Patent Application
20130330185
N.A.
N.A.
1. Field
The disclosure herein relates to an inducer employed to increase the pressure of a liquid introduced to a pump. More particularly, the inducer increases the pressure of a liquid propellant being pumped to the combustion chamber of a rocket engine.
2. Description of the Related Art
Liquid fuel rocket engines typically include tanks containing a liquid oxidizer, such as liquid oxygen, and a liquid fuel, such as liquid hydrogen, collectively called propellants. To reduce the thickness of the tanks and their associated weight, the liquid propellants are usually at a relatively low pressure. The liquid propellants are pumped to a combustion chamber and then ignited to generate thrust. To achieve sufficient thrust, the liquid propellants must be sufficiently pressurized prior to introduction to the combustion chamber. A pump, such as a turbopump, is used to pressurize the liquid propellants. As a first step, an inducer may be located between the propellant tanks and the turbopump to produce an initial pressure increase.
The inducer is an axial flow pumping device, typically a first element of low weight, high performance pumps, for use in liquid propulsion rocket engines. The inducer receives the liquid propellant at a very low inlet pressure and provides sufficient discharge pressure for the next pump stage, usually a radial impeller, to operate safely at high shaft speeds. The inducer must achieve satisfactory discharge pressure at the inducer exit with extremely low pressure at the inlet. The inducer includes a number of rapidly spinning blades to draw the liquid propellant through the inducer. Vortices tend to form on the tips of the blades causing cavitation damage to the blades and a variety of vibrations associated with vortex cavitation instabilities that may be detrimental to the engine operability or life.
It is known that enclosing the tips of the blades in a shroud eliminates the formation of vortices. For example, U.S. Pat. No. 4,642,023 entitled “Vented Shrouded Inducer,” discloses a shroud with a series of holes that allows counterflowing fluid around the outside of the shroud to flow back into the impeller. U.S. Pat. No. 7,070,388, entitled “Inducer with Shrouded Rotor for High Speed Applications,” discloses an inducer rotor with rotor blades terminating at a shroud. The shroud has a variable thickness both to reduce weight and to maintain a uniform gap between the shroud and a housing wall during high speed rotation. U.S. Pat. No. 7,931,441, entitled “Inducer with Tip Shroud and Turbine Blades,” discloses an inducer with two sets of blades arranged axially one after the other. The upstream blades are full size and the downstream blades are half size. The downstream half size blade tips are enclosed in a shroud.
U.S. Pat. Nos. 4,642,023; 7,070,388 and 7,931,441 are incorporated by reference herein in their entireties.
At high rotating speeds, even shrouded blades are subject to a manifestation of cavitation-related hydrodynamic phenomena, including the type referred to as alternate blade cavitation. Alternate blade cavitation manifests as long and short vapor cavities on alternate blades. When there are only two blades, alternate blade cavitation is inherently asymmetric, with a short cavity on one blade and a long cavity on the other blade, resulting in radial load imbalance. A symmetric pattern is only possible with an even number of at least four inducer blades (e.g. 6 or 8 blades also achieve symmetry). Four bladed inducers are utilized in many present day rocket engines. In a four bladed inducer, alternate blade cavitation is characterized by a stable pattern of two large cavities on one opposing pair of blades and two small cavities on the other pair of opposing blades. Such stable patterns result in low radial loads beneficial to bearing life and do not generate traveling pressure instabilities or adverse system vibrations.
However, ultra high suction capability requires a very low inlet flow coefficient (very low ratio of axial inlet velocity to blade tip speed), which in turn requires very low blade angles with respect to the tangential direction, producing a high degree of fluid flow blockage. Higher blade counts exacerbate the blockage problem at low blade angles.
There remains a need for an inducer having the fluid flow capacity of a two or three blade configuration and the cavitation stability of a four blade configuration.
