Two-phase Flow In Microchannels Research Articles

The mechanism by which thermal coupling leads to an asymptotic maldistribution stability boundary for flow boiling in parallel channels

Phase change heat transfer processes are attractive for the efficient thermal management of advanced semiconductor devices, with flow boiling in parallel microchannel heat sinks being one promising solution. However, there are several implementation challenges associated with two-phase flow in parallel microchannels, particularly flow maldistribution, which can adversely affect performance and reliability. Two-phase flow models can be useful in predicting and understanding the instability mechanisms that leads to maldistribution, such that parametric performance trends and safe regions of operation can be identified. Channel-to-channel thermal coupling has been shown to alleviate maldistribution, but it is not yet understood why this effect is limited to some critical lateral thermal conductance above which there is no further benefit. In the current work, a two-phase flow model with a lumped thermal capacitance representation of the heat sink wall is developed to identify the mechanism for this asymptotic maldistribution stability boundary for thermally coupled microchannels. The lumped model allows the nature of stability boundary to be explained via a scaling analysis of the eigenvalues. In such systems, it is identified that a single eigenvalue determines the occurrence of flow maldistribution. Scaling analysis shows that, for small values of thermal conductance, this eigenvalue becomes a quadratic function of thermal coupling, which enhances the stability of microchannels and moves the stability boundary. However, for large values of thermal conductance, the eigenvalue is dependent only on the fluid properties, leading to a limit at which thermal coupling can no further improve stability or affect the stability boundary. The lumped model developed in this work enables mechanistic understanding of the role of channel-to-channel thermal coupling on mitigating flow maldistribution and therefore may offer important guidance on the design of flow boiling systems and heat sinks.

Numerical simulation study of bubble breakup mechanism in microchannels with V-shaped obstacle

Microfluidic gas-liquid two-phase flow in microchannels plays a crucial role in diverse applications, from electronics cooling to biomedical engineering. Precisely controlling microbubble formation and breakup is essential for industrial microfluidic processes. This study investigates microbubble breakup in microchannels with V-shaped obstacles. Four breakup patterns are identified: no breakup, breakup with permanent gaps, breakup with partly obstruction, and breakup with permanent obstruction. The bubble breakup with permanent obstruction could maintain a wider flow pattern range. The bubble breakup process is categorized into three stages: squeezing, transition, and pinch-off. We observe power-law relationships between bubble characteristics and dimensionless time in each stage, and the impact of the capillary number Ca and liquid phase viscosity μL on bubble properties. This research deepens our understanding of gas-liquid two-phase flow in microchannels with obstacle structures and is relevant to analytical chemistry and fine chemical engineering applications.

Bubble Interaction and Heat Transfer Characteristics of Microchannel Flow Boiling With Single and Multiple Cavities

Abstract Flow boiling in microchannels can effectively address the challenges of high power density heat dissipation in electronic devices. However, the intricate bubble dynamics during the two-phase flow in microchannel necessitates understanding the characteristics of complex bubble hydrodynamics. In this study, we perform 2D numerical simulations of flow boiling using the Cahn-Hilliard phase-field method for a 200-µm width microchannel with single and multiple cavities in COMSOL Multiphysics (V5.3). The numerical model successfully captures bubble dynamics, encompassing vapor embryo generation, bubble growth, departure, coalescence, sliding, and stable vapor plug formation. The heat transfer mechanism inside the microchannel is dominated by bubble nucleation and thin-film evaporation. Elevated wall superheats in a single nucleation cavity, and increased mass flux facilitates higher bubble departure frequency and heat transfer performance. Temporal pressure fluctuations are observed inside microchannels in multiple cavities due to bubble coalescence, departure, and subsequent nucleation. Increasing the nucleating cavities from 2 to 5 within the microchannel while maintaining consistent cavity spacing of 100 µm has resulted in nearly 32% enhancement in heat transfer performance. This study offers valuable findings that can help improve the thermal management of electronic devices.

