Boiling and condensation are being considered for operation of thermal control systems (TCSs) in future space vehicles to capitalize upon their high heat transfer coefficients and appreciable reduction in TCS weight and volume. A primary concern in designing these systems is a lack of technical knowhow on the influence of buoyancy and therefore body force on two-phase heat transfer in reduced gravity. In particular, there is keen interest in developing predictive tools for pressure drop and heat transfer and identifying the minimum coolant flow rate that would negate the influence of body force. Two-phase flow in micro-channels provides unique advantages to space systems by greatly increasing flow inertia for a given flow rate, which helps resist body forces effects. This study is the first part of a two-part study addressing the effectiveness of two-phase micro-channels at negating body force effects. Flow boiling experiments are conducted with FC-72 in a test module containing 80 of 231 mu m wide x 1000 mu m deep micro-channels in three different flow orientations: horizontal, vertical upflow and vertical downflow over broad ranges of mass velocity and heat flux. Also investigated are conditions that trigger two different types of two-phase flow instability. In addition, different flow regimes are identified with an aid of high-speed video imaging, and the flow regime data are compared to an exiting flow regime map and used to develop new relations for transition boundaries between regimes. Overall, the present study proves the existence of a mass velocity threshold above which identical heat transfer performances are achieved regardless of orientations. (C) 2014 Elsevier Ltd. All rights reserved.