Flow In Microchannel Research Articles

Enhanced flow boiling by manipulating two-phase flow in Tesla channel heat sink using HFE-7100

Flow boiling of dielectric fluids in copper microchannel heat sinks is highly desirable for cooling large-sized insulated gate bipolar transistor (IGBT) power electronic modules. However, dielectric fluids present challenges in flow boiling because of their unfavorable thermophysical properties. These factors make it difficult to enhance critical heat flux (CHF) without precooling. To address this, we developed a large copper heat sink (10 cm × 5 cm) with Tesla microchannels designed to suppress vapor backflow and promote intense fluid mixing. The microchannels have a high length to hydrodynamic diameter ratio of approximately 220, significantly higher than those in previous studies. Flow boiling experiments using HFE-7100 were conducted for both Tesla and plain-wall microchannels. Tesla channels demonstrated a 26.2 % increase in CHF and a 120 % improvement in heat transfer coefficient (HTC). These enhancements are attributed to the vapor backflow suppression and improved fluid mixing. Moreover, the standard deviation of wall temperature in plain-wall microchannels was 10 times higher than in Tesla channels, highlighting the effectiveness of the periodic Tesla valves in reducing two-phase flow instabilities. Flow pattern visualization was conducted to further understand the mechanism behind vapor regulation, clarifying the role of Tesla valves in controlling vapor backflow. This study demonstrates the potential of dielectric fluids in Tesla microchannels for flow boiling applications, offering a promising solution for cooling large electronics.

Femtosecond laser ablation of nitrocellulose for spatio-temporal flow control in µPADs

Microfluidic paper-based analytical devices (μPADs) are gaining popularity due to their low cost and ease of use, but controlling fluid flow for more complex biochemical assays within these devices remains challenging. This study investigates femtosecond laser ablation of nitrocellulose (NC), a preferred material for µPADs, to create mechanically switchable barriers and flow controllers. We investigated NC ablation using single laser pulses and spatially overlapping pulses that generate lines. Single pulse ablation thresholds were determined for wavelengths of 1,030 nm, 515 nm and 343 nm. Line ablation characteristics were investigated as a function of the temporal and spatial pulse separation and laser wavelength. High aspect ratio grooves (up to 4.26) were achieved under specific conditions. These grooves can be used to define the spatial separation of the flow in separated microchannels or to form a barrier line perpendicular to the microchannel that can modulate the temporal behavior of the fluid flow. This barrier introduced an additional high flow resistance slowing down the flow or, if it was designed to cut through nitrocellulose at the entire depth, completely stopped the liquid flow. It was further shown that a barrier formed in this way could be switched by mechanically bending the µPAD at the barrier position. The femtosecond laser patterning method presented here provides precise spatio-temporal control not only for flow branching and multiplexing, but also for controlling flow speed and switching flow on and off within the same manufacturing process. Our results open up new possibilities for complex, multi-step assays on µPADs.

Characterization of the Dynamic Flow Response in Microfluidic Devices.

The purpose of this study is to characterize the dynamic response of fluid flow in microchannels, which can show significant delay times before reaching steady flow conditions. Two main sources of these delays are numerically and experimentally investigated, the hydraulic compliance which originates from the flexibility of the system components (microchannel, tubing, syringe, etc.), and the compressibility of the liquid dead volume in the setup, also known as the "bottleneck effect". A fluid-structure interaction model is presented for the compliance of rectangular PDMS microchannels that is used to form a numerically based relation for the compliance as a function of the pressure and geometry. This relation is successfully able to predict the dynamics of the flow inside PDMS microchannels in stop-flow experiments. The time delays associated with the bottleneck effect is also shown when using different syringe volumes, microchannel resistances, and liquid types. In these tests, the bottleneck effect has a much larger effect compared to the compliance of the PDMS microchannels. This is true even when using softer PDMS by increasing the monomer-to-curing agent mixing ratio. The characterization that is presented here allows for a simple analysis of microfluidic networks using the hydraulic-circuit approach.

