Showing papers on "Pressure drop published in 2007"

The pressure drop along rectangular microchannels containing bubbles.

Abstract: This paper derives the difference in pressure between the beginning and the end of a rectangular microchannel through which a flowing liquid (water, with or without surfactant, and mixtures of water and glycerol) carries bubbles that contact all four walls of the channel. It uses an indirect method to derive the pressure in the channel. The pressure drop depends predominantly on the number of bubbles in the channel at both low and high concentrations of surfactant. At intermediate concentrations of surfactant, if the channel contains bubbles (of the same or different lengths), the total, aggregated length of the bubbles in the channel is the dominant contributor to the pressure drop. The difference between these two cases stems from increased flow of liquid through the “gutters”—the regions of the system bounded by the curved body of the bubble and the corners of the channel—in the presence of intermediate concentrations of surfactant. This paper presents a systematic and quantitative investigation of the influence of surfactants on the flow of fluids in microchannels containing bubbles. It derives the contributions to the overall pressure drop from three regions of the channel: (i) the slugs of liquid between the bubbles (and separated from the bubbles), in which liquid flows as though no bubbles were present; (ii) the gutters along the corners of the microchannels; and (iii) the curved caps at the ends of the bubble.

Gas Cyclones and Swirl Tubes: Principles, Design, and Operation

Abstract: Introduction.- Basic Ideas.- How Cyclones Work.- Cyclone Flow Pattern and Pressure Drop.- Cyclone Separation Efficiency.- The Muschelknautz Method of Modeling.- Computational Fluid Dynamics.- Dimensional Analysis and Scaling Rules.- Other Factors Influencing Performance.- Measurement Techniques.- Underflow Configurations and Considerations.- Some Special Topics.

Role of the elasticity number in the entry flow of dilute polymer solutions in micro-fabricated contraction geometries

Abstract: We explore the interplay of fluid inertia and fluid elasticity in planar entry flows by studying the flow of weakly elastic solutions through micro-fabricated planar contraction geometries. The small characteristic lengthscales make it possible to achieve a wide range of Weissenberg numbers (0.4 < Wi < 42) and Reynolds numbers (0.03 < Re < 12), allowing access to a large region of Wi-Re space that is typically unattainable in conventional macroscale entry flow experiments. Experiments are carried out using a series of dilute solutions (0.78 < clc* < 1.09) of a high molecular weight polyethylene oxide, in which the solvent viscosity is varied in order to achieve a range of elasticity numbers, 2.8 < El = WilRe < 68. Fluorescent streak imaging and micro-particle image velocimetry ([L-PIV) are used to characterize the kinematics, which are classified into a number of flow regimes including Newtonian-like flow at low Wi, steady viscoelastic flow, unsteady diverging flow and vortex growth regimes. Progressive changes in the centreline velocity profilt are used to identify each of the flow regimes and to map the resulting stability boundaries in Wi-Re space. The same flow transitions can also be detected through measurements of the enhanced pressure drop across the contraction/expansion which arise from fluid viscoelasticity. The results of this work have significant design implications for lab-on-a-chip devices, which commonly contain complex geometric features and transport complex fluids, such as those containing DNA or proteins. The results also illustrate the potential for using micro-fabricated devices as rheometric tools for measuring the extensional properties of weakly elastic fluids. (C) 2007 Elsevier B.V. All rights reserved.

Peristaltic flow and heat transfer in a vertical porous annulus, with long wave approximation

Abstract: In this paper, we study the interaction of peristalsis with heat transfer for the flow of a viscous fluid in a vertical porous annular region between two concentric tubes. Long wavelength approximation (that is, the wavelength of the peristaltic wave is large in comparison with the radius of the tube) is used to linearise the governing equations. Using the perturbation method, the solutions are obtained for the velocity and the temperature fields. Also, the closed form expressions are derived for the pressure–flow relationship and the heat transfer at the wall. The effect of pressure drop on flux is observed to be almost negligible for peristaltic waves of large amplitude; however, the mean flux is found to increase by 10–12% as the free convection parameter increases from 1 to 2. Also, the heat transfer at the wall is affected significantly by the amplitude of the peristaltic wave. This warrants further study on the effects of peristalsis on the flow and heat transfer characteristics.

Flow rate analysis of a surface tension driven passive micropump

Abstract: A microfluidic passive pumping method relying on surface tension properties is investigated and a physical model is developed. When a small inlet drop is placed on the entrance of a microfluidic channel it creates more pressure than a large output drop at the channel exit, causing fluid flow. The behavior of the input drop occurs in two characteristic phases. An analytical solution is proposed and verified by experimental results. We find that during the first phase the flow rate is stable and that this phase can be prolonged by refilling the inlet drop to produce continuous flow in the microchannel.

The science of making more torque from wind: Diffuser experiments and theory revisited

Abstract: History of the development of DAWT's stretches a period of more than 50 years. So far without any commercial success. In the initial years of development the conversion process was not understood very well. Experimentalists strived at maximising the pressure drop over the rotor disk, but lacked theoretical insight into optimising the performance. Increasing the diffuser area as well as the negative back pressure at the diffuser exit was found profitable in the experiments. Claims were made that performance augmentations with a factor of 4 or more were feasible, but these claims were not confirmed experimentally. With a simple momentum theory, developed along the lines of momentum theory for bare windturbines, it was shown that power augmentation is proportional to the mass flow increase generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. The theory predicts an optimal pressure drop of 8/9 equal to the pressure drop for bare windturbines independent from the mass flow augmentation obtained. The maximum amount of energy that can be extracted per unit of volume with a DAWT is also the same as for a bare wind turbine. Performance predictions with this theory show good agreement with a CFD calculation. Comparison with a large amount of experimental data found in literature shows that in practice power augmentation factors above 3 have never been achieved. Referred to rotor power coefficients values of CP,rotor =2.5 might be achievable according to theory, but to the cost of fairly large diffuser area ratio's, typically values of β >4.5.

Viscous flow through microfabricated hyperbolic contractions

Abstract: We study the flow of a Newtonian fluid through microfabricated hyperbolic contractions followed by a sudden expansion, with the aim of investigating the potential of this geometry to serve as an extensional microrheometer. A set of planar converging geometries, with total Hencky strains ranging from 1.0 to 3.7, were fabricated in order to produce a homogeneous extensional flow field within the contraction. The velocity field in various planes of the hyperbolic contraction was quantified by means of microparticle image velocimetry (μPIV) and the pressure drop across the converging geometry was also measured and found to vary approximately linearly with the flow rate. Additionally, an extensive range of numerical calculations were carried out using a finite-volume method to help assess the performance of this geometry as a microfluidic elongational rheometer. The measured velocity fields in the contraction and associated pressure drops compare very well (to within 10%) with the numerical predictions. For the typical dimensions used in the microfluidic devices, the steady viscous flow through the contraction is shown to be three-dimensional and it is demonstrated that regions with nearly constant strain rate can only be achieved using geometries with large total Hencky strains under Hele–Shaw (potential-like) flow conditions.