Flow pattern transition of oil–air two-phase flow in inclined scavenge pipes based on a one-dimensional two-fluid model

The flow pattern of oil–air two-phase flow in aero-engine scavenge pipes significantly affects the stability and reliability of the oil scavenging process. However, most existing flow pattern transition criteria are established based on air–water or refrigerant media, making them difficult to apply directly to aero-engine scavenge pipes with high-viscosity oil, small diameters, and inclined configurations. In this paper, an experimental setup with multiple inclination angles is built for inclined scavenge pipes (ID 14 mm). Within the operating ranges of oil viscosity 0.579 Pa s, liquid flow rate 0.16–9.29 l/min, and air flow rate 0.02–6.35 g/s, inclined angles 30°and 45° are observed using a high-speed camera system. Typical flow patterns, including stratified flow (SS and SW), intermittent flow (I), and annular flow (AN), are identified, and flow pattern maps are constructed. By applying the one-dimensional two-fluid model, the mechanical balances for stratified and AN patterns are constructed, leading to the derivation of the non-dimensional liquid height and film thickness control equations. It shows that the proposed transition criteria can effectively distinguish stratified, annular, and intermittent flow regions at different inclination angles. The theoretical boundaries agree well with the inclined scavenge experimental data and can be well applied to flow pattern prediction. It provides a theoretical basis for the pipeline design of aero-engine scavenge systems.