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The influence of using nanofluids on heat transfer augmentation and characteristics of fluid flow in rectangular cross sectional microchannel heat sink (MCHS) is numerically investigated for single and double stack microchannels at uniform heat flux, q = 100 W/cm2 and Reynolds number ranging from 200 to 1500. In this study, alumina–water (Al2O3-H2O) nanofluid with different nanoparticle volume concentrations from 1% to 5% was used as a coolant for the MCHS. The three-dimensional steady, laminar flow and heat transfer governing equations are solved by finite volume method using ANSYS-Fluent 18.2. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, pumping power and volume reduction. The results reveal that increasing the nanoparticles concentration improves the cooling process. The maximum heat transfer augmentation in the current study was achieved at the highest concentration of 5%. For example, at this concentration the temperature was reduced by 7.3% using single stack microchannel. However, 33.5% temperature reduction was achieved using double stack microchannels. Also, it results in an increase in the heat transfer coefficient by 13.12% compare to that of the pure water. On the other hand, for a single stack at nanoparticle concentration of 5%, both pressure drop and power increased by 10% and 8%, respectively. Because of the significant influence of the nanofluid on the improvement of the cooling process, an estimation of the volume reduction was made. It was found that 62.6% volume reduction for single stake at concentration 5% can be achieved.

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.

The use of acoustic waves for microfluidic aggregation has become widespread in chemistry, biology and medicine. Although numerous experimental and analytical studies have been undertaken to study the acoustophoretic aggregation mechanisms, few studies have been conducted to optimise the device design. This paper presents a numerical investigation of the acoustophoresis of microparticles suspended in compressible liquid. The wall of the rectangular microchannel is made of Polydimethylsiloxane (PDMS), and Standing Surface Acoustic Waves (SSAW) are introduced into the channel from the bottom wall. First, the relative amplitude of the acoustic radiation force and the viscous drag force is evaluated for particles of different radii ranging from 0.1 μ m to 15 μ m . Only when the particle size is larger than a critical value can the particles accumulate at acoustic pressure nodes (PNs). The efficiency of the particle accumulation depends on the microchannel height, so an extensive parametric study is then undertaken to identify the optimum microchannel height. The optimum height, when normalised by the acoustic wavelength, is found to be between 0.57 and 0.82. These findings provide insights into the design of acoustophoretic devices.

Recently published studies have shown that microfluidic devices fabricated by in-house three-dimensional (3D) printing, computer numerical control (CNC) milling and laser engraving have a good quality of performance. The 3-in-1 3D printers, desktop machines that integrate the three primary functions in a single user-friendly set-up are now available for computer-controlled adaptable surface processing, for less than USD 1000. Here, we demonstrate that 3-in-1 3D printer-based micromachining is an effective strategy for creating microfluidic devices and an easier and more economical alternative to, for instance, conventional photolithography. Our aim was to produce plastic microfluidic chips with engraved microchannel structures or micro-structured plastic molds for casting polydimethylsiloxane (PDMS) chips with microchannel imprints. The reproducability and accuracy of fabrication of microfluidic chips with straight, crossed line and Y-shaped microchannel designs were assessed and their microfluidic performance checked by liquid stream tests. All three fabrication methods of the 3-in-1 3D printer produced functional microchannel devices with adequate solution flow. Accordingly, 3-in-1 3D printers are recommended as cheap, accessible and user-friendly tools that can be operated with minimal training and little starting knowledge to successfully fabricate basic microfluidic devices that are suitable for educational work or rapid prototyping.

This paper investigates the effect of silver nanofluid on a flat-plate pulsating heat pipe (FP-PHP). The simulation was performed using FLUENT 15.0 software, for which a three-dimensional model having a microchannel structure in the condensation section was developed. The developed model adopted the volume of fluid (VOF) method to track the internal vapor-liquid interface of the FP-PHP and to observe the state of the two-phase flow. The result revealed the evident presence of various types of bubble flows, including dispersed bubble flows, slug flows, annular flows, and column flows in the evaporation section. Trends in thermal resistance variation during simulation were studied by changing the volume fraction, liquid filling rate, and heating power. The thermal resistance of the FP-PHP containing silver nanofluid was lower and the FP-PHP exhibited stable state when the heating power reached 120 W. The optimal volume fraction of silver nanofluid was 1%, and the FP-PHP containing sliver nanofluid exhibited increased heat transfer efficiency.

