Experimental investigation was carried out on the characteristics of void fraction distribution in a narrow rectangular duct (3.25 mm×43 mm) as well as a conventional circular tube (i.d.50 mm) under inclined conditions to study the influence of inclination on bubble and gas-liquid interface distribution. Air and purified water were used as the test fluids and three inclination angles of 5°

10° and 15° were selected for experiments. Only wall peak distribution was observed in the narrow rectangular duct and no core peak was present

while for the circular tube

both were observed. Inclination had similar effect on wall peak distribution in two channels. Accordingly

with increasing inclination angle

the peak near the upper wall of the tube boosted up

whereas

the one near the lower wall weakened gradually

and even disappeared. As to core peak distribution in the circular tube

central broad peak leaned to the upper part of the tube

and its value increased as inclination angle increased. In addition

the position of wall peak in the narrow rectangular duct was closer to the channel center than that in the conventional circular tube.