Fluid reaction surfaces (i.e. – impellers) – Multiple axially spaced working members
Reexamination Certificate
2002-03-15
2004-02-17
Nguyen, Ninh H. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Multiple axially spaced working members
C416S19800R, C416S23100A, C416S23100A, C415S090000
Reexamination Certificate
active
06692232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a disc turbine rotor assembly comprised of spaced-apart rotor discs, and provides improvements to the coupling of the disc set to the rotor shaft, and spacing means of the disc members which allow local variation and radial expansion under various local operating temperatures without allowing axial deflection, deformation, or excessive warping of the disc material. Said means maintain desired gaps between planar disc surfaces, and additional spacing and positioning means are provided which establish tangential waves in the disc membranes in order to enhance boundary layer effects. Spoke features of the disc rotor stack are staggered so as to capture working fluid momentum during its axial egress.
2. Description of the Related Art
Turbines comprised of spaced-apart rotor discs were first described by Nikola Tesla in U.S. Pat. No. 1,061,142 and 1,061,206. For this reason, these turbines are sometimes referred to as Tesla Turbines, but are alternatively known as Prandtl layer turbines, boundary layer turbines, cohesion-type turbines, and bladeless turbines.
The turbine rotor consists of a stack of spaced apart discs fixed to a rotatable shaft. The rotor assembly is contained in a housing closely fitted to the perimeter of the discs. The discs have vents near the rotor rotational axis, and the housing includes at least one outlet positioned to receive fluid exiting the rotor assembly. In operation, an energetic fluid at pressure and temperature is introduced at the periphery of the rotor assembly and contained in a housing which closely follows the perimeter of the discs. The fluid passes between the discs and exits the stack assembly through the vents, leaving the housing through its outlets.
In operation, the tangential component of the flow of working fluid creates centrifugal force that must be overcome by additional fluid entering the housing. Therefore, during steady state operation, great back pressure is developed at the inlet of the machine, along with a significant drop in pressure between the inlet and the outlet of the machine. This drop in pressure, with its concomitant drop in temperature and expansion of the working fluid, efficiently extracts much of the available thermodynamic energy of the working fluid.
It is therefore understood that the highest operating temperatures of the working fluid exist at the periphery of the rotating disc assembly, and much lower temperatures exist in the axial regions near the outlets of the turbine. This radial temperature difference along the disc membranes, spokes, and other rotating components presents several material-related challenges, including undesired local and general warping of the disc membranes during extended use and especially after thermal cycles caused by intermittent use or periodically varying working fluid temperatures.
The typical failure mode of a rotor assembly composed of a stack of spaced apart discs is that permanent warping causes non-uniform spaces between the discs, even including closed off or occluded sections where warped material of one disc deflects enough to completely close the gap between it and its neighbor. A correspondingly wide gap area created on the reverse side of the warped disc admits of efficiency losses due to a lack of the close spacing required to maintain effective momentum transfer from working fluid passing through the enlarged section of the passage between the discs.
Prior art devices by Tesla and by Possell (U.S. Pat. No. 4,347,033) include pin members extending axially through several discs. This construction suffices when the device is used as a pump or compressor, but operation as a turbine produces local variations in temperature and differential thermal expansion. Disadvantageously, the pin members accumulate and communicate the forces and stresses of these differential material displacements across the several discs and thereby exacerbate the warping problem.
It is therefore advantageous to provide spacing means which maintain a uniform clearance within the entirety of the gap between opposed surfaces of the spaced-apart turbine discs, but it is also advantageous that these means allow local variation and radial expansion under various local operating temperatures, without allowing axial deflection, deformation, or excessive warping of the disc material.
In comparing the use of spacing means versus the simple robustness of a series of thick plates, it is understood that thinner plates with spacing means provide the same total active plate area at a reduced overall assembly axial thickness, and consequently more power is obtained for a given swept volume of a rotor assembly, thus improving efficiency. Assembly weight and material cost are reduced as well.
A generally known prior art method was initially devised and later taught away by Tesla in British patent 186,082, paragraphs 55-65: “Furthermore with the object of cheapening the manufacture I dispense altogether with the former spacing studs . . . accomplishing the spacing by means of small bosses or protuberances which are raised in the plates by blows or pressure.”
Bumps and other features embossed on turbine discs have been employed to limited success as spacing means by individual experimenters and hobbyists to this day. Various inventors, including Conrad, et al, (U.S. Pat. No. 6,183,641) have provided bumps on a first turbine disc surface which do not protrude to closure against any second disc surface. Cafarelli (U.S. Pat. No. 5,470,197) has devised movable means of perturbing the fluid flow between the rotor discs. These inventions, however, do not serve to adequately prevent warping of the disc material or precise and uniform control of the spacing between the discs.
For most turbine applications, maintaining the whole of the disc membranes in flat planar states is desirable for smooth and continuous operation. However, other applications utilize pulsed combustion or pulsed variations of fluid pressures within the turbine. Since the ideal disc spacing for a given fluid at varies with its viscosity, which in turn varies with temperature and pressure, in these applications it is advantageous to present the working fluid with non-planar features within the space between rotor discs so that at any given moment at least some portion of the space between given discs is an optimum space for the fluid condition at any given portion of the pulse.
Another challenge inherent in the design of a disc turbine rotor assembly is the mechanism by which torque imparted to the discs by the working fluid is in turn transmitted to the shaft. Tesla's invention supplies hubs positioned axially fore and aft of the disc stack and includes threaded fasteners by which the assembly is brought into compression. The shaft, discs, and spacing washers deposed between the discs include at least one keyway, typically accepting of a square key. In the mechanical engineer's art, keyways are known to be features which concentrate working stresses and substantially reduce the design safety, requiring larger and heavier sections, increasing mass, cost, and design complexity.
In the typical, current, and prior art construction of a disc rotor assembly, the central vents form a radial array, and thereby form a radial array of material connections from a central disc hub to the planar membrane area where the working fluid transfers its momentum to the disc during operation. During this work, direction of fluid motion generally includes tangential and radial components, with very little axial displacement.
However, during egress through the series of vents, the fluid changes its direction from radial to axial motion. This change of motion inheres a change of momentum and therefore a reaction force, but prior art designs release the fluid with without substantially applying this available force to the rotational output of the turbine rotor. Commonly it is observed as a thrust load within the rotor shaft and opposed at the shaft bearings.
To increase the efficiency
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