In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

Liquid infused surfaces (LIS) achieve non-wetting properties through asperity structures containing pockets of a lubricating liquid rather than air. Although studies have demonstrated several potential applications of LIS, the effect of liquid infused texture on the flow of liquid in microchannels has not been systematically addressed. The present work focuses on the effect of liquid-infused texture geometry and the infused liquid properties on the flow characteristics of a bulk fluid in rectangular and round microchannel geometries. Considering four different textured geometries of transverse ridges, longitudinal ridges, in-lined square posts and staggered squares, a detailed parametric study is presented to assess the resulting drag reduction and friction factor, expressed in terms of the Poiseuille number, on the flow in microchannels. The effect of geometrical parameters is investigated for different asperity solid fraction ( ϕ s ), asperity height ( h s ) and channel thickness ( H ) or tube diameter ( D ), while the effect of the infused fluid is studied in terms of a ratio of its viscosity relative to the viscosity of the flowing bulk fluid. Based on the results of the parametric study an extension of the conventional Moody diagram for non-wetting surfaces is developed. Further, a design of experiments study is conducted to relate the drag reduction and Poiseuille number to the various governing parameters, using which conditions that maximize drag reduction are identified.

This work presents a novel method for the non-invasive, in-line monitoring of mixing processes in microchannels using the Raman photometric technique. The measuring set-up distinguishes itself from other works in this field by utilizing recent state-of-the-art customized photon multiplier (CPM) detectors, bypassing the use of a spectrometer. This addresses the limiting factor of integration times by achieving measuring rates of 10 ms. The method was validated using the ternary system of toluene&ndash, water&ndash, acetone. The optical measuring system consists of two functional units: the coaxial Raman probe optimized for excitation at a laser wavelength of 532 nm and the photometric detector centered around the CPMs. The spot size of the focused laser is a defining factor of the spatial resolution of the set-up. The depth of focus is measured at approx. 85 &micro, m with a spot size of approx. 45 &micro, m, while still maintaining a relatively high numerical aperture of 0.42, the latter of which is also critical for coaxial detection of inelastically scattered photons. The working distance in this set-up is 20 mm. The microchannel is a T-junction mixer with a square cross section of 500 by 500 &micro, m, a hydraulic diameter of 500 &micro, m and 70 mm channel length. The extraction of acetone from toluene into water is tracked at an initial concentration of 25% as a function of flow rate and accordingly residence time. The investigated flow rates ranged from 0.1 mL/min to 0.006 mL/min. The residence times from the T-junction to the measuring point varies from 1.5 to 25 s. At 0.006 mL/min a constant acetone concentration of approx. 12.6% was measured, indicating that the mixing process reached the equilibrium of the system at approx. 12.5%. For prototype benchmarking, comparative measurements were carried out with a commercially available Raman spectrometer (RXN1, Kaiser Optical Systems, Ann Arbor, MI, USA). Count rates of the spectrophotometer surpassed those of the spectrometer by at least one order of magnitude at identical target concentrations and optical power output. The experimental data demonstrate the suitability and potential of the new measuring system to detect locally and time-resolved concentration profiles in moving fluids while avoiding external influence.

In recent years, surface acoustic wave (SAW) devices have demonstrated great potentials and increasing applications in the manipulation of nano- and micro-particles including biological cells with the advantages of label-free, high sensitivity and accuracy. In this letter, we introduce a novel tilted-angle SAW devices to optimise the acoustic pressure inside a microchannel for cancer-cell manipulation. The SAW generation and acoustic radiation force are improved by seamlessly patterning electrodes in the space surrounding the microchannel. Comparisons between this novel SAW device and a conventional device show a 32% enhanced separation efficiency while the input power, manufacturing cost and fabrication effort remain the same. Effective separation of HeLa cancer cells from peripheral blood mononuclear cells is demonstrated. This novel SAW device has the advantages in minimizing device power consumption, lowering component footprint and increasing device density.

