Flow boiling characteristics of carbon dioxide in microchannels

This dissertation presents a systematic and comprehensive study on heat transfer characteristics of CO₂ in microchannels including extensive experimental results on flow boiling of CO₂ and CO₂/oil mixtures in tubes of normal size and microchannels. Moreover, an experimental investigation of capacity and pressure drop for sub-critical CO2 flow boiling and supercritical CO₂ gas cooling in microchannel heat exchangers are performed as well. Furthermore, semi-empirical correlations are developed for predicting heat transfer performance of CO₂ in microchannels.; An extensive review of the literature indicates no previous systematic study in this area, suggesting a lack of fundamental understanding of the phenomena and a lack of comprehensive data that would quantify the performance potential of CO₂ microchannel heat exchangers for engineering applications.; Preliminary studies on CO₂ flow boiling heat transfer in smooth tubes and microchannels reveal the unique flow boiling heat transfer characteristics of CO₂. Compared with R-134a, CO₂ showed outstanding heat transfer characteristics: much higher heat transfer coefficient and lower pressure drop than R-134a. Two-phase flow mal-distribution and its effects on heat transfer process are identified and analyzed.; Based on the outcome of the preliminary study of CO₂ flow boiling in microchannels, the dissertation extends the research to the design and development of microchannel heat exchangers. In the case of microchannel evaporators, it was found that a lower saturation temperature (especially when moisture condensation occurs) improves the overall heat transfer coefficient significantly. For the gas cooler, the temperature and pressure of CO ₂ significantly affect the heat transfer and fluid flow characteristics.; Semi-empirical models are developed to predict the heat transfer performance of CO₂ in microchannels and microchannel heat exchangers. Recommendations for future work in this field are outlined at the end of this dissertation. The research outcomes can be used to build up a solid ground for the practical applications of microchannel heat transfer.