Luwei Yang

DC flow analysis and second orifice version pulse tube refrigerator

This paper analyzes direct current flow (DC flow) due to double-inlet, and introduces a second orifice version pulse tube refrigerator experiment to diminish DC flow. Analysis based on some assumptions shows that DC flow through the double-inlet valve in the pulse tube refrigerator is not generally zero, and the DC flow direction may change when the pressure wave changes. Thus, different schemes should be developed for different DC flow directions. In experiments, through measuring the change of temperature near the hot end of the pulse tube, DC flow direction different from former in pulse tube is decided. The second orifice is arranged from the second-stage reservoir to high pressure of the compressor, instead of the traditional arrangement of from the reservoir to low pressure. Experiments with the newly arranged second orifice show that the lowest temperature is about 4 K lower than without the second orifice.

Development of Stirling-type pulse tube coolers driven by commercial linear compressors

After an introductory section on pulse tube cryocoolers (PTC) this paper reports the state of development of some medium-size PTCs for potential future replacement of commercial Stirling cold fingers. The coolers were designed for operation with the AIM SL200 compressor (nominal input power: 100 W) and Leybold Polar SC7 compressor (nominal input power: 200 W), respectively. Adjustment of phase shift between pressure and mass flow oscillation is accomplished by means of inertance tubes in combination with a reservoir and a second-inlet flow impedance that are attached to the warm end of the pulse tube. Two coolers with U-shaped and one with linear arrangement of regenerator and pulse tube have been built and optimized. Up to now, the smaller U-shaped PTC driven by the AIM compressor at 100 W input power reached a no-load temperature of 45 K, and a cooling capacity of 2.85 W at 80 K is achieved, corresponding to a coefficient of performance of COP = 2.85 %. For the two larger PTCs driven by the Leybold compressor at 200 W of input, the obtained no-load temperature and cooling power at 80 K are 38 K and 6 W for the U-shaped cooler and 44 K and 8.1 W for the linear cooler, corresponding to COPs of 3 % and 4 % at 80 K, respectively. Measurements of the refrigeration temperature as function of the cold head orientation with respect to gravity revealed a small convection-induced temperature variation of several percent. The minimum temperature is achieved with the pulse tube cold end facing downwards.

Medium-size pulse tube coolers with linear compressor

We have designed and constructed two medium-size U-shaped pulse tube coolers for potential future replacement of Stirling cold fingers. The pressure oscillation in the coolers is generated by means of commercial linear compressors (Leybold Polar or AIM SL200) operated at a frequency of 40 Hz. Adjustment of phase shift between pressure and mass flow oscillation is accomplished by means of an inertance tube with buffer volume and a second-inlet flow impedance that are connected to the warm end of the pulse tube. The pulse tube cold head PT 01 operated with Polar compressor reached a lowest temperature of 34.3 K, and a cooling power of 5.2 W at 80 K has been achieved so far at a compressor input power of 180 W. A cooling power of 2.8 W at 60 K has been obtained with 200 W input power. At 100 W compressor input power, the two tested units, consisting of PT01 with Polar compressor and PT02 with SL200 compressor, produced more than 2.5 W of cooling at 80 K. The above results data were obtained with the pulse tube in horizontal position. Measurements of the cold tip temperature as function of the cold head orientation showed a convection-induced temperature variation of several percent. The minimum temperature is achieved with the cold end facing downwards.

Analytical study of the performance of pulse tube refrigerator with symmetry-nozzle

This paper analyzes the mechanism of the symmetry-nozzle instead of needle valve or orifice to improve the performance of pulse tube refrigerator in experiments. Their identifications are to result in a similar reciprocal movement in the hot end of the pulse tube; their differences are that their flow coefficients have different characteristics. The flow coefficient of the symmetry-nozzle has a positive feedback to flow rate while the orifice has little relation to flow rate generally. Further analysis shows that this feature results in large refrigeration when the pressure ratio and mass flow amount are same, which explains the reason for even lower temperature and a larger refrigeration amount when using the symmetry-nozzle.

