Cross-section Microchannels Research Articles

Dimension compensation of printed master molds by a desktop LCD 3D printer for high-precision microfluidic applications.

Recent advances in low-cost liquid crystal display (LCD) 3D printing have popularized its use in creating microfluidic master molds and complete devices. However, the quality and precision of these fabrications often fall short of the rigorous standards required for advanced microfluidic applications. This study introduces a novel approach to enhance the dimensional accuracy of microchannels produced using a desktop LCD 3D printer. We propose a method for dimension compensation, optimize the printing parameters, and provide a straightforward post-treatment technique to ensure high-quality curing of polydimethylsiloxane (PDMS) in master molds made from photosensitive resin. Our investigation assesses the precision of 3D printing across three different scales of square cross-section microchannels by measuring their widths and heights, leading to the determination of optimal printing parameters that minimize dimensional errors. The dimensional errors are further reduced by introducing a series of dimension compensation factors, which correct the nominal dimensions of the microchannels by using the compensation factors in 3D printing. The dimensional accuracy is significantly improved after compensation even in fabricating complex microchannels of triangular cross-sections. Finally, a spiral channel of trapezoidal-like cross-section with tilted edges is fabricated for microfluidic application, and highly efficient particle separation is realized in the channel. The proposed method provides new insights for utilizing desktop LCD 3D printers to achieve high-accuracy microstructures necessary for advanced microfluidic applications.

Numerical Study on the Heat Dissipation Performance of Diamond Microchannels under High Heat Flux Density

Heat dissipation significantly limits semiconductor component performance improvement. Thermal management devices are pivotal for electronic chip heat dissipation, with the enhanced thermal conductivity of materials being crucial for their effectiveness. This study focuses on single-crystal diamond, renowned for its exceptional natural thermal conductivity, investigating diamond microchannels using finite element simulations. Initially, a validated mathematical model for microchannel flow heat transfer was established. Subsequently, the heat dissipation performance of typical microchannel materials was analyzed, highlighting the diamond’s impact. This study also explores diamond microchannel topologies under high-power conditions, revealing unmatched advantages in ultra-high heat flux density dissipation. At 800 W/cm2 and inlet flow rates of 0.4–1 m/s, diamond microchannels exhibit lower maximum temperatures compared to pure copper microchannels by 7.0, 7.2, 7.4, and 7.5 °C, respectively. Rectangular cross-section microchannels demonstrate superior heat dissipation, considering diamond processing costs. The exploration of angular structures with varying parameters shows significant temperature reductions with increasing complexity, such as a 2.4 °C drop at i = 4. The analysis of shape parameter ki indicates optimal heat dissipation performance at ki = 1.1. This research offers crucial insights for developing and optimizing diamond microchannel devices under ultra-high-heat-flux-density conditions, guiding future advancements in thermal management technology.

Multi-objective parameter optimization design of tapered-type manifold/variable cross-section microchannel heat sink

Manifold microchannel heat sinks (MMCHS) have been widely used in the thermal management of high heat flux electronic devices. To improve the temperature uniformity of MMCHS and reduce the hot spot temperature, a tapered-type manifold/variable cross-section microchannel heat sink (TMVC-MCHS) is proposed in this study. Key parameters affecting the heat transfer performance of TMVC-MCHS were first investigated numerically. The results indicate that the width inclination ratio (αw,in) of the inlet manifold has a greater influence on the coolant flow distribution than the height inclination ratio (αh,in), with a ratio of more than 2.5. In addition, the microchannel inclination (β) and width ratio (ε) affect significantly the fluid flow and heat transfer. To further improve the cooling effect of the heat sink, this study adopted XGBoost and multi-objective genetic algorithm (NSGA-II) to optimize the structure. The maximum temperature of the substrate (Tmax) and the total pressure drop (ΔP) are the optimization objectives. Besides, the compromise solution (αw,in = 0.40, β = 0.63 and ε = 0.52) was determined by TOPSIS combined with the entropy weight method which was validated by the CFD method with an error margin of only 3 %. Finally, Compared to MMCHS, the temperature field distribution uniformity of TMVC-MCHS is significantly improved. The maximum bottom surface temperature is drastically reduced from 337.79 K to 320.01 K, and the temperature difference is reduced from 17.5 K to 3.5 K, at the cost of only a 3.5 kPa pressure rise. For inlet flow rates ranging from 1 m/s to 2.5 m/s, the TMVC-MCHS exhibits a hydrothermal performance factor (PEC) improvement of over 35 %, indicating superior comprehensive performance.

