The Study Of Heat Transfer And Expansion Mechanism For Carbon Dioxide Transcritical Cycle

This work is supported by the National Natural Science Foundation of China (No.59876028) and Special Science Foundation of Ph. D Dissertation of Universities (No. D0200105). Moreover, this project is a part of"211 Project"of Tianjin University by Chinese Government (TD211-95-0106).In this study the main technological difficulties of carbon dioxide (CO2) as a refrigerant is discussed and solved. The heat transfer characteristics in the gas cooler and the recovery of the expansion power are studied theoretically and experimentally. Its main contents are summarized as follows:1. The theoretical and experimental investigation of CO2 water to water heat pumpAuthor analyzed theoretically the performance of water-water heat pump for CO2 transcritical cycle with the internal heat exchanger, obtained the variation laws of inner parameters and their different impact to COP of the system, and calculated the optimal rejection pressure and maximum COP. On the basis of theoretical analysis, the CO2 transcritical heat pump is established, and the experiments are conducted. The experimental results verify the validity of theoretical analysis, and gain the performances of the system with the change of internal and external parameters.By the comparison with theoretical results, the COP of this heat pump is low. Author presented the reason for low COP from the viewpoint of both thermodynamics and heat transfer. So, two methods to improve the COP of the system are proposed. One is to develop an expander to recover expansion work, the other is to optimize the heat exchanger.2. The theoretical and experimental study on supercritical carbon dioxideThe thermophysical properties of CO2 near the critical point are calculated with Modified BWR equation. The results shows that these properties, i.e. the specific heat, the density, the viscosity, and thermal conductivity, display extremely large variations near the critical point. Based on these characteristics, author redefined the critical heat transfer region of CO2 and put forward to the principles of heat transfer on supercritical CO2.On the basis of analyzing the heat transfer characteristics of CO2 under the supercritical pressure, we built the experimental apparatus, and conducted the experiments on a horizontal pipe at the cooling process for supercritical carbon...