Single- and multiphase flow in a natural circulation thermosiphon: an experimental analysis

Thermosiphons are circuits in which a flow pattern is driven by buoyancy forces originated from density differences. These equipments are widely applied in industries due to their simplicity and high heat transfer rate. In this paper, a 40.2-mm-inner-diameter and 1-m-high tubular thermosiphon was evaluated at single-phase and multiphase conditions. The experimental unit was set to represent vapor generator systems employed in most industrial applications as an ASTM A192 tube was used at heating section. For single-phase flow, an experimental database regarding temperature and velocity was obtained by thermocouples and particle image velocimetry (PIV), respectively. Natural circulation effects were captured allowing the observation of a preferred path of the flow at the center of the tube. For multiphase flow, liquid-phase velocity was obtained by PIV technique and vapor-phase velocity, diameter, and void fraction were obtained by phase Doppler anemometry (PDA). Results pointed to the establishment of partial boiling regime as bubble presence decelerated liquid velocity. The present study discussed natural convection regime, transition regime that appeared once boiling initiated, and system behavior when subcooled nucleate boiling was established. This represents a robust database regarding phase change and the instabilities that this phenomenon can cause.