小破口失水事故工况下CAP1400核主泵气液两相流模拟

发布时间：2018-08-31 11:58

【摘要】：反应堆冷却剂泵(核主泵)为核反应堆提供驱动压头,是核电站中最核心的设备之一。在发生小破口失水事故后,由于压力条件的改变和冷却剂的汽化,核主泵将会在复杂的汽液两相流工况下运行。由于水蒸汽的变化非常复杂,参考相关研究,本文用空气代替水蒸汽进行汽液两相流数值模拟。根据现有的比较成熟的核主泵水力模型,设计并制造出相应的水力样机,并在水力样机上进行试验研究,通过对比试验数据和模拟结果数据,发现:液相工况下两者具有较高的吻合度,在低含气率工况下的吻合程度优于在高含气率工况,说明数值模拟的方法是可信的。为了解核主泵在气液两相流工况下运行的流动特性,对现有的比较成熟的核主泵模型进行了不同含气率工况下定常和非定常数值模拟。通过定常数值模拟,揭示了不同含气率下核主泵外特性曲线、内部气体分布和速度矢量云图的变化规律:由于离心力和压力梯度的作用造成叶轮进口和背面位置容易出现气体聚集现象;扭曲型径向空间导叶结构容易造成气体滞留现象;类球形的蜗壳结构容易造成局部气体聚集区。通过非定常数值模拟,揭示了含气率对压力脉动影响的规律:含气率和叶轮出口处压力不均匀分布存在一定的正相关性;较高含气率对叶轮出口处压力不均匀分布和叶轮工作面处压力脉动波动幅度都具有加强作用;含气率与导叶内压力脉动波动幅度呈负相关性。为进一步了解核主泵在两相流工况下的运动特性,对核主泵的反转全工况不同含气率下的泵水力性能、流道内部气体体积分布情况进行分析,发现:随着含气率的增大,核主泵扬程曲线和扭矩曲线的下降速度变慢;随着流量的增大,在反转逆流制动工况和反转水泵工况,含气率对于核主泵扭矩特性的影响逐渐减弱,在反转正流制动工况刚好相反;在反转逆流制动工况和反转水泵工况,高含气率主要集中在叶轮叶片进口前区域,在叶轮流道内气体体积分数分布极其不均匀。当一回路发生失水事故后,核主泵将会在汽液两相流工况下运行,此时核主泵的性能将会发生复杂的变化,为探究其性能的变化规律对核主泵进行了小破口失水事故下的瞬态模拟。通过数值模拟,得到了含气率对核主泵非稳态特性的影响规律:随着含气率不断增加、流量值持续减小,核主泵的轴扭矩和效率都出现下降,小的变化规律扬程值出现先增大后减;随着含气率逐渐增大,在叶轮进口背面、中间后盖板处和出口背面处出现较高程度的气体聚集现象,导叶内沿液流方向含气率波动幅度逐渐减小且具有明显的跟随性;瞬态径向力随含气率增大逐渐变小且两类径向力极大值的差值变小,叶轮和类隔舌处瞬态压力出现先增大后减小的变化规律,沿液流方向导叶内压力逐渐增大但波动程度减弱;随着含气率的增大,蜗壳内的低速区不断扩大,说明类球形蜗壳不利于气体的顺利出流。在结合多种优化方法的基础上,采用经过验证的数值模拟方法,对核主泵的叶轮进行优化设计。为了设计出在液相和低含气率下具有优秀水力性能的核主泵水力模型,选取叶轮进口直径Dj、叶片包角φ、叶片出口宽度b2、叶轮出口平均直径D2、叶轮出口安放角β2、叶片出口倾斜角γ和叶片数Z7个对核主泵外特性影响较大的因素进行正交试验设计。通过设计,得到了一组最佳的叶轮几何参数组合,以及7个因素对核主泵在液相和较低含气率工况下外特性影响的主次顺序;根据得到的参数对泵外特性影响主次顺序表,选取出口安放角和包角作为进一步优化的参数,通过进一步优化得到了使核主泵在液相和低含气率下具有优秀水力性能的最优叶轮几何参数组合。
[Abstract]:Reactor coolant pump (nuclear main pump) is one of the most important equipments in nuclear power plant, which provides the driving pressure head for nuclear reactor. In this paper, air is used to simulate the vapor-liquid two-phase flow instead of water vapor. According to the existing mature hydraulic model of nuclear main pump, the corresponding hydraulic prototype is designed and manufactured, and the experimental study is carried out on the hydraulic prototype. In order to understand the flow characteristics of the nuclear main pump under the gas-liquid two-phase flow condition, the steady and unsteady numerical simulation of the existing mature nuclear main pump model under different gas holdup conditions was carried out. The simulation results reveal the variation laws of gas distribution and velocity vector nephogram of the main nuclear pump under different air content: gas accumulation is easy to occur at the inlet and back of the impeller due to the action of centrifugal force and pressure gradient; gas trapping is easy to occur in the twisted radial guide vane structure; the spherical volute is easy to occur; the gas trapping phenomenon is caused by the twisted radial guide vane structure. The unsteady numerical simulation reveals that there is a certain positive correlation between the gas holdup and the uneven distribution of pressure at the impeller outlet, and that the uneven distribution of pressure at the impeller outlet and the amplitude of pressure fluctuation at the impeller working face are both affected by the higher gas holdup. In order to further understand the movement characteristics of the nuclear main pump under two-phase flow conditions, the hydraulic performance and the gas volume distribution in the flow passage of the nuclear main pump were analyzed under different air