Theoretical And Experimental Research On CO₂ Transcritical System And Heat Transfer

The natural refrigerant CO₂ cycle possesses promising application prospect.The key problem is to improve the performance of CO₂ transcritical cycle systemand to put it into practice quickly. This study principally investigates thepotentials and measures to increase the efficiency of the CO₂ transcriticalwater-to-water heat pump system by combining theoretical analysis, computationsimulation and experimental study.Seven different kinds of CO₂ transcritical cycles are analyzed and comparedby thermodynamics in this dissertation. Based on the cycle simulations,correlations of the optimal heat rejection pressures for single-stage CO₂transcritical expander cycle are obtained for specific conditions. A type of cyclefor configuration optimization is brought forward and made performancecomputation and comparison, which is not only can simplify the gas cooler designbut also can organize the cooling medium conveniently. At the same time, thecorrelations for the high pressure and the medium pressure of this cycle versus theexpander efficiency are regressed, respectively, which can provide theoreticalbasis for the system design and operation.The thermo-physical properties of carbon dioxide near the critical region aretheoretically analyzed thoroughly in the three dimensional graphs in thisdissertation. The correlations of the pseudocritical temperature are obtained andthe pseudocritical region is defined, respectively. The heat transfer performance ofsupercritical CO₂ during cooling process is compared with the condensation heattransfer of the conventional refrigerant by thermo-physical properties analysis andquantity comparison. At the same time, the effect of non-condensable gas NC-1 onthe properties of supercritical CO₂ and its heat transfer performance is analyzed.The thermo-physical properties of carbon dioxide in the subcritical region are alsoanalyzed and compared with those of the conventional refrigerants. Thecharacteristics of two-phase flow for CO₂ and its boiling heat transfer mechanismare investigated. The idea for CO₂ evaporator heat transfer surface is presented.All the above studies can help to understand the flow and heat transfercharacteristics of CO₂ fluid and provide indispensable theoretical basis fordesigning high efficiency heat exchangersPerformance analysis for CO₂ heat exchangers is performed by experimentalstudy, and the computation models are created in this dissertation. Themathematical models for CO₂ transcritical throttling valve and expander systemsare developed and verified, respectively, which are utilized to do simulation andcomparison for the two systems. The comparison results show that using expanderinstead of throttling valve can improve the system performance greatly. Theinfluence of the external conditions on the expander system performance is studied,which can provide instruction for the system operation and control. The majorizedobject function is brought forward to analyze the structure sensitivity for the CO₂heat exchangers. Both the majorizing computation results based on the system aswell as the CO₂ heat exchangers themselves show that the small diameter tube andlong tube length should be selected in the CO₂ transcritical cycle. And thesimulation results of the new majorized system indicate that the COP and thecooling capacity are increased on average 15% and 18%, respectively.Based on the majorization computation results and some existing problems inthe primary system, the new heat exchangers and some related components aredesigned and manufactured in order to achieve rational structure and highefficiency operation, and then the new CO₂ transcritical water-to-water heat pumpsystem is set up. The experimental results show that the coefficient of performanceand the cooling capacity of the new system are increased on average 30% or so,and the majorization object is achieved, which also verify the validity of themajorization computation results.