CHAPTER 5 CENTRIFUGAL PUMP IMPELLER VANE …

1 61 CHAPTER 5 CENTRIFUGAL PUMP IMPELLER VANE profile The concept of IMPELLER design and the application of inverse design for the vane profile construction are discussed in this CHAPTER . The vane profile plays a vital role to develop the streamlined flow. In conventional design, the designer uses vane arc method to develop the profile . Due to this approach, the eddy and flow reversal may occur in the flow path. The main focus on inverse design concept is explained here in detail for the vane profile construction. Subsequently, the different vane profile geometry is constructed based on this approach.

2 The design of the CENTRIFUGAL pump IMPELLER is not a universally standardized one. Every firm depends on its designer s experience, expertise and technical intuition to design a good IMPELLER . The fact that the IMPELLER flow physics has not been understood fully has led the designers to fall back on tried and tested old design methodologies. CONVENTIONAL DESIGN IMPELLER dimensions have always been a direct fall down of the head it has to develop and the discharge it has to supply. Previously used empirical formulae and thumb rules have always been the design aid for designers.

3 The different methods developed by highly experienced and accomplished hydraulic engineers like Lebonoff, Kurowzski, Anderson and Lazarkiewicz also have elements of empirical design. 62 DESIGN METHODOLOGY The IMPELLER dimensions are designed based on the head and discharge. The following are the steps involved in designing a CENTRIFUGAL IMPELLER (Figure ): From the head (H) and discharge (Q), the kinematic specific speed (nsQ) is calculated 4/3sQHQnn= ( ) From the head and discharge, the shaft power (Psh) required is calculated.

4 =75 QHPsh unit in hp ( ) Before finding the hub diameter, the shaft diameter (dsh) is found using the formula nP360000d3sh3sh = - Torsional Stress, (kP/cm2) ( ) Figure Pump IMPELLER 63 The hub diameter (dh) is calculated from the empirical relation given below. dh = ( ~ ) dsh ( ) The inlet velocity (u1) is estimated using, u1 = cm1, where cm1 = Kcm1 gH2 ( ) where Kcm1 is the velocity coefficient. From the inlet velocity and the new discharge (Q ) calculated after accounting for volumetric efficiency, the inlet cross section area (A0) is calculated A0 = Q / u1 ( ) From the area, the inlet diameter (d1) is calculated.

5 D1 = 0A4 ( ) Blade inlet angle ( 1) is calculated as tan 1 = 60nduwhereuC1111m = ( ) Breadth of the IMPELLER (B1) at the inlet is 111dAB = ( ) Blade outlet angle ( 2) is assumed to lie within the limits of 15 to 35 , usually of the order of 25 . 64 The outlet peripheral velocity (u2) can be calculated as follows gH2Ku2u2= ( ) where Ku2 is the experimental velocity coefficient. The outlet diameter (d2) and the breadth of the IMPELLER at the outlet are nu60d22 = ( ) 222dAB = ( ) CONSTRUCTION OF VANE profile Having now found the dimensions of IMPELLER like the hub diameter, inlet diameter, breadth of the IMPELLER at the inlet and the outlet, outside diameter and the vane angles, the vane profile or the curve has to be generated.

6 The vane profiles can be of different types and the designer has the discretion to choose the type of curve to be drawn. The vane profile for the IMPELLER of the pump considered as model in this work is a multiple curvature profile with four different radii of curvature with four different centers as shown in Figure The following are the steps involved in tracing the vane profile : The inlet and the outlet circles are drawn. Two axes of reference, one vertical and one horizontal, are drawn. 65 In order to trace the profile with four radii of curvature, four more circles, that is, point 1 to 6, are drawn at equal intervals on the axis.

7 The curve is drawn through A, B, C, D and E based on the positions G, H, I, J and K. Figure Vane profile construction From the point where inlet circle meets the horizontal axis, a line at an angle of inlet vane angle (16 ) is drawn to the length of the radius of curvature of the first arc (47 mm). An arc is drawn with the end point of this line as the centre and with the corresponding radius, till the arc meets the next circle. From the point where the arc meets the next circle, a line is drawn to the length equal to the next radius of curvature and passing through the previous centre.

8 An arc is drawn with the end point of this line as the centre and with the corresponding radius till the arc meets the next circle. 66 This procedure is followed till the four arcs are drawn. The curve drawn along with the dimensions found above, form the basis with which the IMPELLER can be made. Pump IMPELLER dimensions Inlet Diameter (d1) = 75 mm Outer Diameter (d2) = 160 mm Curvature Radius = 47 mm, mm, 81 mm and mm Number of vanes (z) = 6 (4 mm thick) Breadth of IMPELLER (B) = to 8 mm (converging from inlet to outlet) Inlet vane angle ( 1) = 16 degrees Exit vane angle ( 2)

9 = degrees ANALYSIS OF CONVENTIONALLY DESIGNED MODEL The model of the conventionally designed IMPELLER vane profile is as shown in Figure and is meshed using tetrahedral mesh and then read as case in Fluent with actual scaling. The boundary conditions for the analysis are specified as the inlet absolute pressure of 73832 Pa and the outlet mass flow rate of 4 liters per second, which are measured by conducting experiments with the rotational speed of 2880 rpm. The flow analysis is carried out by using Fluent software.

10 It solves the continuity equation, three-momentum equations along with two turbulence equations by segregated solver. Segregated solver is selected as it provides flexibility in solution procedure and the coupling effect between pressure and velocity is not that much significant as of compressible flows. The implicit method and cell 67 based gradient with absolute velocity formulation are followed. The fluid is assigned as water from the database at standard operating condition (properties at standard atmospheric condition) and the flow is considered as steady flow.