ingperformance ofgaseouswateris superiortogaseous CO
2
underhigh temperature
and low pressure.
Abstract
CO2 and water are two commonly employed heat transmission fluids in sev-eral fields. Their temperature and pressure determine their phase states, thus affectingthe heat transfer performance of the water/CO2. The heat transfer characteristics ofgaseous CO2and gaseous water flowing through fractured hot dry rock still need agreat deal of investigation, in order to understand and evaluate the heat extractionin enhanced geothermal systems. In this work, we develop a 2D numerical model tocompare the heat transfer performance of gaseous CO2and gaseous water flowingthrough a single fracture aperture of 0.2 mm in aφ50 × 50 mm cylindrical granitesample with a confining temperature of 200◦C under different inlet mass flow rates.Our results indicate that: (1) the final outlet temperatures of the fluid are very close tothe outer surface temperature under low inlet mass flow rate, regardless of the sam-ple length. (2) Both the temperature of the fluid (gaseous CO2/gaseous water) andinner surface temperature rise sharply at the inlet, and the inner surface temperatureis always higher than the fluid temperature. However, their temperature differencebecomes increasingly small. (3) Both the overall heat transfer coefficient (OHTC) andlocal heat transfer coefficient (LHTC) of gaseous CO2and gaseous water increase withincreasing inlet mass flow rates. (4) Both the OHTC and LHTC of gaseous CO2arelower than those of gaseous water under the same conditions; therefore, the heat mining performance of gaseous water is superior to gaseous CO2under high temperatureand low pressure.
