Three-dimensional numerical study of R134a flow boiling under overload was conducted based on the VOF model. A series of simulations were implemented with a 1.002 mm circular tube under six gravity levels ranging from 1 to 15 g and three directions (θ) ranging from 0° to 180° with mass fluxes of 200, 350, and 500 kg/m2 s, outlet vapor qualities of 0.041, 0.082, and 0.164, and saturation pressure of 0.71MPa. The gravity conditions were introduced into the momentum equation during the simulations. Comparisons among flow boiling characteristics under different gravity levels and directions are made, and significant effects of overload are found. With the increasing overload, the vapor phase distributions of flow boiling gradually turn into forms similar to ellipsoids/spheres, stratified flow, and elongated slugs, at θ = 0°, 90°, and 180°, respectively. Accordingly, the heat transfer coefficient keeps almost stable at θ = 0° and 180°, but decreases obviously at θ = 90°; the frictional pressure drop decreases at θ = 0°, decreases slightly at θ = 90°, but increases at θ = 180°. Making θ away from 90° is a potential method to avoid the flow boiling heat transfer deterioration under overload.

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