EP0672873A1 - Pulse tube refrigerator - Google Patents

Abstract

A hollow tube (1) has a heat exchanger (2) at one end which is connected to an element requiring cooling. The heat exchanger is also connected to a regenerator (4) which links it to a pressure oscillator (7) driven by a piezoelectric unit (72). A second heat exchanger (3) is located at the other end of the hollow tube (1) and is connected to a reservoir (5) through an orifice (6) of cross section 10-50000 um2, this being a little less than the corresponding orifice between the first heat exchanger and the regenerator. A cooling facility (8) is provided at the second heat exchanger.

Description

The invention relates to a pulsed gas cooler and more particularly to a tube micro-cooler.

The most widely used refrigeration devices are based on "Stirling" machines, liquid nitrogen cryostats, Joule-Thomson expansion ports, condensing machines and Peltier elements.

Among the machines operating continuously, only Stirling and Peltier elements are suitable for miniaturization.

The remarkable Stirling refrigerator in this area ("Bat") has a cooling power of 150 mW at 80 K, for an input power of 6 W and a weight of 0.4 kg. On the other hand, the principle of the Stirling cycle generates vibrations, and limits the lifespan to 2000 hours (due to cold moving parts).

Peltier element coolers require a minimum of 3 stages to obtain a power flow at 200 K equal to 50 mW, assuming that the hot radiator is only 300 K. Under these conditions, the electrical power required is around 5 W (or 4 W with 6 stages). Thermoelements making it possible to reach temperature differentials of more than 130 K are rare: they are therefore not efficient enough if the temperature of the radiator is above 60 ° C.

• autonomie et rendement comparables aux machines Stirling ;
• durée de vie d'autant plus grande qu'il n'y a pas de pièce mobile à froid ;
• utilisation en continu pendant toute la durée de vie du système ;
• poids et volume restreints, ajustables en fonction de l'application (température et puissance) ;
• absence de rechargement de fluide, la nature de celui-ci n'étant pas imposée par la température de fonctionnement ;
• possibilité de miniaturisation pour les applications à faible puissance "froide", sans dégradation notable du rendement ;
• vibrations mécaniques limitées en amplitude et sur une bande de fréquence très étroite.

The invention relates to a cooler which allows a significant improvement of machines based on this principle. At equal useful power, it has a number of advantages:

• autonomy and performance comparable to Stirling machines;
• longer service life since there is no moving part when cold;
• continuous use throughout the life of the system;
• weight and volume restricted, adjustable according to the application (temperature and power);
• absence of fluid recharging, the nature of the latter not being imposed by the operating temperature;
• possibility of miniaturization for applications with low "cold" power, without significant degradation of the yield;
• mechanical vibrations limited in amplitude and over a very narrow frequency band.

• un tube creux dont une première extrémité est fermée par un premier échangeur de chaleur et une deuxième extrémité est fermée par un deuxième échangeur de chaleur ;
• un système de pompage alternatif relié à la première extrémité et provoquant une circulation de fluide de la première extrémité du tube à travers le premier échangeur vers la deuxième extrémité ;
• un réservoir relié par un orifice calibré au deuxième échangeur et à la deuxième extrémité de façon à recevoir du fluide provenant de la deuxième extrémité du tube à travers le deuxième échangeur ;
• un radiateur couplé thermiquement au deuxième échangeur ;

The invention therefore relates to a miniature pulsed gas cooler characterized in that it comprises:

• a hollow tube, a first end of which is closed by a first heat exchanger and a second end of which is closed by a second heat exchanger;
• an alternative pumping system connected to the first end and causing a circulation of fluid from the first end of the tube through the first exchanger to the second end;
• a reservoir connected by a calibrated orifice to the second exchanger and to the second end so as to receive fluid coming from the second end of the tube through the second exchanger;
• a radiator thermally coupled to the second exchanger;

the first exchanger being intended to be coupled to an element (load) to be cooled.

• la figure 1, un schéma de principe général du refroidisseur à tube à gaz pulsé connu de l'homme du métier ;
• la figure 2, un mode de réalisation détaillé du refroidisseur selon l'invention ;
• la figure 3, une variante de réalisation du refroidisseur selon l'invention.

The various objects and characteristics of the invention will appear more clearly in the description which follows and in the appended figures which represent:

• Figure 1, a general block diagram of the pulsed gas tube cooler known to those skilled in the art;
• Figure 2, a detailed embodiment of the cooler according to the invention;
• Figure 3, an alternative embodiment of the cooler according to the invention.

