Investigation of flow boiling heat transfer inside small diameter helically coiled tubes

This paper experimentally investigates the effects of tube and coil diameters on flow boiling heat transfer coefficients inside small diameter (<3 mm) helically coiled tubes. Four different helical coils with coil diameters ranging from 30 mm to 60 mm were studied using tube diameters ranging from 1.1 to 2.8 mm. The heat flux varied from 2500 to 12,000 Wm ^-2 and mass velocities from 100 to 450 kgm ^-2s ^-1. Results have shown that decreasing the tube diameter improved the heat transfer coefficient by up to 63% but early dryout (vapour quality as low as 0.4) was observed especially at high heat fluxes (q = 12,000 Wm ^-2). Also, decreasing the coil diameter improves the heat transfer coefficient by up to 150%. New correlation for the boiling heat transfer coefficient inside small tube diameter helical coils was developed which predicted the 227 data points used with mean absolute relative error (MARE) of 16%.