid. When standardized to gas injection rate, computed venturi-local gas-liquid interfacial area showed same dependency on gas injection rate as experimentally determined kLa during aeration of a 1 m3 tank. The model consequently enables in silico characterization of the venturi flow regime, which is critical to the venturi’s mass transfer performance. An experimental factorial design study investigated the influence of venturi geometry on gas-liquid mass transfer and found that the venturi throat diameter is a critical design parameter. The computational model revealed that this was due to its significant impact on generated gas-liquid interfacial area. The ability to characterize venturi flow regimes and qualitatively predict the relative effect of process parameters on kLa by modelling local venturi dynamics rather than the complete reactor system greatly reduces computational complexity, suggesting the model’s usage as an efficient screening methodology to increase venturi gas-liquid mass transfer performance.

Numerical research on bubble formation process in microchannel using diffuse Interface method

Gas-liquid two-phase flow is an important research project in microfluidics. The mechanism of bubble formation still needs more study to control the bubble’s size precisely. As the radius of bubble is about 0.1 mm to 0.6 mm in the simulation, the energy used to overcome the surface tension can’t be neglected. By using the Cahn-Hilliard equation, the diffuse interface method can not only track the fluid interface, but also have a reasonable reduction of the total energy of the system. So the formation of bubble in microchannel is simulated based on the diffuse interface method in this paper. At first, we compare the numerical results with the experiments and find that they agree well with experiments. It indicates that this method can be used to simulate the bubble formation process in the microchannel. Then, the influence of flow rate, gas-liquid flow ratio, surface tension, viscosity, the entrance length and width of the main channel are discussed. The results show that the pressure difference is the main reason for the bubble formation and the change of liquid-gas flow ratio and liquid flow rate have great effects on the formation process. In addition, smaller surface tension can cause the inlet liquid pressure decrease and reduce expansion time to accelerate the formation process. And greater shear force leads to a long bubble neck when the tip of gas phase don’t fill the main channel. Eventually, our work also reveal that bubbles get bigger in a logarithmic tendency with the entrance of main channel widening.

Spraying parameter optimization and microtopography evaluation in nanofluid minimum quantity lubrication grinding

To overcome environmental problems, nanofluid minimum quantity lubrication (NMQL) has been utilized as a burgeoning alternative technique to conventional flood machining, which consumes a large quantity of cutting fluid. Furthermore, spraying parameter optimization is a prerequisite of an effective NMQL grinding and a key to achieving a new sustainable grinding process. However, the spraying parameters of NMQL are multifarious and have varying degrees of influence on lubri-cooling performance. The main objective of this work is to first obtain good spraying parameters in NMQL grinding through the analysis of signal-to-noise ratio and analysis of variance based on orthogonal experiment results. Furthermore, an experimental verification is carried out on the basis of the relative optimization of several groups of spraying parameters from the orthogonal experiments. Power spectral density function (PSDF) and energy spectrum are used to analyze the workpiece and debris microtopography. Consequently, the optimal spraying parameter is obtained. The spraying parameter (palm oil as the base oil, 2 vol% nanoparticle concentration, 0.6 MPa air pressure, and 0.4 gas–liquid flow ratio) achieved the lowest proportionality coefficient (0.245) of PSDF in the low-frequency band and the highest (0.142) in the high-frequency band. Meanwhile, the optimal grinding performance is validated by the workpiece and debris microtopography and long-strip debris morphology.

