PUMPING ELEMENT DESIGN

Abstract

A pumping element includes a blade (20) having a first section proximate a hub and a second section proximate a tip, a cavity height distribution based on a selected incidence angle distribution and a selected blade thickness distribution based on a structural requirement. The resulting cavity height distribution matches the blade thickness at the first section and the second section and is greater than the blade thickness along the blade.

Description

BACKGROUND

The present disclosure relates to a pumping element, and more particularly to design methodology therefor.

Fluid pumps include axial flow pumps and centrifugal flow pumps. Historical design practice typically achieves the required suction performance with some cavitation induced instability. Typical historical design practices such as increased tip clearance, casing treatment, and tip vortex suppression have limited success to minimize cavitation induced instability but often result in reduced suction performance capability.

BRIEF DESCRIPTION OF THE DRAWING

Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows:

FIG. 1 is a developed view of a blade leading edge;

FIG. 2 is a RELATED ART graphical representation of the pumping element design throat thickness and cavity height; and

FIG. 3 is a graphical representation of a pumping element leading edge design approach according to one non-limiting embodiment of the present application.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a schematic view of a blade 20 of a pumping element, inducer, and impeller. Cavitation occurs on pump elements when the static pressure is decreased to a value below that of the fluid vapor pressure. Many types of cavitation are known to occur in fluid mechanics.

The flow coefficient φ shown in Equation 1 defines the relationship between the inlet meridonal velocity C_m, the blade speed U, blade angle β, and incidence angle α

$\begin{array}{cc}\phi =\frac{C_m}{U}=\tan \left(\beta -\alpha \right)& 1\end{array}$

The design philosophy disclosed herein constrains the value of blade angle /3 as a function of incidence angle α to essentially render the incidence angle an independent variable as opposed to the conventional process which considers incidence angle as a dependent variable. The information given in Stripling (1962), Japikse (2001), and Hashimoto (1997) is 15 representative of conventional design practice for selection of blade angle β and incidence angle α. Included by reference herein.

The conventional pump element design methodology typically uses a positive tip incidence angle. For an un-shrouded pumping element, this positive tip incidence angle combined with the tip clearance generates a tip vortex which can travel upstream of the pumping element. This upstream flow is often called backflow. The backflow strength and flowrate are determined by tip incidence angle and the tip clearance. As the backflow strength and flowrate reach a certain level, the backflow will interact with the adjacent pumping element blade and cavitation instabilities will be generated. The cavitation instability mode shapes are determined by the complicity of the backflow and adjacent blade interactions.

The pumping element maximum throat blade thickness from hub-to-tip is usually a linear function of radius (FIG. 2). The minimum and maximum blade thicknesses are determined by structural requirements. The conventional pumping element design process defines the blade leading edge angle by holding the radius (r) times the tangent of the blade angle (β) equal to a constant. This design approach results in the cavity volume being substantial greater than the blade volume (FIG. 2). This results in cavitation induced instabilities. To fix this shortcoming, alternative blade leading edge angle distributions are required.

The new approach to defining the blade leading edge angle distribution requires that the pumping element leading edge blade angle and resulting incidence angle are tailored (FIG. 3). A pumping element includes a blade having a first section proximate a hub and a second section proximate a tip. A cavity height distribution is based on a selected incidence angle distribution. A selected blade thickness distribution is based on a structural requirement. The resulting cavity height distribution matches the blade thickness at the first section and the second section and is greater than the blade thickness along the blade. That is, the incidence angle at the hub (α_h) and tip (α_t) are chosen to match the cavity heights with the first section hub and second section tip blade thicknesses.

With this approach, the cavity volume is substantial less than the conventional pumping element cavity volume and much closer to the blade volume. The reduction in cavity volume results in the reduction of cavitation pumping element instabilities. Additionally, this approach achieved excellent suction performance.

Claims

1. A pumping element comprising:

a blade having a first section proximate a hub, a second section proximate a tip;

a selected incidence angle distribution;

a selected blade thickness distribution based on a structural requirement; and

a cavity height distribution based on the selected incidence angle distribution, wherein the resulting cavity height distribution matches the blade thickness at the first section and the second section and is greater than the blade thickness along the blade.