Interaction between vapor bubbles during flow boiling heat transfer in microchannels

Microchannel flow boiling is an efficient cooling solution for high-power-density miniaturized systems. Many studies on microchannel flow boiling focused on the dynamics of single vapor bubbles, while neglecting the interaction between bubbles, which is important in relevant applications. Here, numerical simulations are carried out to study the interaction between multiple vapor bubbles in microchannel flow boiling. The results show that for different numbers of bubbles in the microchannels with the same initial size and position of leading bubbles, the bubble size in a single-bubble microchannel is larger compared to the leading bubble of multiple-bubble cases because of heat absorption by the vaporization at the rear bubbles. As the initial volume ratio between the leading bubble and the rear bubble decreases, the leading bubble size in the downstream becomes smaller because of the reduced contact with the superheated thermal boundary layer. With increasing the Reynolds number, both the leading and the trailing bubbles increase slightly in size in the upstream of the heated region, because the bubbles at higher Reynolds number move faster and firstly get in contact with the superheated fluid. The increase in the bottom wall thickness increases the growth rate of the multiple bubble sizes with earlier bubble coalescence because of the higher upstream wall temperature by heat conduction in the solid wall.

Flow boiling of HFE-7100 for cooling Multi-Chip modules using manifold microchannels

Two-phase manifold microchannel heat sinks have gained significant interest for their efficient use of the coolant’s latent heat. Despite this, manifold microchannel flow boiling heat transfer has not yet been applied to wide-bandgap semiconductor power modules with multiple chips, primarily due to the complexity of the process and challenges related to electrical insulation. This study introduces a novel embedded manifold microchannel design that ensures uniform mass flow distribution, tailored for the thermal management of multi-chip power modules. The microchannels were laser-etched onto direct bonded copper (DBC) to reduce thermal resistance while maintaining electrical insulation. Thermal test vehicles (TTVs) for multi-chip power modules, featuring two different microchannel widths, were assembled using silver sintering. Experimental tests were then conducted using HFE-7100 as the coolant to evaluate two-phase heat dissipation performance. Results indicate that during flow boiling heat transfer, the temperature difference between chips in a power module strongly correlates with the exit quality. A higher coolant mass flow rate significantly reduces temperature variation between chips, particularly under high chip heat flux. At a coolant mass flow rate of 9 g/s, with a chip heat flux of 357 W/cm2 and total heat dissipation of 536 W, the minimum thermal resistance reached 0.15 cm2∙K/W, yielding a COP of 1391. With a slight sacrifice in thermal resistance, 0.17 cm2∙K/W and 0.20 cm2∙K/W were achieved at mass flow rates of 6 g/s and 3 g/s, respectively. Correspondingly, the COPs reached 2179 and 6749. This research offers valuable insights for applying flow boiling heat transfer with dielectric coolants to cool multi-chip power modules.

Thermal and hydrodynamic characteristics of microencapsulated phase change materials slurry flow in wavy microchannels

Microencapsulated phase change material slurry (MPCS) is a novel cooling fluid with promising performance. This study numerically investigates heat transfer and flow characteristics of MPCS within wavy microchannels. MPCS is modeled as a homogeneous, Newtonian fluid. Five wavy geometries were examined across a Reynolds number range of 50 to 250 (laminar flow regime), varying in amplitude and wavelength. The model results show that with increase in Reynolds number and decrease in the amplitude and radius of curvature of wavy microchannels, the pressure drop and Nusselt number also increase. Furthermore, the results reveal that Dean vortices intensify with increasing wave amplitude and Reynolds number. Conversely, these vortices weaken as channel wavelength and radius of curvature increase. The formation of Dean vortices enhances fluid mixing and consequently improves the thermal performance of the slurry. The study concludes that in wavy microchannels employing MPCS, increasing the Reynolds number, increasing the channel amplitude with respect to wavelength, and decreasing the radius of curvature improves the overall performance. Also, the results reveal that in higher Reynolds numbers, the radius of curvature is the most effective parameter on the overall performance of wavy microchannels.

A microfluidic approach for controllable synthesis of TiO2 submicrospheres

TiO2 submicrospheres are used in dye-sensitized solar cells and Li-ion batteries to enhance energy conversion efficiency and volumetric performance. In this study, a microfluidic chip was designed for the realization of the continuous controllable synthesis of TiO2 submicrospheres. The chip utilizes two T-channels to discrete dispersed phases into droplets, and a Y-channel to fuse the two dispersed phase droplets. The droplets generation process, the influencing factors of droplet length and the fusion process of droplets were investigated. The effects of the flow rate ratio of titanium oxysulfate and ammonia solutions, microchannel geometrical profiles including the Y-channel entrance angle on the size of the synthesized TiO2 were studied. The length of the generated droplets was found to decrease with increasing continuous-phase flow rate, and increase with increasing dispersed-phase flow rate and microchannel cross-section area. When the flow rate ratio of titanium oxysulfate solution and ammonia solution was 1:1, the average diameter of TiO2 spheres synthesized was the smallest, around 300 nm. Increasing the microchannel cross-section and Y-channel entrance angle were beneficial in obtaining larger TiO2 submicrospheres. This proposed microfluidic strategy has the potential for applications required continuous synthesis of TiO2 submicrospheres with controllable sizes.

