| The rapid growth of China’s need for imported natural gas leads to various technical challenges in its transportation and storage. Among them, high power cryogenic cooling with high efficiency and reliability is one of the most urgent key issue to be addressed. Pulse tube coolers (PTCs) which have no moving parts at the cold end, hold great potential in such applications. However, you can not have your cake and eat it, there are mainly two issues which limit a PTC’s efficiency. On the one hand, the match inside a PTC is crucial for improving its efficiency but still poorly understood. On the other hand, the expansion work at the warm end of the pulse tube is always dissipated as heat, which limits its capability of reaching Carnot efficiency even in ideal case, and is especially phenomenal with high cooling capacity at higher cooling temperatures such as liquid natural gas temperature. How to improve the PTC’s cooling efficiency while maintaining its high reliability is crucial to its potential application in this field, which motivates us to carry out the following work in this dissertation.1) A joint Acoustic-Mechanical-Electrical (AcME) coupling method is builtBased on the phasor analysis, the phasor diagrams for the three different parts in a pulse tube cryocooler:acoustic part, mechanical part and electrical part, are obtained. Linked by some vectors in common, a joint Acoustic-Mechanical-Electrical (AcME) coupling phasor diagram is achieved to analyze the coupling characteristics among the entire system. As shown in Figure 0.1, it reveals the intrinsic coupling mechanism between different parts, and provides references for reaching the best performance of a Stirling-type cryocooler.Green lines:Acoustic part; Blue lines:Mechanical part; Red line:Electrical part Figure 0.1 Joint Acoustic-Mechanical-Electrical (AcME)coupling phasor diagramA detailed study focusing on the effects of acoustic impedance in a linear compressor has been performed. An optimized acoustic resistance has been developed without the assumption of resonance, thereby providing more general expressions (see Equation (0.1)). Both calculations and experiments have been carried out to prove the crucial role of an appropriate acoustic impedance in achieving the best performance.The concept of impedance ’sweet spot’ is further introduced here to obtain both the highest efficiency and the maximum power output simultaneously for a given linear compressor, which should satisfy Equation (0.2).In order to ensure high efficiency of both the linear compressor and the cryocooler, several acoustic impedance matching methods are introduced in analogy with the electrical impedance matching network.Based on the AcME coupling, a reverse measurement method of linear compressor parameters like the spring stiffness, the moving mass, the specific motor thrust and the damping coefficient is proposed after a linear compressor assembling, which is expected to provide theoretical basis for engineering application of linear compressors.2) A cascade pulse tube refrigeration method with energy recovery is proposed, which is capable of approaching Carnot efficiency with infinite stagesA pulse tube cryocooler cannot work with Carnot efficiency due basically to the expansion work that has to be dissipated thermally at the warm end of the pulse tube, which limits its ultimate efficiency to be only Tc/Th. In this study, a multi-stage cascade PTC with built-in transmission tubes between sub-PTCs for energy recovery is proposed, as shown in Figure 0.2.The key point of this new configuration is that the acoustic power at the outlet of the former stage can be recovered through the transmission tube which provides proper phase angle to drive a latter stage. The equivalent thermodynamic model of a multi-stage cascade PTC is shown in Figure 0.3.Figure 0.3 Thermodynamic model of a multi-stage cascade PTCFrom Figure 0.3, the cooling efficiency of the cascade PTC can be obtained to be:When n goes to infinity, the COP will become Carnot efficiency:This theoretical analysis with ideal hypothesis is not only beautiful but also meaningful. Although for a refrigerator, there may be many different ways to approach Carnot efficiency, Equation (0.3) shows such a step-by-step method which is theoretically realizable. Each term in Equation (0.3) corresponds to each stage of sub-PTC in Figure 0.2.3) A three-stage cascade pulse tube cooler working at 233 K was designed, fabricated and tested, which shows a 39.9% improvement of cooling efficiency compared with single stage PTCBased on an existing 500 W linear compressor in our lab, the cooling temperature of 233 K (-40℃) is chosen in order to obtain relatively high cooling power. A three-stage cascade PTC which contains three in-line PTC connected by two long transmission tubes in between is designed (see Figure 0.4) and tested.Figure 0.4 Schematic drawing of three-stage cascade PTCExperiments are carried out on three different operating models:single stage, two-stage cascade and three-stage cascade. Results show that, when working at 60 Hz,2.5 MPa and with 500 W electric power input, the single stage PTC obtains a cooling power of 181.3 W at 233 K, the two-stage cascade PTC obtains a total cooling power of 241.6 W (with 175.0 W at 1st stage and 66.6 W at 2nd stage), and the three-stage cascade PTC obtains a total cooling power of 253.6 W (with 164.9 W at 1st stage,70.7 W at 2nd stage and 18.0 W at 3rd stage). If compared with the single stage operation, the cooling efficiency is improved by 33.3% with two-stage cascade operation, and 39.9% with three-stage cascade operation.Figure 0.5 shows the roadmap of this research work. |
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