Experimental study of embedded manifold staggered pin-fin microchannel heat sink

High-heat-flux thermal management of high-power chips in electronic devices is becoming more and more critical. Manifold microchannel heat sink is one of the effective choices. In order to improve the heat dissipation capacity of manifold microchannel heat sink, a silicon-based thermal test chip including staggered pin-fin microchannels is designed and fabricated. Staggered pin-fin microchannels and rectangular microchannels with the same area size are embedded on the opposite side of the silicon-based thermal test chip for liquid-cooled heat dissipation. The experiments are conducted using HFE7100 as the working fluid. The heat transfer performance and the flow characteristic of two distinct microchannel structures are evaluated. It is found that the differences in temperature, convection heat transfer coefficient and pressure drop caused by different microchannel structures are greatly related to the mass flow rate. The heat transfer performance of the staggered pin-fin microchannel heat sink is better than that of the rectangular microchannel heat sink at a larger mass flow rate, but it also brings greater flow pressure drop. The bubble effect caused by the transition from single-phase to two-phase flow in microchannels makes the monotonicity of the convection heat transfer coefficient decrease significantly and the monotonicity of the pressure drop increase significantly. Compared to rectangular microchannel heat sinks, staggered pin-fin microchannel heat sinks reach the two-phase flow at higher heat flux. At the mass flow rate of 5 mL/s and the heat flux of 700 W/cm2, the staggered pin-fin microchannel heat sink can reduce the chip surface temperature by 8 K compared with the chip surface temperature of the rectangular microchannel heat sink.

Pattern identification of kerosene-water two-phase flow in Y-shaped microchannels using terahertz spectroscopy

In this paper, the terahertz time-domain spectroscopy (THz-TDS) is proposed to investigate the behavior underlying kerosene-water two-phase flow in Y-shaped rectangular microchannels. The microchannel is initially saturated with water and water forms an external continuous phase at low kerosene flow velocities (vk), while a dispersed phase is formed at high vk. The flow process of kerosene-water two-phase flow in microchannel was in situ monitored using a THz-TDS peak intensity, and four flow patterns such as droplet flow, plug flow, slug flow and deformed interface flow are identified over a wide range of vk from 2.08 to 43.13 cm/s at selected water flow velocity (vw) of 13.96 cm/s and 27.71 cm/s. Due to the strong dependence of flow patterns on the vw, only droplet, plug and slug flows are observed at high vw. In addition, with the vw increasing from13.96 cm/s to 27.71 cm/s the ranges of vk for droplet flow and plug flow patterns are expended by ∼100%. A parameter β = k/vw, where k is the slop of the peak intensity of THz-TDS against vk, was proposed to characterize the kerosene-water two-phase flow patterns, and each β only corresponded to one phase for various vk at different vw. These results render THz-TDS a feasible technology for investigating the behavior of oil-water two-phase flow inside microfluidic devices and lay the foundation for experimental and numerical investigations on multiphase flow in microchannels for further study.

Examining the interfacial behavior of non-Newtonian gas-liquid two-phase flow in horizontal square microchannels

This study investigates the flow dynamics of a non-Newtonian two-phase flow within a horizontal square microchannel. The experimental apparatus employed an acrylic channel with a side length of 8 × 10−4 m. Nitrogen served as the test gas, and the test liquids consisted of water and CMC (Carboxymethylcellulose) aqueous solutions at concentrations of 0.2 and 0.4 %wt. The superficial velocity of working gas and liquids spanned ranges of 0.26–20 m/s and 0.05–1.0 m/s, respectively. A differential pressure transducer was used to measure pressure gradients, while a high-speed video camera captured the flow behavior for analysis with a custom image processing technique. Nonlinear regression statistical analysis was applied to establish the criteria for flow pattern transition. Furthermore, signal processing techniques such as PSD (Power Spectral Density), DWT (Discrete Wavelet Transform), ANN (Artificial Neural Network), and Kolmogorov entropy were employed to analyze the flow behavior based on the pressure gradient signals from the differential pressure transducers. Our findings reveal the occurrence of bubbly, slug, slug-annular, and churn flow patterns. The nonlinear regression statistical analysis offered ambiguous results for determining flow pattern transitions. However, an empirical coefficient was obtained for the nondimensional liquid height. Moreover, the empirical nondimensional liquid height aligned well with the experimental values. The PSD and DWT analysis corroborated each other in clearly characterizing the flow pattern based on pressure gradient fluctuation and wavelet energy. The Kolmogorov entropy effectively captured the chaotic flow pattern. Furthermore, the ANN analysis demonstrated a solid agreement between the predicted and actual flow patterns.

