Flow Patterns In Microchannels Research Articles

Research on heat transfer and flow distribution of parallel-configured microchannel heat sinks for arrayed chip heat dissipation

The heat dissipation ability and compact structure of microchannel flow boiling show great promise for high-efficiency heat dissipation in the field of chip cooling. In arrayed chip cooling scenarios, multiple microchannel heat sinks are typically arranged in parallel. Random fluctuations in the heat load of each chip can lead to non-uniform flow distribution and even thermal failure of the chip. However, there are still limited studies on the non-uniform flow distribution in the parallel use of multiple microchannel heat sinks. This study presents an experimental and theoretical study on the boiling heat transfer and flow distribution characteristics of two heat sinks in parallel using HFE-7100. The steady-state test results indicated that the flow distribution characteristics in parallel-configured heat sinks were closely correlated with the flow pattern. In the single-phase region, flow distribution in both heat sinks was primarily influenced by the physical properties of the fluid. The effect of heat flux on non-uniform flow distribution was within 7 %. When one of the heat sinks entered the two-phase region, microchannel resistance characteristics changed significantly, leading to a dramatic increase in non-uniform flow distribution, up to 26.0 %. The critical heat flux (CHF) was triggered prematurely, with a decrease of up to 31.4 %. In dynamic characteristics testing, when both heat sinks were in the two-phase region simultaneously, the increase in heat flux led to greater non-uniformity in the flow distribution of the parallel-configured heat sinks compared to the single-phase region. Furthermore, considering the transition boundary of two-phase flow patterns in microchannels, a prediction model for flow distribution in parallel-configured microchannel heat sinks was proposed, demonstrating good predictive capability. The research provides a reference for the design and optimization of two-phase cooling systems for arrayed chip configurations.

Surface charge-dependent slip length modulates electroosmotic mixing in a wavy micromixer

This study explores electroosmotic mixing in microfluidic channel with predefined surface topology, mainly focusing the effect of surface charge-dependent slip length on the underlying mixing dynamics. Our analysis addresses the need for precise control of flow and mixing of the participating fluids at microscale, crucial for medical and biomedical applications. In the present work, we consider a wavy microchannel with non-uniform surface charge to explore the electroosmotic mixing behavior. To this end, adopting a finite-element approach, we numerically solve the Laplace, Poisson–Boltzmann, convection–diffusion, and the Navier–Stokes equations in a steady-state. The model is validated by comparing the results with the available theoretical and experimental data. Through numerical simulations, the study analyzes electroosmotic flow patterns in microchannels, highlighting the impact of surface charge-dependent slip lengths on mixing efficiency. For example, at a diffusive Peclet number of 200, mixing efficiency drops from 95.5% to 91.5% when considering surface charge-dependent slip length. It is established that the fluid rheology, characterized by Carreau number and flow behavior index, non-trivially influences flow field modulation and mixing efficiency. Increased Carreau numbers enhance flow velocity, affecting overall mixing of the constituent fluids in the chosen fluidic pathway. For instance, by increasing the Carreau number from 0.01 to 1.0, a discernible trend emerges with higher flow line density and accelerated velocity within the microchannel. The study also examines the effect of diffusive Peclet numbers on the mixing efficiency, particularly in the convective regime of underlying transport. These insights offer practical guidance for designing microfluidic systems intended for enhanced mixing capabilities. Additionally, the study explores the likelihood of particle aggregation under shear forces, vital in biological non-Newtonian fluids, with implications for drug delivery, diagnostics, and biomedical technologies.

