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Experimental and Calculated Investigation of a Natural Circulation Loop's Thermal-Hydraulic Characteristics

机译：自然循环回路热工水力特性的实验和计算研究

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Abstract— The results from experimental investigation into hydrodynamics and heat transfer in a two-phase natural circulation loop (NCL) under atmospheric pressure are presented. The experiments were carried out for liquids having essentially different properties: water, ethanol, and perfluorohexane C~(6)F~(14)(the product trademark is FC-72). The circulation velocity in the NCL is not known in advance but is a complex function of the specified parameters (heat flux and liquid temperature at the heated section inlet) and of the two-phase flow internal characteristics. The liquid temperatures at the heated section inlet, the wall temperature over the section height, and also the circulation velocity were measured in the experiments at a specified heat flux; in addition, the two-phase flow at the loop riser leg outlet was filmed on video. The experiments and analysis have shown that flow hydrodynamic instability (circulation velocity pulsations) is really unavoidable in a two-phase NCL. Hydrodynamic instability with a high circulation velocity amplitude and with the occurrence of backward flows is typical for regimes involving significant liquid subcooling values at the heated section inlet and for NCLs containing an extended part with single-phase convection. This instability, which is characteristic for experiments with water, is due to the displacement of the boiling incipience section over the section height; the instability also persists at small subcooling values but with a low pulsation amplitude. Under the developed saturated liquid nucleate boiling conditions (at high heat flux values), the circulation velocity and wall temperature pulsations have small amplitudes, and the flow can be regarded as stable. In the experiments with perfluorohexane, the smallest wall temperature and circulation velocity pulsations were pointed out, which is attributed to a relatively high value of reduced pressure. In the experiments with ethanol, instability occurs in the developed nucleate boiling region (q > 35 kW/m_(2)); this instability is caused by periodically alternating two-phase flow structure (regime). A procedure for calculating a low-pressure NCL is developed, in which the two-phase flow’s local parameters (void fraction, phase velocities, and pressure) are calculated according to a modified homogeneous model (taking into account the phase distribution factor and phases slip) and a dispersed-annular flow model taking into account the droplet entrainment and deposition phenomena. A comparison of the NCL calculation results with the experimental data obtained for three different liquids has shown that they are in good agreement with each other.
机译：摘要—给出了在大气压下两相自然循环回路（NCL）中流体动力学和传热的实验研究结果。对具有本质不同特性的液体进行了实验：水，乙醇和全氟己烷C〜（6）F〜（14）（产品商标为FC-72）。 NCL中的循环速度事先未知，但它是指定参数（加热段入口处的热通量和液体温度）和两相流内部特性的复杂函数。在实验中以规定的热通量测量了加热段入口处的液体温度，段段高度上的壁温以及循环速度。此外，还记录了环路立管支腿出口处的两相流。实验和分析表明，在两相NCL中，流动流体动力不稳定性（循环速度脉动）确实是不可避免的。对于在加热段入口处具有显着液体过冷值的状态以及包含具有单相对流扩展部分的NCL，通常存在高循环速度振幅和逆流现象的流体动力学不稳定性。这种不稳定性是水实验的特征，这是由于沸腾初生段在整个段高度上的位移所致。在过冷值较小但脉动幅度较低的情况下，不稳定性也会持续存在。在已发展的饱和液体核沸腾条件下（在高热通量值下），循环速度和壁温脉动幅度较小，并且可以认为流动稳定。在使用全氟己烷的实验中，指出了最小的壁温和循环速度脉动，这是由于减压值相对较高。在用乙醇进行的实验中，不稳定发生在已形成的核沸腾区域（q> 35 kW / m_（2））；这种不稳定性是由周期性交替的两相流动结构（状态）引起的。开发了一种低压NCL的计算方法，其中根据修正的均质模型（考虑了相分布因子和相滑移）计算了两相流的局部参数（空隙率，相速度和压力） ），并考虑了液滴的夹带和沉积现象。 NCL计算结果与从三种不同液体获得的实验数据的比较表明，它们彼此之间具有很好的一致性。

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