Theoretical Simulation And Experimental Investigation On Tube Cooling Heat Transfer Of Supercritical CO₂

Carbon dioxide, as a kind of natural refrigerant, make great contribution toreducing the ozone depletion, reducing the greenhouse and the sustainable developmentof environment. Since the critical temperature of carbon dioxide is31.1℃, the criticalpressure is7.38MPa, when used in the transcritical cycle, carbon dioxide discharges heatin a supercritical state in gas cooler. There is no uniform correlation to accuratelypredict the heat exchangers and pressure drop characteristics of supercritical carbondioxide in-tube cooling. Therefore, this paper adopts theoretical analysis andexperimental research method to study the heat transfer characteristics of supercriticalcarbon dioxide, providing the basis for improving the performance of the gas cooler, aswell as the performance of transcritical cycle carbon dioxide.Based on the variation of specific heat, density, thermal conductivity anddynamic viscosity of supercritical carbon dioxide, the supercritical region can bedivided into two parts: the starting area of supercritical region and the mainsupercritical region. The specific heat, density, thermal conductivity and dynamicviscosity of supercritical carbon dioxide change greatly in the starting area, whichmake an important influence on heat transfer. So that the heat transfer coefficientfluctuates widely in the starting area and reaches the peak at the pseudo-criticaltemperature, but tend to steady in the main supercritical region.Take the tube gas cooler for examp le, use d istributed parameter method andlumped parameter method to establish a gas cooler models, to write EES programmeand simulating calculation. Results show that: The specific heat, density, thermalconductivity and dynamic viscosity of supercritical carbon dioxide change greatly inthe starting area, which make an important influence on heat transfer. So that theheat transfer coeffic ient fluctuates widely in the starting area and reaches the peak atthe pseudo-critical temperature, but tend to steady in the main supercritical region.The heat transfer coefficient increases with increasing mass flow of carbon dioxideand cooling water flow. The trend of heat transfer coeffic ient with pressure is morecomplex, in the starting region increases with pressure, but in the main supercriticalregion, the heat transfer coeffic ient decreases with increasing of pressure on the contrary. Pressure drop grows with carbon dioxide mass flow increasing. Butcompared to the system operating pressure, it is much smaller, only account for1.2%.Then design experiment to study the heat transfer and pressure dropcharacteristics of supercritical carbon dioxide. The experimental results about thesupercritical carbon dioxide law in the starting area and the ma in supercriticalregion agree well with the theoretical analysis and simulation calculations.Finally, comparation of different correlations’ calculating results andexperimental data shows that Churchill correlation calculating results agree wellwith experimental data and the most deviation between Gnielinski correlationexperimental data is only13.8%. Consequently, they are fit for calculating pressuredrop and heat transfer coefficient.