The normal operation of many modern electronic devices such as infrared detectors and high-temperature superconducting devices requires a low-temperature cooling machine with compact structure, high reliability and low energy consumption for low-temperature cooling. Since there are no low-temperature mechanical moving parts, compared with Stirling refrigerator and G-M refrigerator, the pulse tube refrigerator has the advantages of simple structure, no sealing and friction, high reliability, small vibration and simple control.

According to its geometric arrangement, the pulse tube refrigerator can be divided into linear, U-shaped and coaxial pulse tube refrigerators. The coaxial pulse tube refrigerator has a dominant position in the research of the Stirling-type pulse tube refrigerator because of its compact structure and easy coupling with the cooled device.

In 2000, the National Bureau of Standards (NIST) N IST successfully researched the vessel oxygen liquefaction device. The refrigerator can provide 18.8W cooling capacity at 90K, and the input power is 222W. The Netherlands Thales Company has successfully developed 4W@80K, 150W input in 2005. Power LPT9310 commercial coaxial pulse tube refrigerator. Recently, France A ir L iquefier company successfully developed 2. 3W@ 50K, LPTC coaxial pulse tube refrigerator with input power of 160W, and has begun to enter the reliability test stage. China is relatively backward in the development of large-volume coaxial pulse tube refrigerators. At present, the large-volume refrigerators reported in China only have experimental prototypes of 4W@80K compressor input power of 100W developed by the Institute of Physics and Chemistry of the Chinese Academy of Sciences.

In the coaxial pulse tube refrigerator, the refrigerant medium reverses 180 in the flow direction of the cold head portion, and the additional empty volume is easily added in the cold head portion, so the cold head is a key component of the coaxial pulse tube refrigerator. In this paper, a miniature coaxial pulse tube refrigerator with a new integral cold head is designed and fabricated. Different inertia tube combined phase modulation tests and different input power tests are carried out, and the test results are analyzed to lay the foundation for future research. The foundation.

2 New cold head structure The new cold head structure consists of four parts: upper cold head part, lower cold head part, cold end baffle and flow guiding net, as shown. The upper cold head is made of cylindrical copper, and there are 18 grooves widening from the center hole by 0.3 mm, depth 5 mm, and axial length 13 mm. The upper and lower ends of the upper cold head have internal threads for connecting the cold accumulator tube and the lower cold head portion. The lower cold head portion is also made of copper, including the base and the boss. The height of the boss is 2 mm, and the boss has 18 axially symmetrical grooves with a width of 0.3 mm and a depth of 2 mm. The outer circumference of the base is threaded to be threaded to the upper cold head portion. A cold end baffle is installed in the middle of the upper cold head portion, and the cold end baffle is a torus structure made of copper. The outer circumference of the cold end baffle cooperates with the middle hole of the upper cold head portion, and the inner circumference is engaged with the end of the blood vessel. The deflector screen is machined from 100 mesh copper mesh.

After the upper cold head portion and the lower cold head portion are screwed together, a margin is left for welding. Ensure that the channels are aligned before welding the upper and lower cold heads.

3 Experimental device 3.1 Experimental system The experimental platform consists of a compressor system, a chiller system, a temperature measuring system, a cooling capacity measuring system, a pressure measuring system and a vacuum system. Among them, the compressor adopts a moving coil type double piston opposed Stirling compressor developed by the Institute of Technical Physics of the Chinese Academy of Sciences. Control the input power and operating frequency of the compressor with a variable frequency AC power supply and a digital power meter. The compressor outlet pressure waveform is measured using a pressure sensor and a digital oscilloscope. The regenerator tubes and vessels in the chiller system are machined from stainless steel. The outer diameter of the tube is 13. 5mm, the wall thickness is 0. 5mm. The outer diameter of the regenerator tube is 28.1mm, the thickness is 0. 3mm. The 400 gauge stainless steel mesh is filled between the regenerator tube and the vessel, and the filling length is 65mm. Both ends of the vessel have a copper mesh as a layered fluidized screen.

3. 2 experimental instruments and equipment Pacific 115ASX variable frequency AC power supply; Yokogawa WT1600 digital power meter; GW GDS-810C digital oscilloscope; Keithley 2000 multimeter; GW GPS - 2303C DC power supply; VAR IAN Turbo- V70 vacuum molecular pump; thermometer.

