Software for hydraulic design of axial-flow pump impellers and its manufacturing documentations

This article presents a hydraulic design procedure for axial-flow pump impellers, followed by their manufacturing documentations, all in one easy-to-use software named AXPHD V2.0 (AXial Pump Hydraulic Design) developed by one of the authors (Kalmár). After the user determined pump duty, the software offers input data which may be changed interactively. The hydrodynamic singularity method is used to compute the blade profiles on cylindrical surfaces. If the velocity and pressure distributions are accepted, then the body model of the impeller is produced by AUTODESK INVENTOR PROFESSIONAL 2019. Full manufacturing documentation is prepared including shop-drawings for traditional production, numeric modules for CAM, and files for 3D printing. A photo of an impeller made by 3D printing closes the paper.


Introduction
This paper has been written in an attempt to combine the hydraulic design procedure of axialflow pump impellers with the preparation of their laborious production documentation within one easy-to-use software package.
There are many pump makers who use their well-proven techniques for the design and manufacturing of their pumps. However, new problems may arise, new techniques may appear, and so even the oldest firms face changes. It may happen, for example, that for a new duty, pumps are required not with top efficiency, but with extra good cavitation performance, and with very safe operation for about 30 years. In coping with such extra cases, the designer may wish to have a new impeller design within hours. The software named AXPHD V2.0 developed by one of the authors may fulfil this task.
The package is also applicable for jobs in common industrial use e.g., for low head irrigation or drainage pumps.
The software is not suitable for pumps of extra high efficiencies since these require special research. However, the software may provide help even in such cases as an auxiliary tool reducing the time.
Special application of the software emerges in teaching. Getting familiar with the software the student meets all important variables of pump design, and producing a design is a concise job involving both hydraulic and manufacturing aspects.

Computation model
The main data of an axial-flow impeller are shown on Fig. 1. Outer diameter of the blades is DK, the inner (hub) diameter DB. Impeller speed is constant n, and the volume flow rate through the pump is Q.
A flow channel F is shown in Fig. 2 with middle surface of radius r. The blade profile (to be designed) is also shown with absolute velocities cFσ (σ, φ) and cFφ(σ, φ), σ is axial coordinate, and φ is angle measured from the inlet edge of one blade. Incompressible, frictionless flow, with zero radial velocity is assumed in the flow channel. The blade cascade (at radius r) evolved onto a plane is seen in Fig. 3 with the velocity triangles (r, σL, L, T in metres, u, c, w in m/s). The computations are executed for the flow through these blade cascades using the singularity method [1, 4 -7].   Many axial-flow pumps work in practice with wide operating ranges. However, in order to simplify the design, the pump is associated with one single set of the above parameters called nominal or design parameters. The success of a new design depends largely on suitable selection of these parameters. The usual head and discharge ranges of axial-flow pumps are: Of course, there are known units operating at H = 20 m, but these are exceptional. The fluid is clean or raw water: ρ = 1000 kg/m 3 . Some tender documents limit speed n because fear from excessive cavitation, and in selecting nominal data this must be respected. If n has not been fixed, using specific speed nq is advised to be selected not too far from current industrial practice (Table 1): Other design parameters are suggested by the software in Figs 4, 5, 6, based on industrial practice. The pump bowl includes impeller and guide vane row (η is pump efficiency, but without the loss of the discharge bend). The inlet flow coefficient ε is defined by: , where cm is meridional flow velocity upstream of the blades. The outer diameter of the impeller DK is calculated from , while the hub diameter DB accords with Fig.  6.  The software offers also values for , , (Fig. 7). The peripheral velocity is: , and cu (Fig. 3) is calculated from: .

Computation of flow
For the flow (Fig. 2) conservation of mass and circulation-free conditions are required: (2) The partial differential equation system (Equs. (1) -(2)) together with the and boundary conditions is available to determine the unknown functions of cFσ (σ,φ) and cFφ (σ,φ) used to describe the velocity distributions along the middle surface (F) (see Fig. 2).
The middle surface of the flow channel (Fig. 2) evolved onto a plane, results in a straight blade cascade (Fig. 3) where the velocities have the same sizes as in Fig. 2. The camber lines of the profiles are computed (rendering ideal incidence flow to the impeller in pump nominal operating point). The singularities (vortex and source-sink distributions) are then placed on the camber line. The code offers a few distributions which the user may select. Thus, all parameters needed for the design are known, and the computations using the singularity method are carried out. The contour points obtained for the blades are then available, and non-dimensional velocity and pressure distributions are shown for approval. If accepted, the blade coordinates are used to build up the solid model of the impeller as shown below.
The code assumes frictionless flow. It may happen that the user intends to make a loss analysis (not included in the code, but any user's program can easily be adjoined). For such analysis the computed velocity field is easily accessible.

Software Modules
The AXPHD V2.0 design software was developed in the frame of Microsoft Visual Studio Community Edition using the C# language. The Code allows quick input, performs numerical algorithms, stores data, communicates to user written programs (Fortran, C/C, C++, VB, Java), and prepares many things for manufacturing the impeller. The Main menu consist of the following modules: Project management, Specification and definition of hydraulic and geometric characteristics, Implement the hydraulic design of the impeller, Development of the impeller's 3D body model, View and document the results. Project management means selecting a new name or using one from the list of old names. Several impeller variants may be included in one project. Among others the following functions are used: Input, Save, Save as, Archive, Delete. The modules are left by pressing Back to Main menu.
Specification and definition of hydraulic and geometric characteristics. The module opens input Panels enabling easy identification of data. For impeller G162 (Fig. 11)  Implement the hydraulic design of the impeller. The singularity method computes the blade contours on the cylindrical sections only, and their final positions in the 3D space are determined as prescribed in the former module. Figure 8 shows the drawing of the hydraulic surfaces of the blade.  (Fig. 9 a), which is also treated as a body ready for making (Fig. 9 b).

Impeller produced by 3D printing
Files of format stp are suitable for producing 3D printing. The impeller G162, designed for a model pump (Fig. 10) by AXPHD V2.0, has been produced using 3D printing with the help of the specialists from KVINT-R Ltd. The accuracy proved to be excellent, even the threads produced by 3D printing, which fix the blades in the hub, worked appropriately with commercial nuts. This model pump impeller has been prepared for testing in a cavitation test rig of 350 mm impeller diameter.
The work outlined in this paper has been devoted to help pump design. The software is flexible, offers various options for general use, and moreover to these, major changes of the procedure may easily be introduced by user defined programs.