Hydrothermal gasification of sorbitol: H2 optimisation at high carbon gasification efficiencies

Using both experiments and modelling, hydrothermal gasification of sorbitol (SB) aiming at maximal carbon to gas conversion and H2 production was investigated over a wide temperature range (270–350 °C). Kinetics were studied in a continuous tubular reactor using a Pt/γ-Al2O3 catalyst. The addition of N2, resulting in lower H2 concentrations in the liquid phase, was found to have a beneficial effect in terms of higher H2 yield without compromising on the carbon gasification. The highest H2 yield obtained in this work was 4 mol H2/mol SB. Existing reaction schemes for sorbitol gasification were used to derive a path-lumped scheme. A multi-phase reactor model including a path-lumped scheme and gas-liquid-solid mass transfer was developed and parameterized based on datasets with varying temperature, space velocity, inlet gas composition (N2 or H2) and gas-liquid flow ratio. The developed model was used to provide guidelines for the design of an industrial reactor for the gasification of 10 tons/h of 10 wt% aqueous sorbitol. The effect of N2 stripping and industrially attainable kLa values were found to boost the H2 yield from 4 to 12 mol H2/mol SB making it an attractive process for further consideration.

Evaluation of sulfur trioxide detection with online isopropanol absorption method

Measurement of the SO3 concentration in flue gas is important to estimate the acid dew point and to control corrosion of downstream equipment. SO3 measurement is a difficult question since SO3 is a highly reactive gas, and its concentration is generally two orders of magnitude lower than the SO2 concentration. The SO3 concentration can be measured online by the isopropanol absorption method; however, the reliability of the test results is relatively low. This work aims to find the error sources and to evaluate the extent of influence of each factor on the measurement results. The test results from a SO3 analyzer showed that the measuring errors are mainly caused by the gas–liquid flow ratio, SO2 oxidation, and the side reactions of SO3. The error in the gas sampling rate is generally less than 13%. The isopropanol solution flow rate decreases 3% to 30% due to the volatilization of isopropanol, and accordingly, this will increase the apparent SO3 concentration. The amount of SO2 oxidation is linearly related to the SO2 concentration. The side reactions of SO3 reduce the selectivity of SO42− to nearly 73%. As sampling temperature increases from 180 to 300°C, the selectivity of SO42− decreases from 73% to 50%. The presence of H2O in the sample gas helps to reduce the measurement error by inhibiting the volatilization of the isopropanol and weakening side reactions. A formula was established to modify the displayed value, and the measurement error was reduced from 25%–54% to less than 15%.

Absorption and Mass Transfer of CO<sub>2</sub> in Monoethanolamine Solution

This work uses a continuous bubble-column scrubber for the absorption of CO2 with a 5M MEA solution under a constant pH environment to explore the effect of the pH of the solution and gas-flow rate (Qg) on the removal efficiency (E), absorption rate (RA), overall mass-transfer coefficient (KGa), liquid flow rate (QL), gas-liquid flow ratio (γ), and scrubbing factors (φ). From the outlet CO2 concentration with a two-film model, E, RA, KGa, QL, γ, and φ can be simultaneously determined at the steady state. Depending on the operating conditions, the results show that E (80-97%), RA(2.91x10-4-10.0x10-4mol/s-L), KGa (0.09-0.48 1/s), QL(8.74-230.8mL/min), γ (0.19-5.39), and φ (0.031-0.74 mol/mol-L) are found to be comparable with other solvents. In addition, RA, KGa, E, and QL have been used to correlate with pH and Qg, respectively, with the results further explained.

SO2 removal from simulated flue gas using various aqueous solutions: Absorption equilibria and operational data in a packed column

Abstract In this study, SO 2 absorption performance of the novel amino acid salt aqueous solutions such as sodium lysinate and sodium glycinate from simulated flue gas were studied and compared with frequently utilized absorbents including ethylenediamine, 2-amino2-methyl1-propanol (AMP) and disodium hydrogen phosphate. SO 2 -aqueous solution equilibrium data of these absorbents at concentration of 0.1 M in a stirred batch reactor exhibited that amino acid salt aqueous solutions and especially sodium lysinate had higher SO 2 solubility at 298 K. In order to ensure the SO 2 removal efficiency at practical condition, the suggested absorbents were applied in a packed column. The experimental results imply better removal efficiency of amino acid salt solutions consistent with absorption equilibrium data. The effects of parameters such as temperature, SO 2 inlet concentration and liquid–gas flow ratio on removal efficiency were investigated for 1 M concentration of sodium lysinate solution. SO 2 removal efficiency enhanced with increasing gas–liquid flow ratio, but both temperature and SO 2 inlet concentration had opposite effects.

