Patent Grant
7097414
This invention relates generally to fluid motivation and, more specifically, to fluid inducer technology.
Inducers are typically utilized as the first pumping element of centrifugal and axial flow pumps to lower the inlet pressure at which cavitation results in pump head (discharge pressure) loss. Inducers include blades that are designed to operate in a passage with a small positive incidence angle between the fluid angle relative to a blade pressure side angle as the fluid enters an area between the operating blades know as a blade row. A tip clearance between the blade tip and a wall of the passage is necessary to allow the blade tip to operate within the passage.
Camber is then added to blade geometry after the fluid is captured in the blade row to add work to the fluid raising its tangential velocity and static pressure. The small positive incidence angle near the blade tip is selected based on the gross uniform axial velocity. Because of boundary layer losses and a back flow at the blade tip that flows through the tip clearance, the actual incidence angle relative to fluid in the tip clearance is larger due to the momentum exchange and mixing resulting in lower axial velocity of the fluid. The larger incidence angle results in a larger differential pressure across the blade tip near the leading edge, which, in turn, results in larger back flow through the tip clearance. This dynamic feedback mechanism develops a queasy steady state condition at most inlet pressure and flow rate vs. operating speed conditions.
At operating speed, the velocity of the back flow through tip clearance is sufficient to lower the local static pressure below the fluid vapor pressure resulting in vapor bubbles forming in the high velocity region, which collapse as the velocity is dissipated, and the local static pressure increases. Dynamic instability including higher order rotating cavitation (HORC) and higher order surge cavitation (HOSC) appear as pressure oscillations at inlet pressures. HOSC and HORC, occur when the tip vortex (tip clearance) cavitation cavity is approximately equal to 65% of the blade spacing at the blade tip. The frequency of the higher order oscillations are typically on the order of 5 to 8 times shaft speed, depending on the number of inducer blades and other features that make-up the inducer geometry. The dynamic instability only occurs within a limited flow rate verses speed range, which suggests that it is incidence angle sensitive. At low flow rates, the incidence angle is large resulting in a large cavitation cavity at all inlet conditions. At high flow rates, the incidence angle is small resulting in a small cavitation cavity at all inlet conditions.
Head break down results from blockage due to the cavitation sheet that originates on the suction side of the blade leading edge when the local static pressure falls below the propellant vapor pressure. Leading edge cavitation sheets typically progress from alternate blade cavitation to rotating blade cavitation to gross head loss as the inlet pressure is decreased. Problems associated with these characteristics are avoided by maintaining the margin on break down conditions. The HOSC and HORC are not a result of the cavitation sheet that springs from the blade leading edge, but are instead a function of the tip clearance back flow and the tip vortex cavitation cavity length.
As a result, there is an unmet need in the art to minimize the tip vortex cavitation cavity by suppressing the back flow through the tip clearance.
Embodiments of the invention provide a method, device, and turbopump configured to suppress higher order cavitations at an inducer tip in a turbopump. An inducer having a tip is rotated, and a first flow (pump through flow) is induced axially through the inducer at a first axial velocity. An annular fluid flow is introduced axially toward a tip clearance of the inducer substantially parallel to the first fluid flow at a second axial velocity that is greater than the first axial velocity, such that back flow through the tip clearance of the inducer is reduced.
A presently preferred embodiment of the invention includes a rearward-facing step located just upstream of the blade tip leading edge with a radial height equal to or slightly greater than the blade tip clearance. The rearward-facing step can be accomplished by making the inlet duct equal to the inducer diameter or by introducing a gradual convergent section in the duct up stream of the step. An annular flow passage is located in the rearward-facing step to direct an annulus of axial flow along the inducer tunnel into the inducer blade tip clearance. A manifold is provided to supply the flow to the annular flow passage at the required flow rate and velocity. Flow is supplied to the suppressor manifold from a down stream source of sufficient pressure to provide the desire flow rate. Depending on the tip clearance, the flow rate required to decrease the incidence angle to approximately zero will be one to two percent of the inducer through-flow. The required velocity to reduce the incidence angle to approximately zero will be 1.5 to 2.0 times the through-flow axial velocity, depending on the inducer design. Introducing a higher velocity axial flow directed at the blade tip clearance decreases the tip incidence angle to approximately zero which eliminates the tip clearance back flow and incidence angle variation.
In accordance with an aspect of the invention, the second fluid flow is introduced annularly into the tip clearance flow region. Further, the second fluid flow is introduced in an axial flow direction. Also, the second velocity may be approximately equal to the fluid velocity required to reduce the fluid incidence angle relative to the blade pressure side angle to zero.
In accordance with still another aspect of the invention, the second flow is directed to energize a boundary layer flow. Advantageously, the energizing of the boundary layer flow is sufficient to eliminate a tip clearance back flow by optimizing the effective incidence angle at the inducer tip.
