The “Hadoh” group of societies for the study of thermoacoustics and thermoacoustical engineering, established in 1988, was adjourned in 1977. During the 9 years, members of these societies promoted great progress in thermoacoustics and thermoacoustical engineering, especially in the area of pulse-tube refrigeration. The thermoacoustical engineering supported by the Cryogenic Society of Japan is presently spreading into the Japanese Society of Mechanical Engineering and other societies. A wide variety of thermoacoustic phenomena has been studied, and the thermoacoustic theory has developed from “standing wave approximation” to a theory including “progressive waves” in order to understand thermoacoustic devices equipped with regenerators. While thermoacoustic theory succeeded in a qualitative understanding of thermoacoustic phenomena and has been used for the R&D of thermoacoustic refrigerators, there remain important problems; how to control the steady mass flow and how to understand and design heat exchangers of thermoacoustic devices. These are essential and urgent problems in developing practical devices of higher efficiency. A deep insight of thermoacoustics may be one of the keys to the future development of thermodymnamics that consider time.
The basic differences between thermoacoustic theory and traditional theory are reviewed. The first is a difference of image. Traditional theory discusses only enthalpy flow, while thermodynamic theory discusses not only enthalpy flow but also entropy flow and work flow in conjunction with the second law of thermodynamics. Definitions of these flows in the traditional theory are vague and sometimes conflict with both the first and second laws of thermodynamics, while the definitions in thermoacoustic theory are clear and consistent to thermodynamics. The concept of a state variable plays a key part of thermodynamic discussion regarding thermoacoustical phenomena. The last difference is related to transverse averages. Most people who follow traditional theory discuss the transverse averages of oscillating quantities at first, and after that, handle the product of oscillating quantities. Thermoacoustic theory discusses transverse distribution of the oscillating quantities at first and then handles the transverse averages of products of oscillating quantities. Since the product of averages and the average of a product are different in general, results of those following traditional theory are different from those obtained by thermoacoustic theory. This is one of the reasons why thermoacoustic theory succeeds in discussing a wide variety of thermoacoustic phenomena including intrinsically irreversible phenomena.
A single-stage four-valve pulse tube cryocooler with a large cooling power at 80K has been built and tested. The cryocooler has a regenerator stacked with stainless-steel screen disks of 250 mesh and a pulse tube 200mm long. A rotary valve unit directly united to the hot end of the cold head is employed to control the mass flow at the hot ends of the regenerator and pulse tube. Three kinds of pulse tube, 18, 28 and 38mm in diameter, were prepared for investigating the influence of pulse tube size on cooling performance. The cryocooler with the pulse tube of 28mm in diameter delivered a cooling power of 33.5W at 80K, and reached a no-load temperature of 20.5K at a cycle frequency of 1.8Hz. This paper gives the initial experimental results and a brief discussion.
To increase the refrigeration efficiency applying a simple construction is the main subject of research regarding pulse-tube coolers. This paper describes an inter-phasing pulse-tube cooler with a new phase controller concept for increasing the overall refrigeration efficiency. In this pulse-tube refrigeration system, there are two sets of coolers, one of which consists of a pulse tube and a regenerator. The pulse-tube hot ends of two coolers are connected by either an orifice valve or an on-off valve. The two coolers are operated at 180 degrees out of phase using one-valved compressor. By controlling either the mass flow rate through the orifice or timing for the on-off valve, the phase between the pressure and mass flow control each other, and could be optimized in the two pulse tubes. The performance of the inter-phasing pulse-tube cooler was measured and compared with other types of pulse-tube coolers. The equivalent expansion PV diagrams of each cooler are analyzed.
The absorption curves of an acoustic resonator were measured for air, nitrogen, and helium gases. An empirical equation for the curves was proposed. Simple discussion showed that this equation was valid for cases of small dissipation of energy in a resonator. The dissipation was composed of two terms: one due to the incomplete reflection of sound waves at both ends of the resonator, and the other proportional to the length of the resonator indicating the effects of the side wall. The latter was evaluated by employing thermoacoustic theory. The theoretical results showed quantitative agreement with results of experiments. This experiment therefore supports thermoacoustic theory quantitatively.
