Rotary kinetic fluid motors or pumps – With means for re-entry of working fluid to blade set – Turbine regenerative pump
Reexamination Certificate
2001-04-17
2002-11-05
Look, Edward K. (Department: 3745)
Rotary kinetic fluid motors or pumps
With means for re-entry of working fluid to blade set
Turbine regenerative pump
C415S104000, C415S170100
Reexamination Certificate
active
06474938
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject application relates to fuel delivery systems, and more particularly, to a fuel pump for use in conjunction with gas turbine engines.
2. Background of the Related Art
Conventional fuel delivery systems utilize a fuel pump to transfer fuel from a storage tank or reservoir to an engine. Gas turbine engines used in aircraft require the fuel pump to supply the fuel at a high pressure. Limitations inherent in the design of some aircraft, such as helicopters where the engines are located several feet above the fuel tank, result in the delivery of fuel from the reservoir to the inlet of the fuel pump at a relatively low pressure. As a result, the fuel pump used in aircraft applications must be capable of operating under low inlet pressure conditions while supplying fuel at the required high pressure.
In an effort to meet the performance demands placed on aircraft fuel delivery systems, the practice of using an inlet pressure boost pump in conjunction with a main fuel pump has been developed. Typically, the main fuel pump is a high pressure pump such as a gear pump. The main fuel pump receives fuel from the inlet boost pump and supplies high pressure fuel to the gas turbine engine. The boost pump is a low pressure pump which receives fuel from the supply reservoir or fuel tank, increases the pressure of the fuel, and then discharges the fuel to the inlet of the main fuel pump. The function of the boost pump is to adequately charge the high-pressure pump even when the boost pump is subjected to poor inlet conditions such as low Net Positive Suction Pressure (NPSP) and/or high Vapor to Liquid (V/L) ratio.
NPSP corresponds to the absolute pressure of the fuel or liquid at the pump inlet expressed in feet of liquid, plus velocity head, minus the vapor pressure of the fluid at pump temperature, and corrected to the elevation of the pump centerline in the case of horizontal pumps or to the entrance of the impeller for vertical pumps. NPSP
required
is determined by the pump manufacturer and is a function of pump speed and pump capacity. NPSP
available
represents the energy level of the fluid over the vapor pressure at the pump inlet and must be at least equal to the sum of the resistances to flow as follows: (1) the vapor pressure of the liquid in the pump chamber; (2) the suction lift when the liquid level is below the pump level; (3) the pressure required to lift the suction valve and overcome the resistance of its spring; (4) the liquid friction in the suction pipeline; (5) the forces required to accelerate the liquid in the suction pipeline; and (6) hydraulic losses in the pump. Unless the NPSP
available
is at least equal to the NPSP
required
during any operating condition, cavitation will occur. The V/L ratio corresponds to a two-phase inlet flow and equals the ratio of vapor to liquid fuel.
For fixed wing aircraft, a typical minimum NPSP value is 5.0 psid and a typical value for the maximum V/L ratio is 0.45. These requirements are often satisfied with a simple boost pump design that includes an inducer and a centrifugal impeller.
In recent years, side channel pumps such as the model EMC-91 boost stage pump manufacture by Chandler Evans Control Systems of West Hartford, Conn. or similar pumps to that shown in U.S. Pat. No. 4,804,313, which is herein incorporated by reference, have been used as boost-stage pumps in aircraft fuel delivery systems, because they have several performance, size, and weight features attractive to these demanding applications. In particular, side channel pumps perform well under adverse inlet conditions, such as low NPSP and high V/L ratio. Additionally, side channel pumps are self-priming. Thus, they are able to pump large air bubbles without having an adverse effect on pumping efficiency or fluid pressure. Air bubbles are a common problem in helicopter applications as a result of the engines being located approximately six feet above the fuel tank.
However, state-of-the-art helicopter applications have increased the demand on the boost pump and require the pump to handle a bubble mixture flow and an alternating liquid/air flow containing air bubbles as long as twelve inches. This corresponding to an NPSP as low as 1.0 psid and V/L ratio as high as 1.0. Although these requirements can be achieved with conventional side-channel pumps, obtaining these performance goals is a difficult proposition, and when achieved, very little performance margin is available.
The operation of conventional side channel pumps is well understood by those skilled in the art. In general, the fuel enters the pump chamber through side entrance port(s) which axially direct fuel flow into the impeller. The rotation of the impeller within the chamber creates a forced vortex flow pattern therein. Typically, two side channels are adjacent to the rotor chamber about an arc centered at zero degrees. Within this arc, circulating flow enters the channels and establishes a helico-toroidal flow pattern. As a result, the fluid passes through the impeller blades a number of times on its path from the inlet region to the discharge region. Each passage through the blades may be regarded as a conventional stage of head generation, and therefore the equivalent pressure rise of a multi-stage pump is achieved in one revolution of the rotor.
In order to maximize the performance of a pumping element such as a side channel pump, it is important for fuel to enter the pumping element at the lowest possible velocity. Generally, the angular velocity of a rotating element, such as a pump rotor or impeller, is directly proportional to the distance from the center of rotation. Therefore, the lowest angular velocity of a rotating impeller blade, is located at the base of the blade and the highest velocity occurs at the blade tip.
As stated, conventional side channel pumps supply fuel axially through an inlet port(s) disposed within the side of the pump housing, parallel to the axis of rotation. Thus, the supplied fuel has to pass the rotor blades at a velocity proportional to the distance between the port and the center of rotation. This results in a degradation of NPSP and V/L performance because of the high blade speed, especially at the outermost radius of the inlet port.
Another problem associated with conventional side-channel pump design is that the configuration of the impellers is less than optimal, from a performance perspective. More specifically, side channel pumps commonly utilize paddle-wheel type impellers or impellers having blades which are for the most part two-dimensional and positioned radially perpendicular to the impeller rotation. This type of blade is typically selected because it is easy to manufacture. However, NPSP and V/L performance is dependent on incidence angle between the blade surface and the direction of the inlet fuel flow. Therefore, the performance of a paddle-wheel impeller is less than optimal, because the flow entering the pumping chamber axially through the side port(s) is not in angular alignment with the blades.
In response to these difficulties, several NPSP and V/L performance improvements have been made with side channel pumps having impellers designed with blades angled with respect to the direction of rotation, partially rectifying the incidence problem. However, these designs are unpopular because they are difficult and expensive to manufacture.
Another problem associated with conventional side-channel pump configurations is that at times the radial space desired for the inlet port, which is a function of the desired inlet flow rate, and the side channel are greater than the radial space available. As a result, the pump designer is forced to reduce the size of the inlet port and/or side channel below the optimum, corresponding to a reduction in pump performance.
As mentioned previously, the requirement to maximize performance of the fuel pump is married to the goal of achieving lightweight and compact designs in the aerospace industry without sacrificing aircraft performance. Whether a side channe
Bennett George L.
Dalton William H.
Coltec Industries Inc
Cummings & Lockwood LLC
McCoy Kimya N.
Silvia David J.
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