Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2002-06-06
2003-12-23
Doerrler, William C. (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
Reexamination Certificate
active
06666033
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to orifice pulse tube refrigerators, and, more particularly, to orifice pulse tube refrigerators with reduced Rayleigh streaming in the pulse tube.
BACKGROUND OF THE INVENTION
Orifice pulse tube refrigeration is the most rapidly developing field of cryogenic refrigeration today. The high efficiency of a Stirling-based thermodynamic cycle, the lack of moving parts at cryogenic temperature, and the lack of small, easily plugged orifices at cryogenic temperature combine to make this new technology inexpensive and reliable. Furthermore, orifice pulse tube refrigerators can be driven by thermoacoustic heat engines, creating for the first time cryogenic refrigeration with no moving parts. Background information about orifice pulse tube refrigerators is given, for example, by R. Radebaugh, “A review of pulse tube refrigeration,” pages 1191-1205 in Adv. Cryogenic Eng., Volume 35 (1990), and in R. Radebaugh, “Advances in Cryocoolers,” 1997, pages 33-44 in the Proceedings of the Sixteenth International Cryogenic Engineering Conference/international Cryogenic Materials Conference (ICEC16/ICMC), edited by T. Haruyama et al., (Elsevier, Oxford, 1997), all incorporated herein by reference.
A prior art orifice pulse tube refrigerator is shown schematically in FIG.
1
A. One of the key parameters in an operational orifice pulse tube refrigerator is the temporal phase difference between oscillating pressure and oscillating velocity. The reference convention used herein is that x is the distance from the driver along the axis of the refrigerator, that positive velocity is velocity in the positive x direction, and that &thgr; is the temporal phase angle by which oscillating pressure leads oscillating velocity. This temporal phase &thgr; is also called the phase of the complex acoustic impedance Z. It is well known that &thgr; is a function of x, due for example, to the compressibility of the gas in various portions of the refrigerator.
Relative magnitudes and phases are conventionally displayed in a phasor diagram, such as shown in FIG.
1
B.
FIG. 1B
illustrates, for example, the viscous pressure drop across regenerator
12
, which manifests itself as the difference between the pressure phasor P
1,driver
at driver
10
and the pressure phasor P
1,pulse
tube in pulse tube
18
. The difference between the volume flow rate phasor U
1,ambient heat exchanger
at ambient heat exchanger
28
and the volume flow rate phasor U
1,cold
at cold heat exchanger
26
is due to the compressibility of the gas in pulse tube
18
. In
FIG. 1B
, P
1,ambient heat exchanger
can be assumed nearly identical to P
1,pulse tube
, SO &thgr;
ambient heat exchanger
is the angle by which P
1,pulse tube
leads U
1,ambient heat exchanger
, which is approximately 50 degrees in FIG.
1
B. This is a typical value for an orifice pulse tube refrigerator with an inertial impedance (“inertance”).
Continuing to refer to
FIGS. 1A and 1B
, it is well known that the entire phase distribution &thgr;(x) throughout an orifice pulse tube refrigerator, and, in particular, in regenerator
12
, can be controlled by means of inertance
14
and flow resistances
16
,
24
in an acoustic impedance network atop pulse tube
18
of the orifice pulse tube refrigerator. An early published reference to this use of inertance was by S. W. Zhu et al., “Phase shift effect of the long neck tube for the pulse tube refrigerator,” in the Proceedings of Cryocoolers
9
, held June 1996 in New Hampshire. An adjustable version of such an acoustic impedance network with inertance is described by D. L. Gardner et al., “Use of inertance in orifice pulse tube refrigerators,” Cryogenics, Volume 37, pages 117-121 (1997) and G. W. Swift et al., “Pulse Tube Refrigerator With Variable Phase Shift,” U.S. Pat. No. 6,021,643, Feb. 8, 2000, all incorporated herein by reference. In the '643 patent, a variable acoustic impedance network, as shown atop pulse tube
18
in
FIG. 1A
, is described, comprising an inertance tube
14
, a compliance volume
22
, and two adjustable flow resistance valves
16
,
24
.
