Study Of Performance Of Cycle And Heat Transfer Of CO₂ Trans-Critical Cycle

With the objective of optimizing the coefficient of performance (COP) oftrans-critical refrigeration systems, heat transfer of supercritical carbon dioxide cooledin circular tubes was investigated experimentally and theoretically. Both thecharacteristic and influencing factors of the automobile air-conditioning system usingtrans-critical carbon dioxide cycle are analyzed as well. By doing this, we find that theenergy loss of the expansion valve, gas cooler and compressor is larger than any othercomponent in the system. Using an expander instead of throttle components canreduce the energy loss in the throttling process, and the refrigeration capacity andCOP can be improved evidently by adding heat exchanger. The study also shows thatthe efficiency of the cycle is affected by some parameters such as evaporatingtemperature, pressure in the high pressure region and outlet temperature of the gascooler. Among these, the pressure in the high pressure region affect most and theinfluence has an optimum value.The main factors that influencing the convection heat transfer of supercriticalcarbon dioxide such as pressure, mass flux, flow direction, diameter and buoyancy,lubricating oil and heat flux are analyzed. The principium of the heat transfer ofsupercritical carbon dioxide flowing in horizontal is also investigated. It shows thatthe main factors that influence the characteristic of the heat transfer of supercriticalcarbon dioxide can be summarized as the variations of the specific heat and physicalproperties in the flow boundary layer.The heat transfer of supercritical carbon dioxide experiment apparatus aredesigned and constructed. Using these apparatus, the convection heat transfer ofsupercritical carbon dioxide and subcritical carbon dioxide flowing in single tube andmultiple tubes and the performance of heat exchanger can be investigatedexperimentally within a wide range of temperature, pressure and mass flux. Theproblem of the measure of temperature of the fluid in the high pressure tubes and theseal of them are solved.The heat transfer of supercritical carbon dioxide in a horizontal tube during gascooling process was investigated experimentally. Based on this, the influence ofpressure, inlet temperature, mass flux and heat flux on the heat transfer coefficient andpressure drop are discussed. The variation of the quantity of heat transfer, mean heattransfer coefficient, local heat transfer coefficient and pressure drop are presented.The experimental results in this study are compared with four existing correlations forthe supercritical heat transfer coefficient. Finally, the suitable correlation isrecommended for this paper's experimental conditions.The study in this paper provides theoretical foundation and experimentalexperience to the application of carbon dioxide.