局部冷却抑制空化初生机理的数值及实验研究

Cavitation is a common problem in hydraulic machines. Cavitation inception occurs when the local pressure falls sufficiently far below the saturation vapor pressure at which fluid passes from the liquid to gaseous state for a given temperature. Cavitation can negatively affect the performance of hydraulic machines. When the cavitation bubbles collapse, they emit shock waves which are a source of noise. In the meantime, highly localized collapses can erode metals and severely damage the propeller's blades. An efficient cavitation suppression method not only can improve the performance of hydraulic machines and extend their useful lives but also can improve the submarine concealment. The main purpose of this project is to obtain a new method for suppression of cavitation: the vapor pressure of the liquid will be decreased by cooling down the temperature at a small surface area of blade where the local pressure is low. In this project, a numerical scheme recently developed by our research team and boundary conditions for conduction heat transfer will be applied for the simulation of cavitating flows. Therefore, the mechanism of cavitation inhibition using this cooling approach, the optimal cooling location and the influence of cooling temperature can be studied in depth. In addition, experimental studies will also be conducted in a microchannel and a 2D hydrofoil will be tested. A brass tube will be embedded on the surface of hydrofoil and the coolant which passes through the brass tube will be used to keep a portion of the hydrofoil surface cool. As a result, the unsteady cavitating flow field on the 2D hydrofoil before and after local cooling can be grabbed by using a high-speed photography technique with the aid of image processing system. Experimental results will be used to validate the numerical simulations and verify the effectiveness of the proposed cavitation suppression method.

空化是水力机械中常见的一种物理现象。当局部压力低于相应温度下液体的饱和压力时,液体会蒸发为蒸汽,导致空泡初生。空化的发生不仅降低了流体机械的性能,而且空泡溃灭瞬间会发出很大的噪声，同时产生巨大的剥蚀力导致叶片损坏，因此抑制空化初生不仅能够有效提高流体机械的性能,还能延长其寿命,并提高潜艇、军舰等军事装备的隐蔽性。本课题提出了一种抑制空化初生的新方法：即通过冷却叶片表面的局部区域,降低流场中压力较低处的温度,使相应位置处水的饱和压力降低,从而达到推迟或抑制空化初生的目的。本项目将利用课题组提出的可进行空化流场计算的新方法,采用带有热传导的边界条件,研究冷却抑制空化初生的机理；研究冷却位置、冷却强度对抑制空化初生效果的影响；并通过水筒实验,利用高速摄像及图像处理手段,在二维水翼吸力面适当位置,嵌入铜管,导入冷却剂,获取局部冷却前后的空化流场,验证数值模拟结果,证实局部冷却的确能抑制空化初生。

空化是水力机械中常见的一种物理现象。当局部压力低于相应温度下液体的饱和压力时，液体会蒸发为蒸汽，导致空泡初生。空化的发生不仅降低了流体机械的性能，而且空泡溃灭瞬间会发出很大的噪声，同时产生巨大的剥蚀力导致叶片损坏，因此抑制空化初生不仅能够有效提高流体机械的性能，还能延长其寿命，并提高潜艇、军舰等军事装备的隐蔽性。本课题提出了一种抑制空化初生的新方法：即通过冷却叶片表面的局部区域，降低流场中压力较低处的温度，使相应位置处水的饱和压力降低，从而达到推迟或抑制空化初生的目的。本项目采用以压力与焓为自变量的状态方程，建立了适用于空化流场模拟的水和水蒸气状态参数计算的简化方法，在此基础上，采用基于焓、速度与压力为原始变量的预处理方法，将可压流场与不可压流场的模拟统一起来，能进行高速与低速、单相与多相(含相变)、连续与间断(如激波或空泡)等复杂流场的模拟。研究特定翼型在一定的空泡数条件下，不同温度降低量、不同冷却位置对空化初生的影响，分析局部冷却后流场和温度场的结构特征，揭示局部冷却抑制空化初生的机理。在中型的空泡水筒中，利用铜管中通入冰水混合物，降低水翼吸力面上的温度，来冷却水翼表面上最低压力点附近的水。研究温度降低对空泡初生的抑制效果。