A numerical study of bubble growing during saturated and sub-cooled flow boiling in micro channels

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.A CFD study of bubbles growing in a mini-channel with a diameter of 0.64 mm has been done. Coupled level set and volume of fluid (CLSVOF) method is applied to capture the two phase interface. Geo-reconstruct method is used to re-construct the two-phase interface. A constant velocity inlet boundary with mass flux 335 /2 and a heated boundary wall with constant heat flux (10/2 ) is applied. Both saturated and sub-cooled inlet condition are studied. The growth of bubbles and the transition of flow regime differs each other under these two conditions. Sub-cooling significantly lowers the bubble growth rate. However, it does not affect the heat transfer coefficient at the same level due to its complicated heat transfer mechanism

Stretching of a capillary bridge featuring a particle-laden interface: particle sedimentation in the interface

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Colloidal particles adsorbed at fluid interfaces can be subject to external forces, for instance of magnetic, electrical, or gravitational origin. To develop a tool that will enable to study the effect of these forces on interfacial particle transport, we derive a transport equation for the surface particle concentration using the method of volume averaging. This equation is specialised to the problem of particle sedimentation induced by external forces on an axisymmetric capillary bridge stretched with assigned constant velocity between two circular plates. The equation for the interfacial concentration is one-way coupled to the unsteady Stokes equation in the capillary bridge, and solved in the thin-thread approximation, in the limit of small capillary and Bond numbers and for moderate area fractions. We find that owing to the competition between particle settling in one direction, and fluid velocity in the opposite direction, a concentration peak develops between the neck region and the moving plate. Hydrodynamic interactions, modelled through a concentration-dependent hindrance function, have the effect of steepening the shock-like concentration gradients that develop in the interface

Numerical Simulation of Microflows with Moment Method

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.A series of hyperbolic moment equations is derived for the Boltzmann equation with ES-BGK collision term. These systems can be obtained through a slight modification in the deduction of Grad’s moment equations, and such a method is suitable for deriving systems with moments up to any order. The systems are equipped with proper wall boundary conditions so that the number of equations in the boundary conditions is consistent with the hyperbolic structure of the moment system. Our numerical scheme for solving the hyperbolic moment systems is of second order, and a special mapping method is introduced so that the numerical efficiency is highly enhanced. Our numerical results are validated by comparison with the DSMC results. Through the numerical solutions of systems with increasing number of moments, the convergence of the moment method is clearly observed

Pool Boiling Enhanced by Electric Field Distribution in Microsized Space

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this study, the enhancement of boiling heat transfer by electrostatic pressure was experimentally and analytically investigated. A fluorinated dielectric liquid was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. In order to make clear the enhancement mechanisms, three microsized slit electrodes were designed with different slit widths, electrode widths, and total slit lengths over the boiling surface. Slits of several hundred micrometers were formed in the electrode, so as to remove vapor bubbles from the boiling surface by electrostatic pressure. The boiling surface was electrically grounded, and the electrode was placed above the boiling surface at heights of 200 μm to 400 μm. The maximum heat flux was 76 W/cm2 by the application of an electric field of －7 kV/mm, which was 3.5 times over pool boiling without the electrode. The previous analytical equation of pool boiling exhibited the essential feature of the effect of the electric field on the boiling heat transfer, and showed good agreement with the experimental results

Towards the identification of spatially resolved mechanical properties in tissues and materials: State of the art, current challenges and opportunities in the field of flow measurements

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.This work is focused on optical methods that provide tomographic reconstructions of the structure of materials and tissues. Phase information can also be used to measure 3-D displacement and strain fields with interferometric sensitivity. Different approaches are presented, including recent developments in phase contrast wavelength scanning interferometry and a combination of optical coherence tomography and digital volume correlation to estimate elastic properties of synthetic phantoms and porcine corneas. Inversion algorithms based on finite elements and the Virtual Fields Method (VFM) are used to extract mechanical properties from the knowledge of the applied loads, geometry and measured deformation fields. Current efforts into extending these methods into single shot techniques have the potential of expanding the range of applications to study dynamic events such as micro-flows in engineering and biological systems in which scattering particles are transported in a flow, e.g. tribology, microfluidic devices, cell migration or multiphase flows

Fluctuating force-coupling method for interacting colloids

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Brownian motion plays an important role in the dynamics of colloidal suspensions. It affects rheological properties, influences the self-assembly of structures, and regulates particle transport. While including Brownian motion in simulations is necessary to reproduce and study these effects, it is computationally intensive due to the configuration dependent statistics of the particles’ random motion. We will present recent work that speeds up this calculation for the force-coupling method (FCM), a regularized multipole approach to simulating suspensions at large-scale. We show that by forcing the surrounding fluid with a configurationindependent, white-noise stress, fluctuating FCM yields the correct particle random motion, even when higherorder terms, such as the stresslets, are included in the multipole expansion. We present results from several simulations demonstrating the effectiveness of this approach for modern problems in colloidal science and discuss open questions such as the extension of fluctuating FCM to dense suspensions

