A borescope was used to visualize the flow on the inside wall of both smooth and microfin tubes of 9.525 mm O.D. The purpose was to find out the effects of the grooves on the liquid flow in microfin tubes and to explain the mechanism of heat transfer enhancement. The behavior of the liquid film was observed on the inside wall of four test tubes: a smooth tube and three microfin tubes of 6°, 18°, an 44° spiral angle, for mass fluxes 50, 100, and 200 kg/m²s and heat fluxes 5, 10, and 20 kW/m², with test fluid refrigerant 134a. The visualization indicated that the grooves of 18° spiral angle were the most effective in pulling the liquid upward, showing the maximum heat transfer coefficient at mass flux G =50 kg/m²s. At G = 200 kg/m²s, the microfin tube with 6° spiral angle was found to be the most efficient geometry in enhancing heat transfer.

Units: SI