Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2003-04-09
2004-06-29
Doerrler, William C. (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C267S081000, C267S140400
Reexamination Certificate
active
06755027
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to springs and flexure seals and, more specifically, to a spring design that may include an integral dynamic gas seal.
BACKGROUND OF THE INVENTION
The present invention falls generally into the fields of springs and seals. However, one of the preferred applications of the present invention is in a thermoacoustic engine or refrigerator. One such thermoacoustic device is shown in FIG.
1
. This thermoacoustic device
20
is the subject of U.S. provisional patent application Serial No. 60/372,008, filed Apr. 10, 2002, and a co-pending patent application entitled “Compliant Enclosure for Thermoacoustic Devices,” filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference. Another thermoacoustic device is shown in U.S. provisional patent application Serial No. 60/371,967, filed Apr. 10, 2002, and a co-pending patent application entitled “Thermoacoustic Device,” filed Apr. 9, 2003, the entire contents of both of which are incorporated herein by reference.
The thermoacoustic refrigerator
20
includes an outer pressure vessel
50
containing substantially all of the components of the refrigerator, including a compliant enclosure. The compliant enclosure, in turn, houses the thermal components or thermal core
40
. These thermal components
40
include a cold heat exchanger
14
, a regenerator
16
, and a hot heat exchanger
18
. These components
40
are supported by a thermally insulating plate
22
that accommodates the passage of heat exchanged fluids through tubes
24
, which communicate with thermal sinks or loads outside the pressure vessel
50
. A piston
26
is spaced below the thermal components
14
-
18
, with a bellows
28
extending between and interconnecting the piston
26
and the thermally insulated plate
22
. A linear motor or actuator
30
is interconnected with the piston
26
by a moving portion
32
of the motor
30
. Therefore, the motor
30
is operable to move the piston. In the illustrated embodiment, the moving portion
32
of the motor
30
is interconnected with the piston
26
by a rigid tube
36
. A cylindrical spring
34
may be provided between the piston
26
and some stationary portion of the device, such as the insulating plate
22
, to adjust the mechanical resonance frequency of the system.
Most of the thermoacoustic engines and refrigerators that that have been constructed and tested to date, and that either use or produce electricity, operate at “acoustic” frequencies, usually in the range from about 40 Hz to 500 Hz. To operate at these frequencies, the electro-mechanical transduction device (e.g., loudspeaker or linear motor/alternator) requires some supplemental stiffness, such as spring
34
, to allow the moving mass of the electro-mechanical device to resonate at the desired operating frequency. Operation at the resonance frequency has been shown to produce optimally efficient electro-mechanical energy conversion [R. S. Wakeland,
J. Acoust. Soc. Am
. 107(2), 827-832 (2000)].
In some devices, the gas trapped behind the piston, which produces the sound wave in a thermoacoustic refrigerator, provides this supplemental stiffness. The linear harmonic motion of the piston may also be driven by gas pressure oscillations produced by a thermoacoustic engine. The piston is then used to produce electricity when joined to a linear alternator. One such embodiment of a linear alternator is described in U.S. Pat. No. 5,389,844. In other devices, a mechanical spring, usually made of steel, is used to prove the required supplemental stiffness, as taught in U.S. Pat. No. 5,647,216 (linear) and U.S. Pat. No. 5,953,921 (torsional).
Some mechanism must be provided to prevent the passage of gas around the piston that couples the mechanical energy of the electromechanical device to or from the acoustical energy of the thermoacoustic device. Traditionally, either a clearance or flexure seal has been employed. Thermoacoustic device
20
in
FIG. 1
makes use of a flexure seal, which takes the form a bellows
28
.
A clearance seal is produced by a very narrow gap between the circumference of the piston and the cylindrical bore within which the piston must slide. The clearance seal requires very close tolerance (hence expensive) machining of both the piston and the bore. This clearance seal arrangement is known to produce extraneous energy dissipation when some amount of the gas passes through the gap (know as “blow by” loss). The clearance seal can also produce dissipation due to fluid friction produced by fluid (gas) shear caused by the relative motion of the piston and the bore. In addition to the excess dissipation, an asymmetry between the compression and expansion tends to move gas preferentially behind or in front of the piston. This dynamically-induced static pressure difference tends to un-center the piston (known as “piston walking”). Some additional mechanism must be provided to relieve the accumulating differential pressure, such as a relief valve, or an acoustic bypass.
For electrically-driven thermoacoustic refrigerators, the flexure seal employed most successfully to date has been the metal bellows. The metal bellows has advantages over the clearance seal since it does not permit blow-by and the energy dissipated by mechanical losses produced by the metal's flexure is negligibly small. The bellows seal is limited by the fact that the bellows material must be quite thin, typically less than one one-hundredth of an inch (250 micrometers) thick, if it is to be capable of an infinite number of excursions that are on the order of ±10% of its convolved length [for further details, see
Standards of the Expansion Joint Manufacturer's Association, Inc
., 25 North Broadway, Tarrytown, N.Y. 10591]. Since the walls of the bellows must be thin (to reduce deflection stresses), the bellows does not provide the significant elastic stiffness necessary to resonant the moving mass of the electro-mechanical transducer, nor is it always capable of sustaining the large pressure differentials (that produce pressure stresses in the bellows) that would allow it to produce the stiffness required for it to function as a gas spring.
SUMMARY OF THE INVENTION
One purpose of the present invention is to provide a cylindrical spring design that may be more compatible with a new “bellows bounce” resonant cavity than the flat spring designs taught in U.S. Pat. No. 6,307,287. Such a “bellows bounce” resonance cavity is well-suited for compact thermoacoustic engines and refrigerators that utilize electro-mechanical transducers such as moving-magnet motor/alternators. The geometry and function of a “bellows bounce” resonant cavity is described in U.S. provisional patent application Serial No. 60/372,008, filed Apr. 10, 2002, and a co-pending patent application entitled “Compliant Enclosure for Thermoacoustic Devices,” filed Apr. 9, 2003.
A cylindrical spring design according to the present invention can also act as a dynamic gas seal if the gaps between the elastic “spring beams” are closed with a second compliant material, such as an elastomer (e.g., rubber or silicone adhesive), to prevent the passage of the gas that is being compressed and expanded by the motion of the spring. By sealing the gaps with a low-loss compliant material [e.g., Type I rubber as described by J. C. Snowdon,
Vibration and Shock in Damped Mechanical Systems
(J. Wiley & Sons, 1968). Chapter 1], the spring can also replace the bellows that has traditionally been used to provide the flexure seal necessary for vibroacoustic compression and expansion the working fluid (e.g., air, inert gas, or mixture of inert gases) in previous thermoacoustic refrigerators as taught in U.S. Pat. No. 5,647,216.
If the low-loss compliant material used for the dynamic gas seal described above is replaced with a high-loss compliant material (e.g., Type II rubber), then a spring according to the present invention is suitable for use as a vibration isolation mount to decouple machinery vibratio
Doerrler William C.
Gifford Krass Groh Sprinkle Anderson & Citkowski PC
The Penn State Research Foundation
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