FOURIER AND WAVELET ANALYSIS OF PRESSURE FLUCTUATIONS IN GAS-NON-NEWTONIAN LIQUID TWO-PHASE FLOWS IN A MICROCHANNEL

The purpose of this study was to analyze the characteristics of gas-non-Newtonian liquid flow patterns in microchannels using signal processing techniques including power spectral density (PSD) and discrete wavelet transform (DWT) analyses. Square microchannels measuring 0.8 × 0.8 mm were used in this study. Water, 0.1 percent by weight (wt%) xanthan gum (XG) aqueous solution, and 0.2 wt% XG were employed as the working liquids, while nitrogen gas was used as the working gas. The superficial velocities of the liquid and gas were varied between 0.05 and 1 m/s and 0.26 and 7.8 m/s, respectively. The flow patterns were recorded using a high-speed camera, while the pressure drop was measured using a differential pressure transducer. The pressure gradient data were analyzed using signal processing techniques to characterize the flow patterns. Furthermore, PSD and DWT analyses were found to effectively describe the characteristics of the flow pattern.

Controlling gas–liquid segment length in microchannels using a high-speed valve

Segmented flow, one of the most important flow patterns in microchannels, has been widely employed in gas–liquid microreactor systems because it enhances mass transfer through the biphasic interface. The segment length is determined by the channel parameters (e.g., size, material type), fluid flow rate, and fluid properties. Thus, it is difficult to manipulate the gas–liquid segment length without affecting the residence time of the fluid. In this work we successfully achieved regularly segment length control by introducing a high-speed valve into a gas–liquid microchannel system while varying the frequency of the valve installed in the gas-phase tube to allow gas and liquid segment lengths to be controlled within a large range. In contrast, the segment lengths were almost constant under the same conditions without the high-speed valve. Furthermore, a gas/liquid flow rate ratio in the range of 0.7–1.2 was deemed necessary for steady segmented flow. Finally, a correlation was proposed to predict the segment length using parameters such as valve frequency and fluid flow rate, and it matched the experimental data well. This segment length control strategy will improve the performance of gas–liquid reactions in microreactors and advance the design of microreactor systems.

Controlling flow patterns and its evolution behavior in microchannel for intensified separation of rare-earth and Al(III) ions: Hints from the hydrodynamics parameters

Abstract Control of flow patterns and its evolution behavior in microchannel is crucial for microfluidic technology to achieve mass transfer intensification in the micro-scale and selective separation of target ions. In the present work, we found that the non-equilibrium extraction and selective separation behaviors of target metal ions from other co-existing background ions have a close relationship with the change of hydrodynamic parameters of the organic and aqueous two phases and the evolution of flow patterns in microchannel. Taking the separation of Er(III) and Al(III) ions in microchannel as a model system, it is revealed that the transition of the flow patterns from parallel flow to slug flow might originate from the decrease of the linear velocity of two phases flow, the increase of viscosity and the decrease of the interfacial tension between the two phases. The change of those hydrodynamic parameters would result in an obvious increase in specific interfacial areas between the two phases and a decrease in wall film thickness of the slug flow. A higher Capillary number corresponds to a larger specific interfacial area and a higher surface renewal rate between the two phases. Therefore, an intensified separation of Er(III) and Al(III) ions can be attributed to the change of those hydrodynamic parameters in microchannel. The present work highlights a novel approach by controlling the flow patterns of aqueous and organic two phases in microchannel for controllable selective separation of target metal ions.

An experimental investigation of flow boiling characteristics in silicon-based groove-wall microchannels with different structural parameters

