Experimental and model study of two-phase expansion stimulated by rotating impeller

Experiments of two-phase expansion that happened inside a constant volume chamber installed with a rotating impeller were carried out. This proposed method separated liquid flash from vapor expansion of two-phase expansion, facilitating measurement of temperature and instantaneous flash flow rate, while these parameters couldn’t be measured directly in a practical two-phase expander owning to complex solid structures and rotation of the expander rotors. Additionally, the rotation of the rotors always agitates the fluid, inevitably impacting the instantaneous flash evaporation rate. Thus, the rotating impeller was adopted to simulate the rotation of the rotors in a practical expander. Water served as the working fluid, with an initial liquid height of 175 mm and initial injection temperatures of 110 °C and 120 °C inside the constant volume chamber. The rotation speed range of the impeller was set between 125 and 500 rpm. Experiment results showed that agitating the liquid with the impeller could effectively stimulate the instantaneous flash evaporation rate and the total mass of flash evaporation. The two-phase expansion process was divided into the rapid expansion stage and the gradual expansion stage. Du to the high flash evaporation rate, the rapid expansion stage was suggested to generate power in the expanders. Increasing the rotational speed of the impeller leads to a significant increase in indicated power during the rapid expansion stage. However, in practical expanders, the indicated work will decrease due to the limited built-in volume ratio. A thermal non-equilibrium two-phase expansion model was established based on the principle of bubble growth. The proposed model could predict liquid temperature error fell within ± 0.06 %, while the predicted error of vapor pressure was limited to ± 4.9 %.