An analytical gradient method to model interfacial heat and mass transfer in multi-field CFD codes

Phase change at the interface between a liquid and a gas, commonly referred to as bulk condensation/boiling or interface condensation/boiling, is often overshadowed by wall condensation/boiling. However, interfacial phase change plays a critical role in various industrial applications, including boiling flows in microchannels and boiling liquid metal flows. In this article, we introduce a novel analytical model based on the gradient method. This model accounts for interfacial phase change within multi-field codes. It is validated against both analytical and experimental cases involving bulk boiling, demonstrating excellent agreement. Notably, the mesh convergence aligns with the methods employed in single-fluid codes. Additionally, we propose a hybrid approach that combines the high accuracy of this model with the computational efficiency of dispersed methods.

Development of a sensor for liquid film thickness measurements during annular flow in microchannels

Development of a sensor for liquid film thickness measurements during annular flow in microchannels

Numerical Simulations of Flow and Heat Exchange in Zigzag-Shaped Microchannels

Cooling of electronic components is of great importance currently. One of the most important methods of integrated circuit cooling is the use of microchannel heat sinks. The aim of this work is to analyze the cooling capacity of zigzag shaped microchannels. The heat exchange for four different microchannels was tested depending on the flow rate of the cooling liquid (water) and the temperature of the cooled element. The system of differential equations that describe fluid flow and heat transport is presented in the paper. The equations were solved using the finite volume method. The work showed that both the increase in the number of zigzag sections and the increase in the flow velocity cause an increase in the flow resistance. In contrast, an increase in the temperature of the cooled element causes a decrease in fluid viscosity, which results in a decrease in flow resistance. From the point of heat exchange, both the increase in the number of segments, the flow rate, and the temperature of the cooled element increase the cooling capacity of the microchannel. Research shows that zigzag shaped microchannels are excellent cooling elements, especially for geometrically small heat sources such as electronic components.

Unsteady numerical calculation of flow boiling heat transfer in microchannels with Kagome structures

A three-dimensional microchannel flow boiling model was developed based on the VOF model and the Lee model, and the flow and heat transfer in microchannels with Kagome structures were numerically simulated. The characteristics of bubble flow patterns, gas phase distribution, and heat transfer were analyzed under the heat fluxes of 5–8 MW/m2 and flow velocities of 0.2–0.5 m/s. The results show that at a flow velocity of 0.5 m/s, the disturbances caused by fluid impact on the Kagome structure result in bubble break-up, effectively suppressing the formation of large bubbles. However, at a flow velocity of 0.2 m/s, bubbles tend to be adhered on the surface of the Kagome structure, causing local blockage and deteriorating heat transfer. Compared to the flow velocity of 0.5 m/s, the average heat transfer coefficient in the microchannel decreases by up to 75.4 % and the pressure loss decreases by 45.38 % at the flow velocity of 0.2 m/s.

Predictive Model for Cell Positioning during Periodic Lateral Migration in Spiral Microchannels.

The periodic lateral migration of submicrometer cells is the primary factor leading to low precision in a spiral microchannel during cell isolation. In this study, a mathematical predictive model (PM) is derived for the lateral position of cells during the periodic lateral migration process. We analyze the relationship of migration period, migration width, and starting point of lateral migration with microchannel structure and flow conditions and determine the empirical coefficients in PM. Results indicate that the aspect ratio of the microchannel and the Reynolds number (Re) are key factors that influence the periodicity of the cell lateral migration. The lateral migration width is jointly affected by Re, the cell blockage ratio, and the microchannel curvature radius. The inlet structure of the microchannel and the ratio of the cell sample to the sheath flow rate are critical parameters for regulating the initial position. Moreover, the structure of the pressure field at the inlet constrains the distribution range of the starting point of the lateral migration. Regardless of whether the particles/cells undergo 0.5, 1, or multiple lateral migration cycles, the lateral positions predicted by PM align well with the experimental observations, thus verifying the accuracy of PM. This research helps to elucidate the characteristics of periodic lateral migration of cells in spiral microchannels and can provide practical guidance for the development and optimization of miniature spiral microchannel chips for precise cell isolation.