content in the reversed full working condition. With the increase of flow rate, the influence of void fraction on the torque characteristics of the nuclear main pump decreases gradually under the reverse flow braking condition and the reverse pump condition, but the reverse flow braking condition is just the opposite; under the reverse flow braking condition and the reverse pump condition, the high void fraction mainly concentrates on the blades. The gas volume fraction distribution in the impeller passage is extremely uneven in the region before the blade inlet. When the loss of water accident occurs in the loop, the nuclear main pump will operate under the condition of vapor-liquid two-phase flow. At this time, the performance of the nuclear main pump will have complex changes. Transient simulation. The influence of void fraction on the unsteady characteristics of the nuclear main pump is obtained by numerical simulation. With the increase of void fraction, the flow rate decreases continuously, the shaft torque and efficiency of the nuclear main pump decrease, and the small change law of the head value increases first and then decreases; with the increase of void fraction, the back of the impeller inlet and the middle back appear. High degree of gas accumulation occurs at the cover plate and the back of the outlet, and the fluctuation amplitude of gas holdup in the guide vane decreases gradually along the direction of liquid flow with obvious follow-up; the transient radial force decreases gradually with the increase of gas holdup and the difference between the maximum values of the two kinds of radial force decreases, and the transient pressure at the impeller and the tongue separation increases first and then decreases. The pressure in the guide vane increases gradually along the direction of liquid flow, but the fluctuation degree decreases. With the increase of air content, the low-speed region in the volute expands continuously, which indicates that the spherical volute is not conducive to the smooth outflow of gas. In order to design the hydraulic model of nuclear main pump with excellent hydraulic performance in liquid phase and low void content, the impeller inlet diameter Dj, blade envelope angle phi, blade outlet width b2, impeller outlet average diameter D2, impeller outlet placement angle beta 2, blade outlet inclination angle gamma and blade number Z7 are selected as the main factors which have great influence on the external characteristics of nuclear main pump. Orthogonal experimental design was carried out. Through the design, a set of optimum impeller geometric parameters and the order of seven factors affecting the external characteristics of the nuclear main pump in the liquid phase and low air content conditions were obtained. According to the sequence table of parameters affecting the external characteristics of the pump, the outlet placement angle and the envelope angle were selected as further optimization. Through further optimization, the optimal impeller geometric parameters combination was obtained to make the nuclear main pump have excellent hydraulic performance in liquid phase and low void content.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM623

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