MODELING OF TWO-PHASE GAS-LIQUID SLUG FLOWS IN MICROCHANNELS

Even though numerical simulation of two-phase flows in microchannels has been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially the slug-flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble (sometimes called gas pocket or gas slug) may be a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field. Thus, there is a strong motivation to employ numerical simulation methods to avoid some of the experimental difficulties. In this paper, the characteristics of two-phase slug flows in microchannels are calculated with the help of the volume-of-fluid (VOF) method. Formation of the slugs for different superficial velocities, capillary numbers, and gas volume fractions are investigated. The minimum mesh resolution required to capture the liquid film surrounding the gas bubble is reported employing a dynamic mesh adaptation methodology with interface tracking. Results are shown to be in good agreement with experimental data and empirical correlations.

Numerical simulation of boiling behavior in vertical microchannels

High heat flux electronic devices put forward new requirements for heat dissipation, and boiling heat transfer technology is widely used because of its higher heat dissipation capacity. In this study, the volume of fluid method was employed, along with the incorporation of the Lee phase-change mass transfer model, to investigate two-phase flow and heat transfer in vertical upward rectangular microchannels. The heat flux was varied within the range of 10–40 kW/m2, while the mass flux was varied within the range of 200–600 kg/m2 s. With the increase in heat flux, bubble flow, slug flow, churn flow, and annular flow were found successively. A phase diagram was established to predict the flow pattern transition during the boiling process. When the flow pattern changes to the churn and the annular flow, the active nucleation site density increases obviously with the Boiling number (Bo). A new correlation was proposed for two-phase flow boiling heat transfer, suitable for vertical upward channels in microscale fluids. The friction factor obtained using the Darcy friction factor equation agrees well with the simulation results at a high-pressure drop. The instability in microchannels increases with the increase in heat flux, particularly in annular flow, resulting in more severe wall temperature fluctuations.

An improved dissipative particle dynamics method for the liquid-particle two-phase flow in microchannels

Liquid-particle two-phase flow in microchannel widely exists in the fields of biomedical and environmental monitoring, such as the lab-chip device for disease diagnosis. The standard dissipative particle dynamics (DPD) method has been previously employed to study the liquid-particle two-phase flow in microchannel, but it cannot accurately simulate the real process because of the unsuitable DPD parameters. In the present study, an improved DPD method was developed by changing the system energy and fitting the characteristic curve between the random force coefficient and the Schmidt number. In addition, a new logarithmic relationship between the conservative force coefficient and the particle size was found. The result demonstrated that the improved DPD method enabled more accurate simulation on the liquid-particle two-phase flow in microchannels than the standard DPD method. For instance, in the simulation of particle sedimentation, the relative deviation between the value obtained by the improved DPD method and the theoretical value was less than 6% while the relative deviation was more than 20% for the standard DPD method. The simulated result of the particle migration in microchannel was in good agreement with the result obtained by Matas et al, and the relative deviation was less than 1.5%. Therefore, the improved DPD method would have great potentials in the study on the liquid-particle two-phase flow in microchannels.