In accordance with a first embodiment of the disclosure, an inducer includes a hub having an inlet end and an outlet end. At least one full size blade has an inner edge affixed to the hub and an outer edge. This full size blade extends rearwardly from the inlet end in a helical configuration. A partial shroud encloses a first length of the full size blade outer edge adjacent the inlet end. A second length of the full size blade outer edge that is adjacent to the outlet end is free of the partial shroud.
One feature of this embodiment is that the inducer has a low number of full size blades and preferably has two full size blades, offset by 180°, symmetrically disposed about the hub.
Another feature of an inducer with the partial shroud embodiments is that, unlike a fully shrouded low flow-coefficient inducer, this inducer is machinable in one piece, which lowers fabrication cost and preserves dimensional accuracy.
In accordance with a second embodiment, the inducer of the first embodiment further includes short blades symmetrically offset from the two full size blades. These short blades have a short blade inner end affixed to the partial shroud and a short blade outer end extending from the partial shroud towards the hub, but terminating prior to reaching the hub.
Alternatives of this second embodiment include the short partial blades having an inner end affixed to the inducer hub and an outer end terminating before the radius of the full size blade is reached and where there are two sets of partial blades, one set attached to the hub and the other set attached to the shroud.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
With reference to
With reference to
Referring to
The full size inducer blades 18 have a blade length L1+L2, as determined by the total wrap angle at the tip 16, effective to insure tip solidity of at least 1.5, where tip solidity is the ratio of the blade chord length along the tip and the circumferential spacing between blades at discharge. As illustrated in
Referring back to
The partial shroud described herein eliminates tip vortex cavitation and associated vibrations while limiting the length and weight of the shroud to the minimum necessary to accomplish this specific objective without the deleterious effects of a traditional full shroud. The partial shroud may also support the partial blades (reference number 24 in
It is believed that the two short partial blades 24 provide sites for two short cavities 60 to form allowing the two full size inducer blades 18 to develop two stable, long cavities 62. As a result, the two bladed inducer 70 with short partial blades functions as a four bladed inducer with respect to alternate blade cavitation, with the associated benefit of low radial load. The partial short inducer blades 24 only span a fraction of the full blade height. The size of the short partial blade 24 is the minimum necessary to provide a site for sheet cavity stabilization and is typically from 5% to 75% of the full blade height and causes a negligible blade blockage compared to an inducer with four full blades. It is further believed that the partial short inducer blades 24 alternating with full size blades 18 will encourage formation of a benign alternate blade cavitation pattern over a wider range of operating conditions than is observed with conventional four bladed inducers, thereby curtailing the range over which an undesirable rotating cavitation exists. This rotating cavitation phenomenon is linked to inlet pressure oscillations and system vibrations, and is particularly prevalent in 3-bladed inducers.
Partial blades in 3-bladed inducers are expected to convert the undesirable rotating cavitation to the benign alternate blade cavitation resembling that of a 6-bladed inducer, without the excessive blade blockage of the full six blades.
The short partial blade 24 is supported only at the shroud 22 by a fillet 64 (not visible) and has a length smaller, equal to or greater than its height. The leading edge 66 of this short partial blade 24 can be profiled to match the leading edge 68 of the full size blade 18, or may be deliberately different in thickness and blade angle. The trailing edge 73 of the short partial blade 24 is profiled to prevent vibrations by using large enough fillets and blade thickness.
The short partial blades 24 may be cast or machined integral with the shroud or attached, such as by brazing, welding, screwing or riveting.
Alternatively, as shown in
Furthermore, as shown in
The front-end partial shroud in combination with a back flow deflector 23 in
One application of the inducer disclosed herein is the Advanced Upper Stage Engine Program (AUSEP). The inducer enables operation of a liquid oxygen turbopump at an inlet net positive suction pressure (NPSP) of below 4 psi. At this value, the engine weight and size are retained within targeted limits. The inducer may also be used in other liquid rocket engines or general pumping applications wherever a very low inlet NPSP or high suction specific speed is required.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
