Theoretical and experimental investigation of compensation-chamber effects in an open-loop heat pipe

An open-loop heat pipe (OLHP) with a circular flat evaporator was developed by opening the liquid return path of a conventional loop heat pipe (LHP). Condensate is discharged from the condenser as product water, while feedwater is supplied externally to the compensation chamber. Unlike that in a closed-loop LHP, the compensation chamber in the OLHP is directly exposed to the external pressure boundary, and its operating state is jointly governed by the gas pressure, liquid hydrostatic pressure, and heat leakage from the evaporator. A visual experimental prototype and a steady-state thermohydraulic model were used to quantify these effects. The model predictions agreed well with the measured temperatures and condensate output fluxes. Increasing the equivalent feed-line height from 2 to 10 m decreased the vapor temperature from approximately 94–30 °C, thereby extending operation from high-grade to near-ambient heat sources. The measured condensate output flux reached 45 kg/(m 2 ·h) at an evaporator-wall temperature of 52.3 °C, comparable in magnitude to the 55.25 kg/(m 2 ·h) reported for an OLHP supplied by a 60 °C heat source. Near the natural-vacuum limit (the maximum water-column height sustained by atmospheric pressure, approximately 10 m under ambient conditions), small pressure perturbations shifted the compensation chamber between subcooled and locally saturated states. The chamber liquid level controlled the transition between gravity-driven and capillary-gravity co-driven operation; the critical heat input increased from approximately 10–17 and 23 W as the liquid level rose from 1 to 2 and 3 cm, respectively. Heat leakage primarily redistributed energy and altered the chamber thermal state. In particular, axial heat leakage through the joint increased from 0.6 to 2.0 W and produced cooling-dominated, thermally balanced, or leakage-dominated responses depending on joint thermal conductivity. These results clarify how compensation-chamber boundary conditions govern low-pressure capillary evaporation and provide design guidance for OLHP-based low-grade-heat desalination.