Theoretical And Experimental Research On Cooling Heat Transfer Of Supercritical CO₂-Lubricant Oil Mixture

The natural refrigerant CO₂ transcritical cycle technology has a broad application foreground. Improvement of cycle system performance is the key for the CO₂ transcritical cycle technology popularization. Performance of lubricant oil special for CO₂ transcritical cycle and CO₂/PAG mixture were researched. The theoretical and experimental research was used to analyze heat transfer performance of gas cooler. The mechanics of supercritical CO₂ heat transfer under cooling condition was investigated. Heat transfer performance of the new type gas cooler was studied by means of simulated and experimented. And the effect of new type gas cooler on CO₂ transcritical water-to-water heat pump cycle system was researched.New general computing formulas of synthetic lubricant oil basic parameters were put forward by use of alkybenzene lubricant oil data, after different empiric formulas of lubricant oils basic parameter had been analyzed. The new computing formulas of lubricant oil PAG physical properties were presented on the base of existed empiric formulas and a few of PAG physical properties data.The properties of CO₂ and PAG mixture were calculated according to property empiric calculating formulas of refrigerant and lubricant oil mixture. The molecular aggregation theory and non-equilibrium thermodynamics were applied to study properties change in the critical region. The CO₂ properties severe change is resulted in severe change of CO₂ molecular aggregation behavior in the critical region.The effect of lubricant oil on refrigeration system was investigated. The equivalent oil film thickness distribution along length of tube was calculated by continuity equation and momentum equation. The new heat transfer model of gas cooler was proposed according to practical heat exchange process of gas cooler in the CO₂ transcritical refrigeration or heat pump cycle unit. The new heat transfer model was used to simulated heat transfer performance of the different structure gas cooler. And the new experimental gas cooler was designed and made by the simulated and analyzed results. The new experimental gas cooler not only reduces lubricant oil thickness and decreases heat resistance, but strengthens heat convection of heat exchanger fluids CO₂ and water. Thus the whole heat transfer performance of new experimental gas cooler was enhanced at a great degree.The experimental researches on new experimental gas cooler and new CO₂ transcritical water-to-water heat pump cycle were performed. It found that the heat transfer performance of the new experimental gas cooler was better than old one. And experimental research on performance of the CO₂ transcritical water-to-water heat pump cycle system with expander or throttling valve was executed. It was found that the new experimental gas cooler improved the cycle performance of the CO₂ transcritical water-to-water heat pump cycle, at a large degree. The lower the CO₂ outlet temperature of gas cooler is, the better the cycle performance of the CO₂ transcritical water-to-water heat pump system is. The lower the CO₂ outlet temperature of gas cooler is, the smaller the recovery work.