Numerical simulation of the viscous compressible flow in a pulse tube is conducted to study the fundamental mechanism of refrigeration in pulse-tube refrigerators. Axisymmetric two-dimensional Navier-Stokes equations are solved numerically using the finite volume method with an implicit scheme. The simulated model is a 280×15.6mm (L×D) pulse tube driven by an oscillating helium flow. The tube wall is assumed to be an adiabatic wall except for the cold and hot-end heat exchangers. The phase difference in oscillating flow between the tube center and near the tube wall, a typical feature of oscillating flow, is clearly observed by the simulation. The enthalpy flow and gas work are investigated in order to study the mechanism of refrigeration. The simulation results suggest that the boundary layer on the wall might have an important roll of transferring enthalpy from the cold end to the hot end. Additionally, the existence of secondary flow, from the hot end along the wall to the cold end, that may disturb the refrigeration mechanism is also observed. As driving frequency increases, the strength of secondary flow and magnitude of viscous dissipation increases significantly.
This paper describes a simplified loss evaluation method for pulse tube coolers. Assuming an isotherm model, the workflow balance of the pulse tube cooler will be discussed in comparison with an experimental result. The work flow through the valves as well as the equivalent PV work are calculated to figure out the optimum operating condition for a given size of pulse tube. This method makes it easy to determine the basic size of each component and the input power for the required cooling capacity. Work losses of a pulse tube cooler driven by a valved compressor with different phase-shift mechanisms such as double-inlet, four-valve and active-buffer types are discussed. Loss due to enthalpy flow through the regenerator is also taken into account. Results of example calculations indicate performance improvements of 10% and 20% by replacing the phase-shift mechanism from double-inlet method to four-valve and active-buffer methods, respectively. It also indicated that the work loss at the switching valve to the compressor for the double-inlet method and enthalpy loss at the regenerator for the active-buffer method are the dominant losses.
An analytic equation of the mass flow rate through the bypass of double-inlet pulse tube refrigerators was obtained by an analysis of the pressure drop in the regenerator. The work loss through the bypass is discussed based on this equation. The work loss decreases with the increase in void volume ratio of the regenerator over the pulse tube, decreases with the increase in temperature ratio of the room temperature over the refrigeration temperature, and increases with the increase in “double-inlet factor, ” which is introduced in this paper. The work loss ratio at 80K with an ideal double-inlet is about 50, 30, 20 and 10% when the void volume ratio of the regenerator over the pulse tube is 0.5, 1.0, 2.0 and 4.0, respectively. The analytic results were compared with numerical results. A good agreement between them was achieved.
In an orifice pulse tube refrigerator, the dynamic pressure-flow characteristics of the orifice were determined from experimental investigation for the purpose of calculating the accurate PV diagram in the pulse tube. The mass flow calculated from measured pressures on both sides of the orifice involves steady flow that is generated from steady pressure difference produced by acoustic streaming at the orifice. This steady flow compensates for the acoustic streaming and must be subtracted to calculate the PV diagram.
Temperature measurement in a pulse tube is important to understand the gas behavior in particular. However, it has not been measured with few exceptions. In this paper, we propose temperature measurement in a pulse tube with Rayleigh scattering. Rayleigh scattering is elastic scattering of light quanta from molecules or small particles. The gas temperature can be calculated from Rayleigh-scattering light if the gas behaves according to the ideal gas law and the pressure of the gas is known. This method has the following advantages: (1) Temperature is measured without disturbing the flow in the pulse tube; (2) there is little time delay for measurement; and (3) a two-dimensional temperature image is presented. The disadvantages are: (1) Stray light interferences cause some errors in measurement; and (2) an excimer laser is needed. We installed an experimental system in a clean room and measured Rayleigh-scattering light using a CCD camera. A new energy-monitoring method contributed to improvement in measurement precision. We demonstrated that the temperature under static pressure conditions can be measured with a standard deviation of less than 1.7%. Furthermore temperature measurement under a steady flow with a thermocouple agrees approximately with that measured by Rayleigh scattering. Moreover, it was demonstrated that the temperature can be measured under oscillating pressure conditions.
