上游气泡作用下凝水泵内部空化的复杂表征现象与机理研究

Bubbles intensify the cavitation phenomenon in the condensation pump, which serves as the primary motivation of the proposed investigation. Flows passing around a hydrofoil are studied as the first step to explore the interaction mechanism between bubbles and cavity zones. And then the study is extended into the flows in a condensation pump with the contributions of bubbles and blade shape being stressed. Time resolved particle image velocimetry (TRPIV) is used to measure time-dependent velocity distributions near the hydrofoil under non-cavitating conditions. Thereby the background flows are illustrated. Bubbles are released upstream of the hydrofoil, and simultaneously, flow parameters in the loop are adjusted to trigger cavitation. Shadow image technique is used to quantitatively observe bubbly flow patterns, cavity zone profiles, and the interaction behavior associated with bubbles and cavity zones. Bubbles in the model pump are generated through injecting air at the pump inlet pipe. With rotational speed and inlet pressure being geared, cavitation is induced in the pump. Optical endoscopy technique is utilized to visualize the evolution of bubbles and cavitation with the rotation of the pump impeller near blade inlet. Vibration acceleration sensors and miniature dynamic pressure sensors are deployed to measure pump vibration and pressure pulsations, respectively. Impellers with different blade inlet shapes are devoted to the experiment. The mechanism behind the influence of bubbles and body surface on cavitation evolution is expected to be revealed with the presented work. The conditions for the cavitation inception exposed to bubbles are to be defined. And global and transient cavity zone profiles are to be depicted. The interaction between bubbles and cavity zones is to be modeled as well. Thereby, a significant support highlighting the relevant mechanism is anticipated to be provided for deeply understanding complex cavitation phenomena in condensation pumps and establishing corresponding design approaches.

针对凝水泵内气泡加剧空化的现象，从水翼绕流入手探究气泡与空化区的作用机制，进而以泵内流动为载体研究气泡和叶片形状对空化现象的影响。运用时间分辨的粒子图像测速技术测量水翼周围无空化时的瞬态速度分布，描述背景流场特征；在水翼上游释放气泡，调节回路中的流动参数以诱发空化，运用阴影图像技术，对泡状流型、空化区形态、气泡和空化区的相互作用行为进行定量观测。在模型泵吸入段通入空气形成气泡，调节泵转速和泵入口压强，运用光学内窥技术观测叶片进口边附近的气泡和空化区形态随叶轮旋转的变化；应用振动加速度传感器和微型动态压力传感器分别测量泵的振动和流场压力脉动；采用叶片进口边形状不同的叶轮进行实验。通过研究，拟揭示气泡和绕流体边界对空化演化过程的影响机制，定义气泡存在时空化的发生条件，描述空化区的宏观瞬态特征，模化气泡与空化区的相互作用，为深度理解凝水泵内的复杂空化现象和构建相应的设计方法提供机理层面的支撑。

针对凝水泵内的空化问题，以“气”相的存在对空化产生的“汽”相的影响为切入点，开展了从气泡运动、液流中通气产生的气液两相流动、翼型附近的气泡与空化泡相互作用、凝水泵内的空化与泵性能之间的关系等系统性的研究工作。搭建了专门的实验台，开发了图像处理代码，验证了数值计算模型，对不同工况条件下的研究结果进行了分析、对比、归纳与统计。对主要研究结果概述如下：.(1) 补充了对载流液体对气泡群的影响的理解。静止的载流液体中气泡的几何及运动特征与气泡尺寸密切相关；在圆柱尾流中，大尺寸气泡多出现在尾流中心线附近，而尾流边缘的剪切促使气泡破碎。喷嘴尾流中，气泡群的瞬时形态具有尾流的特征，且小气泡对液流变化的响应更为迅速。.(2) 通过向流动的液体中通入气体获得了新的流型。来流速度和通气量是影响气相区形态和演化特征的关键。当通气量较低时，喷嘴尾流抑制气相区的生成，增大通气量后，在喷嘴附近出现附着气相区，且伴随着气泡自该气相区下游端脱落的现象。通过来流速度与通气量的合理匹配，可能获得较强的气相区稳定性。.(3) 解释了凝水泵内空化发生的机理。揭示了叶轮进口流动结构对低压生成的影响机制。通过改变叶片形状提高抗汽蚀性能，扩大了叶轮进口处的流体回转空间，所设计的5长5短叶片方案，其必需汽蚀余量低至1.0 m以下，为当前叶片泵汽蚀性能中的较高水平，且压力脉动的特征频率得到抑制。.(4) 描述了翼型吸力面和尾流中的气–汽–相互作用。当空化数下降时，翼型吸力面出现空化云，气体与空化云之间相互掺混。大攻角时，翼型表面出现脱流，气泡和自然空化泡在脱流区充分混合。低来流速度时，气泡脱落频率随通气量增加而增大；高来流速度条件下，气泡脱落频率无规则变化，多相共存区失稳。.本研究系统地解释了气体、空化、液流之间的相互作用，拓展了对空化多相流动的认识，发现的特征流型为数值模型的有效性验证提供了依据，形成了抗汽蚀设计方法，深化了对复杂流动影响流体机械性能的理解。