Heat and mass transfer analysis of s-PTT nanofluid in microchannels under combined electroosmotic and pressure-driven flows with wall slip using the homotopy perturbation method

The heat and mass transfer of the electroosmotic flow in microchannel transporting viscoelastic nanofluid is investigated considering Brownian motion of nanoparticles and slip boundary conditions. The simplified Phan-Thien-Tanner model is employed to describe the rheological behavior of fluid and the nonlinear Navier model with non-zero slip critical shear stress is considered at walls. The governing nonlinear momentum, mass, and heat transfer equations are solved using the Homotopy Perturbation Method. The study reveals that increasing the fluid elasticity, nanoparticle concentration, and size significantly enhances the flow rate, heat and mass transfer. Additionally, elasticity and Reynolds number decrease the friction factor. Reducing the double-layer thickness and increasing the Reynolds number lead to higher flow rates and fluid velocities. Notably, the findings emphasize the critical role of the slip conditions on the Sherwood and Nusselt numbers.

Compressibility effects in microchannel flows between two-parallel plates at low reynolds and mach numbers: Numerical analysis

Under certain circumstances, flow in microchannels can exhibit compressibility effects even at Reynolds numbers (Re) around (below 2,300) and low Mach numbers (below 0.3). This is particularly true for gases, especially when the flow undergoes significant pressure changes or acceleration within the microchannel. This study investigates the compressibility effects encountered in two-parallel plates microchannels at these low Reynolds and Mach numbers, due to the high-pressure drop associated with the small scale of the microchannels. This uncommon flow is characterized by an exceptionally small channel diameter-to-length aspect ratio (∼10–3), resulting in a friction coefficient that deviates from the typical value for laminar flow between parallel plates (f = 96/Re). Both steady and transient effects on the flow field are examined under low Re subsonic flow, assuming continuum behavior. The ideal gas equation is used to model gas density, while the isothermal Tait-Murnaghan equation models liquid density. For gases, compressibility effects are observed primarily when the inlet pressure ratio exceeds 0.1. The results show that these effects are less pronounced for liquids, even at elevated inlet pressure ratios. Additionally, a flow delay across the channel exhibits a first-order transient response. For liquid flow, this effect depends on the channel resistance, the total fluid volume within the channel, and the liquid's bulk properties, rather than the inlet pressure ratio.

Analogical elucidation of dusty‐hybrid nanofluid flow through the microchannel: An unsteady case

AbstractThis work is the exploration of unsteady flow of dusty fluid in the horizontal microchannel when the channel plates are susceptible to linear radiation. The flow of the dusty fluid is in the existence of magnetic field. Along with dusty fluid, there are immersions of two different types of nanoparticles namely ferrous oxide and silver . The study gives the comparison of nanofluid phase with dusty fluid phase; hybrid nanofluid phase with dusty fluid phase separately. The modeled partial differential equations are non‐dimensionalized and pdsolve command of Maple software tackles these formed partial differential equations directly. Comparative study of nanofluid‐dusty fluid and hybrid nanofluid‐dusty fluid for the flow in microchannel is to elucidate the fact of heat transfer enhancement upon appropriate insertion of nanoparticles in the base fluid; especially the nanoparticles possessing paramagnetic effect for the better performance on the application of the magnetic field. Results deduce that the fluid flow and heat transmission are known to progress with time. Also, the fluid phase is known to reach the steady state earlier than the dust phase. The nanoparticle volume fraction is known to deplete the temperature in both the hybrid fluid phase and nanofluid phase. The particle concentration parameter diminishes the velocity profile, both for fluid phase and dust phase.

Exploring fluid flow in microchannels with branching and variable constrictions

Exploring fluid flow in microchannels with branching and variable constrictions

Experimental study of subcooled flow boiling in microchannel heat sinks integrated with different embedded pin fin arrays microstructures

The optimized microchannel heat sinks could enhance flow boiling for effectively tackling the electronics cooling. The flow boiling experiment for three microchannel heat sinks integrated with different layouts of entrenched pin fins is conducted at flow rate of 273.6–456 kg/(m2.s) and inlet subcooling of 35∼50 K. The overall/local heat transfer features, pressure drop and boiling mechanism are studied. The hybrid pattern presents earliest initial boiling and lower superheat than the microchannel heat sink with uniform pin fins arrangement at moderate and large flow rates. The trend of overall and local HTC (heat transfer coefficient) is similar, which occurs peak at onset nucleation boiling, and then decreasing with increasing heat flux. At the largest flow rate, the hybrid pattern exhibits 2.7–3.5 times peak HTC promotion than other patterns. As for lowest flow rate, the hybrid pattern does not manifest remarkably superior performance due to downstream vapor cores clogging effect. The hybrid pattern shows largest pressure drop, and the smaller inlet subcooling manifests inferior heat transfer and resistance performance. The comprehensive performance factor (CPF) is proposed, and the pattern with uniform small-sized pin fins shows optimal CPF especially for low flow rate, which is considerable compared with the reference heat sink structures until high heat flux. This study may provide some insight into the design of microchannel for flow boiling heat dissipation.

Irreversibility analysis in an electro‐osmotically driven flow using ternary hybrid nanofluids in a microchannel with a porous material

AbstractFlow enhancement is one of the most significant challenges in microfluidics with extremely low permeability. Based on this, electrokinetics remediation for the double‐layer flow of hybrid ternary nanofluid is proposed based on the transport of ions under constant pumping pressure. The entropy generation volumetric rate relation is also modeled. The nonlinear system of equations is formulated, solved, and validated numerically by the collocation method and shooting Runge–Kutta method. Results are shown graphically to explore the impacts of governing parameters, such as the Darcy parameter, viscosity index, electrokinetic, and Joule heating effects, on the velocity, temperature, entropy, and Bejan number profiles. The flow and heat transmission in microchannels may be significantly changed and controlled by the electric double layer. The findings show that as the electrokinetic parameter's magnitude increases, the flow is slowed down, and thermal dispersion is impeded by nanoparticle collisions, which lowers the velocity field and heat transfer profile.

Can boundary slip destabilize rotating microchannel flows?

Deviation from the traditional no-slip boundary condition due to factors like surface roughness and wettability is of paramount importance in microfluidics and nanofluidics, as it is attributable to its significance in drag reduction, flow control and enhancement and improved mixing. Augmentation in mixing, in turn, is known to strongly correlate with potential instabilities in the flow structure. Reported research studies indicate that slip is an inherent flow stabilizer in microfluidics, to the extent that with sufficient slip, the flow becomes linearly stable against all wavelike disturbances for all wavelengths and Reynolds numbers [“The linear stability of slip channel flows,” Phys. Fluids 34,074103(2022)]. Contrary to such intuitive proposition, here we show that slip effects can destabilize microchannel flows under spanwise rotation, delving on the interplay of rotational forces and slippery hydrodynamics. Our results reveal that increasing the slip length decreases the critical rotation speed, indicating lower rotational effort required to destabilize the flow, whereas the critical Reynolds number for the flow remains effectively unaltered for different slip lengths in a spanwise rotating system. As the slip length increases progressively, the critical rotation number (dimensionless rotational speed) for the onset of instability decreases further, then remains constant up to a certain limit, and subsequently declines with additional enhancement in the slip length. This indicates the potential for deploying customized hydrophobic (slippery) substrates to facilitate transitions from stable to unstable modes by simple tuning of the rotational speed—a paradigm that offers great promise in various applications ranging from materials synthesis to biomedical technology.

Kinetic model of multi-component polyatomic gas mixture considering internal energy excitation

In this paper, a kinetic Boltzmann model equation with internal degrees of freedom is established for thermodynamic non-equilibrium multi-component polyatomic gas mixture by using continuous energy levels to deal with rotational energy and vibrational energy. The normalization and conservativeness of the kinetic model are analyzed and proved. Then, numerical algorithm and wall boundary conditions for solving the model equation are given. Then, normal shock-wave structure flow problem and pressure/temperature gradient driven micro-channel flow problem for the two-component gas mixture are used to verify the reliability of the model. The simulation results are in good agreement with those obtained by other methods, which verifies the reliability of the model and algorithm in this paper. Finally, taking 25 N attitude control engine two-dimensional profile as the object, the study for two-dimensional profile nozzle internal and external mixed flow problem is carried out, and the influence of different inlet rarefaction levels on the molecular transports of mixed flow field and the molecular transport phenomena of mixed flow field with different number of components are discussed.