INVESTIGATION ON TWO‐STAGE 300 HZ PULSE TUBE CRYOCOOLER

In the past few years, ultra‐high frequency pulse tube cryocoolers are becoming a research hotspot for their portability and compactness in aerospace and aviation applications. For preliminary research, a two‐stage pulse tube cryocooler working at 300 Hz driven by a thermoacoustic engine is established to investigate the problems due to ultra high frequency, and several results have been derived in our early reports. In order to study the effect of thermal penetration depth, this paper presents the cooler adopting copper mesh as the regenerator, and comparison with stainless steel mesh is given. In addition, the influence of inertance tube on the lowest possible cooler temperature is also tested. Finally, we discuss the improvement for getting a lower temperature.

Study on Kinetics of Color Changes in Thompson Seedless Grapes during Drying Process

The aim of this study was to investigate the characteristic of color changes in Thompson seedless grape during drying at 30°C, 35°C and 40°C air temperatures. The CIE LAB color system was used to measure the surface color of the grapes. It was found that browning began at middle period of drying, where parameters L* and b* decreased and parameter a* increased during drying. Color changes during browning process were described as a first-order kinetic model, where the activation energies for the parameters L*, a*, b*, and total color difference were respectively 40.19, 24.29, 35.43, and 39.46 kJ·mol1.

HIGH FREQUENCY SINGLE-STAGE MULTI-BYPASS PULSE TUBE CRYOCOOLER FOR 23.8K

A below 30K single-stage high-frequency multi-bypass pulse tube cryocooler(PTC) is introduced in this paper. At present, the lowest temperature of 27.46K has been achieved with input power of 100W and 23.8K with input power of 200W. Experiments show that if the area of multi-bypass and the length of inertance tube matching well, a better performance of PTC will be obtained.

Research of Drying Characteristics of Thompson Seedless Grape

Drying characteristics of Thomson seedless grape at different drying temperatures (45°C, 40 °C, 35 °C and 30 °C) were investigated in a Constant Temperature and Humidity Testing Machine. 8 different thin layer mathematical drying models are selected to fitting the date. The logarithmic drying model is satisfactorily to describe the drying process of grapes. In addition, this paper also calculated the moisture effective diffusivity (Deff) of Thompson seedless grape during drying process, and 45 °C has the maximum value 3.0702·10-11 m2/s. The energy of activation (Ea) is 59.03 kJ/mol. This means it will take 3279.44 kJ to remove 1kg water from the grape samples.

Theoretical and Experimental Study on the Falling-Film Evaporator Applied to Mechanical Vapor Compression

Based on a heat pump system of mechanical vapor recompression (MVR) designed and manufactured independently, the heat transfer performance of falling-film evaporator was measured by a combination way of theory analysis and experiment proving as the heat pump operated in practice. After the result of theoretical calculation is worked out, the start thickness and entrance velocity of the liquid film are established by adjusting the flux of raw solution. The result shows there is an optimization that the film thickness at the bottom of the tubes amounts to that of boundary layer of velocity. Additionally, it is a process of falling-film flow with heat and mass transfer between the start and end of falling-film. The last thickness of the falling film is about 0.21~0.44mm. The thickness of falling film when the falling-film flowed and heat exchange was over is compared with each other, and the effect of heat resistance on heat transfer is discussed also. It is showed that an optimal thickness is formed during the process of falling-film flow and evaporation, and disadvantages come up when it is not formed. The falling-film evaporator propelled by the MVR heat pump with low compression ratio carries through a process of strong and high-efficiency heat transfer with phase transition. It is because the states of produced vapor both before compressed and after compressed are saturated. Its heat-transfer coefficient may be as high as 1990 W/ (m2·K). The start and end thickness of falling film become great while the evaporation pressure goes up. It leads to the drop of the heat-transfer efficiency, so there is an optimization to the system in all probability.

Experimental Study of Linear Motor Direct-Driven Thermocompressor

A prototype thermocompressor driven by linear motor was setup and experimental tudied. Peak-peak pressure in the system was influenced by the regenerator, the heating temperature and the gap clearance between piston and cylinder. The optimal frequency of the system was determined by spring constant and total mass of piston, shaft and moving iron. The peak-peak pressure was found to change little with filling pressure when the heating temperature was lower than 250°C, employing with N2 or He4 as the working substance. But when the heating temperature was higher than 250°C, peak-peak pressure increased with filling pressure. Besides, Helium4 was found to perform better than Nitrogen as the working substance. For heating temperature of 650°C and cooling temperature of 40°C, the peak-peak pressure could reach 0.412MPa with pressure ratio of 1.13 when the filling pressure was 2.2MPa, which could be used to drive the pulse tube crycooler (PTC).