An experimental study of flow boiling characteristics of carbon dioxide in multiport mini channels

Due to its unique combination of ecological and personal safety, carbon dioxide (CO₂), a natural fluid, is becoming one of the most promising alternative refrigerants for air-conditioning and refrigeration systems. This paper presents an experimental study of boiling heat transfer and pressure drop of CO₂ flowing in a multi-port extruded aluminum test section, which had 10 circular channels, each with an inner diameter of 1.31 mm. CO₂ was heated by hot water flowing inside copper blocks that were attached at both sides of the test section. Temperatures at the outer surface of the test section were measured using 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured using twelve heat flux sensors to obtain the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO₂ at the inlet and outlet of the test section were measured using two K-type thermocouples. The measurements were performed for pressures ranging from 3.99 to 5.38 MPa, inlet temperatures of CO₂ from -3.08 to 16.96 °C, heat fluxes from 10.1 to 20.1 kW/m², mass velocities from 131.4 to 399.0 kg/m ²s, and vapor quality from 0.0 to 1.0. The results indicate that pressure drop along the test section is very small, two-phase CO₂ flow exhibits a higher heat transfer coefficient than that of the single-phase liquid or vapor flow. It is also shown that the mass velocity and the applied heat flux have significant effects on flow boiling heat transfer characteristics. The measured heat transfer coefficients were compared with correlations reported in the literature and large discrepancies are observed.