- 20 June 2007 -

Advanced design software for pumps

Design software based on the inverse design approach offers a number of advantages for the hydrodynamic design of pump components such as impellers. M. Zangeneh, founding director of UK-based Advanced Design Technology (ADT) and professor of thermofluids at University College London, discusses and illustrates the capabilities of this inverse approach as embodied in ADT's TURBOdesign-1 software. The advanced features of the company's latest release, TURBOdesign-1 v. 3.0, are highlighted.

Many of the leading pump companies in Japan, the USA and Europe currently use the TURBOdesign-1 design code from Advanced Design Technology (ADT) in their design systems. In response to what compelled them to consider TURBOdesign-1, these companies have highlighted the following commercial drivers:

. Global competition. The need to compete globally means that the company's products need to be superior in terms of performance, cost and in meeting the customer's requirements as closely as possible (customized designs).

. Continual price pressure and margin compression. Important to reduce both manufacturing and developments costs.

. Reduced time to market. This will help reduce design and development time and hence costs.

. Skills shortages. Many companies face problems in recruiting skilled hydrodynamic designers and the time required to train a new graduate engineer in hydrodynamic design using conventional design methods is too long and hence considered as an expensive investment.

. Requirement to supply customized products. In many applications customers expect to get designs that meet their requirements in terms of efficiency, suction performance and motor size and no longer accept a choice from a pump series that only half meets their requirements.

. Ease of use. This feature of TURBOdesign-1 not only helps to improve the productivity of designers but also is the main reason why young graduate engineers can be trained very quickly to make them productive in hydrodynamic design.

Helping meet requirements

There are two approaches to the hydrodynamic design of pumps. The direct (or conventional) approach is based on trial and error changes to the blade shape. The designer starts from an initial blade shape and then evaluates the flow field in the impeller or diffuser by the application of a computational fluid dynamics code (CFD), which is then modified based on previous experience. Because of its reliance on previous experience, this approach can inherently restrict the designer to work in a relatively small part of the design space, which limits his/her ability to develop innovative designs quickly.

An alternative approach is the inverse design method, in which the blade shape is computed for a specified distribution of pressure distribution or blade loading. Such an approach offers many advantages:

. Flow-related design parameter. The design parameter is based on pressure or loading distribution, which can be based on aerodynamic principles.

. Design beyond previous experience. The flow-based design parameter allows the designer to use his/her aerodynamics knowledge and feedback from CFD to arrive at a design specification that results in innovative design.

. Database of design know-how. The design specification has generality and can be easily used in the design of similar components. This can allow the generation of valuable design know-how that can be used to generate optimum designs very quickly even if the head, flow rates or rpm change for a given design.

TURBOdesign-1 is based on this inverse approach, with the advantage that it uses blade loading together with thickness distribution as the design input and hence results in a robust design method in which the structural integrity of the designed blade is ensured. The method starts with information available from a basic 1D or mean-line analysis, such as rpm, mass flow rate, Euler head, inlet and exit tip diameters and blade width, which is then used together with the blade loading distribution to generate the blade shape.

Design breakthrough

TURBOdesign-1 has enabled some important breakthroughs in the design of pump stage components. For example, by using this approach in the design of mixed flow and centrifugal impellers, it was possible to derive a simple set of design guidelines for suppression of secondary flows in these impeller types. In another application, the method was used to suppress corner separation in vaned bowl diffusers of the type commonly used in mixed flow pumps. The method has also been used to achieve a supercompact design of mixed flow pumps without significant reduction in efficiency and suction performance, and in suppressing inlet recirculation or rotating cavitation in rocket pump inducers.

In a supercompact pump stage designed by TURBOdesign-1, the pump stage has been redesigned to achieve a 61% reduction in volume. As shown, the stage's tip radius had to be reduced substantially to achieve this aim. As a result, in order to achieve the same head, the pump stage loading coefficient had to be almost doubled from 0.33 to 0.6. Comparing the resulting normalized measured efficiency for the compact stage design (Ns800S) with the conventional stage performance, one can see that the new compact stage performance is very similar to that of the original conventional stage despite having only 39% of the volume.

Graphical user interface

In developing TURBOdesign-1, considerable effort has been spent on creating a very user-friendly graphical user interface (GUI) for the code to further improve the designer's productivity. This interface allows a design case to be built very rapidly from basic 1D design information. Some features of the GUI are as follows:

. The meridional shape can be easily manipulated interactively.

