Heat transfer performance and startup characteristics of separated gravity heat pipe

Separated gravity heat pipes, with distinct arrangements of evaporating and condensing sections, offer low thermal resistance, high heat transfer efficiency, and a simple structure, making them suitable for passive cooling of spent fuel pools through gravity-driven processes. In this study, a steady-state, one-dimensional, two-phase flow model is developed to analyze phase change heat transfer in a separated gravity heat pipe. The research investigates the impact of liquid filling rate and vacuum level on heat pipe operation. The findings reveal a notable increase in pressure drop inside the tube with higher liquid filling rates, which diminishes the adaptability of the heat pipe as pressure rises, resulting in a narrower range of effective liquid filling rates. Specifically, the liquid filling rate of the heat pipe ranges from 38 % to 72 % at P =13.75 kPa and from 44 % to 68 % at P = 15.75 kPa. Additionally, a startup prediction model is formulated based on a thermal resistance network model using the ideal gas assumption. Two startup states-namely, the initial startup state and the subsequent startup state-are proposed for large-scale separated gravity heat pipe arrays. The heat pipe achieves the largetemperature-difference initial startup state at a 50 % liquid filling rate within 7 min before transitioning to the subsequent startup state. The vapor mass, heat transfer coefficient, and working fluid temperature initially increase and then decrease over time, with varying peak times. As the liquid filling rate increases, the maximum vapor mass also increases. The heat transfer coefficient peaks at a 60 % liquid filling rate before subsequently declining. The optimal liquid filling rate of 60 % is identified based on startup time, heat transfer coefficient, and overall heat transfer efficiency.