Tunable viscoelastic size-based particle separation in straight microchannels with triangular cross-sections

In recent years, viscoelastic particle manipulation within microfluidic systems has received much attention due to the ease with which micron-sized objects may be maneuvered and isolated on the basis of size. While several factors, including both fluid and particle properties, regulate the precise locations and trajectories of micron-sized species flowing along a microchannel, the role of channel cross-section shape is critical, since it directly influences the fluid velocity profile and thus the direction and magnitude of hydrodynamic forces. It is therefore surprising that this parameter has not been comprehensively investigated for cell-based separations, especially since most viscoelastic microfluidic systems are only able to efficiently sperate cells over limited size ranges. To address this shortcoming, we present a viscoelastic microfluidic system integrating a triangular cross-section microchannel, for efficient and tunable size-based separations of micron-sized species. We find that particle focusing patterns can be controlled by simple variation of volumetric flow rates, which allows for the efficient separation of particles and cells of variable size. To showcase the efficacy of the approach, we present a size-based separation of various blood components, including white blood cells, platelets, and rare cells. By quantifying the number of particles collected at the outlets, we achieve recovery efficiencies of over 98%.

Thermal stress analysis in pin fin microchannel heat sink

The conjugate heat transfer and the thermal stresses produced within a pin-fin microchannel heat sink are investigated numerically. The pin-fin microchannel heat sink is subjected to a constant heat flux from the bottom surface and cooled by water flow through the channel across the pin fins. Rectangular cross-section microchannel incorporating one raw of square pin fins are considered. The water flowing through the microchannel at Reynolds number varies from 200 to 800. The heat sink dissipates constant heat flux in the range of 75-175 kW/m2. The selected materials used for the solid substrate are Copper, Aluminium, Titanium, and Structural steel. The results are presented as contour plots for the temperature, thermal stress, and deformation distribution. It is found that the heat dissipation and the Nusselt number are increased with increasing Reynolds number, increasing the thermal conductivity of the mate-rial but remain constant throughout various heat fluxes. Thermal stresses are increased with decreasing Reynolds number, increasing heat flux, and increasing Youngs’ Modulus of the substrate material. The total deformation is increased with decreasing Reynolds number, in-creasing heat flux, and increasing the thermal expansion coefficient of the substrate material.

Research on mechanism of gas leakage in microchannels of steel containment vessels for nuclear power plants

Steel containment vessels for nuclear power plants can experience gas leakage due to minute defects such as cracks, corrosion, and aging, leading to gas leakage. A gas leakage model for microchannels is established to elucidate the mechanism underlying gas leakage within microchannels caused by these defects, specifically addressing the issue of unidirectional gas flow. Computational Fluid Dynamics (CFD) and the UK R6 method are employed to calculate the gas leakage rate within microchannels. Furthermore, the characteristics of gas flow within microchannels are explored, including the factors affecting the gas leakage rate. Validation of the calculation results is verified experimentally. The results indicate that the gas mass flow rate exhibits a linear decrease with decreasing internal pressure and a non-linear decline as temperature increases. Additionally, the gas mass flow rate demonstrates a negative correlation with the microchannel length but a positive association to its hydraulic diameter. The primary influencing factors on gas leakage rates are hierarchically ranked as follows: pressure difference, microchannel cross-sectional area, temperature, microchannel length, and microchannel hydraulic diameter.

