Experimental investigation of the flooding phenomenon in a pulsating heat pipe unit cell

This study presents an experimental investigation on characterization and quantification of flooding phenomenon in a pulsating heat pipe (PHP) unit cell at different orientations, evaporator and condenser temperatures. A transparent capillary tube having a diameter of 2.5 mm is used as a PHP unit cell, and n-pentane is used as the working fluid. The meniscus oscillations along with flooding events are recorded using a high-speed camera. Three different flooding mechanisms are recognized, viz., wave transport with film drainage delay, wave transport with slug formation, and droplet entrainment. The flooding frequency is determined for each flooding mechanism at different combinations of the operating boundary conditions. The adverse impact of flooding on the performance of PHP is realized apparently by considering its influence on the amplitude and frequency of meniscus oscillations. The approximate liquid deficit in evaporator caused by flooding is also estimated for one of the mechanisms, viz. wave transport with slug formation. Thus, role of flooding in accession of the operational limit of a PHP is signified. Further, the hydrodynamics of flooding is studied in detail. The superficial velocity of vapor at the onset of flooding and film-vapor interfacial wavelength are determined for a few cases and compared to the respective predictions based on some of the most widely accepted correlations. It is found that none of the considered correlations could predict the flooding velocity of vapor for the present experiments. The study signifies the need to study flooding more intensively for PHPs in near future.