• optic probe
• optical measurements technique
• optical probe
• pulsating heat pipe
• two-phase flow
• void fraction measurements

This dissertation concerns the wide research work, carried on at the Department of Energy, Systems, Territory and Construction Engineering (DESTEC), about Pulsating Heat Pipes (PHPs). PHP is a very promising two-phase flow device for thermal control, whose application seems very suitable for space systems.

The capillary dimensions of PHPs imply a complex physical behavior of the flow inside the channel, thus demanding a wide research on flow patterns into minichannels by means of several measuring techniques.
It has been observed that the flow patterns occurring inside minichannels are the slug flow and, less frequently, the annular flow. The developed measurements techniques regarding those flows, aim to obtain the local void fraction, that is the gas phase of the flow, and other important parameters such as, for example, bubbles velocity.

The method proposed here to investigate on two-phase flows inside minichannels, is the optical measurement technique. A low cost fiber optic probe has been manufactured in the laboratory.
This probe has an input and an output branches and a main branches, whose tip is inserted into a channel with an inner diameter of two millimeters. The probe exploits the Snell's law for light refraction: a laser beam enters inside the probe and then it is detected by a photodiode, the measured luminous intensity depends on which phase interfaces the probe tip.
The signal detected is processed by a control hardware, which in turn is implemented by the computer.
A visualization technique, consisting in the post processing of frame sequences of the flow, is used to check and compare the data obtained by means of the optical probe.

The work proposed here shows the advantages of the optical measurement method in terms of local void fraction detection, concerning especially the optic probes capability to obtain informations about bubbles velocity and, in turn, about flow regimes inside a minichannel, in anticipation of their application on PHPs.