Thermal and hydraulic performance of Al alloy-based 3D printed triangular microchannel heatsink governed by rough walls with graphene and alumina nanofluids as working liquid

Microchannel heat sinks (MCHS) are known for providing enhanced cooling performance but their fabrication requires complex and multi-step processes. The recent development of additive manufacturing has enabled the fabrication of state-of-art monolithic structures that had been impossible to build using conventional methods. In this work, a monolithic cross-flow triangular cross-section MCHS was fabricated from aluminum alloy (AlSi10Mg) using the Direct Metal Laser Sintering (DMLS) process. The microchannel wall surface roughness was measured and the cross-section shrinkage of the microchannels was compared with the initial design hydraulic diameter of 500 µm–1000 µm. The MCHS with an initial design hydraulic diameter of 750 µm possessed a relative wall surface roughness, R a of 7.7%. The triangular cross-section hydraulic diameter underwent a shrinkage of 15.2% and 5.3% in terms of the reduction in angle between adjacent side alloys. Experiments were conducted for Reynolds numbers between 50 and 275 with nanofluids containing graphene and Al2O3 nanoparticles in water/water +10% ethylene glycol; these were compared with their respective base fluids. The Poiseuille number indicated that flow was laminar developed with base fluid and laminar developing with nanofluid as coolant. Despite providing the lowest thermal resistance, the graphene nanoparticles in water created the greatest pressure drop leading to a reduced performance coefficient. Al2O3 nanoparticles in water/water +10% ethylene glycol were found to have 7.7% and 20% better performance coefficients than their respective base fluids.

Liquid plug propagation in computer-controlled microfluidic airway-on-a-chip with semi-circular microchannels.

This paper introduces a two-inlet, one-outlet lung-on-a-chip device with semi-circular cross-section microchannels and computer-controlled fluidic switching that enables a broader systematic investigation of liquid plug dynamics in a manner relevant to the distal airways. A leak-proof bonding protocol for micro-milled devices facilitates channel bonding and culture of confluent primary small airway epithelial cells. Production of liquid plugs with computer-controlled inlet channel valving and just one outlet allows more stable long-term plug generation and propagation compared to previous designs. The system also captures both plug speed and length as well as pressure drop concurrently. In one demonstration, the system reproducibly generates surfactant-containing liquid plugs, a challenging process due to lower surface tension that makes the plug formation less stable. The addition of surfactant decreases the pressure required to initiate plug propagation, a potentially significant effect in diseases where surfactant in the airways is absent or dysfunctional. Next, the device recapitulates the effect of increasing fluid viscosity, a challenging analysis due to higher resistance of viscous fluids that makes plug formation and propagation more difficult particularly in airway-relevant length scales. Experimental results show that increased fluid viscosity decreases plug propagation speed for a given air flow rate. These findings are supplemented by computational modeling of viscous plug propagation that demonstrates increased plug propagation time, increased maximum wall shear stress, and greater pressure differentials in more viscous conditions of plug propagation. These results match physiology as mucus viscosity is increased in various obstructive lung diseases where it is known that respiratory mechanics can be compromised due to mucus plugging of the distal airways. Finally, experiments evaluate the effect of channel geometry on primary human small airway epithelial cell injury in this lung-on-a-chip. There is more injury in the middle of the channel relative to the edges highlighting the role of channel shape, a physiologically relevant parameter as airway cross-sectional geometry can also be non-circular. In sum, this paper describes a system that pushes the device limits with regards to the types of liquid plugs that can be stably generated for studies of distal airway fluid mechanical injury.

