Comparative Investigation on the Heat Transfer Characteristics of Gaseous $\hbox {CO}_{2}$ and Gaseous Water Flowing Through a Single Granite Fracture

Abstract

$\hbox {CO}_{2}$ and water are two commonly employed heat transmission fluids in several fields. Their temperature and pressure determine their phase states, thus affecting the heat transfer performance of the water/$\hbox {CO}_{2}$. The heat transfer characteristics of gaseous $\hbox {CO}_{2}$ and gaseous water flowing through fractured hot dry rock still need a great deal of investigation, in order to understand and evaluate the heat extraction in enhanced geothermal systems. In this work, we develop a 2D numerical model to compare the heat transfer performance of gaseous $\hbox {CO}_{2}$ and gaseous water flowing through a single fracture aperture of 0.2 mm in a $\upphi 50\,\times 50\hbox { mm}$ cylindrical granite sample with a confining temperature of $200\,^{\circ }\hbox {C}$ under different inlet mass flow rates. Our results indicate that: (1) the final outlet temperatures of the fluid are very close to the outer surface temperature under low inlet mass flow rate, regardless of the sample length. (2) Both the temperature of the fluid (gaseous $\hbox {CO}_{2}$/gaseous water) and inner surface temperature rise sharply at the inlet, and the inner surface temperature is always higher than the fluid temperature. However, their temperature difference becomes increasingly small. (3) Both the overall heat transfer coefficient (OHTC) and local heat transfer coefficient (LHTC) of gaseous $\hbox {CO}_{2}$ and gaseous water increase with increasing inlet mass flow rates. (4) Both the OHTC and LHTC of gaseous $\hbox {CO}_{2}$ are lower than those of gaseous water under the same conditions; therefore, the heat mining performance of gaseous water is superior to gaseous $\hbox {CO}_{2}$ under high temperature and low pressure.