Slug flow induced by the combination of a downstream and an upstream two phase flow was experimentally studiedin this paper. A loop, where it is possible to change the flow conditions and the lengths and inclination angles of the pipe sections, was built. Three conductive probes were used for the hold-up measurement in different sections. A data acquisition system based on a microcontroller was also developed. The experimental runs were set for four alternated upstream and downstream sections. Several gas and liquid flow rates to get stratified flow in the first section, were also tested in order to observe the hold-up variation of the resulting slugs. Barnea's criterion predicts acceptably the severe slugging trend for the tested experimental conditions, but for stable slug flow, it gives an incorrect idea of the phenomenon because it doesn't account the void fraction in the column, which produces a value h_(L,min)< 1, and there is still liquid flowing into the separator. On the other hand, the method doesn't consider the existence of a pseudo slug regime in which there is a combination of stable slug flow and sporadic formation of severe slugging. Due to this situation, the flow pattern depends greatly on the amount of liquid draining back that increases the liquid accumulation rate and when the liquid attains its critical level a sudden slug takes place. To improve the method proposed by Barnea, the void fraction has been taken into account. According to the experimental results, there is an important influence of the geometry, that is the length and inclination angles of the sections, in the magnitude of the main slug flow parameters.
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