A Design Tecnique for a High Quantity Axial Flow Pump Impeller using Voznisenki Mean Camber Line Design Method

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1 Student Research Paper Conference Vol-2, No-36, July 2015 A Design Tecnique for a High Quantity Axial Flow Pump Impeller using Voznisenki Mean Camber Line Design Method M.Hamza Khan Department of Aeronautics and Astronautics Institute of Space Technology Islamabad, Pakistan hamzakhanshamsi@yahoo.com Abstract Axial Flow Pumps or AFPs have numerous applications in agriculture, sewage treatment, and domestic water requirement fulfillment. For preliminary design purposes velocity triangles are widely employed, but they are inadequate when it comes to selecting design parameters such as the number of blades required, the stagger angle, the twist angle and the mean camber line of the impeller blade. In this paper a design technique is explained, which upon employment not only gives the above mentioned parameters but also caters for the anti-cavitation characteristics of the blade. To elaborate the technique; the researchers design problem is presented as an example in which Voznisenki method design technique is used to design an impeller which is then analyzed through ANSYS CFX software for verification of agreement with the theoretical design calculations. Keywords Axial Flow Pumps; Camberline Design; Turbo machines I. INTRODUCTION Axial Flow Pumps are those, in which the radial direction of the flow does not change. They have numerous applications in agriculture, sewage treatment, urban and rural water supply and etc. Using circulation, in 1966, Prof. Voznisenki of the Moscow Institute of Power, Russia devised a method for blade profile design by treating the mean camber line of the profile as circular arcs for which the radii are to be calculated [1]. The velocity triangle approach has been a popular tool for turbomachine design for the last two centuries. By using these two design tools a preliminary pump can be designed. In this paper a non-trial and error design technique is developed which utilizes both the Voznisenki Mean Camber Line blade profile design approach and velocity triangles. With the input of flow rate, head requirement and the blade velocity; the blade design parameters: solidity, tip radius, hub radius, twist angle, stagger angle and blade profile camber line can be determined. II. PRINCIPLES OF DESIGN A. Pump Design Requirements Suppose that a pump is to be designed for the following specified requirements 1) Total Head H 2) Flow Rate / Quantity Q 3) Blade Speed U. For an idea of the required pump, the specific speed required, which is given by [1]: s is (1) For Axial Flow Pumps, the specific speed is of a high value from 700 to On obtaining the specific speed from (1), the impeller eye velocity C 0 can be obtained as [1]: (2) B. Selection of hub ratio and calculation of outer and hub diameter In 1951, the French engineer A. Cordier made a relation between the specific speed and hub ratio of high efficiency pumps. These results were graphically charted and are termed as Cordier Diagram [2], which can be viewed in Fig.1. Using the diagram a value for hub ratio can be obtained. The blade tip diameter or the outer diameter D 0 can be calculated as [3]: Now the hub diameter can simply be obtained by multiplying D 0 with. (3) (4) C. Calculation of Unit Head and Unit Flow Rate The unit head K H and unit flow rate K Q can be calculated as [1]: Using these two parameters the hydraulic efficiency of the pump can be estimated using the universal performance charts created by Prof. Staritzky [5] in Fig. 2. (5) (6) 1

2 (7) Fig. 1: Cordier Diagram Fig. 2: Staritzky Diagram for unit head as a function of unit quantity, hydraulic efficiency and specific speed D. Streamline Selection For constant energy transfer over the entire blade span the blade must twist with increasing radius or blade span. To incorporate this twist automatically in the blade design calculations are done for different streamlines with increasing distance from the hub. As per Prof. Voznisenki s method five streamlines are selected. The streamlines are termed in a successive order as r I, r II, r III, r IV and r V. They are determined as follows: E. Selection of Vane/Blade Solidity After selecting the streamlines the blade solidity of the blade can be selected for each respective streamline by the following steps involving velocity triangle calculations: Calculate the blade speed U by: By velocity triangle calculation the relative flow inlet angle is calculated as: The outlet tangential component of the absolute flow velocity is calculated by a simple derivation of the Euler s turbo machinery equation: The relative flow outlet angle is then calculated as (8) (9) (10) 2

3 (11) The difference between the outlet and inlet relative flow angles is then calculated as (12) The vane solidity l/t for the streamlines I to IV are selected per the Howell chart (Fig. 3) obtained from experimental results obtained by the Moscow Institute of Power. For the streamline closer to the periphery the solidity is selected from the results (Fig. 4) obtained by Papir [6] F. Impeller Design To begin the impeller design the numbers of blades required are needed for blade pitch calculation. Prof. Stepanoff has provided a novel way to calculate number of blades required for different specific speeds. Using the Stepanoff charts (Fig. 5) the number of blades required Z can be found out. The blade pitch t can then be simply calculated as: (13) Fig. 5 Stepanoff Chart Fig. 3 Howell Chart for l/t estimation The blade span l is then found out as: (14) The circulation over the entire blade can be calculated by an application of the Kutta-Jowkovski Theorem for cascade blade system [8]: (15) For the individual streamline sections it is Fig. 4 Papir Chart for Periphery Vane Solidity estimation 3

