Visualization and thermal resistance of a sintered wicks structure evaporator in a two phase loop thermosyphon

Abstract

A prototype two-phase loop thermosyphon (TPLT) was built and tested for thermal performance. This experimental study simultaneously investigated the evaporator resistance and flow visualization in operating the TPLT under sub-atmospheric pressure. To facilitate the visualization of flow regime in the evaporator, a transparent glass window was attached to the evaporator. Evaporators with 1-mm-thick sintered copper powder wicks (average diameter, 100 μm) were used. Temperature distribution and evaporator resistance were measured while increasing the input power in a series of 11 steps (20, 60, 100, 120, 140, 160, 180, 200, 220, 240, and 260 W) until the sintered wicks reached completely dried out. Temperature fluctuations and instabilities in the vapour and liquid lines were observed. Heat leakage from the evaporator and intermittent motion in the flow regime were significant factors in generating the implicit boiling instability. As the input power was increased, onset of pool boiling, nucleate boiling, as well as slug and bubbles were observed successively when the liquid level was above the surface of the wicks. When the net input power reached 236 W, the water film suddenly receded toward the wick surface. The process of the meniscus receding during the film evaporation, and the dynamics from the initial condition until dryout on the sintered copper wicks were observed and documented. © 2014 The Japan Society of Mechanical Engineers.