Characteristics study of non-equilibrium flash for two-phase expansion based on equivalent expansion experiments

Abstract

The two-phase expansion inside the expander could be divided into the non-equilibrium flash of superheated liquid and the expansion of the vapor. A higher flash rate would permit higher recovered expansion work. However, it is almost impossible to measure the flash rate inside the expansion chamber. In this paper, an equivalent expansion method s was proposed to study the non-equilibrium flash process for the two-phase expansion. The flash was controlled inside a constant volume flash chamber, which was connected to a vacuum tank to introduce and adjust the flashed vapor rate out of the flash chamber. The flash process inside the chamber was controlled under the working conditions similar to that inside the two-phase expander. By applying the equivalent expansion method, the flash rate could be measured easily. The experiments were carried out with an initial liquid height inside the chamber from 110 to 175 mm, and an initial liquid temperature from 110 to 130 °C with water as the working fluid. The instant flash rate and the PV diagram of the equivalent two-phase expansion were studied. Results showed that the instant flash rate increased and decreased sharply at first, and then the flash became gentle. Thus, the non-equilibrium flash of the two-phase expansion was divided into two stages of the rapid expansion process and the gradual expansion process. It was found that most of the flashed mass was generated during the rapid expansion process. Finally, a non-equilibrium flash model was proposed to simulate the flash process during the two-phase expansion process with the accuracy of ±3.7% predicting the pressure profile and the accuracy of ±2.7% predicting the expansion work.