Abstract:

As a novel single-well heat exchange technology, the super-long gravity heat pipe has broad application prospects in the development and utilization of geothermal resources. The rapid start-up of heat pipe is the basis for efficient and stable operation. However, the current research on the start-up characteristics of super-long gravity heat pipes is still in its infancy. This paper uses a visual experimental platform for super-long gravity heat pipes with a length of 40 m, an inner diameter of 7 mm, and an aspect ratio of 5714 to study the start-up process of the super-long gravity heat pipe and analyze its influencing factors. The results show that the start-up modes of the heat pipe vary with changes in the heating power and fill height, primarily categorized into gradual temperature variation, steady transition, and abrupt temperature change types. As the heating power increases from 100 W to 500 W, the rate of vapor-liquid phase change within the tube accelerates, and the start-up temperature rises from approximately 43.5℃ to 81.4℃. Consequently, the overall start-up time of the heat pipe is reduced by about 51%. When the power increases from 200 W to 300 W, distinct geyser boiling phenomenon begin to occur within the tube, leading to an increase in the resistance to the reverse flow of vapor and liquid. Consequently, the start-up time of the heat pipe does not decrease but instead increases. As the heating power continues to increase, the start-up time decreases. Under a fixed heating power condition (300 W), as the liquid fill height increases from 6 m to 15 m, the start-up time of the heat pipe initially decreases from 6455 s to 3354 s and then increases to 4575 s. Moreover, the shortest start-up time of the heat pipe occurs at the fill height of 9 m (3354 s). When the fill height is set at 3 m and the heating power is increased to 300 W, a significant amount of condensate is carried up to the adiabatic section and into the condenser section. This leads to localized dry-out in the evaporator section, making it difficult for the heat pipe to operate stably and ultimately resulting in start-up failure. Further experimentation revealed that under the same heating power and fill height conditions, shortening the length of the heat pipe's evaporator section can effectively reduce the start-up time (by approximately 56%), increase the start-up temperature, and result in a more stable temperature variation in the evaporator section.

Key words: super-long gravity heat pipe, vapor-liquid two-phase flow, start-up characteristics, geothermal energy, visualization

摘要：

超长重力热管作为一种新型单井换热技术，在地热资源的开发利用上具有广阔的应用前景。热管的快速启动是高效稳定运行的基础，然而，目前对超长重力热管启动特性的研究仍处于起步阶段。利用长40 m、内直径为7 mm、长径比为5714的水工质超长重力热管可视化实验平台，研究了超长重力热管的启动过程，并分析其影响因素。结果表明，热管的启动形式随加热功率和注液高度的变化，主要分为温度渐变型、平稳过渡型以及温度突变型。随着加热功率从100 W增大到500 W，管内气液相变速率提高，启动温度从约43.5℃升高至81.4℃，热管启动时间总体减少约51%；当功率从200 W增至300 W时，管内开始出现明显的间歇沸腾现象，气液逆流阻力增大，热管启动时间不减反增；加热功率继续增大后，启动时间减少。在固定加热功率条件（300 W）下，随着注液高度从6 m增大到15 m，热管启动时间从6455 s先减少到3354 s后增加到4575 s，且当注液高度为9 m时，热管启动时间最短（3354 s）。当注液高度为3 m，加热功率增大至300 W时，出现大量冷凝液被卷携至绝热段上方和冷凝段的现象，导致蒸发段局部干涸，热管难以稳定运行，最终启动失败。实验进一步发现，在相同的加热功率和注液高度条件下，缩短热管蒸发段长度可有效减少启动时间（减少约56%），提高启动温度，使蒸发段温度变化更加平稳。

关键词: 超长重力热管, 气液两相流, 启动特性, 地热能, 可视化