Performance assessment of electro-osmotic flow of rectangular microchannels with smoothed corners

Microchannel heat sinks are a viable alternative to traditional thermal management systems when high fluxes over small surfaces are involved. To avoid high pressure drops especially when liquids are concerned, electro-osmotic flow, a phenomenon which is relevant at the microscales only, can be employed profitably. Joule heating, which occurs every time an electrical current is circulated through a conductor with finite electrical resistance, may hamper the application of electro-osmotic flows significantly; its effects must therefore be investigated, as should the influence of the entry length on the overall transport phenomena which occur in the microchannel, especially so since channels with uniform temperature at the walls tend to be somewhat short, to mitigate heat generation due to Joule heating. In this paper the transport phenomena occurring within a microchannel of rectangular cross-section with uniform wall temperature through which an electro-osmotic flow occurs is studied, while considering the flow fully developed hydrodynamically but thermally developing (Graetz problem). The corners are then smoothed progressively and the effect of this change in the shape of the cross-section over the non-dimensional dissipated power or temperature difference between wall and fluid is investigated using the performance evaluation criteria introduced by Webb. Correlations are suggested for the Poiseuille and Nusselt numbers for all configurations as are criteria to obtain the maximum allowable channel length, i.e. the length of the channel over which the walls start to cool the fluid, owing to Joule heating, in terms of the hydraulic diameter.

Deciphering viscoelastic cell manipulation in rectangular microchannels.

Viscoelastic focusing has emerged as a promising method for label-free and passive manipulation of micro and nanoscale bioparticles. However, the design of microfluidic devices for viscoelastic particle focusing requires a thorough comprehensive understanding of the flow condition and operational parameters that lead to the desired behavior of microparticles. While recent advancements have been made, viscoelastic focusing is not fully understood, particularly in straight microchannels with rectangular cross sections. In this work, we delve into inertial, elastic, and viscoelastic focusing of biological cells in rectangular cross-section microchannels. By systematically varying degrees of fluid elasticity and inertia, we investigate the underlying mechanisms behind cell focusing. Our approach involves injecting cells into devices with a fixed, non-unity aspect ratio and capturing their images from two orientations, enabling the extrapolation of cross-sectional equilibrium positions from two dimensional (2D) projections. We characterized the changes in hydrodynamic focusing behaviors of cells based on factors, such as cell size, flow rate, and fluid characteristics. These findings provide insights into the flow characteristics driving changes in equilibrium positions. Furthermore, they indicate that viscoelastic focusing can enhance the detection accuracy in flow cytometry and the sorting resolution for size-based particle sorting applications. By contributing to the advancement of understanding viscoelastic focusing in rectangular microchannels, this work provides valuable insight and design guidelines for the development of devices that harness viscoelastic focusing. The knowledge gained from this study can aid in the advancement of viscoelastic particle manipulation technique and their application in various fields.

Heat transfer and pressure drop analysis of a microchannel heat sink using nanofluids for energy applications

Abstract The present research aims to enhance heat transfer in straight and wavy profile heat sinks using the same length and hydraulic diameter with different microchannel geometries (triangular, rectangular, trapezoidal, semi-circular, and circular) for uses in electronics, inkjet printing, high heat flux cooling of lasers, and other domains. The nanofluid employed is water/aluminum oxide (water/Al2O3), and the flow regime is laminar. The range of Reynolds number (Re) in this study was 220 ≤ Re ≤ 550, and the concentrations of nanoparticle Al2O3 with Heavy Water (2H2O) were 1.2 % volume. This investigation uses 3-dimensional Computational Fluid Dynamics (CFD) simulation software to investigate the heat transfer characteristics of several cross-sectioned microchannels. The numerical investigation utilizes the finite volume approach, and the CFD analysis is validated with accessible literature with different wavy profiles. According to the CFD simulation results, the microchannel with a circular cross-section has the highest heat transfer performance (up to 18 %) among the other cross-sections. The circular cross-section microchannel seemed to have the most significant increase in coolant temperature (by 9–22 %). The analysis outcomes prove that the microchannel with a circular cross-section has the highest performance for heat transfer; the triangular channel has the lowest performance under the same geometric parameters and boundary conditions. So, it is suggested that a circular microchannel can be used for a heat-carrying capacity of 150 W/cm2, a hydraulic diameter of 500 µm, and a Reynolds number equal to 500.

