Local void fraction and velocity field description in horizontal bubbly flow

Abstract

The internal phase distribution of concurrent, air–water bubbly flow in a 50.3-mm i.d. transparent horizontal pipeline has been experimentally investigated using the hot-film anemometry technique. Liquid and gas volumetric superficial velocities ranged from 3.8 to 5.0 m s −1 and 0.25 to 0.8 m s −1, respectively. The time-averaged local values of the void fraction, bubble-passing frequency, the mean liquid velocity, and the liquid turbulence fluctuations were measured. The experimental results indicate that the void fraction and bubble-passing frequency have local maxima near the upper pipe wall, and the profiles tend to flatten with increasing liquid flow rate. For the horizontal bubbly flow, the observed time-averaged local void fraction can reach 65%, whereas the bubble frequency may reach a value of 1400 s −1. The fact that the local peak void fraction in all cases never exceeds 65% indicates that the maximum bubble packing exists within the channel. It was found that increasing the gas flow rate at fixed liquid flow rate would increase the local void fraction and bubble-passing frequency. The axial liquid mean velocity showed a relatively uniform distribution except near the upper pipe wall, where a sharp reduction in velocity was noticed. The local mean liquid velocity and turbulence fluctuations increased with gas flow rate. An interesting feature of the liquid velocity distribution is that it tends to form a fully-developed turbulent pipe-flow profile at the lower part of the pipe irrespective of the liquid and gas superficial velocities. At very low local void fractions, the turbulent intensity consistently tended to be slightly lower than the single-phase. However, at high void fraction, i.e. high bubble population, introduction of gas strongly enhances the turbulence level as characterized by the turbulent velocity fluctuation and turbulent intensity. In general, it was concluded that the local turbulence intensity is mainly affected by the local void fraction distribution.