Achieving ultra-uniform temperature distribution with a heterogeneous wettability surface for two-phase flow in microchannels

Microchannel-based two-phase schemes have been proposed as an efficient solution for thermal management. However, the non-uniform temperature distribution in flow boiling limits phase-change performance and impedes commercial applications. In this study, we introduce a novel surface featuring heterogeneous wettability, comprising upstream superhydrophobic, middle plain, and downstream superhydrophilic regions. Hierarchical nanowires are employed to achieve the desired wettability, which can promote bubble departure and enhance capillary pumping. This wettability distribution facilitates upstream bubble nucleation and downstream liquid supply, significantly improving temperature uniformity. Moreover, surface contact angle variations effectively contribute to flow behavior improvement by restricting vapor reversal and fostering churn flow. Our results demonstrate that heat aggregation in the middle region and temperature restriction in the downstream region cause the high-temperature region to migrate upstream. This temperature redistribution stimulates axial heat transfer, substantially suppressing inlet subcooling and restricting temperature gradients. Compared to the plain surface, the heterogeneous wettability surface afforded an 87.8% reduction in temperature difference between Tw1 and Tw5 at approximately 120 °C, suggesting an ultra-uniform temperature distribution. Additionally, the average heat transfer coefficient showed a 35.1% enhancement rate at a wall heat flux of 100 W/cm2, and the performance evaluation criterion showed a 44.2% enhancement rate at an effective heat flux of 190 W/cm2. These results indicate that the proposed surface can efficiently regulate temperature distribution, maintain performance stability, and enhance heat transfer. The heterogeneous wettability surface has considerable potential for practical applications in high-power component heat dissipation.

Liquid–liquid two-phase flow in a wire-embedded concentric microchannel: Flow pattern and mass transfer performance

In this work, flow pattern and mass transfer of liquid–liquid two-phase flow in a wire-embedded concentric microchannel are studied using toluene–water system. Droplet flow, slug flow, oval flow and annular flow are observed in the wire-embedded concentric microchannel. The effects of embedded wires and physical properties on flow patterns are investigated. The embedded wire insert is conducive to the formation of annular flow. The flow pattern distribution regions are distinguished by the Caaq (capillary number)–Weorg (Weber number) flow pattern map. When Weorg＜0.001, slug flow is the main flow pattern, and when Weorg＞0.1, annular flow is the main flow pattern. Oval flow and droplet flow are between Weorg=0.001–0.1, and oval flow is transformed into droplet flow with the increase of Caaq. The effect of flow rate, phase ratio, initial acetic acid concentration, insert shape and flow patterns on mass transfers are studied. Mass transfer process is enhanced under annular flow conditions, the volumetric mass transfer coefficient is up to 0.36 s–1 because of the high interfacial area and interface renewal rate of annular flow.

Distribution and Mass Transfer of Gas–Liquid Two-Phase Flow in Comb-Shaped Microchannels

For gas–liquid two-phase flow in comb-shaped microchannels, operating conditions could change the flow pattern and the uniformity of flow distribution and further affect the mass transfer performance. In this study, the hydrodynamics and mass transfer of gas–liquid two-phase flow in comb-shaped microchannels are investigated. Four flow patterns are observed: single-phase or foam flow, slug-bubbly flow, slug flow, and compact slug flow. When the flow pattern is slug flow, the flow distribution and velocity distribution in the microchannel are ideal, and its mass transfer performance is close to that of a single microchannel. Before the appearance of slug flow, the uniformity of flow distribution and mass transfer performance are improved with the increase of the ratio of gas–liquid flow rate. After the appearance of slug flow, the uniformity of flow distribution and mass transfer performance are deteriorated with the increase of the ratio of gas–liquid flow rate.

Hydrodynamics and mass transfer studies of liquid-liquid two-phase flow in parallel microchannels

Parallel or multiple microchannels (PMC) are being tried for high throughput operations. Extremely fewer reports on liquid/liquid two-phase flow in PMC exists in the open literature compared to the gas/liquid flow especially utilizing the new designs. In this work, a bifurcation-based PMC was employed to study hydrodynamics and the mass transfer aspect of solvent extraction of propionic acid. Slug formation, slug volume, specific surface area, extraction efficiency and mass transfer coefficients were evaluated. PMC exhibited better results than the single microchannels (SMC), making them superior in performance than the SMC. The standard deviation for uniform flow distribution is within 2%. A maximum specific surface area was obtained in PMC than the single channels, which is 10513 m2/m3. Similarly, the maximum extraction efficiency of 98% is achieved in PMC at flow rates 16 times higher than the SMCs. The overall pressure drop in PMC is significantly less than in the SMCs. Thus PMC is a suitable choice for process intensification in process industries.

