Regime-dependent evaporator wall temperature distribution in a small diameter thermosyphon

Passive two-phase heat transfer devices, such as thermosyphons, are widely used in applications ranging from geothermal systems to electronics thermal management. With the continuous miniaturization of electronics, there has been a growing attention towards small-diameter thermosyphons. In this study, we investigate the evolution of evaporator wall temperature distribution with increasing heat input in a two-phase closed thermosyphon (TPCT). We establish operating regimes based on the axial wall temperature characteristics, enabling regime identification when direct flow visualization is impractical. Experiments are conducted using air-cooled vertical copper-water thermosyphons of total length 400 mm and 10 mm diameter with three different fill ratios (FRs), 25%, 50%, and 75% at heat input ranging from 5 W (0.1 W/cm 2 ) to 970 W (20.5 W/cm 2 ). At low heat flux, prior to the onset of nucleate boiling (ONB), the heat transfer is dominated by natural convection, resulting in a stratified temperature distribution with maximum wall temperature at the pool bottom and the minimum in the thin-film region. Following ONB, oscillations in the evaporator wall temperature emerge while the estimated bubble departure diameter exceeds the thermosyphon inner diameter indicating the onset of geyser boiling. Increasing the fill ratio intensifies the amplitude of temperature oscillation and evaporator wall temperature non-uniformity. With further increase in heat flux, these temperature oscillations diminish and the evaporator wall becomes nearly isothermal, signifying the transition from geyser boiling to churn regime. At still higher heat inputs, the onset of flooding is characterized by lower temperatures near the top and bottom portions and elevated temperature near the center of the evaporator. To quantitatively characterize these regimes, we introduce a modified Kutateladze number, defined as the ratio of vapor velocity in the thermosyphon to the critical velocity required for the Kelvin-Helmholtz instability to grow. The transition from the geyser boiling to churn is marked by a sudden decrease in the modified Kutateladze number with increasing heat flux, whereas the transition from the churn to a flooding regime occurs when the modified Kutateladze number becomes invariant, regardless of fill ratio.