The thermal conductivity of bulk CNT materials based on three-dimensional randomly oriented CNT networks is rather low comparing with that of the individual carbon nanotube, and one of the important reasons has been considered to be the huge intertube thermal resistance. Though intertube thermal resistance can be reduced via welded junctions between the tubes, it is still unknown whether the formation of the welded junctions can enhance the thermal conductivity of random SWCNTs network. In this approach, the calculations of the average number of thermal contacts per SWCNT are compared by two different methods, and influencing factors of the thermal conductivity of random SWCNTs network are discussed; a hybrid macroscopic analytical model is proposed to predict the overall thermal conductivity of the partially sintered randomly oriented carbon nanotube (CNT) networks. Results show that choosing proper fraction of welded contacts can enhance the thermal conductivity of randomly oriented CNT networks effectively.

3D-IC is getting increasingly attractive, as it improves speed and frequency, and reduces power consumption, noise and latency. However, three dimension (3D) integration technologies bring a new serious challenge to chip thermal management with the power density increased exponentially. Interlayer microchannel liquid cooling is thought as a promising and scalable solution for cooling high heat flux 3D-IC. In this paper, firstly channel number, channel width and height parameters of rectangular channel are optimized by the method of multi-objective parameter optimization under given overall size of 5mm in length and 5mm in width. The results show the total thermal resistances can reach very small under individual constraint condition of volume flow rates, but the pressure drop is too larger to accept. The minimum thermal resistance structure can be got by multi-objective optimization at various constraint conditions. It is found that the channel height and width increase with increasing of flow rates at pumping power less than 0.1W and pressure drop less than 20kPa. Secondly, the zigzag channels are designed on the basis of the optimized rectangular channel structure. The expansion and contraction ratio as an important parameter is optimized by numerical simulation. The thermal enhancement factor and Nusselt number measure the comprehensive performances of heat transfer. The results show heat transfer characteristic is enhanced with the decreasing of expansion and contraction ratio. Besides, the maximum junction temperature and maximum temperature difference are also reduced. 3D-IC with wave channel of β=3/7 is more promising for interlayer cooling.

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.

Amongst various porous media, open-cell metallic foams exhibit distinctive properties: relatively low manufacturing cost, ultra-low density, moderate stiffness and strength, and high surface area-to-volume ratio. They have been, therefore, utilized in a variety of applications such as microelectronics cooling, fuel cells, and compact heat exchangers. For such applications, the knowledge of pressure drop of fluid flowing across the foam is often a key issue, enabling control of fluid flow, heat transfer enhancement, planning and designing chemical engineering processes, optimal flow analysis as well as practical designs. We present in this paper an analytical model capable of predicting the pressure drop of a Newtonian incompressible fluid flowing unidirectionally across isotropic and fully-saturated micro open-cell cellular foams within the Darcy and Forchheimer flow regimes. Analytical exploitations are conducted to determine the foam permeability and inertial coefficient. The analytical model is based on the basis of volume-averaging approach and the assumption of piece-wise plane Poiseuille flow with the modified cubic lattice with spherical node at the junction of struts. To better mimic the foam struts shape, a concave-triangular-shaped strut consisting of two nose-to-nose cones is considered and particular attentions have been paid to both analytically and numerically examine the node shape as well as struts shape effect. Built upon a generalized tortuosity model derived from the modified cubic unit cell, an analytical model of permeability on the basis of a cubic unit cell is developed, valid within a typical engineering range of porosity (ε = 0.86 ∼ 0.98) and pore size (0.254 mm ∼ 5.08 mm). With the effect of Reynolds number considered, the pore-scaled Reynolds number dependent drag coefficient expression is introduced and through this the inertial coefficient is analytically modeled on the basis of flow over bluff bodies, which is found to agree well with experimental data from various sources. The modeling procedure for pressure drop (permeability and inertial coefficient) is based on physical principles and geometrical considerations, and the model predictions agree satisfactorily with existing experimental data. Results show that by building the analytical model on the basis of a cubic unit cell to represent the topology of metallic foams, pressure drops as well as hydrodynamic conditions within both the Darcy and Forchheimer regimes in a Newtonian fluid can be analytically predicted.

In this paper, we present a novel design of bilayer polydimethylsiloxane (PDMS) microchannel formed by bifurcated junction, from which each curved branch lies on the upper and lower layer, respectively. With this 3D platform, we aim to investigate droplet formation and subsequent fission in a multiphase system using non-Newtonian fluids, which are ubiquitous in daily life and have been widely used in industrial applications including biomedical engineering, food production, personal care and cosmetics, and material synthesis. Numerical model has been established to characterize the non-Newtonian effect to droplet fission and associated breakup dynamics when droplet flows through 3D bifurcated junction, where droplets can deform significantly on account of the confining geometric boundaries, and the flow of the surrounding non-Newtonian liquid, both of which control the deformation and breakup of each mother droplet into two daughter droplets. Dispersions of sodium carboxymethyl cellulose in water, and dispersions of polyvinylchloride in dioctylphthalate have been used as model fluids in the study, with the former one possessing shear-thinning behaviour, while the latter one possessing shear-thickening behaviour. The understanding of the droplet fission in the novel microstructure will enable more versatile control over the emulsion formation when non-Newtonian fluids are involved. The model systems in the study can be further developed to investigate the mechanical property of emulsion templated particles such as drug encapsulated microcapsules when they flow through complex media structures, such as blood capillaries or the porous tissue structure, which feature with bifurcated junction.

