Description of phase separation motion in gas‒liquid two-phase flow

Understanding the physics of phase separation between gas and liquid phases as a mixture mass has long been a challenge. In this paper, a phase separation description criterion based on heterogeneous flow model is proposed. A mathematical method similar to Lagrangian coherent structure (LCS) is used to identify the two-phase separation process, which is called relative motion Lagrangian coherent structure (rLCS). The rLCS is able to describe the dynamic evolution of the phase separation process and flow pattern transition in multiphase flows, which is very common in gas‒liquid mixture transportation and industrial processes. The most striking finding of rLCS is that phase separation and phase distribution are not in the same spatial position, that is, the process and the result of separation may not be exactly corresponding as we thought. This new flow structure reflects the underlying dynamic behavior of the multiphase flow field. In addition, the phase separation process has obvious periodicity. This paper reveals the typical phase separation process in the simulation of gas‒liquid two-phase pipe flow and gas‒liquid multiphase pump. These are very important to improve the understanding of multiphase flow processes, and can also lay a solid foundation for future flow control based on multiphase flow characteristics, highlighting the application potential of the new method.