Towards the Development of Flow Pattern Maps for Thermally-Induced Pulsating Two-Phase Flows

Flow boiling instabilities affect the operating range of two-phase thermal management systems. Flow pattern transitions, as a form of instability, has been extensively mapped under conditions of controlled mass flow rate and velocity. In case of a chaotic pulsating flow, like in Pulsating Heat Pipes, the conventional maps present applicability limitations. In this work, flow pattern transitions and bubble break-up events have been investigated in a single loop Pulsating Heat Pipe. Modified Froude, Weber and Bond numbers have been introduced, considering the actual acceleration of the fluid and the length of the bubble as main controlling factors for the transitions. Finally, based on the modified non-dimensional analysis, a novel flow pattern map, valid for a pulsating flow and obtained under the three different gravity levels (from parabolic flight), has been proposed. This new map will be useful to the development of comprehensive designing tools for passive two-phase devices.