4 Fig. 7 f δ m calculation Fig. 6 Γ 1 W lβ estimation (16) The average tangential component of relative velocity is calculated as: hub to periphery usually about 0.5 millimeters to avoid cavitation. The normalized thickness is then used to calculate the mean difference in relative curvature. The relative curvature f is given by (17) The average relative flow angle is then calculated as: The relative flow velocity can then be calculated as (18) Then (19) From chart explained in Fig. 6 the parameter estimated. From these two parameters the curvature calculated. (20) is can be Using the chart provided in Fig. 7, is calculated, from here an iterative procedure begins until the sum of and is equal to. Using the new value of the curvature is found out using Fig. 6. The impeller blade thickness is selected as per wind tunnel test results which usually range from 6 to 4 millimeters. The value is selected such that there is a uniform variation from Fig. 7 β estimation 4

5 A Design Technique for a High Quantity Axial Flow Pump Impeller using Voznisenki Mean Camber Line Design Method (21) The curvature difference per mean thickness is found out using the chart in Fig. 8. Thus the curvature difference is obtained by the product of the normalized thickness and the curvature difference per mean thickness. The next value for the relative curvature f 1 is calculated by the sum of the curvature f and curvature difference. The impeller curvature is calculated as (22) Finally the arc radius R for the mean camber line can then simply calculated as: (23) The camber line profile achieved can then be dressed with a thick profile after appropriate wind tunnel tests. III. EXAMPLE The researcher s design problem was to design an axial flow pump that can impart 1800 gpm/ m 3 /sec for a 3.66 meter head. A. Pump Specifications The following design specifications were calculated for the required head and flowrate: = 4.6 m/sec D. Impeller Design = 97 mm = 109 mm The results are presented in a tabular form, to save space only the first two streamlines are presented. Sr.No Parameter I II 1 Radius (mm) Impeller eye Velocity (m/sec) C 0 3 Tangential Velocity U (m/sec) 4 Relative inlet Flow Angle (deg) Solidity (l/t) Average Tangential Relative Velocity (m/s) = 0.5 (Cordier Diagram) 8 Stagger Angle (deg) = m = m 9 Average Relative Velocity(m/s) B. Flow Coefficients = Chapligin s Postulate = C. Streamline Selection 12 Curvature (rad/deg) 0.223/ / 8.6 = 61 mm = 73 mm = 85 mm

6 15 16 Curvature (rad/deg) 17 Blade Thickness(mm) 18 Normalized Thickness 19 Relative Curvature IV. CONCLUSION A non trial-and-error method for designing an impeller blade for an axial flow pump is presented. The method utilizes the velocity triangle design approach and the Voznisenki mean camber line design technique. Using the head and flow rate required by the pump, an impeller blade can be designed by calculating the blade design parameters: solidity, tip radius, hub radius, twist angle, stagger angle and blade profile camber line. As seen from the example the CFD results are in agreement with theoretical design calculations, thus proving that the Voznisenki Method is an excellent choice for the preliminary impeller desigm V. REFERENCES [1] Voznisenski, I.N. (1952). Life, action and selection of the work in the area of Hydraulic Machine development and automatic regulation. Moscow Pub. House. [2] A Valan Arasu (2012). Turbo Machines (2nd ed.). Vikas Publishing House. p [3] Rama S.R. Gorla, Aijaz A. Khan (2003). Turbomachinery Design and Theory (illustrated ed.). CRC Press. p. 59. [4] Merle C. Potter, David C. Wiggert, and Bassem H. Ramadan (2011). Mechanics of Fluids (4th ed.). Cengage Learning. p [5] S M Yahya (2005). Turbines Compressors and Fans (3 ed.). Tata McGraw-Hill Education [6] Abromavich, S.F. (1950). Application of N.E Joukovski s method and Research on flow over cascade of profiles with finite thickness [7] Joukovski, N.E. (1960) Calculation of flow over cascades of Turbomachine, Moscow Publishing House. [8] Papir, A.N. (1965). Axial flow Blades for ships (Fundamental theory andcalculation Ship Building Publication, Leningrad [9] Srinivasan, K.M. (1966). Comparative Analysis of Design of Axial Flow Pumps. Ph.D.Thesis Deptt. of Hydraulic Machines, Leningrad Polytechnic, Leningrad USSR [10] Staritski, V.G (1955). Calculation of Interaction between Cascades of Impeller blades and diffuser blades in Axial flow Machines Fig.8 The designed impeller Fig.9 ANSYS-CFX CFD results for the impeller designed by Voznisenki Method 6