The groove-wall microchannel possesses high performance for flow boiling heat transfer. However, the structural parameters of grooves have not been studied comprehensively. This paper presents an experimental study of the effects of groove-wall microchannel aspect ratio, groove spacing ratio and groove depth on the flow boiling. The silicon-based channel aspect ratios of 1, 2.5 and 4 with the same channel height of 200 μm, groove spacing ratios of 2, 5 and 8, and groove depths of 15, 30 and 45 μm, are investigated. Experiments are performed using deionized water as the working fluid with mass fluxes of 446-963 kg/m2·s and heat fluxes of 36.3-502.8 W/cm2. As a comparison, flow boiling experiments of plain-wall microchannels are conducted. A high-speed camera is used to record the flow patterns in microchannels, providing an in-depth understanding of the heat transfer. The average heat transfer coefficient is found to vary significantly with the channel aspect ratio, while it varies moderately with different spacing ratios or groove depths. At the aspect ratio of 2.5, the groove-wall channel achieves the best improvement of heat transfer. The critical heat flux is easily triggered at the aspect ratio of 4 and it becomes significantly high at the aspect ratio of 1 in both channels. Encouragingly, reduced pressure drops are achieved over most test cases. The high aspect ratio groove-wall channel is found with the ability to reduce more pressure drop. Similar pressure drops are observed for channels with different groove spacing ratios or different groove depths. This work provides a guidance for the optimal design of the groove-wall microchannels to enhance flow boiling.

Resistance characteristics analysis of droplet logic gate based on lattice Boltzmann method

In this paper, the geometric models of microfluidics NON-AND and AND-OR droplet logic gate devices are designed, and the operations of NON-AND and AND-OR logic are implemented in the designed models. Based on the color-gradient model (R-K model) of lattice Boltzmann method (LBM), the two-phase flow pattern in microchannel is simulated. Based on the “equivalent length method”, the Laplace resistance at the droplet phase interface and the local resistance at the right-angle bend are equivalent to the additional pipe length required for continuous phase flow in the channel to generate the same on-way resistance increment, and the resistance characteristics when slug flow droplets exist in the microchannel are quantitatively analyzed. The method is further applied to the analysis of the resistance characteristics of the droplet logic gate, and the quantitative explanation of the droplet path-choosing behavior is obtained. Compared with the “hydroelectricity analogism method”, the “equivalent length method” is more concise and easy to understand. The research results in this paper provide theoretical basis for the design of other forms of droplet logic gate devices.

Using deep learning to recognize liquid–liquid flow patterns in microchannels

AbstractIn this work, an automatic liquid–liquidtwo‐phase flow pattern recognition platform was developed to help circumvent the difficulties in labor‐intensive hydrodynamics studies. Trained by about 30,000 of human‐labeled flow pattern images, a convolutional neural network was built with the expert‐level ability in the flow pattern recognition tasks and then coupled with automatic pump feeding system and online high‐speed camera monitoring system to realize the high‐throughput experimentation platform for microchannels. Effects of important factors such as flow rate, viscosity, interfacial tension, and so on were studied, and different flow pattern maps were obtained. With these thousands of flow pattern data in hand, we eventually drew the generalized liquid–liquidtwo‐phase flow map and then proposed the relatively prudent criteria for slug flow operation window in the microchannel. This study extended the applications of artificial intelligence on microreactor technology or microfluidics, and in particular facilitated understanding complex hydrodynamics and flow patterns.

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75°. The effect of water to oil flow rate ratio (Qwat/Qoil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Qwat/Qoil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Qwat/Qoil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.

Experimental characterization of two-phase flow patterns in a slit microchannel

At present, there is revolutionary development of microchannel systems. In such systems, it is extremely important to determine two-phase flow patterns depending on the gas and liquid flow rates. However, in most works there are no clear criteria for turning from one regime to another. In the present work, we have developed a new technique that makes it possible to measure the local void fraction, size of characteristic regions of the liquid films on the upper and lower walls of microchannel, frequency of bubble formation, and other quantitative characteristics. The two-phase flow patterns in a microchannel of 50-μm height and 10-mm width have been studied experimentally. Based on the developed methodology, criteria that accurately determine the boundaries between the two-phase flow regimes have been created. The features of a two-phase flow in a microchannel have been determined, and a new flow regime map has been created.

