7,904 research outputs found
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Micromixing and microchannel design: Vortex shape and entropy
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.In very recent years microdevices, due to their potency in replacing large-scale conventional laboratory instrumentation, are becoming a fast and low cost technology for the treatment of several chemical and biological processes. In particular microfluidics has been massively investigated, aiming at improving the performance of chemical reactors. This is because of the fact that reaction is often an interface phenomenon where the greater the surface to volume ratio, the higher the reaction speed, and microscale mixing increases the interfacial area (in terms of mixing-induced-by-vortices generation). However, microfluidic systems suffer from the limitation that they are characterized mostly by very low Reynolds numbers, with the consequence that (i) they cannot take advantage from the turbulence mixing support, and (ii) viscosity hampers proper vortex detection. Therefore, the proper design of micro-channels (MCs) becomes essential. In this framework, several geometries have been proposed to induce mixing vortices in MCs. However a quantitative comparison between proposed geometries in terms of their passive mixing
potency can be done only after proper definition of vortex formation (topology, size) and mixing performance. The objective of this study is to test the ability of different fluid dynamic metrics in vortex
detection and mixing effectiveness in micromixers. This is done numerically solving different conditions for the flow in a classic passive mixer, a ring shaped MC. We speculate that MCs design could take advantage from fluidic metrics able to rank properly flow related mixing
Cosmological quantum entanglement
We review recent literature on the connection between quantum entanglement
and cosmology, with an emphasis on the context of expanding universes. We
discuss recent theoretical results reporting on the production of entanglement
in quantum fields due to the expansion of the underlying spacetime. We explore
how these results are affected by the statistics of the field (bosonic or
fermionic), the type of expansion (de Sitter or asymptotically stationary), and
the coupling to spacetime curvature (conformal or minimal). We then consider
the extraction of entanglement from a quantum field by coupling to local
detectors and how this procedure can be used to distinguish curvature from
heating by their entanglement signature. We review the role played by quantum
fluctuations in the early universe in nucleating the formation of galaxies and
other cosmic structures through their conversion into classical density
anisotropies during and after inflation. We report on current literature
attempting to account for this transition in a rigorous way and discuss the
importance of entanglement and decoherence in this process. We conclude with
some prospects for further theoretical and experimental research in this area.
These include extensions of current theoretical efforts, possible future
observational pursuits, and experimental analogues that emulate these cosmic
effects in a laboratory setting.Comment: 23 pages, 2 figures. v2 Added journal reference and minor changes to
match the published versio
Fluid and thermal behaviour of multi-phase flow through curved ducts
Fluid flow through curved ducts is influenced by the centrifugal action arising from duct curvature and has behaviour uniquely different to fluid flow through straight ducts. In such flows, centrifugal forces induce secondary flow vortices and produce spiralling fluid motion within curved ducts. Secondary flow promotes fluid mixing with intrinsic potential for thermal enhancement and, exhibits possibility of fluid instability and additional secondary vortices under certain flow conditions. Reviewing published numerical and experimental work, this thesis discusses the current knowledge-base on secondary flow in curved ducts and, identifies the deficiencies in analyses and fundamental understanding. It then presents an extensive research study capturing advanced aspects of secondary flow behaviour in single and two-phase fluid flow through curved channels of several practical geometries and the associated wall heat transfer processes.As a key contribution to the field and overcoming current limitations, this research study develops a new three-dimensional numerical model for single-phase fluid flow in curved ducts incorporating vortex structure (helicity) approach and a curvilinear mesh system. The model is validated against the published data to ascertain modelling accuracy. Considering rectangular, elliptical and circular ducts, the flow patterns and thermal characteristics are obtained for a range of duct aspect ratios, flow rates and wall heat fluxes. Results are analysed for parametric influences and construed for clearer physical understanding of the flow mechanics involved. The study formulates two analytical techniques whereby secondary vortex detection is integrated into the computational process with unprecedented accuracy and reliability. The vortex inception at flow instability is carefully examined with respect to the duct aspect ratio, duct geometry and flow rate. An entropy-based thermal optimisation technique is developed for fluid flow through curved ducts.Extending the single-phase model, novel simulations are developed to investigate the multiphase flow in heated curved ducts. The variants of these models are separately formulated to examine the immiscible fluid mixture flow and the two-phase flow boiling situations in heated curved ducts. These advanced curved duct flow simulation models are validated against the available data. Along with physical interpretations, the predicted results are used to appraise the parametric influences on phase and void fraction distribution, unique flow features and thermal characteristics. A channel flow optimisation method based on thermal and viscous fluid irreversibilities is proposed and tested with a view to develop a practical design tool