Optimization analysis of multi-layered microchannel heat sink

Microchannel heat sinks with a large specific surface area are widely applied to the heat dissipation of microelectronic devices. In this work, a multi-layered microchannel heat sink with a size of was proposed based on the microchannel of the Printed Circuit Heat Exchanger (PCHE). The thermal resistance network model was established for the multi-layered heat sink with a semi-circular cross-section microchannel. Then, the Genetic Algorithm(GA) was employed to optimize the structure and operation parameters of the heat sink based on the thermal resistance network model. The optimized parameters and the corresponding thermal resistance under various power consumption were obtained. The optimization results show that the thermal resistance decreases rapidly with the increase of power consumption. When the power consumption is large, the trend of thermal resistance reduction is gradually flat. When the power consumption increases to 0.8 W, the optimal value of channel layers is up to 12, and the thermal resistance of the heat sink is 0.322 °C/(W/cm2). As the power consumption further increases from 0.8 W, the optimal value of layers remains constant at 12, and the ratio of the channel radius to the plate thickness remains at about 0.56. Three-dimensional numerical simulation is performed to verify the thermal resistance network model, and the results of the thermal network model agree well with the simulation.

Fabrication of Circular Cross-Section Microchannels with 3-D Lattice Arrangement and Their Use as On-Off Valves

In this paper, circular cross-section microchannels with 3-D lattice arrangements are designed and fabricated using the Mosquito method in order to construct on-off valves. The 3-D microchannels with on-off valves consist of two types of lines: the flow lines for chemical liquid flow and the control lines to activate the valves. We confirmed that both a circular cross-section and a PDMS with low elastic modulus used as the microchannel material contribute to a valve that can be closed with a lower pressure. Then, we demonstrated liquid flow to evaluate the functionality of the valve. Fluorescein solution was flown into a flow line. We found that the fluorescence intensity decreases at the intersection between the flow and control lines when the flow line is closed by the inflation of the control line, experimentally confirming the functionality of the valve microchannels fabricated via the Mosquito method.

A review of frictional pressure drop characteristics of single phase microchannels having different shapes of cross sections

Abstract This paper theoretically studied pressure drop variation in microchannels having different cross sections (circular, rectangular, square, trapezoidal, triangular, elliptical, parallel plate, co-centric circles, hexagonal, wavy, smoothed or rounded corners cross sections, and rhombus) for single phase Newtonian fluid (gas and liquid) flow. Based on 41 years (approximately) prior literature (1981–till now), 249 articles were studied and number of correlations of pressure drop calculation in microchannels with or without friction factor equation for four cross sections i.e., rectangular, square, circular, trapezoidal, wavy and triangular is collected and also mentioned their limitations at one place. Other than these four cross sections, there is very few experimental/numerical works was present in the literature. A comparable study was performed for laminar as well as turbulent friction factor to calculate the pressure drop with the help of classical theory for gas and liquid flow in microchannels with circular and rectangular cross sections. Results show wonderful outcomes i.e., correlations of laminar pressure drop study can be extendable for transition and turbulent regime in both types (circular and rectangular) of cross sections of microchannels. In different types of flow regime, it is suggested that for each type of cross section (circular and rectangular) we can go for single correlation for gas/liquid system. It is also investigated that the macro channels pressure drop equations can be used for microchannels up to the certain values of Reynolds number. Basically, this paper provides all possible equations of friction factor related to the microchannels that helps to calculate the pressure drop, is collected at one platform also compared their deviation with conventional channels.

