Flow Visualisation in a Transparent Thermosyphon: Influence of Internal Pressure

Heat pipes and thermosyphons are generally recognized as a reliable heat transfer device for many different applications however clarity is needed to describe the thermal mechanisms driving the transfer of heat. This experiment involved the construction and testing of a transparent, sapphire thermosyphon, with water as the working fluid. Through high speed imagery and corresponding temperature and pressure measurements, the dominant mechanisms of boiling and condensation were visualized in the evaporator and condenser. The tests were carried out at various internal pressures, which ranged from P = 4 - 25 kPa, at a constant heat input of Q = 50 W. Of particular interest was the pulsating nature of the thermosyphon at lower pressure levels. The high speed images show, at lower pressures, a liquid plug and vapor slug were seen to oscillate between the evaporator and condenser. At the higher pressures tested, film condensation becomes more dominant in the condenser and the frequency and amplitude of liquid oscillations is reduced. The overall thermal resistance of the system was also calculated, and ranged from Rtot = 0.44 K/W at low pressure, to Rtot = 0.10 K/W at higher pressure.