A mechanistic model for the prediction of swirling annular flow pattern transition

Abstract

The accurate prediction of flow patterns and their transition is extremely important for proper design, operation and optimization of two-phase flow systems, since the parameters such as pressure loss and heat and mass transfer are strongly dependent on the flow pattern. So far, the non-swirling gas-liquid flow in straight pipes have been widely studied and various mechanisms that lead to flow pattern transition have been clarified and modeled. However, the dynamics of gas-liquid flow under swirling condition are not well understood, and no detailed models are available for the prediction of swirling flow pattern transition. To address this, in our previous work (Liu and Bai, 2018), a visualization experiment aimed at classifying flow regimes in swirling gas-liquid flow was presented and three typical swirling flow regimes, i.e., swirling gas column flow, swirling intermittent flow and swirling annular flow were classified and defined, respectively. As the swirling annular flow can be regarded as a special case of conventional annular flow (i.e., when tangential velocity does not equal zero), in present paper, a mechanistic model for the prediction of the swirling annular flow pattern transition was developed considering its physical interest and great practical significance. Two physical mechanisms that lead to the transition from swirling annular flow to other flow patterns were revealed and modeled, respectively. The model was evaluated against a wide range of swirling and non-swirling experimental data and based on this model, the effects of different parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the boundary of flow pattern transition were presented. Results revealed that the range of swirling annular flow enlarges with the increase of the working pressure and swirl angle but narrows with the hydraulic diameter. Taking these influencing factors into account, a generalized formula for the prediction of the swirling annular flow pattern transition was proposed. Compared with existing empirical correlations for annular flow, the newly developed correlation provided more accurate and reasonable prediction of flow pattern transition for both swirling annular flow and annular flow.