Microstructured optical fibers (MOFs) achieve their desired performance via a pattern of holes that run trough the whole length of the fiber. The variation of the hole pattern allows the production of a variety of optical effects. However, the cross-sectional hole structure can be different from that designed in the preform, due to the combined effects of surface tension and internal pressure. The present paper focuses on the comparison between experiments and numerical calculation of a six hole-optical fiber taking into account the effects of surface tension and internal hole-pressure, since those are of essential importance during drawing. It is shown that the numerical computations deliver reliable results for practical applications and can be used as a predictive tool for fiber development, as long as the inner pressure or the temperature do not exceed too high values
Abstract:The design of high-efficiency fans is often based on the experience of the designer. In order to determine its main dimensions, fan designers use the Cordier diagram. For a given operating point (i.e. flowrate and pressure, and a rotating speed), the optimum diameter of highefficiency fans can be found in the Cordier diagram. The Cordier diagram is an empirical diagram based on measurements. It delivers a relation between flowrate, pressure, rotating speed, and diameter. However, the Cordier diagram does not provide any information on the blade shape (i.e. the angles and the blade width). In order to fill this gap, there are design rules based on the experience of the designer and some analytical performance parameters in the literature. One very common performance parameter is the reaction, which is the ratio between the static and the total pressure rising from the impeller inlet to its outlet. These design rules and performance parameters are, however, of limited use. Therefore, the total-to-static ideal efficiency is introduced to yield, together with the speed and diameter numbers σ and δ, the essential parameters that distinguish the different turbomachines in the Cordier diagram. Based on the integral parameters of the flow and the geometry of turbomachines, a performance analysis of turbomachines is performed and the Cordier diagram is theoretically derived.
Abstract:The use of high-speed radial impellers is very common in blowers for industrial application. It is also very common to manufacture these impellers using circular arc blades. The design process as well is almost always based on former impeller series and experimental data available. In this work, a method is presented to improve the efficiency of radial impellers with a combined analytical and numerical method. This method is based on an extended analytical formulation of the flow in radial impellers, allowing optimizing efficiency in the design stage. It is complemented by the mathematical implementation of a well-known qualitative principle of efficiency optimization according to Carnot. Finally, the torque-speed characteristic of the motor is included in the design stage. The blade shapes are computed using an inverse method. The design is then validated by means of computational fluid dynamics (CFD) computation with a commercial solver. Finally, a prototype was built and measurements were carried out in a test rig. It is also shown that the design method provided very good predictions leading to an efficiency increase of 13 per cent and a maximum flowrate increase of 11 per cent. The design point was also met. It is also shown that the numerical computations and measurements are in good agreement. An analysis of the CFD results is also presented, giving an insight view into the substantial flow information within the old and the new impellers. The method presented is a combined analytical and numerical method suited to design high-efficiency radial impellers considering also the torque-speed characteristic of the motor without the need of a previous impeller series or knowledge of experimental data.
The present contribution deals with thermofluidynamical features occurring during the drawing of capillaries for microstructured optical fibres. Here, the process stability depends strongly on flow and thermal processes taking place as a preform is heated and drawn in the furnace. This is the case particularly for hollow fibres for which the existence of the inner hole directly depends on material parameters such as the surface tension and the rheological properties and on process parameter such as hole internal pressure and the process temperature. A fluid-mechanics model suggested in the literature [8] that makes use of asymptotic analysis based on small aspect ratio of the micro capillaries, has been revisited and improved recently and the leading-order equations have been then examined in some asymptotic limits by Luzi et al. [7]. Starting from the novel class of solutions of the simplified equations of motion the present paper focuses on the effect of both surface tension and internal hole pressure since those are of essential importance during drawing. Thus, comparisons with experimental data are performed, in order to validate the analytical model developed in [7], which will be briefly presented here. The theoretical model gives very accurate predictions both when the internal hole is pressurized or when no pressure is applied, as long as the temperature does not reach too high values
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