Experimental study on the influence of gas injection on flashing in open natural circulation systems under stable boundary conditions

As an efficient passive heat exchange system, natural circulation systems are widely used in various fields, including passive containment cooling system in nuclear power plants. Flashing is the most common two-phase flow state in natural circulation systems. To enhance the circulation capacity and stability of natural circulation systems, this study conducts experiments on the effects of gas injection on flashing two-phase flow using water and air as mediums. The research analyzes the impact of different gas injection positions and volumes on the flow rate, flow pattern, flow instability, and void fraction of the flashing flow. The results indicate that gas injection enhances the circulation flow rate of the loop and improves system stability. The further the gas injection position is from the upstream of the flashing point, the better the enhancement effect on the circulation flow rate. As the gas injection volume increases, the circulation capacity of the loop strengthens; however, once a certain injection volume is reached, the circulation capacity no longer increases, with a maximum enhancement of up to 80% in circulation flow rate. The introduction of gas injection leads to significant fluctuations in the axial distribution of the void fraction in the rising section, but has a negligible effect on the bubble fraction at the exit of the rising section. The paper analyzes the pressure drops and driving force fractions in the rising section before and after gas injection, and presents the mechanisms by which different gas injection positions and amounts affect the flow in the loop. These findings provide important insights for enhancing and stabilizing natural circulation systems, contributing to improved design and operation of passive safety shell cooling systems. •The experimental results of flashing flow and the influence of gas injection on flashing characteristics were obtained by setting up an interference-free flow Experimental bench.•The effects of different gas injection positions and different gas injection volumes on the position of flash onset, flow pattern, vacuole share and flow characteristics of two-phase flashing flow were investigated.•Combining the visualization image and the measurement instrument, the distribution of the vacuole share of the two-phase flow under the gas injection condition is obtained.