【作者】 杨杰；

【导师】 王建军；

【作者基本信息】 哈尔滨工程大学 ， 核科学与技术， 2019， 硕士

【摘要】 射流是在反应堆工程领域广泛存在的一类物理现象,自由射流是对一般射流过程的简化,研究自由射流流场中流体瞬时速度的变化规律,对于了解湍流的模化方法具有重要理论意义,也对与射流相关的反应堆安全系统设计和事故后果分析具有工程应用价值。本文利用平面激光粒子成像测速技术（PIV）,对圆形出口射流的瞬时速度场进行实验研究,获得不同出口雷诺数条件下自由射流速度场、脉动速度以及雷诺应力和轴向速度梯度的分布规律,并对自由射流主体段内的湍流粘性系数进行模化,使用不同湍流模型对自由射流流场进行了数值模拟,与实验结果进行了对比研究,评价了现有湍流模型在预测自由射流流场时的性能。本文首先介绍了搭建的高精度自由射流PIV实验台架,根据日本可视化协会提出的误差评估方法,对实验测量系统进行了误差分析,确定实验台架和测量系统的保守实验误差为2.391%。选取聚酰胺树脂颗粒作为实验示踪粒子,研究了示踪粒子浓度和时均场叠加帧数对实验测量结果的影响,给出了相应推荐值。然后开展了不同雷诺数条件下自由射流实验,利用PIV技术获得了自由射流轴线无量纲速度的衰减规律,确定了自由射流起始段长度与出口雷诺数之间的关系,分析了不同出口雷诺数条件下,距离出口不同位置处轴向无量纲时均速度的变化规律,发现当出口雷诺数大于2400后,射流主体段内轴向时均速度呈现良好的自相似性,与Gortler理论解符合良好,其射流卷吸量的分布与Tollmien理论解符合良好。之后通过对瞬时速度的统计分析,对不同出口雷诺数条件下的轴向脉动速度和径向脉动速度所代表的脉动强度进行了研究,结果表明:在射流起始段内,轴向脉动强度和径向脉动强度在不同横截面均为双峰分布,峰值位于r/b_1/2≈±1处,射流主体段为单峰分布,峰值位于中心轴线即r/b_1/2=0处;对不同横截面无量纲雷诺应力与轴向速度梯度的研究表明,无量纲雷诺应力和轴向速度梯度均为中心对称双峰分布,且存在一定的自相似性。此外,根据雷诺应力和轴向速度梯度的分布对自由射流湍流粘性系数进行了模化,得出湍流粘性系数的分段表达式。最后,本文分别使用标准k-ε、RNG k-ε模型和大涡模拟（LES）的方法,对出口雷诺数为531.5和2444.9工况进行了数值模拟研究,并将计算结果与实验值对比。结果表明,当出口雷诺数高于临界出口雷诺数时,标准k-ε模型的计算结果更接近实验值;当出口雷诺数低于临界出口雷诺数时,LES模拟结果更接近实验值。更多还原

【Abstract】 Jetting phenomenon could be found in many places in nuclear reactor engineering.A free jet could be considered as a simplification of a jet.The study on the instaneous velocity field is beneficial for the modelling of turbulent flow,which is also meaningful for the design of safety-related system and the analysis of the consequences of accidents relevant to jet phenomenon.In this thesis,the particle image velocimetry（PIV）technique with planar laser was incorporated to measure the instaneous velocity for a round free jet.The velocity field,the fluctuation of velocity and the Reynolds stress were derived from the measurement and analyzed in detail.The numerical simulation of the flow field of the round free jet was conducted by using different turbulence models or modelling methods.The comparison between the simulation results and experimental measurement was made and the models were evaluated.The free jet experimental facility was set up with high resolution PIV measurement system installed.Based on the methodology to assess the measurement error proposed by the Japan Visualization Association,the bounded relative error was then estimated as within2.391%.Polyamide resin was selected as tracing particles for PIV measurement.The sensitivity analysis on the concentration of tracing particles and number of images to calculate time-average quantities were performed.The acceptable values were then recommended.The free jet experiments were carried out under different inlet Reynolds number（Re）conditions.The decay behaviors of dimensionless velocity along the axis of round jet were measured by using PIV technique.The relationship between the developing section and the Reynolds at the nozzle was identified.Through analysis on the radial dimensionless velocity at different axial locations,the self-similarity phenomena could be found in full-developed section of the free jet when Re was over 2400.The results agreed well with the prediction from Gortler theory.And the entrainment of free jet agreed well with the prediction from Tollmien theory.By conducting statistics analysis of instaneous velocity,the fluctuation of local velocity with respect to time-averaged velocity in both axial and radial direction was quantified.The results showed that in developing section,the ratio of fluctuation over time-averaged velocity in either direction had two peaks located at r/b_1/2≈±1;in full-developed section,the same ratio presented the only peak at the centerline.Non-dimensional Reynolds stress and gradient of time-averaged velocity showed double crest distribution and self-similarity could also be found.The turbulent viscosity coefficient was given by Reynolds stress and gradient of the experiment date.Finally,the numerical simulations of cases with Re=531.5 and 2444.9 respectively were conducted by using different turbulence models of standard k-ε,RNG k-εand large eddy simulation method as well.The results were compared against experimental measurement.It was shown that standard k-εworked well for the relative high Re case and LES took the advantage for the relative low Re case.更多还原