The prediction offlow pattern transitions is extremely important to understand the coupling of thermal andfluiddynamic phenomena in two phase systems and it contributes to the optimum design of heat exchangers. Twophaseflow regimes have been extensively studied under controlled massflow rate and velocity. On the otherhand, less effort has been spent in the literature on the cases where theflow motion is purely thermally inducedand consequently the massflow rate or the velocity of the phases are not known a priori. In the present work,flow pattern transitions and bubble break-up and coalescence events have been investigated in a passive twophase wickless capillary loop, where the massflow rate is intrinsically not controllable. Modified Froude, Weberand Bond numbers have been introduced, considering the actual acceleration of thefluid and the length of thebubble as merit parameters for the transitions. The proposed nondimensional investigation was developed byanalysing experimental data obtained with ethanol and FC-72, as workingfluids, different heat input levels(from 9 to 24 W) as well as three different gravity levels (through a parabolicflight campaign). A new empiricaldiabaticflow pattern map for accelerated two-phase capillaryflows is presented, together with quantitativecriteria for the calculation of theflow regime transitions, defining the physic limits for the bubble coalescenceand break-up. This kind of new regime maps will be useful to the further development of comprehensive de-signing tools for passive two-phase wickless heat transfer devices.
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