The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.
a is a vector diagram of a flow at the inducer tip where the relative velocity of the flow, based on the through flow, nearly aligns with the blade angle;
b is a vector diagram of a flow at the inducer tip where the relative velocity of the flow departs significantly from the blade angle due to boundary layer flow and tip clearance back flow;
c is a vector diagram of flows at the inducer tip where the relative velocity of the flow is optimized to align with the blade angle by introducing suppressor flow;
By way of overview, embodiments of the invention provide a method, device, and turbopump configured to suppress higher order cavitations at an inducer tip in a turbopump. An inducer having a tip is rotated at a tangential velocity and a first flow is induced axially through the inducer at a first axial velocity. A second fluid flow is introduced toward the tip clearance of the inducer substantially parallel to the first fluid flow at a second axial velocity that is greater than the first axial velocity, such that back flow through the tip clearance of the inducer is reduced.
Referring to
To suppress high order oscillations, the vortex suppressor 10 defines an annular manifold 30. The annular manifold 30 includes an annular vent 27 to direct a second fluid flow 24 generated by conducting fluid from the annular manifold 30 to the inducer tip clearance 21 substantially parallel to the first fluid flow 8.
The annular vent 27 is defined by the inlet duct 5 to direct the second fluid flow 24 into the tunnel housing 7 through a rearward-facing step 33 with a radial thickness that is equal to or greater than the dimension d. The step 33 overlays the inducer tip clearance 21 in a manner to occlude the inducer tip clearance 21 from the first fluid flow 8 thereby introducing, instead, the second fluid flow 24 to fill the inducer tip clearance 21.
Referring to
The magnitude of incidence angle α is a function of the magnitude and direction of each of a fluid axial velocity 39 (VA), a blade tip tangential velocity 42 (VT), and a pressure side blade angle β. The incidence angle α, is defined by the relationship:
α=β−θ (1)
where blade angle β is an angle of a blade pressure side surface and fluid relative angle θ. The blade pressure side surface, in this case is the leading surface of the inducer blade 15 at the inducer blade tip 18 traveling with a tangential velocity VT. The blade angle β is established by the blade geometry with reference to the blade tip tangential velocity 42 (VT). The fluid relative angle θ is an angle expressing the relationship between the fluid axial velocity 39 (VA) and blade tip tangential velocity 42 (VT) and is defined as:
The incidence angle α=β−θ is typically selected to be a small positive value to optimize the suction performance and is generally based on an assumption of a uniform axial flow velocity VA across the inducer blade 15. Prior industry practice has allowed no accounting for boundary layer effects but rather has designed with optimization of the greatest part of the inducer blade 15 in mind.
a is a vector diagram 36a of the fluid flow at the inducer tip at based on a uniform through flow velocity 39a, i.e. where the fluid velocity relative to the blade 45 nearly aligns with the blade angle. Because the relationship between the fluid axial velocity 39a, set forth as VA, to the blade tip tangential velocity 42, set forth as VT, determines the fluid velocity relative to the blade 45. The fluid velocity relative to the blade 45 determines the fluid relative angle θ that is sufficiently aligned with blade angle β thereby preventing a significant backflow.
b is a vector diagram 36b of the fluid flow at the inducer tip where the tip clearance back flow is mixed with the first flow 8 boundary layer lowering the axial velocity 39a (VA) at the blade tip such that the fluid velocity relative to the blade 45 is not aligned with the inducer blade 15 resulting in a larger incidence angle α. As the magnitude of the incidence angle α increases so too does the occurrence of HOSC and HORC.
c is a vector diagram 36c of flows at the inducer tip where the relative velocity of the flow is optimized to align with the blade angle β. In
Referring to
Referring to
At a block 75, the inducer is rotated in the inducer tunnel. At a block 78, a flow of fluid is introduced. Inclined blades of the rotating inducer receive the fluid and as the inducer rotates, the fluid is propelled axially through the inducer blades. The movement of the fluid upstream of the inducer in the inlet duct defines a boundary layer in which the viscosity of the fluid causes the flow of the fluid to slow in proximity to a wall of the duct. The slowing of the fluid in the boundary layer causes cavitation at the inducer blade tip at suitably high rotational speeds.
At a block 81, a second flow of fluid is introduced into the boundary layer. The second flow of fluid energizes the boundary layer by being introduced at an axial velocity in excess of the first flow velocity thereby overcoming the slowing of the boundary layer. By observing the presence of the cavitation at the inducer blade tips, generally evidenced by high order oscillation, the speed of the second fluid flow can be optimized to minimize cavitation. Generally, introducing the second fluid flow at a velocity to reduce the fluid incidence angle relative to the blade to zero will suitably suppress the cavitation at the inducer blade tip.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3307776 | White | Mar 1967 | A |
| 5762034 | Foss | Jun 1998 | A |
| 6244817 | Ngo | Jun 2001 | B1 |
| 6379110 | McCormick et al. | Apr 2002 | B1 |
| 20020192073 | Japikse | Dec 2002 | A1 |
| Number | Date | Country |
|---|---|---|
| 3524297 | Jan 1987 | DE |
| 0 606 475 | Jul 1994 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20050129500 A1 | Jun 2005 | US |