In this paper, we present the results of a systematic numerical study of the flow and mixing modes of fluids in micromixers of various configurations, in particular, an analysis of passive micromixers, the most widely used in practice, as well as the main methods to intensify mixing. The advantages of microstructure reactors can significantly reduce reaction times and increase productivity compared to traditional bulk reactors. Four different geometries of micromixers, including the straight T-shaped microchannel, were considered. The effect of the geometrical patterns of micromixers, as well as of the Reynolds number on flow regimes and mixing efficiency were analyzed. The Reynolds number varied from 1 to 300. Unlike other studies, the efficiency of the considered mixers was for the first time compared with the cost of pressure loss during pumping. As a result, the efficiency of the most optimal micromixer in terms of hydraulic mixing and the optimal operation ranges were determined. It was shown that the maximum normalized mixing efficiency in the entire range of Re numbers was noted for mixer, in which a vortex-based intensification of mixing occurs due to the flow swirling in cylindrical chambers. This mixer allows mixing the fluids 600 times more efficiently than a straight T-mixer, while all other conditions being equal.

Thermal performance enhancement in microchannel heat sinks has recently become a challenge due to advancements in modern microelectronics, which demand compatibility with heat sinks able to dissipate ever-increasing amounts of heat. Recent advancements in manufacturing techniques, such as additive manufacturing, have made the modification of the microchannel heat sink geometry possible well beyond the conventional rectangular model to improve the cooling capacity of these devices. One such modification in microchannel geometry includes the introduction of secondary flow channels in the walls between adjacent mainstream microchannels. The present study computationally models secondary flow channels in regular trapezoidal and parallel orientations for fluid circulation through the microchannel walls in a heat sink design. The heat sink is made of silicon wafer, and water is used as the circulating fluid in this study. Continuity, momentum, and energy equations are solved for the fluid flow through the regular trapezoidal secondary flow and parallel secondary flow designs in the heat sink with I-type, C-type, and Z-type inlet–outlet configurations. Plots of velocity contours show that I-type geometry creates optimal flow disruption in the heat sink. Therefore, for this design, the pressure drop and base plate temperatures are plotted for a volumetric flow rate range, and corresponding contour plots are obtained. The results are compared with corresponding trends for the conventional rectangular microchannel design, and associated trends are explained. The study suggests that the flow phenomena such as flow impingement onto the microchannel walls and formation of vortices inside the secondary flow passages coupled with an increase in heat transfer area due to secondary flow passages may significantly improve the heat sink performance.

International audience; The survival and function of thick tissue engineered implanted constructs depends on pre-existing, embedded, functional, vascular-like structures that are able to integrate with the host vasculature. Bioprinting was employed to build perfusable vascular-like networks within thick constructs. However, the improvement of oxygen transportation facilitated by these vascular-like networks was directly quantified. Using an optical fiber oxygen sensor, we measured the oxygen content at different positions within 3D bioprinted constructs with and without perfusable microchannel networks. Perfusion was found to play an essential role in maintaining relatively high oxygen content in cell-laden constructs and, consequently, high cell viability. The concentration of oxygen changes following switching on and off the perfusion. Oxygen concentration depletes quickly after pausing perfusion but recovers rapidly after resuming the perfusion. The quantification of oxygen levels within cell-laden hydrogel constructs could provide insight into channel network design and cellular responses.

In this paper, a new experimental setup was proposed to realize the constant-heat-flux boundary condition based on a counter flow microchannel heat exchanger with the same heat capacity rate of the hot and cold streams. This approach provides a constant fluid temperature gradient along the surfaces. An analytical two-dimensional model was developed to describe the heat transfer processes in the hot stream and the cold stream, respectively. In the experiments, DI-water was employed as the working fluid. Laser induced fluorescence (LIF) method was used to measure the fluid temperature field within the microchannel. Different combinations of flow rates were studied to investigate the heat transfer characteristics in the microchannel. The measured mean temperature distribution matched well with the proposed analytical model. A correlation for Nusselt number (Nu) was proposed based on the experimental Reynolds number (Re

The ability to enhance heat transfer rates of shear-thinning fluids in microchannel devices is evaluated numerically for steady and time-dependent flows in the range of 400 < Re

