Operational characteristics and oscillatory phenomena in two-phase closed thermosyphons

• Published in: Applied thermal engineering 2025-01, p.125401, Article 125401
• Main Authors: Yilgor, Ilyas, Shi, Shanbin
• Format: Article
• Language: English
• Subjects: Flooding; Geyser boiling; Heat pipe; Intermittent boiling; Oscillations; Two-phase closed thermosiphon; Two-phase flow
• ISSN: 1359-4311
• DOI: 10.1016/j.applthermaleng.2024.125401

•Quasi-steady and transient experiments were conducted on a two-phase thermosyphon.•Experiments include condenser coolant flow rate and evaporator power input transients.•Reported data includes internal temperatures/pressures, flow visuals, and film thicknesses.•High fill ratios and coolant flow rates result in higher axial temperature drops.•Oscillations and disturbances are studied based on their amplitudes and frequencies. Two-phase closed thermosyphons (TPCTs) are passive phase change heat transfer devices that have found a large variety of applications due to their efficiency and compact form factor. Due to complex flow structures within thermosyphons, a comprehensive experimental database consisting of internal flow temperatures, pressures, pressure drops, flow visuals, and film thicknesses, is needed for model development and verification, as well as robust identification of operational characteristics, potential instabilities, and oscillatory phenomena. This article presents such a database for a water filled thermosyphon with a variety of fill ratios (25 % to 105 %), input powers (200 W to 600 W), and condenser coolant flow rates (0.02 to 0.15 kg/s). The experimental cases investigated include quasi-steady states, condenser heat transfer coefficient transients, and power transients, with an emphasis on describing oscillatory phenomena and disturbances. It was found that high fill ratios (≥ 60 %) and high condenser coolant flow rates (≥ 0.10 kg/s) result in higher axial temperature drops across the thermosyphon through increased subcooled liquid presence and the formation of a liquid plug at the condenser endcap. The sustained liquid plug isolated that portion of the condenser from the rest of the thermosyphon, resulting in temperatures that were near the condenser coolant temperature of 35 °C. Furthermore, intermittent or geyser boiling oscillations observed at low powers were characterized based on their amplitudes and frequencies, which were found to be strongly influenced by the fill ratio. It was found that the amplitude of oscillations were reduced by an order of magnitude for both evaporator pressure drops and evaporator exit temperatures as the input power was increased from 200 W to 600 W. Lastly, high frequency temperature and pressure oscillations of ∼ 0.1 Hz were identified at the onset of flooding, along with a lower secondary frequency for higher fill ratios of ∼ 0.01 Hz.