This study explores the complex flow boiling CHF mechanisms encountered at different orientations relative to Earth's gravity when the fluid is supplied as a two-phase mixture. Using FC-72 as working fluid, different CHF regimes are identified for different orientations, mass velocities and inlet qualities. Low mass velocities are shown to produce the greatest sensitivity to orientation, while high mass velocities greatly reduce this influence, especially for high inlet qualities. It is also shown that the influence of orientation can be negated by simultaneously satisfying three separate criteria: overcoming the influence of gravity perpendicular to the heated wall, overcoming the influence of gravity parallel to the heated wall, and ensuring that the heated wall is sufficiently long to ensure liquid contact. These criteria are combined to determine the minimum mass velocity required to negate gravity effects in both terrestrial and space applications. Exceeding this minimum is of paramount importance to space systems since it enables the implementation of the vast body of published CHF data, correlations and models developed from terrestrial studies for design of thermal management systems for space applications. (C) 2013 Elsevier Ltd. All rights reserved.