Investigation into the Formation Mechanism and Distribution Characteristics of Suspended Microparticles in MQL Grinding

A large number of patents have recently been devoted to the development of minimum quantity lubrication (MQL) grinding techniques that can significantly improve both grinding fluid alternatives in terms of environmental consciousness, energy saving, cost effectiveness, and sustainability. Among these patents, one is for a supply system for MQL grinding that turns the lubricant into pulse drops with fixed pressure, constant pulse frequency, and the same drop diameter. The drops are produced and injected into the grinding zone in the form of jet flow under high-pressure gas and air seal. With the growing demands of our environment, MQL has become widely used in the grinding and cutting machining processes. Based on the dangers of suspended air microparticles from machining to health and the environment, the sources, classification, and characteristics of suspended air microparticles were analyzed in this research. Taking account the features of microparticles, we explored the dangers posed by suspended microparticles to health and identified the common diseases attributed to these microparticles. With the use of the machining process, we analyzed the formation mechanism of suspended air microparticles and conducted research on the distribution characteristics of suspended air microparticles formed from the atomizing nozzle during MQL grinding. The relationship between jet parameters and microparticle size of sprays was explored. The results showed that atomized microparticle size has a linear relationship with nozzle caliber and gas-liquid flow ratio. Atomized microparticle size is directly proportional to the nozzle caliber but inversely proportional to gas-liquid flow ratio. We also analyzed the distribution rules of spray droplets and determined the amount of spray microparticles and volume distribution function. Keywords: Grinding, jet parameters, minimal quantity lubrication (MQL), spray droplets, suspended microparticles.

Formation characteristics of Taylor bubbles in a microchannel with a converging shape mixing junction

The bubble formation in a square microchannel with a converging shape mixing junction has been investigated under gas–liquid Taylor flow using a high-speed camera. A typical bubble formation process was found to consist of two steps including the expansion and rupture steps. The bubble length could be approximated as the product of the rupture time and two-phase mixture velocity. Significant influence of liquid viscosity and two-phase mixture velocity on the bubble length was observed, although a linear dependence of the bubble length on gas–liquid flow ratio is present for a given two-phase mixture velocity or liquid viscosity. This indicates that shear stress plays an important role in determining the bubble length in the current microfluidic device even at low Capillary numbers where the squeezing regime is expected to predominate. An empirical correlation expressing the bubble length as a function of gas–liquid flow ratio, liquid viscosity and two-phase mixture velocity was developed to describe the experimental results. The bubble frequency was found to reach a maximum as gas–liquid flow ratio is increased from 0.5 to 1. The cross-sectional shape of Taylor bubble was close to be square at low capillary numbers, which is in agreement with the literature results.

Analysis on Influencing Factors of the Gas-liquid Mixing Effect of Compressed Air Foam Systems

Abstract Compressed Air Foam System is a kind of new high efficiency firefighting systems. It produces high quality foams by injecting compressed air into foam solution. The gas-liquid mixing effect determines properties of foams. Properties of foams determine the efficiency of extinguishing. This paper introduced the status of compressed air foam system gas-liquid mixing technology, categorized and analyzed existing gas-liquid mixing modus, finally put forward factors influencing mixing effect. Stability of foams was used as criteria to evaluate the gas-liquid mixing effect. Gas-liquid mixing modus, gas-liquid mixing area and turbulence enhancer were three factors determined by mixing chamber structure. Gas-liquid flow ratio, gas-liquid mixing pressure and gas-liquid mixing speed were another three factors determined by working parameters. These factors were analyzed and corresponding optimization advices were put forward in order to better gas-liquid mixing effect. This paper could provide some guidance to the design of mixing chambers and optimization of mixing parameters.