Forced Convective Heat Transfer and Fluid Flow Characteristics in Curved Ducts
Fluid flow through curved ducts is influenced by the centrifugal action arising from duct curvature and has behaviour uniquely different to flow within straight ducts. In such flows, centrifugal forces induce secondary flow vortices and produce spiralling fluid motion within curved ducts. Secondary flow promotes fluid mixing with intrinsic potential for thermal enhancement and, exhibits possibility of fluid instability and additional secondary vortices under certain flow conditions. Reviewing the published work on numerical and experimental studies, this chapter discusses the current knowledge-base on secondary flow in curved ducts and, identifies the deficiencies in analyses and fundamental understanding. The chapter then presents an extensive research study capturing advanced aspects of secondary flow behaviour and associated wall heat transfer processes for both rectangular and elliptical curved ducts.This study develops a new three-dimensional numerical model incorporating helicity approach and curvilinear mesh that is validated against published data to overcome current modelling limitations. Flow patterns and thermal characteristics are obtained for a range of duct aspect ratios, flow rates and wall heat fluxes. Results are analysed for parametric influences and construed for clearer physical understanding of the flow mechanics involved. The study formulates two analytical techniques whereby secondary vortex detection is integrated into the computational process with unprecedented accuracy and reliability. The vortex inception at flow instability is carefully examined with respect to the duct aspect ratio, duct geometry and flow rate. An entropy-based thermal optimisation technique is developed and tested for fluid flow through curved rectangular and elliptical ducts
Analysis of secondary flow characteristics and hydrodynamic instability in fluid flow through curved ducts
This paper presents an investigation on the unique flow characteristics associated with fluid flow through curved ducts, which are fundamentally different to those in straight fluid passages. In curved ducts, the flow is subjected to centrifugal forces that induce counter-rotating vortices in the main axial fluid stream and give rise to spiralling fluid motion, commonly known as secondary flow. The study develops a novel three-dimensional computational fluid dynamics analysis whereby the laminar developing fluid flow in a curved rectangular duct is modelled. The flow characteristics are identified for a range of flow rates and duct aspect ratios at several duct curvatures. The contours of secondary flow and axial velocities are obtained to recognise the influence of flow/geometrical parameters on the secondary flow. Comparisons are made between the numerical predictions and the available experimental data. It is observed that, with increased duct flow rate, the secondary flow intensifies and beyond a certain critical flow condition, leads to hydrodynamic instability. The fluid flow structure is then significantly altered with the appearance of additional pair (or pairs) of vortices, termed as Dean Vortices, at the outer wall of the curved duct. This flow behaviour is also highly influenced by the duct aspect (height to width) ratio. The paper develops and presents a new approach for predicting the onset of Dean vortex generation
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Flow and Temperature Fields in Cooling Devices with Embedded Serpentine Tubes
The turbulent flow (Re = 5124) and conjugate heat transfer in heat-sink designs of the tube-on-plate type are numerically investigated. The cooling configurations employ a serpentine tube partially (or fully) embedded inside the plate. Two-and four-pass configurations are investigated. A constant heat flux is applied at the bottom surface of the heat-sink plate. The SST k-ω model is used for the prediction of the turbulent flow and heat transfer. Two pairs of longitudinal vortices, as well as secondary flow separation, have been found to set in at the tube curved section. The combined secondary flow pattern enhances heat transfer at the tube sections over a considerable distance downstream of the 180° bends. In the last part of the analysis, the overall performance of the two configurations is compared using a number of evaluation criteria suitable for heat exchanging devices. The four-pass configuration with fully embedded tubing exhibits the best thermal (energetic) and exergetic performance
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