Bubble dynamical behavior and thermal non-equilibrium during flow boiling in U-turn bends of hairpin tubes

A series of experiments was conducted to investigate the flow boiling of R141b in hairpin tubes. A focus was addressed on visually observing flow boiling and two-phase flow in U-turn bends of a vertical downward hairpin tube with inner diameter of 6 mm at liquid velocities of 0.098, 0.147 and 0.196 m/s and heat fluxes in the range of 4624.88–18499.53 W/m2. Bubble dynamical behavior was divided into four zones based on flow pattern and bubble size, or secondary flow, transition, inertia and buoyancy zone, and each of them was combined with unique interaction between two phases. Furthermore, thermal non-equilibrium of two-phase flow was defined and introduced to understand bubble dynamical behavior and the U-turn bend effects on local transportation mechanism. Normally a U-turn bend could re-construct flow pattern and redistribute the thermal non-equilibrium of flow boiling two-phase flow, mainly release the thermal non-equilibrium.