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

Multiscale modeling of physical and biological systems

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute

The hybrid grid implemented DSMC method used in 2D triangular micro cavity flows

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.In this study a new hybrid grid is implemented in a 2D DSMC solver to be used in 2D triangular micro cavity flows. Currently DSMC is the prominent method to analyze micro scale gas flows which are rarefied. Because of the computational cost, DSMC solvers are generally used in rarefied gas conditions in which continuum based solvers are useless. If the efficiency of DSMC solvers is improved, the application range of these solvers can be increased further where the continuum based solvers dominate. Indexing the particles according to their cells is one of the main steps in the DSMC method. Either the particles are traced cell-by-cell along their trajectories or coordinate transformation techniques are used in this step. The first option requires complex trigonometric operations and search algorithms which are computationally expensive. But it can be used in both structured and unstructured grids. Although the second option is computationally more efficient, it demands specially tailored structured grids which are more geometry dependent compared to the unstructured grids. Here it is shown that a novel hybrid grid structure can be used successfully in 2D DSMC solver to analyze triangular shaped lid-driven micro cavity flows. Hybrid grids used in this study are much less dependent of the geometry like unstructured grids. Additionally, hybrid grids like structured grids facilitate coordinate transformation techniques in order to increase the efficiency of the particle indexing step in the DSMC method

Thrust effectiveness of micronozzle

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Thrust of the divergent part of axially symmetric micronozzle is under the study. It’s input to total thrust is considered by means of analysis of relative thrust determined as divergent part’s thrust related to nozzle’s thrust without divergent part when gas issuing into vacuum. An inviscid one-dimensional flow is used as start condition of analysis. For this case, it is shown from conservation laws that divergent part of infinite length has finite relative thrust depending only on sort of issuing gas. Analysis of the influence of shear stress on thrust of divergent part with the use of theory of laminar boundary layer shows that optimal nozzle wall angle at the exit increases and optimal length decreases with decreasing of nozzle’s dimension in comparison with initial inviscid case. This conclusion is approved by results of numerical simulation of flow inside nozzles with throat diameter 10 micrometers and various form of divergent part based on Navier – Stokes equations with both no-slip and slip wall conditions. Detailed analysis of flow shows advantage of micronozzles with wall form far from traditionally used in “large” thrusters.This study is partially supported by the Russian Foundation for Basic Research (Project 11-08-00422-а)

Similarities and differences between flow boiling in microchannels and pool boiling

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This paper explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of the similarities and differences will help in overcoming some of these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels

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

Mechanics of blood flow in capillaries

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Blood is a concentrated suspension of red blood cells (RBCs). Motion and deformation of RBCs can be analyzed based on knowledge of their mechanical characteristics. Models for single-file motion of RBCs in capillaries yield predictions of apparent viscosity in good agreement with experimental results for diameters up to about 8 μm. In living microvessels, flow resistance is also strongly influenced by the presence of a ~ 1-micron layer of macromolecules bound to the inner lining of vessel walls, the endothelial surface layer. Two-dimensional simulations, in which each RBC is represented as a set of interconnected viscoelastic elements, predict that off-center RBCs take asymmetric shapes and drift toward the center-line. Predicted trajectories agree closely with observations in microvessels of the rat mesentery. Realistic simulation of multiple interacting RBCs in microvessels remains as a major challenge for future work.This work was supported by NIH Grant HL034555

Reliability of large-eddy simulation

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute