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.
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.
The use of high speed radial impellers is very common for industrial fans and blowers. The aerodynamic design of these machines can be made through direct or inverse means. The design process is almost always direct, based on the existing impeller series and available experimental data [1]. This paper presents the design results and optimization of high speed radial impellers with combined vaneless radial and volute diffusers used for industrial fans, through a combined analytical–numerical method validated by measurements of prototypes at the test rig. The reduction of the velocity from the outlet of the impeller to the outlet of the vaneless diffuser and hence the velocity at the inlet of the volute has a significant influence on the turbulence intensity and hence on the noise and therefore was a major parameter in determining the outer diameter of the circular vaneless diffuser. The designs where then validated by means of CFD computation, and following the numerical results, prototypes for both the volute and the impeller were built and experimentally investigated. The experimental results confirmed the numerical ones and it was shown that the optimized impeller had an absolute increase in efficiency with 70% at the operating point. One has to mention, that in this design, because of the special medical application, the operating point has to be unusually chosen shifted from the maximum efficiency. The results presented show the potential and advantages of the combined analytical and numerical method suited to perform a coupled design of high efficiency radial impellers and spiral casings. It shows also that new designs and improvement of existing designs are possible without the need of a previous impeller series or knowledge of experimental data.
The use of high speed radial impellers is very common in fans for industrial applications. The most common design case is the one with constant speed. In that case, one assigns the corresponding value to the speed n, hence the speed no longer matters in the further design procedure: it is given and it is constant. However, in many cases the speed is not constant, since it is governed by the torque-speed characteristic of the driving motor. In such a case it is necessary to consider the motor characteristic already at the design stage. In the present work a design method was developed in order to perfectly match the torque-speed characteristic of the radial impeller to the torque-speed characteristic of the driving motor. In such a way it is possible to design an impeller-motor unit with maximum efficiency. The extended impeller mean-line-design formulas presented in Epple [6] were complemented with the equations describing the motor torque-speed-characteristic. Both sets of equations where combined and solved in order to achieve a prescribed operating range at a maximum efficiency. In order to validate the design method, it was applied to an industrial fan which should be improved. That radial fan with spiral casing consisted of the main radial fan and a motor cooling axial fan at the other end of the shaft. This later fan was rotating at a too low speed leading to cooling problems of the motor. Hence, a new fan had to be designed which had to deliver the same hydraulic performance but at higher rotating speeds. This had to be done, however, on the given motor. That could only be done when properly designing an impeller matching its torque-speed characteristic to the torque-speed characteristic of the motor: it was an excellent task to validate the combined impeller-motor design procedure. Under these constrains six designs where performed and validated with a commercial CFD solver. The three best designs according to the CFD results were built as prototypes and measured at a standard test rig. The best design delivered the prescribed head-flow characteristic at an even improved hydraulic efficiency. The higher speed was also properly achieved. The design procedure is described and explained in detail and a detailed CFD analysis is presented, complemented by the experimental data obtained at the test rig. A final comparative analysis of the combined impeller-motor design method, the CFD analysis and the measurements is presented.
Radial diffusers are devices to increase the static pressure of a radial impeller-diffuser-unit (IDU) and in many cases also its efficiency. A new design method for the coupled Impeller-Diffuser-Layout is proposed. This new design method is presented and the resulting theoretical differences of the vaned and vaneless diffusers are shown. It is known, also, that at high flow rates, the vaned diffusers will choke, i.e. the maximum flow rate of an IDU will be much less as the one of the impeller alone or an IDU with vaneless diffuser. In order to avoid the flow rate decrease in the vaned diffuser due to this blockage or choking, a new kind of diffuser is proposed: the slotted diffuser. The theoretical principles of chocking and the solution with the slotted diffusers are explained. In order to have an in depth understanding of its working principle, three IDU are numerically examinated: with vaneless diffuser, regular vaned diffuser and the new slotted diffuser. In general the slotted diffuser delivers approximately the same pressure and efficiency but a substantial higher flow rate than the vaned diffuser. The vaneless diffuser has the highest flow rate, but the lowest pressure. An in detail analysis of the gap losses between the exit of the impeller and the entry of the vaned regular and slotted diffusers is presented, unrevealing an major loss source in vaned IDU. Flow patterns of the different diffuser types are shown illustrating in a clear manner the working principle of these diffusers and their respective advantages and disadvantages. Finally, in order to validate the theoretical and numerical results, prototypes were built and measurements performed at a norm test rig according to DIN 24 163. Pictures of the prototypes as well as of the test rig are shown. The experimental results are in good agreement with the predictions of the numerical simulations confirming the theoretical and numerical investigations.
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