Experimental analysis on the heat transfer performance of gas-liquid Taylor flow in a square microchannel

The flow and heat transfer behaviors of gas–liquid Taylor flow in a 1 × 1 mm square microchannel are experimentally investigated in this paper, focusing on evaluating the slug length, void fraction β, and mixing velocity on the overall performance of Taylor flow in the microchannel. T-shaped junctions of varying widths were employed to control the slug length in the Taylor flow. The experimental results indicate that reducing the junction width can heighten the bubble generation frequency and shorten the bubble/slug length. This reduction in slug length can enhance the heat transfer performance of the Taylor flow in fixed cross-section microchannels, but the degree of enhancement is significantly influenced by the void fraction. When β = 0.5, reducing the slug length can significantly improve the heat transfer coefficient while maintaining a reasonable pressure drop increment, resulting in the best overall performance. The friction pressure drop increases significantly when β = 0.67, and the level of overall performance is low. When β = 0.33, the short slugs have little effect on the performance of Taylor flow, and the change of heat transfer and the friction coefficient is only within 3%. In general, increasing the mixing velocity can improve the overall performance of the Taylor flow. However, at a relatively low void fraction, the pressure drop increase outweighs the heat transfer coefficient, leading to a decline in overall performance. Finally, a correlation is proposed to predict the Nu number under Taylor flow in the three-sided heated rectangular microchannel, with an average error of 11.77%.

Accurate modeling of blood flow in a micro-channel as a non-homogeneous mixture using continuum approach-based diffusive flux model

This paper presents a continuum approach for the blood flow simulation, inside the micro-channel of the few micrometers characteristics dimension, within the context of the finite volume method on unstructured grids. The velocity and pressure fields, for the blood flow, are obtained here by solving the Navier–Stokes equations. A particle transport equation, based on the diffusive flux model, provides the hematocrit distribution (i.e., the red blood cells volume-fraction). The momentum conservation equation for a non-Newtonian fluid model is coupled with the particle transport equation through the constitutive blood viscosity model, and this blood viscosity is dependent on hematocrit and shear rate. The continuum approach for blood flow inside the micro-channel of the length scale of a few micrometers to a few hundred micrometers is expected to break down. Interestingly, the present approach provides meaningful insights into biophysics with less computational cost and shows a good match with the experiments and mesoscale simulation with a maximum average deviation of 11% even at the characteristic dimensions of 10–300 μm. A correlation is proposed for additional-local shear rate in terms of the hematocrit and the ratio of red blood cells diameter to the channel diameter, which helps us to demonstrate an increase in the accuracy and also eliminates the issues of unphysical hematocrit reported in the earlier studies available in the literature. The study is extended to provide new results inside a square and rectangular cross section micro-channels, under a range of inlet parameters.

Microparticle separation using dielectrophoresis-assisted inertial microfluidics: A GPU-accelerated immersed boundary-lattice Boltzmann simulation.

In this study, the migration of microparticles towards the inertial equilibrium positions in a straight microchannel with a square cross sectionin the presence of an inhomogeneous oscillating electric field was examined. The dynamics of microparticles were simulated using the immersed boundary-lattice Boltzmann method of fluid-structure interaction simulation. Moreover, the lattice Boltzmann Poisson solver was applied to calculate the electric field required for calculation of the dielectrophoretic force using the equivalent dipole moment approximation. These numerical methods were implemented on a single GPU coupled with the AA pattern of storing distribution functions in memory to speed up the computationally demanding simulation of microparticles dynamics. In the absence of an electric field, spherical polystyrene microparticles migrate to four symmetric stable equilibrium positions corresponding to the sidewalls of the square cross-sectional microchannel. The equilibrium distance from the sidewall was increased by increasing the particle size. The equilibrium positions near electrodes disappeared and particles migrated to the other equilibrium positions far from the electrodes by the application of the high-frequency oscillatory electric field at voltages beyond a threshold value. Finally, a two-step dielectrophoresis-assisted inertial microfluidics methodology was introduced for particle separation based on the crossover frequencies and the observed threshold voltages of different particles. The proposed method exploited the synergistic effect of dielectrophoresis and inertial microfluidics methods to remove their limitations, allowing the separation of a broad range of polydisperse particle mixtures with a single device in a short time.