• un tube 1 aux extrémités duquel sont disposés des échangeurs de chaleur 2 et 3, en communication avec un espace régénérateur 4 d'une part, et d'autre part avec un réservoir 5 au travers d'un orifice 6 ;
• un oscillateur de pression 7 qui maintient une onde de pression dans le tube 1 en agissant au travers du régénérateur 4 ;
• un radiateur 8 pour évacuer la chaleur transmise par l'échangeur 3.

The system of FIG. 1 comprises:

• a tube 1 at the ends of which are arranged heat exchangers 2 and 3, in communication with a regenerative space 4 on the one hand, and on the other hand with a reservoir 5 through an orifice 6;
• a pressure oscillator 7 which maintains a pressure wave in the tube 1 by acting through the regenerator 4;
• a radiator 8 to dissipate the heat transmitted by the exchanger 3.

Under these conditions, the exchanger 2 located on the side of the regenerator 4 yields heat to the gas which cools down on expansion, before being thermalised by regenerator 4: it therefore constitutes the cold point of the cooling system.

A load shedding circuit 9 can be used to improve the performance of the system by directly diverting part of the flux transmitted to the regenerator 4 to the tube 6.

According to the invention, such a cooler can be produced in miniature form and can be intended to cool an element of small dimensions. The use of microelectronics techniques makes it possible to maintain a low production cost, a reduction in "dead" volumes (thermodynamically, and even from the standpoint of space) and technological compatibility with the majority of electronic circuits (infrared detectors, sensor magnetic field or low noise amplifier).

FIG. 2 represents a detailed embodiment of such a pulsed tube micro-cooler according to the invention.

In Figure 2, we have used the same references as those in Figure 1 to designate the same elements.

It can therefore be seen in FIG. 2 that the cooler according to the invention is made compact and can be miniaturized due to the arrangement of its constituent elements in the following manner:

The tube 1 is attached to the regenerator 4. The exchanger 2 is at the upper outlet end of the tube 1 and communicates with a charging device 20 designed to be coupled with a device to be cooled (not shown) and to be capable of capture the heat from this device. The upper end of the regenerator 4 communicates with the exchanger 2. The opposite end of the regenerator 4 has a cavity 71 closed by a device capable of compressing the air contained in the cavity. This device, according to FIG. 2, is a membrane 70. A device such as a piezoelectric 72 periodically presses on the membrane 70 to compress the gas contained in the cavity 71.

The production of the membrane can take other embodiments. It can be made of piezoelectric material and carry, on its two faces, electrodes. It will be able to deform and act as a pump.

The oscillation of the membrane can also be generated by an electrostatic effect. For example, in this case the membrane is provided with a voltage control electrode and a second voltage control electrode is provided either inside the cavity (on the face 73 for example) or outside of the cavity on a fixed part not shown.

The exchanger 3 communicates with a reservoir 5 which is also attached to the tube 1.

The exchanger 3 is thermally integral with a thermal evacuation device making it possible to evacuate the heat to the outside. This thermal evacuation device can consist of a plate 30 and a cooling radiator 8 thermally coupled to the plate 30. The plate 30 and the cooler 8 are made of a material which conducts heat well.

A material with low thermal conduction (for example fused silica or polymer resin) is used to make the tube 1, the regenerator 4 and the reservoir 5. The exchangers 2 and 3 are made with a material with high thermal conduction and preferably usable by microfabrication technologies (for example silicon, sapphire, beryllium oxide, aluminum nitride, CuW or Mo).

The integration of the exchanger 2 with the load (for example on the back of the substrate of an electronic circuit) constitutes an intimate thermal coupling between the gas and the load, if the thermal conduction of the load is high. Otherwise, the load will be placed on the outer surface of the exchanger 2, large enough to ensure good thermal contact. Figure 3 shows an example of arrangement in the latter case. It also also shows an embodiment of the load shedding circuit 9 (of FIG. 1) in the form of a wall separating the tank into two zones, one of which also plays the role of resonant cavity for the oscillator system.

In this embodiment of FIG. 3, the reservoir 5 is disposed at the end of the tube 1 and includes the membrane 70. According to a preferred embodiment, the walls of the reservoir 5 are made of a material which is a good thermal conductor and constitutes the cooling surface of the device. to which is optionally attached a cooling radiator such as a finned cooler 8.

