Study of helically coiled adiabatic capillary tubes for transcritical CO2 expansion

摘要

Transcritical expansion through coiled adiabatic capillary tubes having CO₂ as the refrigerant is numerically simulated, and the performance is compared with the flow of R22 through such tubes. Mori and Nakayama friction factor correlation in combination with Churchill equation, which is fitted into the form of Blasius type equation, is employed to estimate the friction factor. Results indicate that the reduction in the mass flow rate with a coiled capillary tube compared with a straight one is more pronounced in case of R744 in comparison to R22. The mass flow rate of refrigerant increases as the coil diameter increases, but changes little beyond a coil diameter of 180 mm. Variation in gas-cooler exit temperature does not lead to any significant change in variation in the mass flow rate with coil diameter. A shorter capillary tube will be required to match the requisite system mass flow rate when coiled capillary tubes are employed in lieu of a straight one.