FIG. 2
, which is a reproduction of
FIG. 7
from the '643 patent, shows the broad range of &thgr;
ambient heat exchanger
accessible by this method. The points on
FIG. 2
show some typical values of acoustic impedance Z at the top of the pulse tube, experimentally accessed by adjusting the two valves
16
,
24
; all points between these points are also accessible. Absent viscous effects in inertance
14
, all values of Z between the two horizontal dashed lines would be accessible, and the experimental reality is not far from that ideal. The values of &thgr; represented by these points range from about zero to 80 degrees (the angle between the horizontal axis and a line from the origin to a given point).
The three large circles on
FIG. 2
are contours of constant power dissipation in the acoustic impedance network
14
,
16
,
22
,
24
, and, hence, of constant gross cooling power at cold heat exchanger
26
. Then, an operating point for the orifice pulse tube refrigerator is uniquely defined by, and is often chosen by, selecting a gross cooling power, i.e., at which circle one wants to operate, and a value of &thgr;. The actual net refrigerating power is the gross cooling power minus the sum of heat leaks to cold heat exchanger
26
. Imperfect operation of regenerator
12
and imperfect operation of pulse tube
18
are two sources of potentially large heat leaks, but proper design can minimize these. Efficient refrigeration also requires little viscous dissipation in regenerator
12
.
It is well known that refrigeration occurs only if &thgr; lies between plus 90 degrees and minus 90 degrees in regenerator
12
, and that both regenerator heat leak and viscous dissipation are minimized by keeping &thgr; as close to zero degrees as possible throughout regenerator
12
. In a cryogenic orifice pulse tube refrigerator, typically &thgr; is between zero and minus 45 degrees at the ambient end of the regenerator, passes through zero somewhere within the regenerator, and is positive and less than 45 degrees at the cold end of the regenerator. However, the sensitivity of regenerator efficiency to the exact values of &thgr;(x) is not too strong, and a regenerator with &thgr;(x) shifted by 10 or even 20 degrees from the optimal values may not have a noticeable loss in efficiency with respect to either viscous dissipation or heat leak.
The temporal phase &thgr; also plays an important role in the efficiency of the pulse tube of the orifice pulse tube refrigerator. Pulse tubes are susceptible to an internal, toroidal steady convection, called Rayleigh streaming, that is superimposed upon the desired oscillatory motion. Rayleigh streaming reduces the efficiency of orifice pulse tube refrigerators because the streaming convects heat from ambient heat exchanger
28
atop pulse tube
18
to cold heat exchanger
26
at the bottom of pulse tube
18
, thereby reducing the cooling power of the orifice pulse tube refrigerator. Rayleigh streaming is caused by boundary-layer processes at the side walls of the pulse tube, which are controlled by various parameters including phase angle &thgr;, the taper angle of the pulse tube, and properties of the working gas, as described by J. R. Olson et al., “Acoustic streaming in pulse tube refrigerators: Tapered pulse tubes,” Cryogenics, Volume 37, pages 769-776 (1997) and G. W. Swift et al., “Tapered pulse tube for pulse tube refrigerators,” U.S. Pat. No. 5,953,920, Sep. 21, 1999, all incorporated herein by reference. All other variables being fixed, there is at most one value of &thgr; that stops Rayleigh streaming.
Rayleigh streaming is extremely sensitive to the value of &thgr;, as shown in
FIG. 3
, from G. W. Swift et al., “Performance of a tapered pulse tube,” pages 315-320 in Cryocoolers
10
, edited by R. G. Ross Jr. (Kluwer Academic/Plenum Publishers, 1999), incorporated herein by reference. This experimental evidence shows that a 3 degree change in &thgr; away from the
Swift Gregory W.
Wollan John J.
Doerrler William C.
The Regents of the University of California
Wilson Ray G.
LandOfFree
Method and apparatus for fine tuning an orifice pulse tube... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for fine tuning an orifice pulse tube..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for fine tuning an orifice pulse tube... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3181185