Experimental study on two-phase flow pressure drop during ethanol–water vapor mixture condensation in microchannels

Ethanol-water vapor mixtures condensation shows different flow patterns in microchannels due to the equivalent surface free energy differences. The transition point of streak flow pattern was introduced in the paper which was found closely related to the frictional pressure drop of ethanol–water vapor mixtures condensation flow in microchannels. Characteristics of two-phase pressure drop for ethanol–water vapor mixtures condensation flow in silicon microchannels of streak and smooth annular flow pattern were investigated. Four types of multi-port trapezoidal and triangular microchannels with hydraulic diameter in the range of 126–155 μm were employed. The pressure drop was determined under steam mass flux between 259.2 and 504.8 kg m−2 s−1 when the inlet ethanol weight concentration varied from 1% to 60%. The experimental pressure drop results were compared with existing correlations of flow condensation. The transition point basing on the streak flow pattern of binary vapor condensation was proposed to replace the traditional Reynolds number criterion of the laminar flow state to turbulent flow state for the frictional factor and the C parameter for Kim and Mudawar’s universal pressure drop model, which is in good agreement with the results the experiment results.

Fundamental Issues Related to Flow Boiling and Two-Phase Flow Patterns in Microchannels – Experimental Challenges and Opportunities

ABSTRACTFlow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data.

Numerical simulation of isothermal gas−liquid flow patterns in microchannels with varying wettability

Microchannel technology has attracted great interest due to its potential for process intensification and design of compact devices. In this study, isothermal gas−liquid flow patterns (Taylor and stratified) developed in a rectangular microchannel were numerically evaluated through the volume of fluid (VOF) model. Hydrophilic (θw=25°) and hydrophobic (θw=105°) microchannels were investigated. The numerical results were compared with experimental data. A comparison between numerical and experimental data and some available correlations showed good agreement. At similar feeding conditions, the model was able to capture the different gas−liquid flow patterns expected when the wall contact angle is varied. Thus, this work elucidated the use of CFD for the evaluation of the impact of variations of the wall wettability on isothermal gas−liquid flow patterns developed in microchannels. This procedure can be used as a source of preliminary information on the flow morphology expected in microfluidic devices subjected to superficial treatment.

Solvent Extraction and Stripping Studies in Microchannels with TBP Nitric Acid System

Solvent extraction experiments are carried out in a t-junction serpentine microchannel and a split-and-recombine microchannel with TBP-nitric acid system. Stripping experiments have been carried out in the t-junction serpentine microchannel. The effects of residence time and O/A ratio on overall volumetric mass-transfer coefficients are discussed. Flow patterns in microchannels are observed to have more insight into experimental trends. For the same average velocity, the split-and-recombine microchannel is found to give higher overall volumetric mass-transfer coefficient. Overall volumetric mass-transfer coefficients observed for the two microchannels are compared with overall volumetric mass-transfer coefficients reported for conventional extractors.

Prediction of Two-phase Slug Flow Patterns in Microchannel with T-junction

Prediction of Two-phase Slug Flow Patterns in Microchannel with T-junction

Gas–liquid two-phase flow patterns in microchannels with reentrant cavities in sidewall

Two-phase flow patterns in two horizontal microchannels with uniform reentrant cavities in sidewall were experimentally studied, using deionized water and air as working fluids. The hydraulic diameter of microchannels in the constant rectangular cross-section is 133.3μm. Two structures (fan-shaped and triangular) of reentrant cavity are arranged to be mounted in sidewall with 0.25mm spacing to develop two new microchannels with variable cross-sections. The liquid and gas superficial velocities in the microchannels cover the 0.1–0.5m/s and 0.1–10m/s ranges, respectively. High-performance CCD digital imaging camera with a Nikon microscope is used to determine the flow patterns in the two new proposed microchannels. Two distinct flow patterns were observed: intermittent flow and separated flow. Evolution characteristic for different flow patterns along the flow direction are presented. Flow pattern maps using gas and liquid superficial velocities as coordinates and the correlations to describe flow pattern transition for the newly proposed microchannel test sections are obtained, respectively.

