Nano liquid-metal fluid as ultimate coolant

We proposed for the first time the concept of the nano liquid-metal fluid, aiming to establish an engineering route to make the highest conductive coolant. Using several widely accepted theoretical models for characterizing the nano fluid, the thermal conductivity enhancement of the liquid-metal fluid due to addition of more conductive nano particles was predicted. Further, the effects of particle size, cluster of nano particle, solid-like layer due to adsorption, volume fraction and particle types were evaluated. Having the highest conductivity, being electromagnetically drivable, the liquid metal with low melting point is expected to be an idealistic base fluid for making super conductive solution which may lead to the ultimate coolant in a wide variety of heat transfer enhancement area.     

弯曲狭窄管内血液流的分析

与血管狭窄有关的异常血液动力学特征在血管疾病的发生和发展过程中起着重要的作用,由于血管狭窄和弯曲的综合影响,将会出现一系列有趣的流体力学现象,本文研究具有对称狭窄的弯曲小动脉内定常血液流动,在一定的假设条件下,直接从支配血液流动的Navier-Stokes方程求出问题的摄动解,由此求得弯曲狭窄管內血液流动的轴向速度、二次流速度及压力梯度等分析表达式,并进一步求得轴向和周向血管壁切应力。本文的结果是先前有关狭窄直管和弯曲均匀管流动研究的拓广。     

Swirl effects on variable-density jet mixing in confined flows

An experimental investigation on swirl effects on inhomogeneous confined jet mixing in a combustor configuration is reported. The confined swirling flow was simulated by a swirler with a central jet mounted in a cyclindrical tube. Helium and air jets set at different velocities were injected into the confined swirling air flow. The resulting flow fields due to two vane swirlers with constant vane angles of 35° and 66° were compared. Results show that the 35° vane swirler produces a solid-body rotation core with a slope about twice that created by the 66° vane swirler. It is the behavior of this solid-body rotation core that determines jet mixing rather than the swirler vane angle. Consequently, the coaxial jet decays much faster, the mixing is more intense, and the turbulence intensities are higher for the 35° vane swirler. In view of these results, combustor designers should be more concerned with behavior of the solid-body rotation core produced by the swirler, instead of the swirler vane angle.     

On the existence of self-similar solutions of the equations governing unsteady flow through a porous medium

In this paper the conditions for the existence of self-similar solutions of the equations governing unsteady flows through a porous medium are presented and discussed. The first two sections deal with the case of non-Newtonian fluids of power-law behavior; the third section analyzes the case of non-Darcy gas flows. The boundary and initial conditions occuring currently in a large class of fluid mechanics problems, of practical interest in engineering, are considered.     

Condensation from steam—air mixtures in a horizontal annular flow channel

Experiments were performed on the condensation of steam from steam-air mixtures in annular flow at a cooled inner tube. The range of investigation was varied for laminar and turbulent flow for 1.5 × 10³ Re 1.3 × 10⁴ and inlet concentrations 0.59 p_steam/p_total 0.95. The measurements, performed at an open test loop at p_total ≈ 0.96 bar, allowed local heat and mass transfer coefficients to be evaluated for various inlet lengths in the 2 m long annulus. The steam concentration was measured locally inside the annulus with a newly developed dew-point probe. The heat flux was measured locally using the temperature gradient in the cooled inner tube.

Near the inlet region the experiments showed a slightly higher heat flux at the bottom of the tube compared to the top, although it is expected to be smaller there owing to a thicker liquid film. Far downstream from the inlet region the heat transfer at the top was higher than at the bottom. The reasons for these effects are discussed, yielding a better understanding of the thermal and fluid processes involved in condensation from vapor-gas mixtures. The measured data allow the development of correlations for predicting the local Nusselt and Sherwood numbers in a horizontal annular-flow chanbel.     

A numerical exploration of secondary separation in starting flow around a flat plate by the discrete vortex model

In the present work, a further numerical simulation of the starting flow around a flat plate normal to the direction of motion in a uniform fluid has been made by means of the discrete vortex method. The secondary separation occurring at rear surface of the plate is explored, and predicted approximately using Thwait's method. The calculated results show that in the early stages of the flow secondary separation does occur. The evolution of flow field, the vortex growing process and the characteristics of secondary vortices have been described. The time dependent drag coefficients, the vorticity shed from the edges and rear surface, and the separation positions are calculated as well as the distributions of velocity and pressure on the plate. In the case of flow normal to the plate, the calculated secondary vortices are weak. Their existence will change the local velocity distributions and affect pressure distributions. However, the effect on drag coefficient is negligible.     

A computer model of pulse wave and input impedance in human arteries

To predict the propagation of pressure and flow pulses in arterial system and the variation of vascular input impedance, a branched and tapered tube model is studied through one-dimensional transient flow analysis. Coupling the continuity and momentum equations yields a group of quasilinear hyperbolic partial differential equations which can be solved numerically by using the method of characteristics. Several boundary conditions of the arterial system are also simplified suitably. The propagation of the pulses of the arterial system and the vascular input impedance is calculated on computer by using the dimensions and the physiological data of the arterial system. The results point out that the pressure and flow pulses of the arterial system and the vascular input impedance produced by this theoretical model is consistent quite well with the experimental results published.     

