Abstract: Design of gaseous MEMS devices is currently very limited by restrictions of the experimental visualisation techniques. Flow behaviour on such scales resultant from the geometrical confinement is significantly affected by slip phenomena where curvature of the wall and gradients of density and temperature influence flow patterns. This paper presents a numerical study of steady low Reynolds (Re << 1) and low Knudsen (Kn << 0.1) numbers in heated microchannel systems with curved walls. The flows, that will be described here, are characterised by vortex structures despite their very low Reynolds numbers. The performed investigation gives new insight to the flow behaviour for such conditions with an extensive topological study of the resultant flow structures. As an application we propose gaseous microfluidic systems, such as a flow diode. This study is unique in its application of Navier-Stokes set of equations with a secondary slip wall boundary condition formulation for non-isothermal domains.
Abstract: Due to the existence of a velocity slip and temperature jump on the solid walls, the heat transfer in microchannels significantly differs from the one in the macroscale. In our research, we have focused on the pressure driven gas flows in a simple finite microchannel geometry, with an entrance and an outlet, for low Reynolds (Re<200) and low Knudsen (Kn<0.01) numbers. For such a regime, the slip induced phenomena are strongly connected with the viscous effects. As a result, heat transfer is also significantly altered. For the optimization of flow conditions, we have investigated various temperature gradient configurations, additionally changing Reynolds and Knudsen numbers. The entrance effects, slip flow, and temperature jump lead to complex relations between flow behavior and heat transfer. We have shown that slip effects are generally insignificant for flow behavior. However, two configuration setups (hot wall cold gas and cold wall hot gas) are affected by slip in distinguishably different ways. For the first one, which concerns turbomachinery, the mass flow rate can increase by about 1% in relation to the no-slip case, depending on the wall-gas temperature difference. Heat transfer is more significantly altered. The Nusselt number between slip and no-slip cases at the outlet of the microchannel is increased by about 10%.
Abstract: Motivation of this work has its origin in the boundary layer control for aeronautics and turbomachinery. For that purpose boundary layer can be modified by perforated plates with holes of specific sizes. The questions which rise in such configuration are related to the existence of optimal size of the holes and the influence of microscale phenomena on the global flow patterns. This paper concentrates on the issue of the entrance effects on the microchannel flow. It is shown that mass flow rate is only insignificantly influenced by slip effects. Global parameters such as pressure difference and geometrical shape in more pronounced way alter flow behavior. In this paper we concentrate on the numerical investigation of the microchannel flow for Kn < 0.01 and Re < 500. The channel length is finite. Hence, entrance and outlet effects on microchannel flow can be studied.
Abstract: We study the impact of inlet channel geometry on microfluidic drop formation. We show that drop makers with T-junction style inlets form monodisperse emulsions at low and moderate capillary numbers and those with Flow-Focus style inlets do so at moderate and high capillary numbers. At low and moderate capillary number, drop formation is dominated by interfacial forces and mediated by the confinement of the microchannels; drop size as a function of flow-rate ratio follows a simple functional form based on a blocking-squeezing mechanism. We summarize the stability of the drop makers with different inlet channel geometry in the form of a phase diagram as a function of capillary number and flow-rate ratio.
Abstract: Mesoscale flows of liquid are of great importance for various nano- and biotechnology applications. Continuum model do not properly capture the physical phenomena related to the diffusion effects, such as Brownian motion. Molecular approach on the other hand, is computationally too expensive to provide information relevant for engineering applications. Hence, the need for a mesoscale approach is apparent. In recent years many mesoscale models have been developed, particularly to study flows of gas. However, mesoscale behaviour of liquid substantially differs from that of gas. This paper presents a numerical study of micro-liquids phenomena by a Voronoi Dissipative Particle Dynamics method. The method has its origin from the material science field and is one of very few numerical techniques which can describe correctly molecular diffusion processes in mesoscale liquids. This paper proves that correct prediction of molecular diffusion effects plays predominant role on the correct prediction of behaviour of immersed structures in the mesoscopic flow.
Abstract: The normal shock wave turbulent boundary layer interaction still draws a great deal of attention as a flow phenomenon. This is due to its profound importance to numerous applications. The understanding of phenomena is crucial for future aims connected with the interaction control. Experimental investigations of the interaction have been carried out since the 1940s. They were aimed however at the determination of such general flow features as: pressure distribution, shock wave configuration or oil visualization of separation structures. In order to better understand the phenomenon, measurements of the entire field are required. At present, such measurements do not exist. A great help is expected from numerical simulations in this respect. There is enough experimental data to check the general features of the flow obtained from calculations. This thesis presents numerical simulations of flow that is assumed: steady, three-dimensional, compressible, viscous and turbulent. Its general aim is to present to what extend the modern numerical methods are able to predict the flow in shock wave turbulent boundary layer interaction including shock induced separation structures. These structures are very sensitive to channel geometry and may be useful in the understanding of separation's development. In order to illustrate the abilities of numerical simulations, one aim of the presented thesis is to investigate the effect of the span-wise depth of the nominally two-dimensional test section. The presented results cast some light on the common problems experienced by typical comparisons of two-dimensional simulations to wind tunnel tests having a three-dimensional nature. The first Chapter presents the basic theory of elementary structures. Considerations of elementary structures of the flow along with their dependencies are necessary for a better understanding of the separation flow structures induced by the boundary layer shock wave interaction. The classification of elementary structures will be presented. In addition, the possible occurrence of bifurcation will also be studied. The second Chapter will be devoted to studying specific cases of transonic turbulent flow. The analysis of numerical results will be bounded to the shock wave structure. Studies shall include: the influence of the numerical scheme, three-dimensional effects connected with the changing width of the channel, a comparison to experiment and the influence of the symmetric boundary condition on the flow prediction in the channel. Finally, the boundary layer influence on the 1-foot structure will also be presented. Chapter three will present the separation structures. Here too a comparison to experiments will be done. Changes in separation structures connected with the width of the channel will be studied. The influence of the symmetry boundary condition will be shown. Finally, the specification of the basic flow structures will be done.