Enhanced flow boiling heat transfer and suppressed boiling instability in counter-flow stepped microchannels

Flow boiling in microchannels is one of the most effective ways to solve the heat dissipation problems caused by the aggressive miniaturization of electronic components. In this study, counter-flow stepped microchannels (CSMC) are designed to address the main issues currently faced by boiling two-phase flow in microchannels: premature triggered critical heat flux (CHF) and severe boiling instability. Flow boiling experiments are conducted in three CSMCs with different step depths of 100 μm, 200 μm, and 300 μm, respectively. Deionized water is used as the working fluid, and the mass flux ranges from 118 kg/m2·s to 370 kg/m2·s. Combined with the visualization results captured by a high-speed camera, the boiling heat transfer and instability characteristics in three CSMCs are investigated and compared with the traditional parallel-flow straight microchannels (PMC). The results show that the CHF and average heat transfer coefficient (HTC) of CSMCs are increased by 50.0%-105.6% and 35.8%-90.3%, respectively, while the two-phase pressure drop is decreased by 61.7%-77.7% compared to PMC. The design of the step structure can disrupt the boundary layer, change the flow velocity between different steps, and promote fluid mixing, all of which are more pronounced in the case of deeper step depth. Therefore, with the increase in step depth, the CHF and average HTC are increased, and the two-phase pressure drop decreases. More importantly, the flow boiling instability in CSMC is well suppressed. The oscillations of the wall temperatures are eliminated during the flow boiling process in CSMC. Compared with PMC, the magnitudes of pressure oscillations in CSMCs are decreased by 52.8%-84.9%, and the rises in inlet temperatures caused by flow reversal are also achieved in a 6.1 °C-36.6 °C reduction. Similarly, the suppression effect on boiling instability increases with the step depth. The visualization results revealed that for CSMC with the largest step depth, the upstream expansion of bubbles is not observed throughout the flow boiling experiment. All above encouraging results indicate that the demonstrated counter-flow stepped microchannels would provide a good reference for future microchannel design.

Flow boiling heat transfer characteristics of two-phase flow in microchannels

A set of experimental platforms with widths of 0.5, 1.0, 1.5, and 2.0 mm was established to explore the mechanism of flow boiling bubble dynamics in microchannels, focusing on heat transfer characteristics, pressure loss, and two-phase flow pattern identification. Bubble flow, restricted bubble flow, and dry area were observed in all four channels. The appearance of flow pattern was related to flow rate and channel width. Under the condition of the same channel width, the initial heat flux of subcooled boiling gradually increased with increase in flow rate, and this change trend was close to the linear trend. Under the same flow rate, the initial heat flux of subcooled boiling increased with decrease in channel width. This condition was due to the faster flow rate of the working medium in the narrow channel, resulting in decrease of heating time. The increase in bubble generation frequency directly led to the increase in the wall heat transfer coefficient and the decrease in the bubble separation diameter. Mathematical analysis showed that under the condition of small flow, reduction of channel size led to reduction of the total wall heat transfer coefficient. In this condition, reduction of channel size cannot enhance heat transfer. With increasing volume flow rate, the range of hydrodynamic control area increased and the index decreased. When the flow rate was large, the total heat transfer coefficient increased greatly with the decrease in channel size. The theoretical values were in good agreement with the experimental data.