Blood coagulation is a critical and complex reaction that involves various chemical substances, such as prothrombin, fibrinogen, and fibrin. The process can be divided into three main steps, namely the formation of the prothrombin activator, conversion of prothrombin to thrombin, and conversion of fibrinogen to fibrin. In this study, an ANSYS simulation is carried out to determine the prothrombin time (PT) of blood, the chemical changes that occur during coagulation and the anticoagulation factor. The addition of deionized water to the microchannels before the addition of blood and reagents results in a two-phase flow. The evaluation of this two-phase flow is necessary, and dynamic simulations are required to determine the PT. The chemical rate constant and order of the chemical reaction are derived from the actual prothrombin time. Moreover, the genetic algorithms in PYTHON and ANSYS are used to estimate chemical reaction parameters for a 20 s PT. The blood and anticoagulant exhibit increased dynamic behavior in the microchannel. In addition, particles are added to the microchannel and the dynamic mesh method is used to simulate the flow behaviors of the red and white blood cells in the microchannel. The PTs for different volumes of blood are also reported.

The modeling of transfer processes is a step in the generalization and interpretation of experimental data on heat transfer. The developed two-dimensional model is based on a homogeneous mixture model for boiling water flow in a microchannel with a new evaporation submodel. The outcome of the simulation is the distribution of velocity, void fraction and temperature profiles in the microchannel. The predicted temperature profile is consistent with the experimental literature data.

We report a microfluidic chaotic micromixer with staggered herringbone grooves having a geometry optimized for fast mobile-phase modification at the interface of a two-dimensional liquid chromatography system. The volume of the 300 mm mixers is 1.6 microliters and they provide mixing within 26 s at a flow rate of 4 mL min(-1) and 0.09 s at a flow rate of 1000 mL min(-1). Complete mixing is achieved within a distance of 3 cm along the 5 cm-long microchannel over the whole range of flow rates. The mixers can be used to mix aqueous phosphate-buffered saline solutions with methanol or acetonitrile at different ratios (1 : 2, 1 : 5 and 1 : 10). We also describe in detail an improved fabrication protocol for these mixers using a two-step soft photolithographic procedure. Mixers are made by replication in poly(dimethylsiloxane).

Gravity-driven cooling systems employing microchannel flow boiling can become a scalable and viable long term solution for the cooling of datacenter servers. In order to design this emerging type of cooling system, a new dynamic simulation tool using interconnected PDEs is described in the first part of the present paper followed by the validation of the modeling for both steady and dynamic regimes using a first-of-a-kind 15 cm-height thermosyphon test bench. A corrected error of 0.4 K was found for 7 different steady-state mean chip temperatures and heat load disturbances were predicted with the same 2-stage process presented here as the experimental results. The code was then used to predict the behavior of a mini-thermosyphon that would fit within the height of a 2U server. Multiple steady-state solutions were found with both stable and unstable operating states. A sensitivity study demonstrated that a 30% increase in the riser internal diameter led to 10-60% increase in the mass flow rate depending on the heat flux. Then, simulations with unbalanced heat loads showed that the flow gets self redistributed into the CPU with the highest heat load, e.g. 1.7 times larger flow rate for a heat flux 3 times larger. Finally, heat load and water coolant flow rate disturbances were also simulated and discussed. (C) 2016 Elsevier Ltd. All rights reserved.

International audience; Amorphous silicon based microchannel plates are being developed to overcome performance limits of conventional microchannel plates. They offer a new flexibility and ease of fabrication. A comprehensive AMCP model is being constructed to analyze the performances of AMCPs. It includes Monte Carlo simulation of secondary electron emission distribution as a function of energy and angles and finite element analysis multiphysics software to compute electron trajectories. The paper presents the results of Monte Carlo simulations of secondary emission functions in silicon and the high secondary emissive material Al 2 O 3. We discuss the gain and potential performance as a function of geometry of such devices. The validity of the Eberhardt model for the analysis of AMCPs is also addressed.

In this paper, I will present an abridged overview of what is presented in my keynote lecture. I will recount some of the initial history of microchannel two-phase flow and boiling work by others that enticed me to join into the development of this field and will then focus on some my own team's work at the EPFL in Lausanne during the last 20 years, covering work on microchannel flow boiling, new flow visualization/image processing techniques, flow stabilization, heat transfer models, flow pattern maps, micro-two-phase cooling systems, and numerical modeling of bubbly/slug flows (not all in this paper), with much effort to get micro-two-phase cooling into practice. The lecture gives a visual view of microchannel flow boiling, some prediction methods and some prototype units.

