Pressure drop and heat transfer for flow boiling inside horizontal smooth and internally helical-grooved small-diameter tubes.

Experiments were performed to examine the pressure drops and heat transfers of single-phase flow and flow boiling inside smooth and internally helical-grooved small-diameter horizontal tubes with outer diameter of 4 mm. The helical-grooved tube had 25 grooves, a helix angle of 15°, and a fin height of 0.1 mm. The singlephase flow experiments employed water as the test fluid and the pressure drop experiments were performed under adiabatic conditions. For the flow boiling of R32, the heat transfer and pressure drop were measured at the saturation temperature of 15°C for mass velocities of 100–400 kg/(m2s) and heat fluxes of 5–20 kW/m2. The pressure drop and heat transfer characteristics of the helical-grooved tube for single-phase and boiling flows were examined and compared with those of the smooth tube, and the pressure drops were also compared with the predictions of existing correlations.