In FIG. 3, the plate 20 to be cooled to which the component to be cooled is coupled contains in its body the heat exchanger 2. It is this heat exchanger 2 which makes the tube 1 communicate with the regenerator 4. The component to be cooled is placed or fixed on the upper face of the plate 20. This arrangement was applied to the embodiment of FIG. 3 but could also be applied to the embodiment of FIG. 2.

The exchanger 3 is placed on the face opposite to the exchanger 2 to avoid a thermal short circuit between the two exchangers.

The means 7 for maintaining pressure oscillations in the tube are, in these examples, also integrated into the cooling system: a membrane is kept in oscillation by electrostatic effect, the control circuit can also be integrated into the structure, in particular if the exchanger 3 is made of silicon.

Sizing:

   où V_rés est le volume du réservoir, f = ω/2π est la fréquence de l'oscillateur de pression, P₀ est la pression moyenne du gaz, x est le taux de modulation de la pression ; τ est la constante de temps d'équilibre de la pression du réservoir au travers de l'orifice 6 d'impédance R₀ : $$\text{τ = R₀.3/5.}\frac{{\text{V}}_{\text{rés}}}{{\text{R.T}}_{\text{b}}}$$

   où T_b est la température de l'échangeur chaud, R la constante des gaz parfaits, et R₀ est déterminé par la nature du gaz et la géométrie de l'échangeur. Pour un trou cylindrique de longueur I et de diamètre D, et un gaz de viscosité η $$\text{R₀ = 128 η I/πD⁴}$$

Assuming that the gas used is monoatomic (helium), the relation linking the power q extracted at the level of the cold exchanger to the dimensions of the system is:

where V _res is the volume of the reservoir, f = ω / 2π is the frequency of the pressure oscillator, P₀ is the average pressure of the gas, x is the rate of modulation of the pressure; τ is the equilibrium time constant of the reservoir pressure through the orifice 6 of impedance R₀: $$\text{τ = R₀.3 / 5.}\frac{{\text{V}}_{\text{res}}}{{\text{RT}}_{\text{b}}}$$

where T _b is the temperature of the hot exchanger, R the constant of ideal gases, and R₀ is determined by the nature of the gas and the geometry of the exchanger. For a cylindrical hole of length I and diameter D, and a gas of viscosity η $$\text{R₀ = 128 η I / πD⁴}$$

By taking q of the order of 50 mW and ωτ >> 1, we find D = 30 µm for I = 50 µm.

   où M est la masse molaire du gaz. Ceci permet d'exprimer la capacité de l'oscillateur de pression par le débit volumique D_V correspondant à la température ambiante T₀ :

Furthermore, the mass of gas injected into the tube during the compression phase is given by the following approximate relationship:

where M is the molar mass of the gas. This makes it possible to express the capacity of the pressure oscillator by the volume flow D _V corresponding to the ambient temperature T₀:

The mechanical power to be supplied on the order of P₀D _V , is therefore related to q by the relation.

pour les faibles valeurs de x.Its inverse, corresponding to the yield, can be approximated by $$\frac{{\text{2xT}}_{\text{at}}}{\text{T₀}}$$

for low values of x.

Therefore the mechanical power of the pressure oscillator necessary to obtain 50 mW at 200 K is of the order of 0.4 W if a pressure modulation rate of 10% is chosen. Take P₀ = 1 Atmosphere, then D _v = 4 cm³ / s. The volume displaced per period at the level of the pressure oscillator, for a frequency of 100 Hz, is therefore 40 mm³. From the compression ratio chosen equal to 10%, it can be deduced that the total volume of gas to be used is of the order of cm³.

If the orifice 6 is integrated into the exchanger 3 by etching slots or holes in the exchanger, these will be sized using suitable formulas of hydrodynamics, so as to obtain the desired impedance R₀ . Optimization of this geometry will ensure heat transfer from the gas to the maximum exchanger.

In general if we consider that the exchanger 3 has only one hole and that this hole is cylindrical, its diameter will be of the order of 50 µm (D = 30 µm in the previous calculation) and any way less than 100 to 200 µm. It will therefore be produced using microelectronics techniques. If this hole is not cylindrical, its impedance R₀ and therefore the surface of its cross section must be equivalent to that of the hole cylindrical, that is to say of the order of 500 μm² and in any case less than about 50,000 μm² = 0.05 mm² (less than 10,000 to 50,000 µm²).

If n holes are provided, their overall heat exchange surface must be equivalent to that of a single hole within the meaning of the previous formula R₀ = 128 ηI / πD⁴N. For example, instead of a cylindrical hole with a diameter of 100 μm, one could have 256 holes with a diameter of 25 for each.

