Traveling-wave thermoacoustic engines with internal combustion

Details Traveling-wave thermoacoustic engines with internal combustion Traveling-wave thermoacoustic engines with internal combustion

2003-03-13

2004-05-11

Nguyen, Hoang (Department: 3748)

Power plants

Motor operated by expansion and/or contraction of a unit of...

Unit of mass is a gas which is heated or cooled in one of a...

C060S526000, C060S721000

Reexamination Certificate

active

06732515

ABSTRACT:

FIELD OF THE INVENTION
The present disclosure relates generally to the fields of thermoacoustics and combustion and, more particularly, to thermoacoustic devices.
BACKGROUND
Thernoacoustic devices have been used as heat engines and heat pumps. As shown in
FIG. 1
, one mechanism for manipulating thermoacoustic waves is a conventional traveling wave thermoacoustic driver
100
having a hot heat exchanger
130
and a primary cold heat exchanger
140
, which are used to generate a temperature gradient across a regenerator
120
. The conventional thermoacoustic driver
100
contains a compressible fluid that is capable of sustaining acoustic oscillations. To convert thermal energy into acoustic energy, acoustic traveling waves are introduced through the top of the conventional thermoacoustic driver
100
. At substantially the same time, the primary cold heat exchanger
140
is cooled by passing an ambient temperature (or externally chilled) fluid
180
through pipe
160
, and the hot heat exchanger
130
is heated by passing externally heated fluid
170
through pipe
150
. The hot heat exchanger
130
and the primary cold heat exchanger
140
set up a temperature gradient in the regenerator
120
, which is interposed between the hot heat exchanger
130
and the primary cold heat exchanger
140
. The regenerator
120
comprises packing material that is fine enough so that the working fluid in the regenerator
120
is essentially in thermal equilibrium with the packing around it, but not so fine as to prevent the passage of acoustic waves through the regenerator
120
.
Pressure oscillations produced by the acoustic traveling wave induce the compressible fluid in the regenerator to move down towards the hot end of the temperature gradient, or up towards the cold end of the temperature gradient. Consequently, when the compressible fluid moves down, the hotter regenerator packing heats and expands the compressible fluid; when the compressible fluid moves up, the colder regenerator packing cools and contracts the compressible fluid. As the acoustic traveling wave passes through the compressible fluid, it imparts time-dependent pressure and velocity oscillations to a small volume of the fluid at the wave's location. Since traveling waves are intrinsically phased such that the peak velocity and the peak pressure occur at substantially the same time, the processes undergone by the small volume of the fluid in the regenerator mimic the thermodynamic cycle of a Stirling engine. The thermodynamic cycle, therefore, results in conversion of thermal energy into mechanical energy. In other words, the traveling wave causes the compression, expansion, and fluid movement, which adds pressure and momentum to the waves, thereby amplifying the acoustic traveling wave as it passes through the regenerator.
As is known in the art, if the direction of the acoustic traveling wave is reversed from the hot heat exchanger
130
to primary cold heat exchanger
140
, then the conventional thermoacoustic driver
100
may be used as a heat pump for refrigeration, air conditioning, or other cooling or heating applications. Since the operation of the conventional thermoacoustic driver
100
is known in the art, further discussion of the conventional thermoacoustic driver
100
is omitted here.
FIG. 2
is a diagram showing a cross-sectional view of a thermoacoustic Stirling heat engine (TASHE)
200
having a conventional thermoacoustic driver. As shown in
FIG. 2
, the TASHE
200
comprises a resonator
220
, a variable acoustic load
210
, and a thermoacoustic driving section
300
. In one working example, the TASHE
200
is filled with helium at approximately thirty bars mean pressure. The use of high-pressure helium increases the acoustic power density of the TASHE
200
, which permits acoustic effects to prevail over heat conduction losses.
FIG. 3
is a diagram showing, in greater detail, the thermoacoustic driving section
300
of the TASHE
200
from FIG.
2
. The thermoacoustic driving section
300
of the TASHE
200
comprises a toroidal acoustic feedback loop (or torus)
315
having a regenerator
330
interposed between a primary cold heat exchanger
325
and a hot heat exchanger
335
. As described with reference to
FIG. 1
, the primary cold heat exchanger
325
, the regenerator
330
, and the hot heat exchanger
335
are configured to amplify acoustic traveling waves that propagate clockwise through the torus
315
. At the junction
350
, a portion of the amplified acoustic energy travels to the right towards the resonator
220
and the acoustic load
210
, while the remainder is fed back, through the torus
315
, to the cold end of the regenerator
330
to be amplified within the regenerator
330
. Thus, when the acoustic traveling waves propagate clockwise through the torus
315
, the thermoacoustic driving section
300
functions as a heat engine. Conversely, a counter-clockwise propagation of acoustic traveling waves through the torus
315
attenuates the acoustic traveling waves, thereby resulting in a heat pump configuration in which heat is pumped from the primary cold heat exchanger
325
to the hot heat exchanger
335
.
Additionally, the torus
315
contains an inertance section
305
and a compliance section
310
. These sections
305
,
310
, along with the regenerator
330
, define the properties of the acoustic waves in the thermoacoustic driving section
300
. Each of these components
305
,
310
and
330
, are much shorter than an acoustic wavelength, though their specific geometries create the traveling wave acoustic phasing within the regenerator
330
. They are also geometrically configured to reduce the acoustic velocity within the regenerator
330
, thereby reducing viscous losses that would normally accompany the passage of an acoustic traveling wave through a conventional thermoacoustic driver
100
, as shown in FIG.
1
.
The thermoacoustic driving section
300
of the TASHE
200
further comprises a secondary cold heat exchanger
345
, which, in conjunction with the hot heat exchanger
335
, defines a thermal buffer tube
340
. The thermal buffer tube
340
provides thermal isolation between the hot heat exchanger
335
and the rest of the TASHE
200
beyond the cold heat exchangers
325
,
345
. Since the TASHE
200
is described in greater detail in U.S. Pat. No. 6,032,464 to Swift et al., further discussion of the TASHE
200
is omitted here.
One drawback of the TASHE
200
is that acoustic streaming in the thermoacoustic driving section
300
results in a convection current that travels clockwise around the torus
315
, carrying thermal energy away from the regenerator
330
and out the secondary cold heat exchanger
345
. Since this degrades the performance of the engine, it is desirable to eliminate or minimize any clockwise mean flow around the torus
315
and through the regenerator
330
. As a result, the thermoacoustic driving section
300
of the TASHE
200
comprises a hydrodynamic mass-flux suppressor (or jet pump)
320
that is adjustable to minimize or eliminate any net flow of the compressible fluid around the torus
315
. The operation of the mass-flux suppressor
320
relies on turbulence and the viscous dissipation of kinetic energy, so its use in suppressing the clockwise convection current is also accompanied by some dissipation of acoustic energy.
Also, in the TASHE
200
, conduction of heat through the walls of the torus
315
can result in significant energy losses. These energy losses are due to heat conduction radially through the walls into the insulation or atmosphere surrounding the torus
315
, and also due to axial heat conduction along the walls of the torus
315
between the hot heat exchanger
335
and the cold heat exchangers
325
,
345
, essentially bypassing the regenerator
330
. For higher internal gas pressures as are typically present in the TASHE
200
, greater wall thickness is required, which results in greater axial conduction losses. Additionally, cross-flow heat exchangers
325
,
335
,
345
, which are typically used due to g

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