Mechanism Analysis And Visual Experimental Research On Expanding And Phase Change Process In CO₂ Rolling Piston Expander

Based on the key issues of CO₂ heat pump cycle system with expander, paper focuses on researching the theoretical analysis and experimental observation of internal phase change and expansion process in expander.The conventional complex calculation approach of GWP value had been simplified on basis of"group contribution method", and the GWP value of complex structure refrigerants were estimated by use of this method. The results are consistent with reference datum. The analysis approach, some macro nature is comprehensive contribution performance of group characters, was conducive to predict and judge the future uncertain nature of the new refrigerant. According to the evaluation standards comparative analysis of refrigerants various performance, paper suggested the"nature"evaluation criteria concept for refrigerants. And based on the view of sustainable development point, the future development direction of refrigerant was analyzed and predicted. It is established that CO₂ etc natural refrigerants should be the main development direction of the refrigerant.Paper regards as CO₂ expander is one of the key components for improving the performance of CO₂ heat pump system, internal expansion in expander involves the transformation from the supercritical to subcritical, so it has the complex mechanism. Therefore, paper discussed phase transition process in expander, firstly from the view of theoretical analysis. a new integrated suitable theory andmodel was put forward based on the relevant theories and different models. According to the integrated model, Papers simulated and caculated the phase transition and expanding process of expender, the expanding process had been divided into three stages according to parameters change and bubble-driven force: initial nucleation, free expansion and limited expansion; each stage could been analyzed by use of their own models, and thereby the whole research process of expansion could be simplied. The results show that the theoretical calculations value was anastomotic consistant with the visual observational datum.In order to directly observing the whole rapid decompression process in expander, paper designed two visual experimental devices. One is high-pressure CO₂ test block. To distinguish between lubricant drip and bubble, the fluorescent tracer is added to the mixture. The decompression process of CO₂ and PAG lubricant mixture solution was experimental observed using high speed camera. The research results found that greater decompression rate, more nucleation rate, and smaller diameter of the stability bubble. The other is the rebuilded CO₂ rolling piston. The LED light source were added in expander, and the internal high-pressure quartz windows was installed on cylinder, and the cover and piston of expander were processed and grinding in order to increase those reflectance. The high-speed photographic observation of internal phase change and expansion process was carried out. A lot of continuous picture of internal phase change process in expander were obtained, and the track a singe bubble could be caught through image processing. It provided the measured datum for the theoretical analysis of expansion processFinally, considering that there is not refrigerant condensation process in transcritical cycle,"non condensable gas"is appended CO₂ expander on the view of phase change and nucleation theory. It has no negative impact on compression and heat process, and plays a positive role as increasing nucleation of bulk phase in expansion and evaporation process. From the experimental test results of adding the appropriate ratio of two"no condensable gas", the methods had improved the performance of expander.