| Cryocoolers working at liquid nitrogen temperatures have played important role in the fields of power grids, military, medical applications, transportation and low-temperature physics. The Stirling pulse tube cryocooler (SPTC) has a high potential to fulfill the increasing demands of High temperature superconducting (HTS) applications due to its advantages of high reliability, high efficiency, compactness, and low mass, thus gradually become a hot topic in the filed of cryocooler. The high power SPTC has already made some progress in the past few years. Nevertheless, comparing with the commercial high power cryocooler like G-M cooler and Stirling cooler, the SPTC has still to be further improved both on efficiency and reliability. Extending SPTCs to high cooling capacity is not a simple matter of scaling up the existing small-scale SPTCs. There are several stubborn problems only related to high power SPTCs, such as flow inhomogeneity and temperature inhomogeneity in the regenerator, impedance matching between compressor and cooler, the design of high power heat exchanger. These problems restrict the developments of high power SPTCs. With the aim of further improving the cooling performance of SPTC working at liquid nitrogen temperatures, in this paper, the flow and heat transfer characteristics of large-scale high power high frequency regenerators has been studied, the mechanism of temperature non-uniformity and related losses in the regenerator has been revealed, the methods to realize high efficient regeneration under the condition of high power and high frequency has been investigated. Based on the efforts listed above, the cooling efficiency of SPTC working at liquid temperatures is supposed to be improved, thus to brandly extend the use of cryocoolers in industry applications. In this paper, the theoretical and experimental research include three parts as listed below:1. Developed a numerical model of single-stage high power SPTC working at liquid nitrogen temperatures, and further investigated the working mechanism of high power SPTC:In order to fully and correctly understand the refrigeration and losses mechanism of large scale high power SPTC. A single stage SPTC numerical model was established based on simulation software Sage. The influence of regenerator and pulse tube dimensions together with regenerator matrix on the flow and heat transfer characteristics inside the SPTC were investigated on the basis of simulation. The high power SPTCs are supposed to adopt regenerators with small length to diameter ratio in order to guarantee the dissipation ability of acoustic power and simultaneously reduce the friction losses. The higher the refrigeration temperature is, the shorter the optimum regenerator is with respect to the object of cooling power. Therefore, the length of regenerator was optimized in this paper to improve the cooling performance at liquid nitrogen temperatures. Moreover, the effect of charge pressure and operating frequency on the cooling performance was also studied to guide the experimentally optimization of SPTC. Based on calculation, the SPTC can reach a no-load refrigeration temperature of31.5K and provide a cooling power of508W at80K.2. Theoretically investigated the temperature non-uniformity in the regenerator of SPTC, derived the analytical governing equation of relative DC flow caused by temperature non-uniformity, and developed a simulation model with parallel regenerators to numerically study the effect of temperature non-uniformity:Temperature non-uniformity in the regenerator will introduce significant DC flow losses, thus to reduce the cooling performance of high power SPTC. In this paper, the mechanism of temperature non-uniformity, the influence of temperature non-uniformity on the regenerator performance and the method of inhibiting temperature non-uniformity was comprehensively concluded and summarized. Theoretical study shows that even tiny DC flow occurred in the regenerator will skew the temperature profile and lead to transverse temperature non-uniformity. On the other hand, the transverse temperature non-uniformity will enlarge the amount of DC flow. Based on thermodynamic analysis, the governing equation of relative DC flow was derived. The result illustrates that either non-uniform temperature profile or porosity can generate DC flow. The effect of axial temperature gradient, the regenerator length, the regenerator matrix and charge pressure on the DC flow were also discussed. A numerical model with parallel regenerator was built to investigate the influence of non-uniformity porosity on the temperature non-uniformity and the cooling performance, which will guide the inhibition of DC flow losses and improvement of cooling performance.3. Developed the hybrid regenerator fillings with enhanced thermal conductance, further investigated the working mechanism of hybrid fillings, and optimized the composition of the regenerator fillings:In order to degrade the regenerator temperature non-uniformity in the SPTC and reduce the DC flow losses, the hybrid regenerator fillings with enhanced thermal conductance was proposed. Part of the original stainless steel screens were replaced by materials with higher thermal conductance like copper and brass. Seven different hybrid fillings were tested and compared, the experimental results revealed that the temperature non-uniformity in the regenerator was significantly reduced by enhancing the thermal conductance of regenerator fillings. By optimizing the composition of hybrid fillings, the SPTC can reach a no-load refrigeration temperature of41.3K. A cooling power of424.5W can be obtained at78.2K with an input power of8.9kW, the relative Carnot efficiency is13.5%, which is the best among the domestic reported similar SPTCs. |