Nanofluid flow and heat transfer in channel entrance region

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The present work uses the continuum description of nanofluid flow to study the flow, heat and mass transfer in the entrance and developing region of channels or tubes, where the viscous and heat conduction layers are thin and the heat transfer is much more intense than fully developed flow. Instead of supplementing the formulation with thermodynamic properties based on mixture calculations, use is made of recent molecular dynamical computations of such properties, specifically, the density and heat capacity of gold-water nanofluids. The more general formulation results, within the Rayleigh-Stokes (plug flow) approximation and perturbation for small volume fraction, show that the nanofluid density-heat capacity has an enormous effect in the inertia mechanism in causing the nanofluid temperature profile to steepen. The nanofluid thermal conductivity though has an explicit enhancement of the surface heat transfer rate has the almost hidden effect of stretching the nanofluid temperature profile thus giving the opposite effect of enhancement

Microfluidic multiscale model of transport phenomena for engineering and interdisciplinary education applied to elements of a stirling engine

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Microfluidic model based on elementary mathematical tools and basic corpuscular physics is applied to flow configurations simulating the Stirling engine. Universality and mathematical simplicity of the model is main objective of its development. This to facilitate its application not only in micro and standard macro, single- and multiphase flows in engineering but in biology, medicine and interdisciplinary sciences as well. As dynamics of disperse systems it promotes the common physical background of multiple, apparently unrelated phenomena. Main feature of the method - compared with standard methods - is departure from differential notation where possible to ensure suitability for analysis of discontinuous systems. Physical quantities are determined directly at required scale by choice of reference volumes/surfaces and use of the mean value theorem (MVT) of integral calculus where required. Thus the method is applicable to discrete particles and avoids higher order requirements of Navier-Stokes solutions. Besides saving one integration step it generally facilitates the analysis considerably. Newton’s second law is used explicitly as single equation of motion. Together with conservation laws it is applied to non-relativistic motion of particle systems in range from individual particles, atoms, molecules or even electrons, over to macroscopic particle sets in solid or flowing systems of traditional mechanics, up to celestial bodies of classical astro-physics. The basically microfluidic model was used to derive all definitions and equations of standard continuum fluid mechanics and multiphase flows. Compared with standard methods the here used model has the singular ability to describe consistently all phenomena related to one of most inspiring technical devices: to Stirling engine

MicroRNA sensors

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.We have developed a technology for the profiling of miRNA expression in intact cells. The approach is based on sensor oligonucleotides, which upon entering the cell, bind specific miRNA targets, are cleaved as a result of this binding, and produce a fluorescent signal that is proportional to the abundance of the miRNA target. Specifically, the sensor oligonucleotides are completely complementary to a target miRNA species, are non-stabilized around the seed region (the region cleaved by the miRNA-RISC), and are labeled with a fluorescent dye and a quencher at their 5’- and 3’- end respectively. Upon entering the cell, these oligonucleotides engage the target miRNA by complementary base pairing. This leads to recruitment of the RNA induced silencing complex (RISC) to the duplex. The complex cleaves the sensor oligonucleotide and the miRNA is free to “catalyze” subsequent clevage reactions. The cleavage of the sensor oligo leads to separation between the dye and the quencher, and a resultant fluorescent enhancement that can be measured. We have demonstrated the feasibility of this method for the sensing of the pro-metastatic miRNA-10b in cell-free extracts and intact cells using human and murine breast adenocarcinoma cell lines. The miRNA epigenome represents a fundamental molecular regulator of metastasis. Consequently, developing tools to understand metastatic changes at the miRNA level can lead to the mapping out of a comprehensive and systematic atlas of cancer progression. The described technology is potentially transformative because it addresses this important issue. Furthermore, the technology has broad implications and can be utilized in any model system or clinical scenario to answer questions related to microRNA function. Specifically, the technology can help distinguish, assess, and/or monitor cancer stages and progression; aid the elucidation of basic mechanisms underlying cancer initiation and progression; facilitate early cancer detection and/or cancer risk assessment; and facilitate/accelerate the process of drug discovery

Kinetic calculation of rarefied gaseous flows in long tapered rectangular microchannels

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Gaseous flows in microsystems have attracted considerable attention in fluid dynamic communities over the last few years. When the size of the device is in the range of microns, the molecular mean free path becomes comparable with the device size, and the details of the molecular interactions need to be taken into account. The proper description of such microflows requires the consideration of the velocity distribution function of the molecules and kinetic equations. The scope of the present paper is to discuss the determination of the behavior of pressure driven rarefied gas flows in microchannels at the kinetic level. As a new application of the methodology, preliminary results are presented for pressure driven flows of single gases through long rectangular tapered microchannels, which have constant widths but varying depths along the axis of the channel. The kinetic calculation is based on the solution of the linearized Bhatnagar-Gross- Krook (BGK) equation and refers to the determination of the mass flow rate through the channel and the axial distribution of the pressure. The BGK equation is solved by the discrete velocity method. It is shown that the mass flow rate exhibits the diodicity effect, which means that the flow rate depends on the orientation of the channel. If the gas flows from the larger cross section towards the smaller one, the flow rate is larger than in the opposite situation. The pressure profile strongly varies near the small cross section, and it has a quite different character than in the case of channels with uniform cross sections. The tapered microchannel might be useful for separating the different gaseous components in engineering applications