As regards the exchanger 2, it must have a lower impedance than that of the exchanger 3. It must therefore have holes of cross-sectional dimensions (diameter) greater than those of the holes of the exchanger 3. Nevertheless , the holes in exchanger 2 will be as small as possible, so they will be very slightly larger than those in exchanger 3.

This system allows to locally cool any load requiring a low cooling power, with a high efficiency. It will be able to cool electronic circuits where the temperature intervenes either by thermal activation, for example cooling of infrared detector of the diode type; either by thermal noise, for example cooling of amplifier type device, infrared detector of pyroelectric type, or other devices and materials whose characteristics depend on temperature, for example magnetic field detectors (MMM).

It can be mounted in a multi-stage system, and even combined with Peltier elements serving as the first stage.

Claims (19)

Miniature pulsed gas cooler comprising: - a hollow tube (1) of which a first end is equipped with a first heat exchanger (2) intended to be coupled to an element (charge) to be cooled, and a second end is equipped with a second heat exchanger ( 3); - a regenerator (4) coupling the first end of the tube (1) to a pressure oscillator (7); - a reservoir (5) connected to the second exchanger and to the second end so as to receive fluid coming from the second end of the tube through the second exchanger; - a radiator (8) thermally coupled to the second exchanger (3); characterized in that the second exchanger (3) has at least one hole communicating the tube (1) with the reservoir (5), the surface of the cross section of this hole being less than a value of the order of 10 000 at 50,000 µm²; the first exchanger (2) has at least one hole communicating the tube (1) with the pumping system, the dimension of the cross section of this hole being slightly greater than that of the hole of the second exchanger (3). Cooler according to claim 1, characterized in that the holes are cylindrical and that the second exchanger (3) has at least one hole with a maximum diameter of the order of 100 to 200 µm. Cooler according to claim 1, characterized in that the exchangers have several holes, the overall impedance of which is equivalent to that corresponding to a single hole of section less than 50,000 µm² (or 0.05 mm²). Cooler according to claim 1, characterized in that the pressure oscillator comprises an alternative pumping system (7) connected to the regenerator (4) and causing a circulation of fluid from the first end of the tube through the first exchanger to the second end Cooler according to claim 4, characterized in that the tube (1), the reservoir (5) and the pumping system (7) are joined and in that the tube (1) is made of thermal insulating material. Cooler according to claim 5, characterized in that the tube (1), the regenerator (4), the tank (5) are made of material with low thermal conduction and in that the exchangers (2, 3) and the radiator are strong thermal conduction. Cooler according to claim 5, characterized in that the calibrated orifice (6) is integrated in the second exchanger (3). Cooler according to claim 7, characterized in that the calibrated orifice (6) consists of slots or holes made in the second exchanger (3). Cooler according to claim 1, characterized in that each exchanger comprises a plate (20, 30) of material which is a good thermal conductor, one zone of the plate having perforations connecting the two opposite faces of the plate and serving as a heat exchanger, the rest of the plate being at least partially thermally coupled either to the load to be cooled or to the radiator depending on whether the corresponding exchanger of the plate is the first or the second exchanger. Cooler according to claim 9, characterized in that the plates are arranged in two parallel planes and delimit a volume in which the tube (1) and the reservoir (5) are arranged. Cooler according to claim 10, characterized in that the regenerator (4) is juxtaposed with said volume. Cooler according to claim 5, characterized in that the pumping system comprises a membrane (70) closing a space (71) communicating by the regenerator with the first end of the tube. Cooler according to claim 12, characterized in that it comprises a piezoelectric device (72) generating a periodic movement of the membrane (70). Cooler according to claim 12, characterized in that the membrane (70) comprises a first voltage control electrode and in that a second control electrode is located opposite the first electrode on a fixed part of the cooler. Cooler according to claim 14, characterized in that the second electrode is located in the cavity on a surface (73) facing the membrane. Cooler according to claim 12, characterized in that the regenerator (4) is coupled to the space (71) closed by the membrane (70) in a plane substantially parallel to the plane of the membrane and has a section along this surface plane significantly less than the surface of the membrane. Cooler according to claim 12, characterized in that the reservoir (5) communicates with the face of the membrane opposite to that closing the space (71). Cooler according to claim 6, characterized in that the material with low thermal conduction is silica or a polymer resin. Cooler according to claim 6, characterized in that the material with high thermal conduction is silicon, sapphire, beryllium oxide, aluminum nitride, CuW or molybdenum.

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