Philipp Epple

Numerical Solution and Experimental Validation of the Drawing Process of Six-Hole Optical Fibers Including the Effects of Inner Pressure and Surface Tension

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.

Design of radial impellers: A combined extended analytical and numerical method

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.

Influence of Surface Tension and Inner Pressure on the Process of Fibre Drawing

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 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. 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, 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.

Comparison Between an Analytical Asymptotic Fiber Drawing Model With Full Navier-Stokes Solution Taking Into Account the Effects of Inner Pressure and Surface Tension

A fluid-mechanics model that make use of asymptotic analysis based on small aspect ratio of capillaries has been compared with the full 3-D set of the N.-St. equations, for modelling the fabrication of capillary tubes. The final asymptotic equations are solved numerically and then compared with the N.-St. solutions, obtained with a commercial finite elements solver. The present paper focuses on the comparison of the solution of the two methods 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 analytical asymptotic method delivers reliable results for practical applications, as long as the inner pressure or the temperature does not exceed too high values.

Asymptotic Analysis of Flow Processes at Drawing of Single Optical Microfibres

Microstructured optical fibres (i.e. fibres that contain holes) have assumed a high profile in recent years, and given rise to many novel optical devices. The problem of manufacturing such fibres by heating and then drawing a preform is considered for both the cases of annular microfibres and annular capillaries. A fluid-mechanics model suggested in literature that uses asymptotic analysis based on the small aspect ratio of capillaries is analysed and revised. The leading-order equations are examined in some asymptotic limits, many of which give valuable practical information about the control parameters that influence the drawing process. Additionally, the solution obtained for a single capillary provides a suitable basis for describing more complicated fibre structures.

Analytical and numerical investigation of the optimum pressure distribution along a low-pressure axial fan blade:

Abstract In the field of axial flow turbomachines, the two-dimensional cascade model is often used experimentally or numerically to investigate fundamental flow characteristics and overall performance of the impeller. The core of the present work is a design method for axial fan cascades aiming to derive inversely the optimum blade shape based on the requirements of the impeller and not using any predefined aerofoil profiles. While most design strategies based on the aerofoil theory assume constant total pressure at all streamlines, i.e. free-vortex flow, this paper investigates the possibility of varying the total pressure along the blade and based on that, an analytical expression of the outlet blade angle is determined. When computing the blade profile at a specified radius, critical parameters reflecting on the flow characteristics are observed and adjusted (i.e. sufficient lift and controlled deceleration of the flow on the contour) so that the resulting profile is derived for minimum losses. The validation of this design strategy is given by the numerical results obtained when employed as an optimization tool for an industrial fan: 10–20 per cent absolute increase in the static efficiency of the optimized impeller.

Compact Test Rig Design for Fans and Blowers

In order to develop high efficiency fans and blowers the design methods are being improved continuously. The same is valid for the modern CFD (Computational Fluid Dynamics) programs, which are used in the design process in order to validate the designs. However, at the end of the development process measurements at test rigs are needed in order to verify the final design. CFD is substituting to some extend EFD (Experimental Fluid Dynamics), but still EFD is the final way to evaluate a design.Usually the universities and the industry have some test rig where measurements are done. These test rigs in general are unique and built according to a corresponding standard. However, these standards, as for example the German DIN 24 163 [1] or the European DIN EN ISO 5801 [2] prescribe only the main proportions of such test rigs while several important features are not described in detail. In particular the aerodynamic theory behind the standard is very often omitted. For example, the measurement of the pressure characteristic of a fan is performed at a pressure tab at the test chamber wall and not at the fan itself. How to assure that the pressure measured at the wall tab corresponds to the fan pressure?In this work the relevant theory behind the design of test rigs was worked out in detail for the relevant test rig features where the standards do not explain the fluid mechanical aspects. On this basis pressure and suction side test rigs were designed and completely simulated with a commercial CFD program, Ansys CFX.The goal was to develop compact test rigs according to the European DIN EN ISO 5801 [2] standard. It is shown in this work how the size of the test chamber influences the measuring results. Furthermore an in detail CFD study of a series of flow measurement devices, as inlet and Venturi nozzles, was performed. In such a way it was possible to show the influence of the dimensions of these devices on the accuracy of the measurements.Finally two test rigs were built, one for suction side and the other one for pressure side measurements. Compared to other test rigs in this category in use in the industry in Germany it was possible to reduce the size of these test rigs by a factor of about two complying with the measurement uncertainty of the DIN EN ISO 5801 standard.Copyright

Analysis of Propeller Design Methods and Validation With the CFD Computation of a Propeller-Pump

There is a large variety of axial propellers available, ranging from very small devices with only millimeters in diameter to ship propellers being several meters in size. Also they are applicable in several different ways, from low pressure propellers when actually used as fans or high pressure differences when used for propulsion purposes. Several theories have been developed to calculate and predict propeller performance. The basic theory is the linear momentum theory which takes only the axial motion into account. The theory can be refined taking the rotational motion and hence the angular momentum into account and also segmenting the propeller into several blade elements, to which classical airfoil theory can be applied. However, common literature does not include any precise verification of these theories. The present work shows with CFD computations the validation and hence accuracy of the blade element theory and its predecessors on a specific axial machine, namely the blood assist Reitan catheter propeller-pump. The propeller-pump is evaluated in a large operational range, using a commercial CFD code. The theory is then applied to the CFD results calculating the stream tube, in which all the necessary parameters, like interference factors, are evaluated. Those will deliver the machine characteristics thrust, torque and efficiency according to these theories. Comparison of this data to the CFD values shows good agreement, especially when segmenting the propeller and therefore using multiple stream tubes. Thus, the validity of these theories and its range of applicability was verified showing in detail how these theories can be employed as reliable design tools coupled with CFD verification.Copyright

An Extended Analytical and Numerical Design Method With Applications of Radial Fans

The use of high speed radial impellers is very common in fans for industrial application. It is very common also to manufacture the radial impellers for these fans with circular arc blades. The design process is also 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 a new extended analytical formulation of the flow in radial impellers allowing optimizing efficiency in design stage. The blade shapes are computed with an inverse method. The design is then validated by means of CFD computation. Finally a prototype was built and measurements were carried out in a test rig. It is shown also that the design method delivered very good predictions leading to an efficiency increase of 13% of efficiency and a maximum flow rate increase of 11% absolute. 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 insight in the substantial flow information inside the old and the new impeller. The method presented is a combined analytical and numerical method suited to design high efficiency radial impellers without the need of a previous impeller series or knowledge of experimental data.Copyright

Advances in Fluid Dynamics of Turbomachinery

1Department of Fluid Machinery and Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2Fluid Mechanics and Turbomachinery, Coburg University of Applied Sciences and Arts, Friedrich-Streib-Strasse 2, 96450 Coburg, Germany 3Department of Mechanical Engineering, Andong National University, 1375 Gyeongdong-ro, Andong 760-749, Republic of Korea 4Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China