Bionic microchannels for step lifting transpiration

Those various cross-sectional vessels in trees transfer water to as high as 100 meters, but the traditional fabrication methods limit the manufacturing of those vessels, resulting in the non-availability of those bionic microchannels. Herein, we fabricate those bionic microchannels with various cross-sections by employing projection micro-stereolithography (PµSL) based 3D printing technique. The circumradius of bionic microchannels (pentagonal, square, triangle, and five-pointed star) can be as small as 100 μm with precisely fabricated sharp corners. What’s more, those bionic microchannels demonstrate marvelous microfluidic performance with strong precursor effects enabled by their sharp corners. Most significantly, those special properties of our bionic microchannels enable them outstanding step lifting performance to transport water to tens of millimeters, though the water can only be transported to at most 20 mm for a single bionic microchannel. The mimicked transpiration based on the step lifting of water from bionic microchannels is also achieved. Those precisely fabricated, low-cost, various cross-sectional bionic microchannels promise applications as microfluidic chips, long-distance unpowered water transportation, step lifting, mimicked transpiration, and so on.

Numerical Study About Nanofluids of Spherical and Tube-Shaped TiO2 Nanomaterials on the Thermal Performance and Entropy Generation of Different Cross-Section Microchannel Heat Sinks

We numerically evaluated the thermal performance of microchannel heat sinks, considering rectangular, hexagonal, and circular cross-sections. Moreover, as a passive heat transfer augmentation technique, dimples were added to improve the rectangular microchannel heat sinks. These simulations use nanofluids based on TiO2 nanoparticles or nanotubes dispersed in bidistilled water as working fluids. The mathematical model considered variable thermophysical properties of the nanofluids; for this purpose, polynomial fittings correlate the dependence of the thermophysical properties on the temperature. We considered a heat flux of q″ = 50 W/cm2 at the microchannel’s lower surface as a boundary condition along with laminar flow conditions. The numerical simulations allowed the Nusselt numbers and entropy generation calculation, which were the basis for the thermal performance calculation. Regarding the effect of TiO2 nanoparticles shape, spherical TiO2 nanoparticles based nanofluids using rectangular microchannels improve the Nusselt number. Moreover, the frictional entropy decreases with nanofluids based on TiO2 nanotubes, but the thermal entropy decreases with nanofluids based on TiO2 nanotubes. Incorporating dimples in the rectangular microchannel enhances the Nusselt numbers and lowers the entropy generation. Considering the Reynolds number range and from the perspective of Nusselt number and entropy generation, we concluded that the microchannels must be operated at a high Reynolds number to improve the microchannel heat sinks thermal performance.

Annular liquid film thermohydraulic and intermittent dryout in microchannel flow boiling

The annular flow regime in microchannels is commonly described as the steady flow of a vapor core with a surrounding liquid film along the channel length. However, limited studies have suggested temporal discontinuity in the liquid film coverage of the microchannel wall. The lack of experimental tools to directly interrogate the liquid flow characteristics and the local surface heat flux has resulted in proliferation of different hypotheses concerning the physics of this process. Surface de-wetting due to high shear stresses at the vapor–liquid interface and liquid film instability and rupture due to perturbation growth at the vapor–liquid interface have been suggested as the underlaying causes of this phenomenon. In this study, a recently developed microsensor array capable of measuring the surface local heat flux and liquid film thickness and velocity is utilized to decipher thermohydraulic characteristics of the annular flow regime and intermittent dryout with unprecedented details. The studies are conducted in a 600 μm square cross-section microchannel using FC-72 as the test fluid at 70–80% exit vapor quality. The results show that liquid film thinning because of evaporation rather than de-wetting and film rupture is responsible for intermittent surface dryout. Studies at different mass and heat fluxes suggest that for a given surface heat flux, there is a mass flux threshold above which intermittent surface dryout vanishes. Furthermore, the experimental liquid and vapor flow velocities and liquid film thickness are used to calculate the liquid–vapor interfacial shear stress. The results suggest that under the studied conditions, the liquid–vapor interfacial shear stress has little impact on the surface intermittent dryout. This study enhances predictable and reliable design and operation of two-phase heat sinks at high exit vapor qualities.