10-08-2013, 12:57 PM
Study of Pulse Tube Cryocooler
Introduction
The pulse tube refrigerator (PTR) is a cryocooler which is capable of reaching
temperature of a few tens of Kelvin in a single stage and a few Kelvin in two stages. Unlike
ordinary refrigeration cycles which utilize the vapor compression cycle, a PTR implements the
oscillatory compression and expansion of gas within a closed volume to achieve the desired
refrigeration. Pulse tube refrigerator has the advantages of long–life operation, high reliability
and low vibration over the conventional Cryocooler, such as G-M and Stirling coolers because of
the absence of moving parts at their low temperature end. Due to its associated advantages, pulse
tube refrigerators have several applications such as cooling of infrared sensors, night vision
equipment‟s, SQUID, cryopumping etc. All pulse tube refrigerator units operate as closed
systems where no mass is exchanged between the cryocooler and its environment.
Pulse Tube Cryocooler
Description
The moving displacer in the Stirling and Gifford-McMahon cryocoolers has several
disadvantages. It is a source of vibration, has a limited lifetime, and contributes to axial heat
conduction as well as to a shuttle heat loss. In the Pulse Tube Cryocooler the displacer is
eliminated. The proper gas motion in phase with the pressure is achieved by the use of an orifice,
along with a reservoir volume to store the gas during a half cycle. The reservoir volume is large
enough that negligible pressure oscillation occurs in it during the oscillating flow. The oscillating
flow through the orifice separates the heating and cooling effects just as the displacer does for
the Stirling and Gifford-McMahon refrigerators. The orifice pulse tube refrigerator (OPTR)
operates ideally with adiabatic compression and expansion in the pulse tube.Thus, for a given
frequency there is a lower limit on the diameter of the pulse tube in order to maintain adiabatic
processes.
Improvement in performance of pulse tube cryocooler
Cryocooler is one of the most active field of research today in cryogenic technology. Several
such coolers have been developed in support of space mission, where high reliability and low
weight are primarily concerns. One produced by TRW Inc. in Redondo Beach, Calif, provides
1.5W cooling at 115K,requires 38.2W of input power and weight only2.3Kg,not including the
control electronics. Indeed, this is an example of the potential miniaturization of cryocooler-
particularly pulse tube that will make it easier to incorporate Cryocooler in varieties of
application, especially superconducting electronics.
Working Principle of the Pulse Tube Refrigerators
The pulse tube refrigerators (PTR) are capable of cooling to temperature below 123K.Unlike the
ordinary refrigeration cycles which utilize the vapor compression cycle as described in classical
thermodynamics; a PTR implements the theory of oscillatory compression and expansion of the
gas within a closed volume to achieve desired refrigeration. Being oscillatory, a PTR is a non-
steady system that requires time dependent solution. However like many other periodic systems,
PTRs attain quasi-steady periodic state (steady-periodic mode). In a periodic steady state system,
property of the system at any point in a cycle will reach the same state in the next cycle and so
on. A Pulse tube refrigerator is a closed system that uses an oscillating pressure (usually
produced by an oscillating piston) at one end to generate an oscillating gas flow in the rest of the
system. The gas flow can carry heat away from a low temperature point (cold heat exchanger) to
the hot end heat exchanger if the power factor for the phasor quantities is favorable. The amount
of heat they can remove is limited by their size and power used to drive them.
Types of Pulse Tube Refrigerators
Pulse tube refrigeration systems can be classified as either a Stirling type or a GM type according
to the method of pressurization and expansion as shown in Fig.6.1 (a) and (b). For a Stirling type
pulse tube shown in Fig.6.1 (a) a piston cylinder apparatus is directly coupled to the hot end of
the regenerator so that the pressure fluctuations are directly generated by the piston movement.
In Stirling type PTR, the frequency of the compressor is the same as that of the pulse
tube. The heat of compression by the compressor must be removed to the environment by a heat
exchanger between the compressor and the regenerator, commonly known as after cooler or
precooler. These aspects are the same in Stirling type and G-M type refrigerators. These are used
for PTRs in the higher temperature ranges of about 50K.The typical operating frequency of
Stirling type PTR is 10-120Hz.
Double-inlet pulse tube refrigerator
In the double-inlet pulse tube refrigerator (DIPTR) the hot end of the pulse tube is
connected with the entrance (hot end) of the regenerator by an orifice adjusted to an optimal
value shown in Fig.6.1(a) and (b) for Stirling type and GM type DIPTR respectively. The double
inlet is a bypass for the regenerator and hence reduces the cooling power. In addition, the valve
is a dissipative device, which leads to a deterioration of the performance.
Compressor
The main function of the compressor is to supply gas pressurization and depressurization in the
closed chamber. Electrical power is applied to the compressor where this electrical work is
converted into the mechanical energy associated with sinusoidal pressure waves, resulting in gas
motion. In an ideal compressor, the electrical power provided to the compressor must be equal to
f ∮PdV, where the integration is performed over an entire cycle, P is the compressor pressure,
and f is the compressor frequency. In an actual system, however, the above mentioned power
(the PdV power) is always less than the actual measured electrical power due to the associated
irreversibility. Usually reciprocating nature of compressor is used in case of Stirling model; it
may also be a dual opposed piston type.
Regenerator
The regenerator is the most important component in pulse tube refrigerator. Its function is to
absorb the heat from the incoming gas during the forward stroke, and deliver that heat back to
the gas during the return stroke. Ideally, PTC regenerators with no pressure drop and a heat
exchanger effectiveness of 100% are desired, in order to achieve the maximum enthalpy flow in
the pulse tube. The performances of the real regenerators are of course far from ideal. Stainless
steel wire screens are usually selected as the regenerator packing material, since they offer higher
heat transfer areas, low pressure drop, high heat capacity, and low thermal conductivity. A
typical regenerator housing is shown in Fig.6.6 (a) At present, there exist many version of pulse
tube refrigerator .Three main version of the pulse tube refrigerator in historical order are the
basic,the orifice and the double inlet type refrigerator.
Conclusion
In this chapter we have described the conclusion on the basis of the research which was carried
out in the various papers. After the study, the paper Ref no.10 describes the results based on CFD
for modeling the pulse tube system.
It states that a single stage Stirling type ITPTR and OPTR systems (that includes a
compressor, an after cooler, a regenerator, a pulse tube, cold and warm heat exchangers, an
inertance tube/orifice valve and a reservoir) in a fully-coupled system operating in steady
periodic mode, without any arbitrary assumptions other than ideal gas and no gravity effect
shows that the performance of the ITPTR is superior to that of OPTR. The sole external
boundary conditions imposed on the model are a sinusoidal oscillating piston face velocity along
with one of the thermal boundary conditions, adiabatic, isothermal or known heat flux at the cold
end heat exchanger. In order to observe the difference of refrigeration performance between
inertance type pulse tube and orifice type pulse tube refrigerators, pressure wave of same
amplitude and frequency are applied. The physical dimensions of other components are kept
same except that inertance tube is replaced by an orifice valve. For each system three separate
simulations are analyzed. One simulation assumes an adiabatic cold-end heat exchanger (CHX);
another assumes a known cooling heat load, and the last assumes a pre-specified CHX
temperature. Each simulation started with an assumed uniform system temperature, and
continued until steady periodic conditions are achieved.