Patent Grant
12060889
None.
None.
The present invention relates generally to pumps, compressors, and turbines, and more specifically centrifugal pump impellers and radial turbines.
To achieve significant pressure-rise in a pump traditionally requires multiple impellers; also known as stages. This is both cost prohibitive and weight penalizing as the system part count increases with each subsequent impeller. Utilizing a single impeller to achieve the pressures that multiple impellers produce requires extremely high rotor speeds, which result in highly stressed impeller disks and blades, which ultimately limit the maximum pressure and life of the impeller. This stress is principally a function of the impeller's angular momentum which forces the mass outwards radially, creating high stress concentrations at regions where minimal movement occurs relative to the forces exerted and the adjoining geometry.
Traditional designs of high pressure-rise impellers rely on reducing the mass of the impeller disk, in turn lowering the stresses of the impeller along the disk, blades, hub, and shroud. While this weight reduction works to an extent, a point of diminishing returns is achieved where the impeller cannot support the torque and pressure exerted upon itself by the fluid it is pumping. This condition is defined as the tip speed limit of an impeller and is historically a combination of impeller material density and fluid dependent due to temperature, density, and viscosity of the acting medium on the impeller hydro surfaces.
A centrifugal impeller that employs an internal pressurized cavity can reduce the stresses in the impeller disk, blades, shroud and hub as the stiffness of the rotor is not compromised while weight is reduced across the component and peak stress regions are shifted across the part. This is due to the pressure acting in the same way a balloon does to support itself via pressure acting against the deflections naturally experienced under high rotor speed. The impeller can be a centrifugal pump or a radial inflow turbine. The pressurized cavity can be pressurized by a number of orifices that open into a pressurized section of the impeller, either thru the hub, the aft wall, the forward wall, or the tip in order to adjust the pressure in the cavity from relatively low to relatively high. the impeller can include annular rings extending between the aft wall and the forward wall thru the pressurized cavity to stiffen the impeller. A plurality of beams or baffles can also be used to stiffen the impeller. Radial and tangential stiffening ribs on the aft and forward walls can also be used to stiffen the impeller.
The amount of pressure that an impeller can generate is limited by one principle feature, the tip speed of the impeller. As the tip speed is increased, the discharge pressure increases with the square of this velocity change. Consequently, the stresses in the impeller increase by the square of this tip speed change as well. Failure for conventional designs is limited by the life margin, wherein the stresses in the part will cause the impeller to fatigue and fracture given enough operating time. In high performance expendable systems, such as rocket engines, the life is not the issue but the maximum speed at which they can operate. In either of these cases, long life or high performance, decreasing the stress in the impeller allows for higher tip speeds, and in turn higher pressures. With the utilization of the pressurized impeller disk the same life and structural margins can be achieved in comparison to the historical designs while developing greater pressures.
The pressurized impeller disk of the present invention is a way to both stiffen and lighten the impeller disk. Stress in the impeller increases with speed via three main factors: 1) the increasing pressure of the fluid, 2) the torque generated on the impeller via the fluid pumping, and 3) the increasing angular momentum of the impeller. Since the pressure is a desirable outcome and the torque a byproduct of this, the only way to combat the stresses is via angular momentum. Angular momentum has two main components: velocity and weight. Since velocity is a fixed value, only weight can be addressed. By reducing the weight, the stresses can be reduced proportionally. Historically, this is achieved by thinning the impeller disk to reduce stresses until such a point that the pressure loading and deflections overcome the stiffness imparted by the disk, resulting in an increasing stress with further thinning of the material. This is conceptually shown as a parabolic stress/weight curve with an ideal design point at the inflection for minimum stress. This approach results in an optimal design but produces high stress concentrations at principle locations in the impeller due to the planar nature of the disk. By employing an internal pressurized cavity, the weight can be shifted and reduced strategically. This allows for strain energy to be directed more controllably to create a more uniform stress profile across the entire impeller, resulting in a significantly reduced parabolic stress/weight curve and inflection point. This allows for lower stress, longer life, and higher performance impellers. This technology is not limited to just impellers for pumps either, radial inflow turbines, essentially a centrifugal pump in reverse, also benefit equivalently from this approach. Thus, for purposes of the description of the present invention, an impeller can be a centrifugal pump or a radial inflow turbine.
An embodiment of the pressurized impeller disk 10 is shown in
The pressurized cavity (16) of
In cases where an axially persistent cavity cannot sustain the desired stress requirements,
Internal stiffening ribs are shown in
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