The study investigates the characteristics of oscillations and associated thermo-hydrodynamic phenomena for startup, limit state, and intermediate transitional behavior of a pulsating heat pipe (PHP) with the help of its single branch. Experiments are performed on a transparent single branch PHP (SBPHP) at different orientations, evaporator and condenser section temperatures using n-pentane as the working fluid. The entire two-phase flow behavior is recorded with the help of shadowgraph based high speed imaging synchronized in real-time domain with pressure data acquisition at evaporator end. Instability threshold is determined in terms of evaporator temperature at different tube orientations and condenser temperatures. The oscillation startup mechanism along with the associated intermittency is studied in detail. The SBPHP is found to have characteristic variations in oscillation amplitude and frequency as it undergoes a gradual transition from its oscillation threshold to limit state. The mechanism by which the limit state creeps-in and intumesces further is an intrinsic function of liquid-vapor distribution in SBPHP, which in turn depends on the applied boundary conditions. The operational limit is governed by evaporator dryout exceeding permissible limits marked by the steep rise in the dry length of evaporator. As SBPHP approaches or operates at its limit state, evaporator is found to have different regimes across its length, viz., mild convective evaporation combined with subcooled nucleate boiling, saturated nucleate boiling, forced convective boiling, film breakdown and permanent dryout. The liquid film distribution, pressure fluctuations, and heat transfer components in evaporator at the limit state have also been studied.

中文翻译：

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单支路脉动热管极限状态运行振荡阈值的实验研究

该研究借助脉动热管 (PHP) 的单支路，研究了脉动热管 (PHP) 的启动、极限状态和中间过渡行为的振荡特征和相关热流体动力学现象。使用正戊烷作为工作流体，在不同方向、蒸发器和冷凝器部分温度的透明单分支 PHP (SBPHP) 上进行实验。借助基于阴影图的高速成像，记录整个两相流行为，该高速成像与蒸发器端的压力数据采集在实时域中同步。不稳定阈值根据不同管方向的蒸发器温度和冷凝器温度来确定。详细研究了振荡启动机制以及相关的间歇性。研究发现，当 SBPHP 从振荡阈值逐渐过渡到极限状态时，其振荡幅度和频率具有特征变化。极限状态蠕变和进一步膨胀的机制是 SBPHP 中液汽分布的固有函数，而这又取决于所应用的边界条件。操作限制由蒸发器干燥超过允许限制（以蒸发器干燥长度急剧上升为标志）来控制。当 SBPHP 接近或在其极限状态运行时，发现蒸发器在其长度上具有不同的状态，即温和对流蒸发与过冷核沸腾、饱和核沸腾、强制对流沸腾、薄膜破裂和永久干燥相结合。还研究了极限状态下蒸发器内的液膜分布、压力波动和传热分量。