Experimental study of heat transfer characteristics among the two-phase expansion process

Different from a gas expander, there is a phase change from liquid to vapor called the flash inside the two-phase expander. But, same with the gas expander, only the vapor phase expands and pushes the rotation of the expander to output power in the two-phase expander. It means that the amount of the vapor is increased during the expansion by the flash. Obviously, the flash rate of the working fluid from liquid to vapor is critical to the expansion power recovery. In this paper, an experiment system with water as the working fluid was established to study the heat transfer characteristics of flash evaporation during the two-phase expansion process. The flash was controlled inside a flash chamber, under the initial temperature from 100 ℃ to 130 ℃, and the initial liquid level height from 110 mm to 175 mm. The pressure evolution of the flash evaporation was limited by the pressure regulating valve. The obtained results showed that the level of the instant liquid superheat was positively associated with the flash rate of evaporation. The temperature drop rate behaved negatively correlated with the instant superheat, while higher the instant superheat resulted in faster liquid temperature reduction. Both the liquid-vapor heat transfer coefficient and the instant flash rate were increased with the increase of instant superheat. Generally, the peak of instant superheats occurred earlier than that of instant flash rate and instant heat transfer coefficient. The instant superheat level was increased and affected by the initial temperature and initial liquid level height.