Enhanced heat exchanger tubes with subsurface structures have been investigated in the pool boiling mode. The tubes, one smooth and 4 enhanced (variant 1 to variant 4), are made of carbon steel St35.8. The experiments are carried out with iso-butane N35 as working fluid at saturation temperatures in the range from T = 243 K to 293 K and heat fluxes q between 2 kW/m² and 100 kW/m². The experimental results showed significant improvement of the heat transfer of the structured tubes compared with that of the smooth reference tube. The best heat transfer coefficients α, found for tube variant 4, are better by a factor up to 4 with a maximum of 14 kW/m²K for heat fluxes between 40 kW/m² and 60 kW/m².
The boiling behaviour of the working fluid iso-butane N35 was visualised with a high speed video system, and the software IKEDBV for digital image processing was used for the evaluation of the video data concerning important boiling parameters, e.g. bubble departure diameter.