Recent work has demonstrated that as the bulk convective velocity in subcooled nucleate flow boiling increases, the heat transfer tends to become independent of flow orientation with respect to gravity. There is significant interest in developing heat exchangers for next generation spacecraft that operate in the gravity-independent flow boiling regime. In order to develop such heat exchangers it is important to understand the effect of gravity on the critical heat flux and to determine whether a gravity on the critical heat flux and to determine whether a gravity-independent flow boiling critical heat flux regime exists. This work describes subcooled flow boiling experiments where the critical heat flux is measured over a range of flow orientations with respect to gravity: 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. It has been found that at low bulk flow velocities there is a large variation of critical heat flux with different flow orientations. At large convective velocities, the variation of critical heat flux with different flow orientations is significantly diminished. It appears that with further increases in bulk flow velocity, a gravity-independent critical heat flux regime exists, although the current experimental facility was not capable of operating at those flow conditions.

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