Recognition and Primary Test on Secondary Flow in T-Type Outlet

This work presents a recognition and primary test on alternative secondary flow in T-type outlet. The phenomenon belongs to a kind of dean vortex. Dean vortexes always are found in 90 bend pipe, and a pair of vortexes presents an alternating cyclical vibration in some condition. In recent experiments, a phenomenon of unstable back-flow occurs in T-type outlet, and the phenomenon in experiment is shown in pictures. The numerical study is performed by Fluent CFD, in which a pure gas flow and gas-particle flow are simulated in a riser with a T-type outlet. Euler-Euler two-fluid model and RNG k-ε model was adopted. The momentary result of flow filed in T-type outlet is achieved either in pure gas condition and gas-solid condition. In same gas and particle velocity, 3 volume friction was chosen, such as 0.01（Gs=138 kg/m2·s), 0.03（Gs=414 kg/m2·s), 0.08(Gs=1104 kg/m2·s). In the results of pure gas, the basic flow field in T-type outlet possessed a pair of Dean Vortexes. In a certain condition, there existed a pair of switching vortexes, which alternatively occupied the main space of T-type outlet. The changing process of the switching vortex along with time was discussed. When gas velocity was 10 m/s, gas flow state cycle time was 0.16 s. In the results of gas-solid condition, flow states cycle time is 0.6, 0.9 and 1.1 when the particle volume friction was 0.01, 0.03 and 0.08. It mean that, a higher particle volume friction lead to a long flow state cycle time. Ignoring the calculation error, the flow states cycle time of gas-solid flow is much higher than the one of pure gas. This indicated that, affected by the delay effect from particle, gas-solid flow showed a lower frequency in switching than pure gas flow. And the flow states cycle time increased with the particle volume friction grow.