Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2003-04-09
2004-09-21
Esquivel, Denise L. (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
Reexamination Certificate
active
06792764
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to thermoacoustic devices and, more specifically, to thermoacoustic engines, heat pumps, and refrigeration devices. However, the present invention has applicability outside the field of thermoacoustics, and is therefore not limited to thermoacoustic devices.
BACKGROUND OF THE INVENTION
During the past two decades, there has been an increasing interest in the development of thermoacoustic cooling engines (pumps) for a variety of commercial, military and industrial applications. Interest in thermoacoustic cooling has accelerated rapidly with the production ban of chlorofluorocarbons (CFC's). Thermoacoustic refrigerators can be constructed such that they use only inert gases, which are non-toxic and do not contribute to ozone depletion, nor to global warming. Exemplary prior art designs for thermoacoustic engines and refrigerators are shown in the following patents: U.S. Pat. Nos. 4,489,553, 4,722,201, 5,303,555, 5,647,216, 5,953,921, 6,032,464, and 6,314,740.
Most of the thermoacoustic engines and refrigerators that consume or produce electrical power require the containment of the thermoacoustic components and the gaseous working fluid within a rigid-walled enclosure (pressure vessel). The pressure vessel is typically either a rigid-walled adiabatic compression volume or a rigid-walled acoustic resonator of either the standing-wave or Helmholtz type. Within these rigid enclosures, oscillatory pressures are produced. If one is building an electrically-driven thermoacoustic refrigerator, there is usually a piston that is actuated by some electro-mechanical transducer (e.g., loudspeaker or other motor mechanism) contained within, or attached to the rigid enclosure. Motion of that piston produces the required pressure oscillations. The piston is coupled to the rigid enclosure, containing the thermoacoustic components, by some means that provides a dynamic pressure seal against leakage of the gaseous working fluid around the piston. In all known cases except one, this dynamic pressure seal is either a clearance seal (e.g., a close-fitting surface that surrounds the piston) or a flexure seal, such as a bellows or diaphragm. The only exception is the Torsionally-Resonant Toroidal Thermoacoustic Refrigerator (T-RTTAR) (see U.S. Pat. No. 5,953,921). The T-RTTAR approach does not employ a dynamic pressure seal, but requires that the entire rigid enclosure be oscillated at the operating frequency.
In a thermoacoustic prime mover (engine), the pressure oscillations are generated thermoacoustically when thermal energy (heat) is supplied to the engine. To extract electrical power from such thermoacoustically-induced pressure oscillations within the rigid enclosure, a piston, which is connected to an electromechanical transducer, is driven by the pressure oscillations. Again, a dynamic pressure seal is required to suppress the flow of the gaseous working fluid around the piston.
Adiabatic Compression Volume
The simplest implementation of a rigid enclosure used to contain the thermoacoustic components and the gaseous working fluid for a thermoacoustic device is one for which each of the rigid enclosure's dimensions is small compared to the acoustic wavelength. An example of such a device is shown in
FIG. 1
, which is taken from U.S. Pat. No. 6,314,740 to DeBlok (originally FIG.
2
). The acoustic wavelength, &lgr;, is given by the formula &lgr;=a/f, where the sound speed in the gas or gas mixture within the resonator is a, and piston oscillation frequency is f. One or more pistons, oscillating sinusoidally at frequency f, use electrical power to produce the pressure oscillations or utilize those pressure oscillations to produce electrical power. The relationship between the pressure oscillations and the enclosure volume changes caused by the motion of the piston is controlled by the adiabatic gas law if the smallest dimension of the rigid enclosure, L
typ
(usually the length or diameter of the rigid enclosure), is small compared to the acoustic wavelength, L
typ
<<&lgr;. For higher power thermoacoustic devices, the wavelength, &lgr;, is typically on the order of one meter.
Another consideration in the design of thermoacoustic devices is the thermal penetration depth, &dgr;
&kgr;.
, which serves as a characteristic length that describes over what distance heat can diffuse through the working fluid during an acoustic cycle. The rigid enclosure's smallest dimension is always large compared to the thermal penetration depth, &dgr;
&kgr;
. That is, L
typ
>>&dgr;
&kgr;
. In most thermoacoustic applications, the thermal penetration depth, &dgr;&kgr;, is typically on the order of 100 micrometers (100 &mgr;m).
δ
κ
=
κ
πρ
c
p
f
(
1
)
The thermal penetration depth depends upon the density &rgr;, thermal conductivity &kgr;, and isobaric specific heat c
p
of the gaseous working fluid, as well as on the frequency of operation f.
Where the acoustic wavelength is large compared to the rigid enclosure's dimensions, the pressure oscillations everywhere within the enclosure are to a very good approximation constant (i.e. p
l
(t)=p
l
sin(2 &pgr;ft) regardless of position), and to the extent that the volume of gas within the enclosure is large compared to the product of the interior surface area and the thermal penetration depth, the pressure oscillations are to a good approximation governed by the adiabatic gas law, pV
&ggr;
=constant. The polytropic coefficient of the gaseous working fluid, &ggr;, is the ratio of the specific heat of the gas at constant pressure, c
p
, to the specific heat of the gas at constant volume, c
v
,
γ
=
c
p
c
v
(
2
)
For small changes in the rigid enclosure volume, V
o
, that contains the gaseous working fluid at the mean (static) pressure, p
m
, the magnitude of the oscillatory pressure, p
l
, can be expressed in terms of the magnitude of the change in the volume of the rigid enclosure, &dgr;V, using the adiabatic gas law.
p
1
=
γ
p
m
δ
V
V
0
(
3
)
The motion of the piston produces the change in the volume of the rigid enclosure. The magnitude of the oscillatory pressure, p
l
, can be related to the magnitude of the piston motion, y
o
, using the area of the piston, A
pist
. The time-dependent, sinusoidal displacement of the piston is given by y
pist
(t)=y
l
sin(2 &pgr;ft), which has a displacement amplitude, y
l
. The internal volume of the rigid enclosure is V
o
, if it is measured when the piston is at its neutral or equilibrium position, y
pist
=0. At the neutral piston position the gas pressure within the rigid enclosure is equal to the mean pressure p
m
,
p
1
=
γ
p
m
A
pist
V
0
y
1
.
(
4
)
The above result (Equation 4) demonstrates that for a given volume defined by the rigid enclosure volume, V
o
, and piston area, A
pist
, the magnitude of the pressure oscillations, p
l
, is increased as the magnitude of the piston displacement, y
l
, is increased. To simplify comparison of the performance of an adiabatic compression volume to the performance of a standing-wave resonator, it is convenient to characterize the piston's motion by expressing its motion as producing an oscillatory volume flow rate of amplitude dV/dt=2 &pgr;fy
l
A
pist
. The result of (Equation 4) can then be expressed as an acoustic impedance Z
ac
=p
l
/(dV/dt),
Z
ac
≡
p
1
(
ⅆ
V
/
ⅆ
t
)
=
γ
p
m
2
π
f
V
0
.
(
5
)
FIG. 1
illustrates an earlier design that uses this adiabatic compression volume approach. As shown, a piston is joined to a rigid enclosure by a flexible bellows. An electromechanical actuator
2
is attached to the piston-bellows combination
3
, which is joined to the rigid enclosure
1
that contains the thermoacoustic elements of this refrigeration system. An acoustic phase control bypass
10
is formed by an in
Garrett Steven L.
Poese Matthew E.
Smith Robert W. M.
Wakeland Ray S.
Drake Malik N.
Esquivel Denise L.
Gifford Krass Groh Sprinkle Anderson & Citkowski PC
The Penn State Research Foundation
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