. The thickness distribution can be constant or specified from a file or modified interactively.

. The blade loading also can be modified by using a three-segment combination of two parabolic sections and a straight line section or arbitrarily modified by using B-splines. The arbitrary modification by B-splines has been found to be useful for cavitation control in pumps.

. Accurate leading and trailing edges can be generated with different elliptic distributions or under and over filing.

. Accurate surface pressure distributions can be obtained that correlate closely with CFD data. This information can be used to improve the cavitation performance of the blade.

. The code allows for detailed comparison of one or two different blade geometries in terms of blade shapes, blade angle, wrap angle, thickness and curvature both in streamwise and spanwise directions.

. Blade geometry export to most commercially available CFD, CAD and FEA codes (including export functionality for FLUENT, CFX, STAR-CD and CCM+), IGES surface and point data formats, as well as STL formats for rapid prototyping.

In addition to the above, TURBOdesign has a special script version that allows very easy coupling of the code with automatic optimization algorithms.

Version 3.0 suite

The development of TURBOdesign version 3.0 has been based on intensive consultation with customers, who expressed a requirement for better integration of the design code into existing CAE (computer-aided engineering) systems. Many customers have a large database of existing pumps defined in terms of blade geometry and wish to translate this database with ease into the TURBOdesign environment. Another important requirement was the facility to make simple changes to the blade geometry once the program generates the blade shape. This feature allows designers to meet any possible manufacturing or other related geometrical constraints.

In response to these requirements, detailed planning was undertaken in order to ensure that the TURBOdesign software can be fully integrated into existing CAE systems in a seamless manner. In order to achieve these aims, three main components were developed, as outlined below.

CAD conversion module

This allows the translation of IGES surface data into native TURBOdesign point data. This module allows automatic generation of all the required TURBOdesign data files from an existing IGES file.

Q3D analysis module

By using this quasi-3D code, the blade loading for a given blade geometry can be computed with ease. This will then allow the designer to relate better the features of the optimum blade geometry to the hydrodynamics of the flow and derive more general design know-how from the existing database. Also this blade loading can then be brought into TURBOdesign-1 and modified to improve the hydrodynamic design.

Direct design module

This module will allow for the small manipulation of the blade shape computed by TURBOdesign-1 to meet any possible constraints in terms of manufacturing.

The converter requires the user to specify the surface patches that make up the hub, shroud and blade surfaces. Once these surfaces are specified interactively by the user, the point data is generated automatically, including all the necessary files to run TURBOdesign such as the thickness and meridional geometry file.

The point data can then be read into the quasi-3D module which, together with basic design specifications such as rpm, mass flow rate and inlet total pressure, is used to compute the loading distribution on the blade. An example of a blade geometry converted to point data and its resulting loading distribution computed by the quasi-3D code is shown in Figure 6; the loading distribution on the highlighted streamline is shown in the lower left window.

The loading distribution can then be read into TURBOdesign-1, where it can be modified interactively and this can then be used to compute an improved blade shape.

Concluding remarks

TURBOdesign enables the designer to control the flow field in the impeller and diffuser by careful control of 3D pressure fields, through the specified blade loading distribution. Thus, it is possible to achieve an innovative design for turbomachinery blades having, for example, high efficiency, high suction performance and very compact machine size. Once optimum input data, based on solid physical background, are found for TURBOdesign, it is possible to apply these results to similar designs or at least give a good baseline design to start an optimization process. This feature is especially useful for the systematic series development of pumps covering a wide range of flow coefficients (or specific speeds). The design guidelines or expertise thus obtained are expected to be more universal and operator-independent, and are easily transferred to the next generation.

The use of TURBOdesign also enables the optimization of one of the design parameters while keeping other parameters the same. The effects of adopting a different meridional geometry, for example, can be evaluated independently while keeping the blade loading distribution and other design parameters the same.

In addition to the above features, TURBOdesign version 3.0 will offer seamless integration to existing CAE systems and will allow easy import of existing design databases into the TURBOdesign environment.

ADT is currently involved in a consortium with Engenious Japan in the development of a design system where TURBOdesign is coupled with Design of Experiment, Response Surface Modelling and Multi-objective Genetic Algorithms. This methodology, which will become available in TURBOdesign next year, will allow rapid development of new know-how in terms of optimum blade loading for multi-objective and multi-point applications.

www.adtechnology.co.uk

 

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