Visualized investigation of gas-liquid stratified flow boiling of water in a natural circulation thermosyphon loop with horizontal arranged evaporator

The gas-liquid stratified flow boiling heat transfer of water in a natural circulation thermosyphon was investigated experimentally. The flow boiling test was carried out in a visual thermosyphon loop system with a 0.8-m-long, horizontally arranged, evaporator tube with an inner diameter of 15.0 mm and under a heat load of 1.0 kW. The development of the gas-liquid stratified flow in the evaporator and its effects on thermal performance of the thermosyphon loop were analyzed. The results show that flow pattern in the evaporator was gradually developed from the slug/stratified-wavy flow to the stratified-wavy/stratified-mist flow along the flow direction. Simultaneously, a periodic transition of the flow pattern was observed. A countercurrent flow at the exit of evaporator has emerged to prevent the development of the stratified-wavy flow to the mist flow which subsequently resulted in a drop in superheat. Moreover, two scales of dynamic instabilities were observed. The large scale was found to be the typical pressure drop type oscillation. The small scale, however, was found to be highly related to the flow pattern transition. Due to the countercurrent flow and backflow, a continuous density oscillation was observed. Finally, a modified correlation was developed for the local heat transfer coefficient that could account for the effect of the subcool and flow pattern transition.