【作者】 叶辛欧文；

【导师】 程旭；

【作者基本信息】 上海交通大学 ， 核能科学与工程， 2012， 硕士

【摘要】 燃料棒是压水堆的核心部件之一,大多数核燃料棒均按照方形或三角矩阵排列布置,在棒与棒构成的子通道间冷却剂形成轴向流动,相邻的子通道之间通过缝隙相连,而缝隙的大小由棒束间圆心距与棒径比(P/D)所确定。采用较小P/D的紧密堆芯燃料栅元布置方式能够减少水装量,降低水铀比,使得堆芯中子能谱硬化,从而提高燃料的转换比与功率密度,而紧密栅元内的流场存在很多独特的流动行为,其中就包括棒束子通道间冷却剂强烈的交混作用。为了进一步了解紧密栅棒束内特殊的流场结构,本文以水作为工质,构建了P/D=1.1的双排六棒束方形排列试验段,对通道内的流动进行了试验与数值研究,具体内容如下:试验方面采用可视化测量手段结合流场示踪方法,在Re=2000~ 40000范围内拍摄了紧密栅内棒壁间瞬态流动的可视化信息,捕捉到了大尺度类周期性脉动结构,并获得了该脉动流的相关特征参数,结果表明当Re≥5000时,中心棒壁间隙处大尺度脉动流伴随湍流的转变而发生,并在Re=5000～40000的范围内呈现很强的周期性,大尺度脉动的波长在平均值为65mm左右的范围内存在小幅的变化,且与Re无关,振动频率则与Re呈线性正相关趋势。数值模拟方面首先以已有试验数据为基准,建立了相应的计算模型,采用非稳态雷诺时均法(URANS)结合周期性边界条件,对湍流涡粘性以及雷诺应力两类湍流模型作出了系统的评估,结果表明SSG雷诺应力模型在模拟该流动时的精度高于涡粘性模型,从而确定了湍流各项异性假设在模拟棒束子通道内流动的重要作用;其次建立了与试验段截面尺寸相同的计算模型,对模型通道内的流动进行了相同工况下的非稳态计算,结果成功模拟出了棒壁间隙处的大尺度周期性脉动流,但该脉动流的波长、频率和斯特鲁哈数等参数与试验值存在偏差,同时研究了截面其它监测点处的速度脉动情况,发现中心狭缝处的脉动频率和幅值最大,而子通道中心点的脉动频率和幅值相对较小,且当雷诺数较大时,速度脉动曲线存在着一定规律的变形;最后初步探究了大尺度脉动的发生机理,通过压力准测与Q因子准则,确认了相干结构与大尺度脉动的关系,并确定了雷诺数在狭缝内大尺度脉动的发生过程中的阈值作用。更多还原

【Abstract】 Fuel rod is one of the essential components of the pressure water reactor, the majority of the fuel rods are arranged in rectangular or triangular array, which form the axial flow of coolant fluid in the subchannel constitute of rods, while the adjacent subchannels are connected by the gap, which is determined by the ratio of the pitch between the rods’center and the diameter of the rod(P/D). Adopting the tight lattice arrangement of the rods whose P/D is relatively small could cut down the water needed, reduce the mass ratio of water and uranium, harden the neutron power spectrum of the core in order to enhance the utilization of the fuel. However, many special flow behaviors also exist in the tight lattice, including the strong mixing effect of coolant between subchannels. In order to study the special structure of flow in tight lattice, experiment investigation and numerical simulation are performed in a 2x3 square channel with P/D= 1.1, and followings are some detailed contents.Visual measurement method and dye tracers are adopted in the experiment, while the Reynolds number is varied from 2000 to 40000. Large scale quasi-periodic oscillation is observed in the narrow gap between rod and wall, while some characteristic parameters of oscillation are obtained. The results show that the large scale oscillation occurs at Re≥5000 which appears periodic behavior strongly and is accompanied by the transition from laminar flow to turbulent flow. The wavelength of the oscillation is 65mm on average, which is also independent of Reynolds number, whereas the oscillation frequency increases proportionally with Re.On the aspect of numerical simulation, firstly, the simulation method of Unsteady Reynolds Averaged Navier-Stokes equations(URANS), combined with periodic boundary condition, are adopted and the results are compared to the existed experiment data in order to systematically assess the validation of two types of turbulent models, and the result shows that the SSG turbulent model is more accurate than any vortex viscous models, which ensure the significance of anisotropy hypothesis of turbulent model in simulation the turbulent flow in subchannel; secondly, the computing model which has the same cross section as the experiment is built to simulate the transient flow behavior in tight lattice, and Large scale quasi-periodic oscillation in the gap between rod and wall is successfully obtained, while the calculated characteristic parameters don’t agree much well with the measured data. At the same time, the velocity vibration of other monitor points are also investigated, and the results show that both frequency and amplitude are maximum in the center point of the cross section, and are minimum in the center point of subchannel, moreover, some regular pattern of distortions are found in the velocity oscillation profile. Finally, the mechanism of large scale quasi-period oscillation is preliminarily studied, and the relationship between coherent structure and large scale quasi-period oscillation is verified through both static pressure and factor Q criteria, which confirms the threshold effect of Reynolds number in the occurrence of large scale periodic oscillation.更多还原