As for pulse tube refrigerators, it is difficult to understand the phenomena in the pulse tube because gas in the tube oscillates, exchanging heat with the wall. So it is necessary to measure the temperature profile in the pulse tube and to compute the enthalpy flow in order to understand the phenomena. We adopted a measurement using Rayleigh scattering in order to measure the temperature profile in the pulse tube. We made a pulse tube refrigerator with a quartz-glass pulse tube. The pulse tube refrigerator was refrigerated in air, with only the regenerator covered with insulators. Therefore the pulse tube works as a heat exchanger. Under the above conditions, the temperature profile in the pulse tube has been measured and visualized. Enthalpy, work and heat flows have been calculated from the measured temperature profile. The calculated work flow plus the calculated heat flow is equal to the calculated enthalpy flow. The calculated enthalpy flow shows the level of heat exchange in the pulse tube.
In this study, the behavior of the gas in a pulse tube is visualized using a shuttle (light resinous ball), acrylic pulse tube, and acrylic vacuum chamber. In the case of an orifice pulse tube refrigerator, the lowest temperature was 173K at 3Hz. This paper presents the influence of gas displacement and phase difference between gas displacement and pressure in a pulse tube on the performance of a pulse tube refrigerator. Experimental results show that, in the case of an orifice pulse tube refrigerator, not only must gas phase difference be magnified but also gas displacement must be optimized because larger gas displacement results in larger regenerator loss. In the case of a double-inlet pulse tube refrigerator, we observed a DC flow in the pulse tube and measured the DC flow rate under specific conditions. DC flow rate was 0.3×10-6m3/s when the Cv value of the bypass valve was 0.03. By holding down the DC flow rate to 0×10-6m3/s with the use of a check valve and DC flow control valve, the performance of the double-inlet pulse tube refrigerator was improved. Furthermore, gas displacement and phase difference could be measured under no DC flow in the double-inlet pulse tube refrigerator. Experimental results show that optimized gas displacement in the double-inlet pulse tube refrigerator is smaller than the optimized gas displacement in the orifice pulse tube refrigerator, and optimized phase difference in the double-inlet pulse tube refrigerator is larger than the optimized phase difference in the orifice pulse tube refrigerator. Therefore, the performance of the double-inlet pulse tube refrigerator is better than that of the orifice pulse tube refrigerator.
We tried to observe the flow behavior in typical pulse tube refrigerators (i.e., basic, orifice and double-inlet pulse tube refrigerators) using a smoke-wire flow visualization technique, and discuss the fundamental flow behavior in those pulse tube refrigerators. The pulse tube is made of a transparent acrylate tube having the effective length of 280mm, an inner diameter of 16mm and a wall thickness of 4mm. The regenerator is composed of a stainless-steel housing and a bakelite tube, 18mm in diameter and 168mm long packed with 770 discs of 100-mesh stainless-steel screen. The reservoir has a volume of about 3×10-4m3, which is 5.5 times larger than the pulse tube volume. Air is used as the working gas and pressure oscillation is generated by introducing pressurized air of about 0.2MPa into the rotary valve and releasing it to the atmosphere. Visualization is done under the conditions of frequency of 5Hz and compression ratio of about 1.2. The flow behavior throughout the pulse tube during one cycle is cleared. It was found that typical velocity profiles of the viscous oscillating flow are observed for all pulse tube refrigerators, and that the orifice and double-inlet types have larger velocity and displacement compared to the basic one. It was also found that the thickness of velocity boundary layer for oscillating flow is estimated to be about 30% of the radius of the pulse tube. Secondary streaming in the pulse tube, which flows in the direction from cold- to hot-end in the core region and the opposite direction in the boundary region, was recognized for all refrigerators.
A GM-type two-stage pulse tube refrigerator equipped with three rotary valves was developed, and the lowest temperature of 10.3K was obtained when lead shots were used as the second-stage regenerator material. From the measured pressure oscillation at both sides of the orifice plate connected to the pulse tube hot ends, the mass flow rate through the orifice was estimated and the equivalent PV work calculated. Based on the cooling capacity and calculated PV work, the refrigeration losses were found to depend on the temperature difference of the regenerator and showed a linear relation for both stages.
To reduce the quantity of liquid helium evaporation, we applied a pulse-tube refrigerator to the neck tube of a 1, 000-gallon liquid helium container. The contact spot of the pulse tube can be adjusted over a distance of 10cm to attain the optimum cooling point. The cooling performance of the pulse-tube refrigerator is the lowest operating temperature of about 50K and cooling power of 2W (at 80K) with power consumption of 700W. We have achieved a reduction in the liquid helium boil-off rate of 10%.