3. 3 Experimental method In the experiment, the operating frequency and input power of the compressor are controlled by the variable frequency AC power supply. The pressure at the outlet of the compressor is measured by a pressure sensor and the waveform is displayed on a digital oscilloscope to measure the corresponding average pressure and amplitude. The main measurement parameters of refrigeration performance are cooling temperature and cooling capacity. In order to reduce the heat leakage from the cold head, the cold head is placed in a vacuum hood and wrapped with a multi-layer aluminum-plated film.

(1) Measurement of cooling temperature In the experiment, a platinum resistance thermometer was attached to the cold head of the pulse tube refrigerator with a low temperature glue, and the platinum resistance value was read by Keithley 2000, and the collected platinum resistance value was converted into a temperature value by a Labview program. Display and save in real time.

(2) Measurement of cooling capacity The basis for measuring the cooling capacity is the principle of heat balance. In the experiment, a micro-resistance heating sheet was attached to the cold head of the pulse tube refrigerator, and the DC power source was connected through a vacuum electric joint by a two-wire connection method.

After the chiller reaches the minimum temperature, the DC power supply outputs a certain power. After the temperature is stable, the voltage and current values ​​on the heating sheet are recorded to calculate the heating power, and the obtained power is the cooling capacity at this temperature. Repeat the measurement in turn to obtain the amount of cooling at different temperatures.

4 Experimental results and analysis 4.1 Influence of different inertial tube combinations on the performance of the refrigerator The inertial tube has been widely studied and applied as a new phase modulation method. At present, for the 60K 80K temperature zone, the pulse tube refrigerator with a cooling capacity of 16W mainly adopts the inertia tube phase modulation mode.

During the experiment, we mainly carried out experiments on the phase modulation of four different inertial tubes. The inertial tubes were flanged and sealed with O-rings. In each set of inertial tube experiments, the compressor input power is 100W, and the refrigeration system inflation pressure is 3. 0M Pa. The specific inertial tube specifications are as shown.

Shown is the curve of the minimum temperature of the cold head with the operating frequency under different inertia tube phase modulation. Under different combinations of inertia tubes, the chiller has an optimal frequency corresponding to it. For the same set of inertial tubes, the minimum temperature change of the cold head is small with the change of operating frequency. In the range of 4 Hz near the optimal frequency, the temperature change of the cold head does not exceed 2K. Different inertial tube combinations are strongly correlated with the cooling performance. For example, the inertia tube combination No. 3 and No. 4 differ by about 5K, mainly because different inertial tube combinations have different effects on the mass flow of the cold end of the vessel and the phase angle adjustment of the pressure wave. Among the four inertial tube combinations, the inertia tube of No. 1 has the best phase modulation effect.

Shown is a typical cooling curve for inertial tube combination No. 1 phase modulation. The pressure of the cold head of the pulse tube refrigerator was reduced to a minimum temperature of 62 K after 37 minutes, and the input power of the compressor was 100 W. The inflation pressure and the operating frequency were 3.52 MPa and 41 H z.

4. 2 The effect of different input power on the performance of the refrigerator is shown as the graph of the cooling capacity as a function of the cold head temperature. The operating frequency is 41H z, and the inflation pressure is 3. 52MPa. As shown, when the input power is 134W, the minimum temperature of the cold head reaches 56K. At 80K, when the input power is 100W, 120W, 134W, the cold head cooling capacity They are 2. 5W, 3. 5W, 4W.

The actual efficiency of the compressor currently used is 70%, and the efficiency of the currently reported linear compressor is up to 85%. Assuming that the efficiency of the compressor is 85%, the pulse tube refrigerator can input power at the compressor 110. 3W. Provide 4W cooling capacity at 80K. In this case, the efficiency of the pulse tube refrigerator relative to the Carnot cycle can reach 10%, which is close to the performance of the Stirling refrigerator of the same specification.

5 Conclusions This paper designed and fabricated a miniature coaxial pulse tube refrigerator with a new cold head structure. By optimizing the inertial tube combination, the pulse tube refrigerator achieves a 4W cooling capacity at 80K at an input frequency of 41 Hz at an input power of 134 W. The prototype is slightly less efficient than the commercial Stirling refrigerator, but it has simple manufacturing, low cold head vibration, high reliability and possible long life. The next step is to further increase the cooling capacity and efficiency of the linear compressor.

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