Experimental Study on Frictional Pressure Drop Characteristics of Wire-wrapped Bundles in Transition Flow
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摘要:
绕丝棒束组件低流速下摩擦阻力是钠冷快堆堆芯热工水力研究的重点内容之一。通过对现有组件摩擦阻力系数公式应用于过渡流存在的问题、现有组件摩擦阻力实验在低流速时出现的不同摩擦阻力特性的分析讨论，以及对一个37棒组件低流速摩擦阻力的实验研究，提出了组件内流动的转捩特性和提高经验公式在低流速时计算准确性的有效策略。研究结果表明：绕丝棒束组件内流动转捩并不是同时发生，而是在部分子通道的局部先发生，随着流速的增加扩散到整个组件，只有当一定数量子通道都发生转捩时，棒束的平均摩擦阻力才会显著增加。绕丝棒束组件摩擦阻力系数公式应尽可能使用小的层流向过渡流转捩的临界Re和插值指数，使公式尽早开始转捩，并平滑向过渡流过渡。
Abstract:The investigation of frictional pressure drops in wire-wrapped bundles at low flow velocities is crucial for thermal-hydraulic studies of sodium-cooled fast reactor cores. This study aims to refine empirical formulas for predicting friction factors in wire-wrapped bundles by addressing their limitations in accurately calculating friction factors during transition regime. By analyzing existing empirical formulas and meticulously observing frictional pressure drop experiments conducted under low-flow conditions that reported in the literature, the study proposes and demonstrates the mechanism of the laminar-to-transitional regime transition in wire-wrapped bundles. Additionally, experimental research was conducted on the frictional pressure drop of a 37-rod wire-wrapped bundle. A novel high-precision differential pressure measurement technique, the photographic liquid column manometer, was employed to enhance frictional pressure drop measurement accuracy. This innovative method achieves a measurement uncertainty of less than 2 Pa within a range of 0 to 300 Pa. The mass flow rate of the fluid was measured and converted to obtain the flow velocity through the assembly, further improving the accuracy of flow velocity measurements. The analysis and experiments reveal that the transition from the laminar to transitional regime does not occur uniformly but initiates locally in certain subchannels before spreading across the assembly as flow velocity increases. Significant increases in the frictional pressure drop occur only after a sufficient number of subchannels have undergone transition. This perspective is supported by extensive existing literature, which report that in assemblies with fewer rod bundles, smaller P/D and H/D values, and constructed from hard metal materials such as stainless steel, the friction factor at the initial stage of the transitional regime shows a significant increase or remains constant with increasing Reynolds number. Under these conditions, the high consistency of the P/D ratio within the assembly causes the fluid to transition within a narrow range of flow velocities, leading to a marked increase in the friction factor with Reynolds number. The research results also indicate that the prediction accuracy of the critical Reynolds number for the laminar-to-transitional flow transition significantly impacts the calculation accuracy of empirical formulas at low flow velocities. Formulas using smaller predicted values of the critical Reynolds number show better agreement between predicted values and the measured results in this study. Therefore, the study recommends adopting smaller critical Reynolds numbers and interpolation indices in empirical formulas to facilitate an early onset and smooth transition of laminar-to-transitional regime behavior in friction factors, thereby improving the accuracy of calculated friction factors.
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图 1 绕丝棒束组件几何结构
a——组件几何结构；b——取压孔结构；c——测量组件取压孔轴向位置
Figure 1. Diagram of tested wire-wrapped rod bundle assembly
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图 5 组件不同轴向位置1个绕丝螺距长度产生的摩擦阻力压降
Figure 5. Axial pressure drop of a single wire helical pitch at various axial positions
表 1 压差和质量流量测量的不确定度
Table 1 Measurement uncertainties of pressure drop and mass flow rate
参数 测量范围 不确定度 压差 0～100 Pa 1 Pa 100～200 Pa 2 Pa 200～500 Pa 3 Pa 500～1 600 Pa 6 Pa 质量流量 0～0.3 kg/s 0.25% 0.3～0.5 kg/s 0.48% 
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