Characteristics of Time-Varying Void Fraction in Isothermal Air-Water Cocurrent Flow in a Horizontal Capillary Tube.

An experimental investigation was carried out on the varying properties of void fraction averaged over a tube cross section in an isothermal fully developed air-water two-phase mixture flowing in a horizontal capillary tube of 1.0mm, 2.4mm and 4.9mm inside diameter, using the constant current method. The flow pattern was observed by video and photograph with streak flashing. Calibration of the void meter was performed using a nonconductive plastic model in bubbly flow, intermittent flow and annular flow. It was found that the scatter of the data of the void fraction was within 20% in each flow region. We obtained correlations between (1) void fraction and gas volume flow rate quality, (2) bubble volocity and total volume flux, (3) lengths of slugs and gas-liquid flow ratio and (4) mean liquid film thickness and pipe diameter

Determination of Steam Quality Using an Orifice Meter

Abstract Steam generators used to inject steam into petroleum reservoirs are designed to produce wet steam, usually about 80 percent quality. For economical operation the steam quality should be continuously monitored, which can be done using an orifice meter. In addition to using data for flow of the generator effluent through an orifice, the method uses the flow rate of water to the generator as measured independently, such as by a positive displacement meter. An improved calculation method has been demonstrated at pressures of 129 psig and 300 psig with qualities, of 32 to 100 percent. The calculation method is based on a literature correlation that includes data at pressure up to 929 psia. The method is expected to be useful at higher pressures as long as the volumetric gas-liquid flow ratio and the ratio of mass flow rate to pressure are above specified minimum values. Introduction Many references have mentioned or discussed the problem of two-phase flow through an orifice. However, most of the literature has been inaccurate or has not considered all of the important variables in this operation. Calculation methods for wet steam shown in two well known orifice meter handbooks are not accurate. For example, Diehl and Beck use the steam coefficient factor F. and state that the orifice differential is approximately linear with quality at a given pressure and constant total mass flow rate of wet steam. This is incorrect. The square root of the differential is approximately linear with quality. An accurate and detailed treatment of measuring two-phase flow with an orifice was made by Murdock. He published experimental data that both he and W. H. Osborne had obtained, as well as a correlation of the data, which has a standard deviation of only 0.75 percent. The data included two-phase systems of steam-water, air-water. natural gas-water, natural gas-salt water and natural gas-distillate at pressures of 15 to 929 psia. Two other recent references give data that are reasonably consistent with Murdock's correlation. However, the data in these references are not complete enough to permit exact comparisons. This paper illustrates the application of Murdock's correlation to measuring steam quality in thermal recovery operations, and further defines the limitations of the method. Experimental Procedure To evaluate the orifice method for measuring steam quality, orifice data were obtained at various qualities. The qualities were measured simultaneously by an independent method, and the values obtained by the two methods were compared. The effect of varying the total mass flow rate of water plus steam was studied. The equipment used is shown in Fig. 1. The following procedure was used at each total mass flow rate. At a measured rate, water was heated to within 3 F of the saturation temperature. Steam was passed through a cyclone to remove water. The steam from the cyclone was mixed with the preheated water and flowed to the orifice, flow control valve and condenser. The total liquid flow rate from the condenser was determined by weight. The feed rate of saturated water was determined by the calibrated water tank gauge, and the dry steam rate was calculated by difference. The orifice was calibrated by flowing dry steam at an orifice differential of 100 in. of water. Data were then obtained at various water and steam flow rates, with the total mass flow rate of water plus steam being the same (within about 1 percent) as the flow rate observed at 100 in. differential at 100 percent quality. Calculation Method The flow rate of saturated or superheated steam can be calculated from orifice data using the equation (1) The term whg is expressed in lb/hr. The subscript h denotes hourly rate, and the subscript G is gas phase. JPT P. 587ˆ