Two-phase Flow In Microchannels Research Articles

Theoretical and experimental study of two-phase flow in micro-channels grooved into horizontal pipes

Theoretical and experimental study of two-phase flow in micro-channels grooved into horizontal pipes

Effects of Gas-Liquid Inlet/Mixing Conditions on Two-Phase Flow in Microchannels

In order to know the effects of gas-liquid inlet and mixing conditions on two-phase flows in microchannels, adiabatic experiments were conducted with changing a combination of inner diameters for the mixer and the microchannels. In the experiments, horizontal circular microchannels of 100, 176 and 251 μm diameter were used as the test channel, and water and nitrogen gas as the working fluids. It was observed that both “quasi-homogenous flow” and “quasi-separated flow” occurred if the area ratio of the microchannel to the mixer was below a certain value. Furthermore, void fraction and two-phase frictional multiplier were found to be smaller in the quasi-homogeneous flow than the quasi-separated one.

A numerical method for two-phase flow in micro channels and its application to droplet control by electrowetting on dielectric

With the development of micro chemical and medical analysis in MEMS, micro-fluid-control chip becomes more and more popular. Based on discrete droplet control system, chemical and medical analysis is applied in the micro chip lab. The change of voltage between the dielectric layer of the electrode controls the wettability of a droplet on a dielectric solid surface and then makes it possible to create,transport,merge and cut the micro droplets. This paper presents a numerical method to simulate the droplet motion by electrowetting on dielectric (EWOD). The second-order projection method is carried out to solve N-S equations and level set function. The interface is captured as zero level set. Dynamic contact angle for fluid contacting a solid surface by the electric potential is used to evaluate the wettability of fluid by EWOD. The numerical scheme is based on staggered MAC grid. Numerical simulations are used to capture the interface between two kinds of fluids in micro channels, the shapes of micro droplets resting on the horizontal solid surface under different electric potential and the transportation of a micro droplet in a parallel-plate channel when electrodes are activated.

Lattice Boltzmann Simulations of CO2 Bubble Dynamics at the Anode of a μDMFC

This paper presents the bubble transport phenomenon at the anode of a micro-direct methanol fuel cell (μDMFC) from a mesoscopic viewpoint. Carbon dioxide bubbles generated at the anode may block part of the catalyst/diffusion layer and also the flow channels that cause the μDMFC malfunction. Lattice-Boltzmann simulations were performed in this paper to simulate the two-phase flow in a microchannel with an orifice which emulates the bubble dynamics in a simplified porous diffusion layer and in the flow channel. A two-dimensional, nine-velocity model was established. The buoyancy force, the liquid-gas surface tension, and the fluid-solid wall interaction force were considered and they were treated as source terms in the momentum equation. Simulation results and parametric studies show that the pore size, the fluid stream flow rate, the bubble surface tension, and the hydrophilic effect between the fluid and the solid wall play the major roles in the bubble dynamics. Larger pore size, higher methanol stream flow rate, and greater hydrophilicity are preferred for bubble removal at the anode diffusion layer and also the flow channels of the μDMFC.

Continuous-Flow Fractionation of Animal Cells in Microfluidic Device Using Aqueous Two-Phase Extraction

Monitoring of live cells is important in the field of medical science, diagnostics, biology, and the pharmaceutical industry. In this study, live and dead CHO-K1 (Chinese Hamster Ovary) cells were fractionated by continuous-flow extraction in a microfluidic device using immiscible aqueous two-phase extraction technique. The polymer solutions offered stable two-phase flows in microchannel without diffusive mixing. The fundamentals of aqueous two-phase extraction can support stable and reproducible recovery and separation of biomolecules in microfluidic devices. Polyethylene glycol 8000 (PEG 8000, 4%) and dextran T 500 (5%) were selected as model polymer solutions. The appropriate flow rates of polymer and cell solutions were suggested. The fractionation efficiency of live and dead CHO K-1 cells from the culture broth was compared in normal macroscale system and microfluidic device. The optimum pH for the fractionation was 6.6 in both the normal and micro-scale systems. The loss of target live cells by sedimentation was circumvented in microfluidic device because of the negligible effect of gravity on the sedimentation. Most live cells were distributed to PEG-rich phase, while dead cells were found at the interface of two polymer solutions in microchannel. In this case, the recovery and fractionation efficiency of live cells in the PDMS-based microfluidic device was 100% and 97.0%, respectively.

Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.

1016 マイクロチャンネル内気液二相流のボイド率と圧力損失特性(GS-2,5 管内流・非定常流,研究発表講演)

Recently, gas-liquid two-phase flow in microchannels is paid increasing attention in many fields. We investigated characteristics of two-phase flow in microchannels experimentally in the slug flow region. Measured void fraction agreed with the homogenous flow model. Pressure drop cannot be described in conventional models but had an influence of surface tension. We propose a correlation of pressure drop considering the loss due to the surface tension.

OS9-04 自励振動型マイクロヒートパイプの基礎研究(OS9 動力エネルギーシステムにおける熱流動)

Experimental study on self-oscillation two-phase flow in microchannel has been performed. A single winding flow path consists of 20 turns microchannels fabricated on a silicon wafer the size of which was 30mm×33mm. Test fluid was R141b and the liquid fractions were 0, 61.9, 72.4 and 81.2%. Flow oscillation was observed when the bottom of channels was heated and the top cooled. The effective thermal conductivity during the flow oscillation was by 3.3 times as large as that without oscillation at the maximum case.

403 シリコンウェーハ上のマイクロチャンネルによる自励振動型ヒートパイプ(OS4 マイクロ流れの基礎と応用)

Experimental study on self-oscillation two-phase flow in microchannel has been performed. A single winding flow path consists of 20 turns microchannels fabricated on a silicon wafer the size of which was 30mm×33mm. Test fluid was R141b and the liquid fractions were 0, 61.9, 72.4 and 81.2%. Flow oscillation was observed when the bottom of channels was heated and the top cooled. The effective thermal conductivity during the flow oscillation was by 3.3 times as large as that without oscillation at the maximum case.

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406×2.032mm2 cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Features unique to two-phase micro-channel flow were identified and employed to validate key assumptions of an annular flow boiling model that was previously developed to predict pressure drop and heat transfer in two-phase micro-channel heat sinks. This earlier model was modified based on new findings from the adiabatic two-phase flow study. The modified model shows good agreement with experimental data for water-cooled heat sinks.

Visualization of Wettability Effects on Microchannel Two-Phase Flow Resistance

Visualization of Wettability Effects on Microchannel Two-Phase Flow Resistance

Hydrodynamic instabilities of near-critical CO2 flow in microchannels: Lattice Boltzmann simulation

Motivated by systematic CO2 evaporation experiments which recently became available (J. Pettersen, “Flow vaporization of CO2 in microchannel tubes,” Doctor technicae thesis, Norwegian University of Science and Technology, 2002), the present work constitutes an exploratory investigation of isothermal flow of CO2 near its liquid–vapor critical point through a long 5 μm diameter microchannel. A modified van der Waals constitutive model—with properties closely approximating those of “real” near-critical CO2—is incorporated in a two-dimensional lattice Boltzmann hydrodynamics model by embedding a dimensionless parameter X, with X→1 denoting the “real” fluid. The hydrodynamic phenomena resulting by imposing a constant pressure gradient along a periodic channel are investigated by considering two regimes in tandem: (1) transition from bubbly to annular flow with a liquid film formed at the channel walls and (2) destabilization of the liquid film by the Kelvin–Helmholtz instability. Due to numerical constraints, intrinsic modeling errors are introduced and are shown to be associated with discrepancies in the relative vapor–liquid interfacial thickness, which is expressed by X. The effects of these errors are investigated both theoretically and numerically in the physical limit X→1. Numerically determined flow patterns compare qualitatively well with direct visualization results obtained by Pettersen. Overall, the characteristics of isothermal near-critical two-phase flow in microchannels can be reproduced by the appropriate modification of the thermophysical properties of CO2.