The flow rate of a pressure-driven liquid through a microchannel may be enhanced by texturing its no-slip boundaries with grooves aligned with the flow. In such cases, the grooves may contain vapour and/or an inert gas and the liquid is trapped in the Cassie state, resulting in (apparent) slip. The flow-rate enhancement is of benefit to different applications including the increase of throughput of a liquid in a lab-on-a-chip, and the reduction of thermal resistance associated with liquid metal cooling of microelectronics. At any given cross-section, the meniscus takes the approximate shape of a circular arc whose curvature is determined by the pressure difference across it. Hence, it typically protrudes into the grooves near the inlet of a microchannel and is gradually drawn into the microchannel as it is traversed and the liquid pressure decreases. For sufficiently large Reynolds numbers, the variation of the meniscus shape and hence the flow geometry necessitates the inclusion of inertial (non-parallel) flow effects. We capture them for a slender microchannel, where our small parameter is the ratio of ridge pitch-to-microchannel height, and order-one Reynolds numbers. This is done by using a hybrid analytical–numerical method to resolve the nonlinear three-dimensional (3-D) problem as a sequence of two-dimensional (2-D) linear ones in the microchannel cross-section, allied with non-local conditions that determine the slowly varying pressure distribution at leading and first orders. When the pressure difference across the microchannel is constrained by the advancing contact angle of the liquid on the ridges and its surface tension (which is high for liquid metals), inertial effects can significantly reduce the flow rate for realistic parameter values. For example, when the solid fraction of the ridges is 0.1, the microchannel height-to-(half) ridge pitch ratio is 6, the Reynolds number of the flow is 1 and the small parameter is 0.1, they reduce the flow rate of a liquid metal (Galinstan) by approximately 50 %. Conversely, for sufficiently large microchannel heights, they enhance it. Physical explanations of both of these phenomena are given.

The entrance region constitutes a considerable fraction of the channel length in miniaturized devices. Laminar slip flow in microchannel plate fin heat sinks under hydrodynamically developing conditions is investigated semi-analytically and numerically in this paper. The semi-analytical model for the pressure drop of microchannel plate fin heat sinks is obtained by solving the momentum equation with the first-order velocity slip boundary conditions at the channel walls. The simple pressure drop model utilizes fundamental solutions from fluid dynamics to predict its constitutive components. The accuracy of the model is examined using computational fluid dynamics (CFD) simulations and the experimental and numerical data available in the literature. The model can be applied to either apparent liquid slip over hydrophobic and superhydrophobic surfaces or gas slip flow in microchannel heat sinks. The developed model has an accuracy of 92 percent for slip flow in microchannel plate fin heat sinks. The developed model may be used to predict the pressure drop of slip flow in microchannel plate fin heat sinks for minimizing the effort and expense of experiments, especially in the design and optimization of microchannel plate fin heat sinks.

Porous hydrogel scaffolds are ideal candidates for mimicking cellular microenvironments, regarding both structural and mechanical aspects. We present a novel strategy to use uniquely designed ceramic networks as templates for generating hydrogels with a network of interconnected pores in the form of microchannels. The advantages of this new approach are the high and guaranteed interconnectivity of the microchannels, as well as the possibility to produce channels with diameters smaller than 7 μm. Neither of these assets can be ensured with other established techniques. Experiments using the polyacrylamide substrates produced with our approach have shown that the migration of human pathogenic Acanthamoeba castellanii trophozoites is manipulated by the microchannel structure in the hydrogels. The parasites can even be captured inside the microchannel network and removed from their incubation medium by the porous polyacrylamide, indicating the huge potential of our new technique for medical, pharmaceutical, and tissue engineering applications.

A numerical investigation is carried out on the effects of flow maldistribution on the temperature uniformity and overall thermal resistance in double-layered microchannel heat sinks. Different flow maldistribution models accounting for the effects of some typical header designs are considered together with different combinations of the average inlet velocity in the two layers of microchannels for a given total mass flow rate. The numerical simulations are carried out using an in-house FEM procedure previously developed by the authors for the analysis of cross-flow microchannel heat exchangers.

