Y. L. Ju

A pulse tube refrigerator below 2 K

Up to now, all pulse tube refrigerators operating at the liquid helium temperature range use 4He as the working fluid. However, the lambda transition of 4He is a barrier for reaching temperatures below 2 K. Theoretical analysis in this paper shows that, using 3He, the temperature limit is below 2 K, and the efficiency of a 4 K pulse tube refrigerator can be improved significantly. A three-stage pulse tube refrigerator is constructed. A compressor with input power of 4 kW and a rotary valve are used to generate the pressure oscillations. With 4He, a minimum average temperature of 2.19 K was reached. Replacing 4He by 3He, at the same valve settings and operating parameters, the minimum average temperature goes down to 1.87 K and the cooling power at 4.2 K is enhanced about 60%. After fine tuning of the valves, a minimum average temperature of 1.78 K was obtained. This is the lowest temperature achieved by mechanical refrigerators.

Numerical simulation and experimental verification of the oscillating flow in pulse tube refrigerator

An improved numerical modeling for simulating the oscillating fluid flow and detail dynamic performance of the orifice and double-inlet pulse tube refrigerator has been developed in this paper. The governing equations that include the pressure gradient, inertia, viscous and convection terms are based on the conversion of mass, energy and momentum for oscillating flow in refrigerator. The full implicit time-dependent and upwind second-order finite difference scheme are used to discrete the governing equations. Simulation results and predicated performance are compared with the experimental data. Good agreement has been found between the two. Detail time-dependent axial wall temperature distribution, transient gas temperature, mass flow rate and dynamic pressure variations in the pulse tube refrigerator have been obtained in this paper. The simulation model is useful for understanding the physical process occurring in the pulse tube refrigerator, and also for predicting the effect of the orifice and double-inlet valve on the refrigeration power and efficiency of pulse tube refrigerator.

GEM operation in helium and neon at low temperatures

We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne and Ne+H2 at temperatures in the range of 2.6–293 K. In He, at temperatures between 62 and 293 K, the triple-GEM structures often operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved by the Penning effect in the gas impurities released by outgassing. At lower temperatures, the gain–voltage characteristics are significantly modified probably due to the freeze-out of impurities. In particular, the double- and single-GEM structures can operate down to 2.6 K at gains reaching only several tens at a gas density of about 0.5 g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be reestablished in Penning mixtures of Ne+H2: very high gains, exceeding 10 4 , have been obtained in these mixtures at 50–60 K, at a density of 9.2 g/l corresponding to that of saturated Ne vapor near 27 K. The results obtained are relevant in the fields of two-phase He and Ne detectors for solar neutrino detection and electron avalanching at low temperatures.

Dynamic experimental investigation of a multi-bypass pulse tube refrigerator

We designed and constructed a dynamic experimental apparatus to measure the instantaneous velocity and pressure in the multi-bypass pulse tube refrigerator (MPTR). Some experimental results of the instantaneous measurements of velocity and pressure in an MPTR with two-bypass tubes during actual operation are presented. D.c. flow phenomena are observed in this MPTR. Reasons are given for the multi-bypass version improving the performance of the pulse tube refrigerator.

NUMERICAL SIMULATION OF NATURAL CONVECTION IN AN ENCLOSURE WITH DISCRETE PROTRUDING HEATERS

A numerical study is presented for natural convection flow and heat transfer in a rectangular enclosure cooled from above with five discrete protruding heaters mounted on one vertical wall. The governing equations for natural convection and conjugate conduction within protruding heaters were solved using a finite volume method over a range of Rayleigh numbers from 5.0 × 105 to 1.0 × 107. Results showed the flow pattern and temperature profile within the entire enclosure for various power inputs, the correction of local Nusselt number versus modified Rayleigh number was presented. Comparison of the numerical results with experimental data showed good agreement. Address correspondence to Dr. Yonglin Ju, Cryogenic Laboratory, Chinese Academia Sinica, P. O. Box 2711, Beijing 10080, Peoples Republic of China.

Nonideal-gas effect in regenerators

Reformulating the theory of pulse-tube operation from the ideal-gas situation to a real gas, changes the properties drastically. The nonideal-gas properties have a profound effect on the energy balance in the regenerator and on the expression for the cooling power. The temperature profiles in the regenerator are strongly affected by the thermal properties of the fluid. We show that, for large flow rates, the dissipation in the regenerator is proportional to the flow rate and cannot be reduced by reducing the thermal conduction coefficient or the regenerator geometry.

Computational study of a 4 K two-stage pulse tube cooler with mixed Eulerian-Lagrangian method

Abstract A new mixed Eulerian–Lagrangian numerical model for simulating and visualizing the internal processes and the variations of dynamic parameters of a two-stage pulse tube cooler (PTC) operating at 4 K temperature region has been developed. We use the Lagrangian method, a moving grid, to follow the exact tracks of gas particles as they move with pressure oscillation in the pulse tube to avoid any numerical false diffusion. The Eulerian approach, a fixed computational grid, is used to simulate the variations of dynamic parameters in the regenerator. A variety of physical factors, such as real thermal properties of helium, multi-layered magnetic regenerative materials, pressure drop and heat transfer in the regenerator, and heat exchangers, are taken into account in this model. The time-variations of gas temperature, pressure, mass flow rate, enthalpy flow in a cycle, in first- and second-stage regenerators are presented in the paper. More attention is paid to the effects of different regenerative materials on the performance of the 4 K two-stage PTC.

Gas Purity Effect on GEM Performance in He and Ne at Low Temperatures

The performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne, He+H2 and Ne+H2 was studied at temperatures in the range of 3-293 K. This paper reports on previously published measurements and additional studies on the effects of the purity of the gases in which the GEM performance is evaluated. In He, at temperatures between 77 and 293 K, triple-GEM structures operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved through the Penning effect as a result of impurities in the gas. At lower temperatures the gain-voltage characteristics are significantly modified probably due to the freeze-out of these impurities. Double-GEM and single-GEM structures can operate down to 3 K at gains reaching only several tens at a gas density of about 0.5 g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be re-established in Penning mixtures of Ne+H2 : very high gains, exceeding 104, have been obtained in these mixtures at 30-77K, at a density of 9.2 g/l which corresponds to saturated Ne vapor density at 27 K. The addition of small amounts of H 2 in He also re-establishes large GEM gains above 30 K but no gain was observed in He+H2 at 4 K and a density of 1.7 g/l (corresponding to roughly one-tenth of the saturated vapor density). These studies are, in part, being pursued in the development of two-phase He and Ne detectors for solar neutrino detection

Multistage pulse tubes

In this paper we address the question of when and how multistaging in pulse-tube refrigerators improves the performance. A two-stage pulse-tube refrigerator is treated as an example. In order to avoid complicated mathematical or numerical calculations we assume that the only irreversible process in the regenerator is heat conduction and that the average enthalpy flow in the regenerator is zero. We derive analytical expressions for the position of the first stage connection to the regenerator in the case of maximum cooling power and in the case of the minimum temperature.

The experimental investigation of a two-stage pulse tube refrigerator

Abstract A two-stage pulse tube refrigerator in combination with a rotary valve and a valved compressor has been constructed. Its performance and the effects of the operating parameters were investigated. A minimum temperature of 11.5 K and a cooling capacity of 1.3 W at 20 K were obtained at a frequency of 4.2 Hz and a system pressure of 1.5 MPa. The 2nd-stage regenerator was only filled with lead shot.