マイクロチャンネル内気液二相流の圧力損失 : 流路寸法と液物性値の影響(OS7-1マイクロ・ナノフルイディクス(マイクロ・ナノ流体工学))

マイクロチャンネル内気液二相流の圧力損失 : 流路寸法と液物性値の影響(OS7-1マイクロ・ナノフルイディクス(マイクロ・ナノ流体工学))

ADIABATIC GAS–LIQUID FLOW IN MICROCHANNELS

This article presents a review of adiabatic two-phase flow in minichannels and microchannels. Differences between them are identified and explained based on this review and our own research. Several channels of decreasing diameter were used in our experiments to determine the effect of the channel size on the two-phase flow of nitrogen gas and water. The effect of channel geometry was examined by characterizing the two-phase flow in a circular and square microchannel of similar size. Only slug flow was observed in the microchannels. Four new sub-classes of slug flow were subsequently defined. A new correlation was developed for the time-averaged void fraction data in the microchannels. The two-phase pressure drop in microchannels was predicted by treating the two phases as being separate with a large velocity difference. Regarding the effect of microchannel geometry, the transition boundaries on the two-phase flow regime maps were shifted for the slug flow subcategories.

Ordered and disordered patterns in two-phase flows in microchannels.

We show that wetting properties crucially control the patterns in two-phase flows of immiscible fluids in microchannels. Ordered patterns, continuously entrained by the flow, are obtained when one phase completely wets the walls, while disordered patterns, intermittently adhering to the channel walls, are unavoidably produced when wetting is partial. A lower limit for the channel sizes capable of generating well structured objects (drops, pears, pearl necklaces, ...) is presented.

Flow boiling heat transfer in two-phase micro-channel heat sinks––II. Annular two-phase flow model

This paper is Part II of a two-part study devoted to measurement and prediction of the saturated flow boiling heat transfer coefficient in water-cooled micro-channel heat sinks. Part I discussed the experimental findings from the study, and identified unique aspects of flow boiling in micro-channels such as abrupt transition to the annular flow regime near the point of zero thermodynamic equilibrium quality, and the decrease in heat transfer coefficient with increasing quality. The operating conditions of water-cooled micro-channels fell outside the recommended range for most prior empirical correlations. In this paper, an annular flow model is developed to predict the saturated flow boiling heat transfer coefficient. Features unique to two-phase micro-channel flow, such as laminar liquid and vapor flow, smooth interface, and strong droplet entrainment and deposition effects, are identified and incorporated into the model. The model correctly captures the unique overall trend of decreasing heat transfer coefficient with increasing vapor quality in the low vapor quality region of micro-channels. Good agreement is achieved between the model predictions and heat transfer coefficient data over broad ranges of flow rate and heat flux.

Two-phase flow patterns in parallel micro-channels

Micro-channel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro-electronic devices. Some of the important aspects associated with two-phase flows in micro-channels, is to study the bubble behavior and flow regimes in diabatic, parallel micro-channels. Most of the reports in the literature present data of only a single channel and mostly adiabatic. This does not account for flow mixing and hydrodynamic instability that occurs in parallel micro-channels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air–water and steam–water flow in parallel triangular micro-channels. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air–water flow different flow patterns were observed simultaneously in the various micro-channels at a fixed values of water and gas flow rates. In steam–water flow, instability in uniformly heated micro-channels was observed. This work develops a practical modeling approach for two-phase micro-channel heat sinks and considers the discrepancy between flow patterns of air–water and steam–water flow in parallel micro-channels.

Two-phase flow in microchannels

Gas–liquid two-phase flow patterns are visualized with a microscope for air–water flow in circular tubes of 20, 25 and 100 μm i.d. and for steam–water flow in a 50 μm i.d. circular tube. The superficial velocities cover a broad range of J L=0.003–17.52 m/s and J G=0.0012–295.3 m/s for air–water flows. Several distinctive flow patterns, namely, dispersed bubbly flow, gas slug flow, liquid ring flow, liquid lump flow, annular flow, frothy or wispy annular flow, rivulet flow, liquid droplets flow and a special type of flow pattern are identified both in air–water and steam–water systems, and their special features are described. It has been confirmed that two-phase flow patterns are sensitive to the surface conditions of the inner wall of the test tube. It has been evidenced that a stable annular flow and gas slug formation with partially stable thin liquid film formed between the tube wall and gas slugs appeared at high velocities under carefully treated clean surface conditions. At lower velocities, dry and wet areas exist between gas slug and the tube wall. The cross-sectional average void fraction was also calculated from photographs, showing a good agreement with the Armand correlation for air–water flow in lager tubes.

Gas–liquid two-phase flow in micro-channels

Gas–liquid two-phase flow in micro-channels

B163 細管内気液二相流の特性を利用した液体輸送における圧力損失の検討

B163 細管内気液二相流の特性を利用した液体輸送における圧力損失の検討