Integrated microchannel cooling is a very promising concept for thermal management of 3D ICs, because it offers much higher cooling performance than conventional forced-air convection. The thermo-fluidic simulations of such chips are usually performed using a computational fluid dynamics (CFD) approach. However, due to the complexity of the fluid flow modelling, such simulations are typically very long and faster models are therefore considered. This paper demonstrates the advantages of TIMiTIC—a compact thermal simulator for chips with liquid cooling—and shows its practical usefulness in design space exploration of 3D ICs with integrated microchannels. Moreover, thermal simulations of a 3D processor model using the proposed tool are used to estimate the optimal power dissipation profile in the chip and to prove that such an optimal profile allows for a very significant (more than 10 °C) peak temperature reduction. Finally, a custom correlation metric is introduced which allows the comparison of the power distribution profiles in terms of the peak chip temperature that they produce. Statistical analysis of the simulation results demonstrates that this metric is very accurate and can be used for example in thermal-aware task scheduling or dynamic voltage and frequency scaling (DVFS) algorithms.

The red blood cell (RBC) aggregation phenomenon is majorly responsible for the non-Newtonian nature of blood, influencing the blood flow characteristics in the microvasculature. Of considerable interest is the behaviour of the fluid at the bifurcating regions. In vitro experiments, using microchannels, have shown that RBC aggregation, at certain flow conditions, affects the bluntness and skewness of the velocity profile, the local RBC concentration, and the cell-depleted layer at the channel walls. In addition, the developed RBC aggregates appear unevenly distributed in the outlets of these channels depending on their spatial distribution in the feeding branch, and on the flow conditions in the outlet branches. In the present work, constitutive equations of blood viscosity, from earlier work of the authors, are applied to flows in a T-type bifurcating microchannel to examine the local viscosity characteristics. Viscosity maps are derived for various flow distributions in the outlet branches of the channel, and the location of maximum viscosity magnitude is obtained. The viscosity does not appear significantly elevated in the branches of lower flow rate as would be expected on the basis of the low shear therein, and the maximum magnitude appears in the vicinity of the junction, and towards the side of the outlet branch with the higher flow rate. The study demonstrates that in the branches of lower flow rate, the local viscosity is also low, helping us to explain why the effects of physiological red blood cell aggregation have no adverse effects in terms of in vivo vascular resistance.

In this paper, a combined numerical and experimental approach for the estimation of the average friction factor along adiabatic microchannels with compressible gas flows is presented. Pressure-drop experiments are performed for a rectangular microchannel with a hydraulic diameter of 295 &mu, m by varying Reynolds number up to 17,000. In parallel, the calculation of friction factor has been repeated numerically and results are compared with the experimental work. The validated numerical model was also used to gain an insight of flow physics by varying the aspect ratio and hydraulic diameter of rectangular microchannels with respect to the channel tested experimentally. This was done with an aim of verifying the role of minor loss coefficients for the estimation of the average friction factor. To have laminar, transitional, and turbulent regimes captured, numerical analysis has been performed by varying Reynolds number from 200 to 20,000. Comparison of numerically and experimentally calculated gas flow characteristics has shown that adiabatic wall treatment (Fanno flow) results in better agreement of average friction factor values with conventional theory than the isothermal treatment of gas along the microchannel. The use of a constant value for minor loss coefficients available in the literature is not recommended for microflows as they change from one assembly to the other and their accurate estimation for compressible flows requires a coupling of numerical analysis with experimental data reduction. Results presented in this work demonstrate how an adiabatic wall treatment along the length of the channel coupled with the assumption of an isentropic flow from manifold to microchannel inlet results in a self-sustained experimental data reduction method for the accurate estimation of friction factor values even in presence of significant compressibility effects. Results also demonstrate that both the assumption of perfect expansion and consequently wrong estimation of average temperature between inlet and outlet of a microchannel can be responsible for an apparent increase in experimental average friction factor in choked flow regime.

A numerical simulation of a rectangular microchannel with partially superheated wall area is carried out in order to gain better understanding of the physical mechanism behind boiling in such confined geometries. Variation of the initial bubble position and boundary conditions for temperature and velocity fields is performed to extract the distinct influence on the vapor bubble growth. A significant difference in the motion and growth of the vapor bubble is observed depending on the operating conditions.