The Marangoni convections in microchannels are of interest in various applications such as heat transfer, material and microfluidics. In this paper, the Marangoni asymmetrical instability at a T-junction in a microchannel is investigated. The T-junction is formed by a main channel which water going through and a side channel which is open to the ambient. A convex meniscus is formed in side channel near the T-junction, evaporating/volatizing into the ambient. The consumption of water due to the evaporation is compensated by the supply from the main channel. It is found that for weak evaporations, the evaporation-induced Marangoni convections are symmetrical. However, when the evaporation reaches a critical intensity, the symmetrical Marangoni convection becomes unstable and evolves into an asymmetrical one, with one single vortex under the meniscus. More interestingly, the vortex creates a steady convection through the main channel from its one end to the other, just like a pump. The pumping flow rate is found linearly correlated with the Marangoni number at the T-junction.

A carbon nanotube (CNT) aerogel is a low-density network of small diameter single-walled CNTs held together by van del Waals forces. Due to the excellent mechanical, thermal, and electrical properties of individual CNTs and the potential to fuse the junctions in the aerogel, CNT aerogels are candidates for ultralight structural media, radiation detectors, thermal insulators, and electrical conductors. Using molecular dynamics (MD) simulation, we predict the thermal conductance of the junction formed between two CNTs. To access the range of conditions present in the aerogel, we test the effects of different boundary conditions, the CNT lengths, and the rotational angle of the CNTs. A 3-D network model of the aerogel is built that will be used with the MD predictions to estimate the aerogel thermal conductivity.

Using a T type probe, the effect of the interstitial material (interposer) on the thermal contact resistance of a junction has been estimated by measuring an individual carbon fiber with different interposers, including the solidified metallic powder, lubricant grease, and dry contact as a comparison. For the metallic powder, the thermal contact conductance was obtained to be 3.0 M W m −2 K −1 by changing the fiber length when the same contact between the fiber and the hot wire was maintained. However, this method can only be applicable to the solidified contact, and the stability of the operating temperature is a must in each length measurement. To estimate the thermal contact resistance of the lubricant Apiezon N grease, even a dry contact, an improved T type probe was employed, by applying an alternative current to the hot wire. This method was verified by measuring the same type of carbon fiber in the frequency range of 0.1 to 1Hz based on a Labview-based virtual lock-in measurement system. The same value of the thermal effusivity of the test fiber was obtained with different interposers, and the thermal contact conductances for the dry contact and high vacuum grease were found to be 0.10 M W m −2 K −1 and 0.26 M W m −2 K −1 , respectively.

This paper reports flow visualization experiments of fluid mixing in a microchamber on a rotating disk. The two centrifuge-driven sample fluids were brought in contact at the Y-junction microchannel and then flowed to a circular mixing chamber where the main course of mixing took place. The flow images were acquired using a micro-image-capturing unit in synchronization with the rotational motion to allow only one shot of the targeted object on the rotating disk per revolution. Both the visualization and quantification of flow images show that the mixing efficiency of the two fluids depends not only on the rotational speed but also on the depth of the channels. It is found that the mixing efficiency generally decrease with increasing rotational speed in the lower speed range (≤ 420 rpm). Beyond this lower speed range, the mixer with a larger channel depth h = 300 μm shows an increase of mixing efficiency with increasing rotational speed to reach as much as 83% at 1200 rpm. For the mixer with a smaller channel depth h = 200 μm, however, the mixing efficiency continues deceasing or becomes flat with increasing rotational speed. It is also found that the counter-clockwise rotation produces a better mixing efficiency than the clockwise rotation in the high speed range.

This paper aims to investigate a mechanism of microdroplet formation using “multicolor confocal micro particle image velocimetry (PIV)” technique. The present system can measure dynamical behavior of multiphase flow separately and simultaneously. It also enables to identify the interactions between two immiscible fluids. We have applied this system to measure the water droplet formation at a micro T-shaped junction. We have also succeeded in dispersing fluorescent tracer particles into both phases. The interaction between the internal flow of to-be-dispersed water phase and of continuous oil phase is measured as a liquid-liquid multiphase flow. As a result of PIV measurement and interfacial geometry scanning, the relationship between flow structure of each fluid and interfacial geometry is clarified. It indicates that the gap between the tip of discontinuous flow and capillary wall, and interface area play an important role in the flow structure and shear stress on the interface.