Experimental and numerical studies of liquid-liquid two-phase flows in microchannel with sudden expansion/contraction cavities

The addition of micro cavities is a promising passive method to enhance mass transfer performance in microchannels. Experimental and numerical methods are adopted to investigate the flow characteristics of velocity field, droplet shape and vorticity for the liquid–liquid two-phase flows in microchannels with micro cavities. The results indicate that the droplet expands in cavities for slug and transitional flow patterns with droplet length longer than the channel width, while the expansion is not obvious for droplet flow patterns. Dimensionless expansion width increases with decreasing Capillary number and with increases in droplet length. For long droplet with length more than 2.5 times of the channel width, the expansion width is nearly the same with increasing droplet length. Due to the sudden decrease and increase of cross-section areas in cavities, the droplet moving speed fluctuates in the cavities, leading to the pulsating flow in cavity-shaped microchannels. When the droplet passes through the cavity structure, the continuous phase near the wall enters the cavity and induces small vortexes in the cavities. The intensity of vortex is more pronounced with longer droplet due to the blocking effect of droplet expansion in cavities. The vorticity magnitude in cavities is always higher than the value in straight microchannel, indicating that micro cavities can enhance the fluid mixing in the microchannel.

Distribution of liquid-liquid two-phase flow and droplet dynamics in asymmetric parallel microchannels

The numbering-up of microchannels in parallel is usually engineered to achieve high-throughput production of droplets, and the flow distribution of fluids and uniformity of droplets in parallel microchannels are core issues for droplet formation. In this work, the distribution of liquid–liquid two-phase flow and droplet dynamics in asymmetric parallel microchannels are investigated by using a high speed camera, and the factors affecting the stability and uniformity of droplets are explored. The dynamic behavior of droplet swarm in the rear cavity and its feedback effect on droplet generation at T-junctions are revealed. The microfluidic devices with various configurations are designed to investigate its effects on flow distribution. In addition, based on the principle of hydrodynamic resistance for the liquid-liquid two-phase flow in microchannels, the prediction for the distribution of liquid-liquid two-phase flow under slug flow is proposed.

Experimental study of Taylor bubble flow in non-Newtonian liquid in a rectangular microchannel

The flow characteristics of gas and non-Newtonian fluid in a rectangular T-microchannel with cross section of 150 μm × 50 μm are experimentally studied. Four flow patterns are observed, including bubble flow, Taylor flow, transitional flow and annular flow. The effects of two-phase flowrate, the CMC/SDS concentration, and microchannel size on the flow pattern maps and Taylor bubble/liquid slug length are compared. The bubble length increases with gas/liquid flowrates ratio, whilst the change of slug length is opposite. The bubble length decreases with liquid viscosity or surface tension. The slug length increases with the liquid viscosity, but decreases with surface tension. Experimental data is analyzed in relationship to JG/JL, Re and Ca using dimensional analysis. Two empirical equations are given respectively to predict the Taylor bubble/slug lengths. Predictions are largely consistent with the experimental results with maximum 25% variance. This provides an important reference for the controllable operation of two-phase flow in rectangular microchannels.

Numerical Investigation of Heat Transfer Enhancement due to Two-Phase Flow in Microchannel

Microchannels play an important role in cooling applications of electronic devices, especially since the size of electronic devices is becoming smaller and the energy generated per unit volume is increasing. In order to understand the development of two-phase oil-air flow in a microchannel, a numerical investigation is performed that aims to uncover the effects of two-phase mixture on the heat transfer and pressure drop characteristics. The methodology consists of numerical simulation in a T-junction microchannel mesh that was built in Gambit software. Its dimensions are 1 mm thickness and 52 mm in length. Oil enters through one of the inlets and air enters through the other inlet. After the junction, the air-water two-phase flow forms Taylor bubbles. The two-phase flow is simulated by a computational fluid dynamics code developed in Python for use in general purpose and open source codes. The heat transfer is simulated with constant temperature wall boundary condition. Pressure drop is also studied as it can severely inhibit the efficiency of microchannel devices. The heat transfer characteristics show the effectiveness of the two-phase mixture for microchannel cooling purpose. Gas phase-liquid phase pressure drop are numerically simulated and compared with Martinelli and similar correlations, and the results indicate satisfactory and close correlation. Internal circulation is seen to enhance the heat transfer coefficient and the Nusselt number is 2-3 times higher compared to flow where there is only the single phase liquid.

Research on Flow Boling Heat Transfer Characteristics of Two-Phase Flow in Micro-Channels

Research on Flow Boling Heat Transfer Characteristics of Two-Phase Flow in Micro-Channels