Three-dimensional study of extrusion processes by Boundary Element Method.

In this paper we report an implementation of a Boundary Element Method (BEM) dealing with three-dimensional extrusion problems of viscous fluid. Die shapes with and without sharp corners and their influence on the final shape of extrudate have been studied: both shrinkage and swelling of the extrudates relative to the original die shapes have been discovered. The first half of this paper deals with the problem of implementing isoparametric boundary elements. It is followed by results of test problems, and finally the results of extrusion study are presented.     

Transient spherical flow of non-newtonian power-law fluids in porous media

The transient spherical flow behavior of a slightly compressible non-Newtonian, power-law fluids in porous media is studied. A nonlinear partial differential equation of parabolic type is derived. The diffusivity equation for spherical flow is a special case of the new equation. We obtain analytical, asymptotic and approximate solutions by using the methods of Laplace transform and weighted mass conservation. The structures of asymptotic and approximate solutions are similar, which enriches the theory of one-dimensional flow of non-Newtonian fluids through porous media.     

Zum Einfluß von Füllstoffen auf das rheologische Verhalten von hochmolekularen Polyethylenschmelzen

Zusammenfassung Zur Sichtbarmachung der Strömungsvorgänge im Bereich der konvergenten Strömung vor einer Kapillaren wurde ein Verfahren entwickelt, das die Eigenschaften bestimmter teilkristalliner Polymere bei der Kristallisation unter Scherung bzw. aus einer vorgescherten Schmelze nutzt. Diese zeigen im erstarrten Zustand sogenannte Fließlinien, die aus in bestimmter Weise auf reihenförmig angeordnete Keime aufgewachsenen Kristallitstrukturen bestehen. Solche linienförmigen Aggregate sind in der Lage, ein Stromlinienbild der Strömung in der Schmelze im erstarrten Polymer wiederzugeben. Für ein LDPE vom Spritzgußtyp konnte die Entwicklung der Einlaufwirbel mit wachsender Schergeschwindigkeit dargestellt werden. Die hochmolekularen Polymertypen erwiesen sich schon bei geringsten Durchsätzen als instabil, die Einlaufwirbel verschwanden dabei vollständig, und die Bilder des erstarrten Polymers legen einen Mechanismus der Instabilitäten nahe, bei dem jeweils unterschiedliche Teilvolumina zeitweise in die Kapillare einströmen, bei Überschreiten einer kritischen Deformation jedoch abreißen und zurückschnellen, während andere Volumenelemente ihren Platz einnehmen. Das Verhalten der Suspensionen unterscheidet sich diesbezüglich nicht von dem der ungefüllten Schmelzen.Ein signifikanter Einfluß der Füllstoffe zeigt sich jedoch beim Einströmen in Kapillaren. Während die ungefüllten Schmelzen nach sehr kurzer Zeit ein laminares Strömungsprofil ausbilden, findet man mit zunehmender Füllung im Bereich stabiler Einströmbedingungen einen sehr hohen Einlaufdruckverlust bei kurzen Kapillaren, nicht jedoch bei Lochdüsen. Dies deutet auf einen Verfestigungsmechanismus bei der Ausbildung des laminaren Strömungsprofils hin, der durch Aggregierungsvorgänge im Bereich der hohen Dehnbeanspruchung in der konvergenten Einlaufströmung bewirkt wird.

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For the visualization of polymer flow in the entry region of a capillary, a method has been developed that uses distinct crystallization phenomena of the partially crystalline polymer itself solidifying from a presheared melt. The solid polymer shows so-called flow lines made up of transcrystalline overgrowth on rows of nuclei capable of showing a picture of the past melt-flow streamlines. The development of circulating flow in the entry region of capillaries could be shown with an injection-molding type LDPE. The higher molecular-weight types appeared to show unstable flow with disappearing circulating flow in the corners even at very low flow rates. The bulk polymer seems to flow into the capillary from certain parts of the reservoir until a critical strain is reached, then snaps back, giving way for the same flow type from another part of the reservoir. The behaviour of the suspensions does not differ in this respect from that of the pure melts.A significant filler influence is, however, shown within the capillary. While unfilled polymer melts reach steady flow conditions only one or two diameters downstream of the capillary entrance, an entry pressure loss increasing with increasing filler content is found in short capillaries (l/d = 5 – 15), but not in no-land dies (l/d 1). This seems to indicate a flow hardening mechanism in the region where the steady laminar flow profile develops and which might result from the very high extensional strain (next to instability) in the converging flow of the capillary entrance.

Zweiter Teil einer vom Fachbereich Chemietechnik der Universität Dortmund genehmigten Dissertation.