Theoretical Analysis And Experimental Investigation On The CO₂ Cooling Heat Transfer Near Critical Point

Due to the increasingly serious requirement of energy-saving and environmental protect,natural refrigerant CO₂ with excellent thermal properties and environment-friendly features in the field of transcritical cycle technology has been re-valued. In the CO₂ transcritical cycle, the gas cooler is a different component with that in traditional refrigeration or heat pump cycle. In the gas cooler, the physical properties of CO₂ working fluid change dramatically in the near-critical region, which has great effect on heat exchange. So in this paper, theoretical analysis and experimental method on the cooling heat exchange characteristics of CO₂ near the critical point is applied to provide a reference design for the high efficiency heat exchanger.The exergy analysis and entropy analysis of the transcritical CO₂ cycle point out the gas cooler is an important component affecting system performance, the exergy loss of which accounts for almost half of the system. The main factors are the pressure, the gas cooler outlet temperature and other parameters, in particular the change characteristic in the vicinity of the critical point. In this paper, Matlab programming software is utilized to analyze the relation of density, specific heat, thermal conductivity, dynamic viscosity and other physical parameters with the variation of pressure and temperature, and the molecular aggregation theory and critical point fluctuation theory is used to analyze the reason for the change. Simultaneously, based on the reference results of the heat exchange of CO₂ near the critical region, a heat exchange correlation of CO₂ near the critical region is obtained.The cooling heat release in a horizontal pipe of CO₂ critical region is studied, by using existing experiment. And the characteristics and laws of exothermic process in CO₂ critical region is obtained. With the increase temperature of CO₂, heat transfer coefficient increases rapidly, near the pseudocritical temperature peak, and then rapidly reduced. With the mass flow increase of CO₂ and water, heat transfer coefficient is larger, the peak is higher. The peak of heat transfer coefficient ruduce with increasing pressure. The range summary of coefficients in CO₂ heat transfer correlation is determined.A gas cooler casing model is established by combining distribution parameters and lumped parameter. Four appropriate heat transfer correlations and experimental correlations are selected by using programming simulate tube heat exchangers. Experimental correlations can express the heat transfer rules in transcritical CO₂ near critical region well.