The effects of viscosity, surface tension, and flow rate on gasoil-water flow pattern in microchannels

A microchannel was fabricated with glass tubes to investigate the effect of viscosity, surface tension, and flow rate on the liquid-liquid two-phase flow regime. Water and gasoil were selected as aqueous and organic working fluids, respectively. The two fluids were injected into the microchannel and created either slug or parallel profile depending on the applied conditions. The range of Reynolds and capillary numbers was chosen in such a way that neither inertia nor interfacial tension forces were negligible. Xanthan gum was used to increase viscosity and Triton X-100 (TX-100) and Sodium Dodecyl Sulfate (SDS) were used to reduce the interfacial tension. The results demonstrated that higher value of viscosity and flow rate increased interfacial area, but slug flow regime remained unchanged. The two surfactants showed different effects on the flow regime and interfacial area. Addition of TX-100 did not change the slug flow but decreased the interfacial area. In contrast, addition of SDS increased interfacial area by decreasing the slug’s length in the low concentrations and by switching from slug to parallel regime at high concentrations.

Probing single cells using flow in microfluidic devices

Enabling fluids to be manipulated on the micron-scale, microfluidic technologies have facilitated major advances in how we study cells. In this review, we highlight key developments in how flow in microfluidic devices is exploited to investigate the behavior of individual cells, from trapping and positioning single cells to probing cell deformability. Exploiting the properties of fluids and flow patterns in microchannels makes it possible to study large populations of single cells at micron-length scales with increased throughput and efficiency.

Experimental Investigation of Silicon-Based Micro-Pulsating Heat Pipe for Cooling Electronics

A simultaneous temperature measurement and flow visualization experiment was performed to investigate the thermal and flow behaviors of a silicon-based micro-pulsating heat pipe (micro-PHP) with trapezoidal microchannels with a hydraulic diameter of 352 μm. FC-72 and R113 were used as working fluids. Variations in temperature versus time at different locations of the micro-PHP under different power inputs and typical flow patterns in microchannels were recorded. The evaporator wall temperature, or the maximum localized temperature, of the micro-PHP at moderate filling ratios was measured and compared to those derived from the empty microdevice (0% filling ratio). Experimental results showed that a micro-PHP embedded in a semiconductor chip could significantly decrease the maximum localized temperature. At a power input of 6.3 W, reductions in the evaporator wall temperature of about 42.1°C (or 34.1%) and 41.9°C (or 33.9%) were obtained for the micro-PHP charged with R113 at filling ratios of 41 and 58%, respectively. When the micro-PHP charged with FC-72, a maximum power input of about 9.5 W associated with a heat flux up to 10.7 W/cm2 was reached at a moderate rise in wall temperature of the evaporator. The visualization study demonstrated that the evaporation, adiabatic, and condensation sections of the micro-PHP were largely occupied by annular, slug, and slug–bubbly flows, respectively, at a steady state characterized by sustained self-exciting oscillations of working fluid. However, no local nucleate boiling was detected in the micro-PHP at the power input range, which was different from the results reported for traditional PHPs.

Quantitative prediction of flow patterns in liquid–liquid flow in micro-capillaries

Flow patterns in microstructured reactors (or microchannels) play an important role in dictating the mass transfer rates. In the present work, experiments were carried out to investigate the two phase (liquids) flow patterns in microchannels with different cross sections and contacting geometries. The pattern formation was analysed and conditions were classified in the three regions: surface tension dominated (slug flow), transition (slug-drop and deformed interface flow) and inertia dominated region (annular or parallel flow). A criterion for liquid–liquid flow pattern transition was developed using Capillary and Reynolds numbers based on the work of Dessimoz et al. [1] for gas–liquid systems. Finally, it was applied to the literature data and good agreement was obtained. The criterion is suitable for capillaries with hydraulic diameter up to 3 mm independently of cross section form and is an important predictive tool for the rational design of micro reactors for liquid–liquid reactions.