绕空化器自然空化与通气空化的对比

本文采用高速全流场显示技术和动态应变式测力系统相结合对比了绕圆盘空化器自然空化流场与通气空化流场流动结构和影响因素。结果表明,自然空化和通气空化在流场形态上有着相同的发展过程:初生空化、带状空化、团状空化、半透明超空化和透明超空化;空化数是自然空化主要影响因素之一,通气量是通气空化主要影响因素之一。通气能够促进绕空化器空化流动发展程度。     

绕空化器自然空化流动的研究

本文采用高速全流场显示技术、片光源流动显示技术和应变式测力系统相结合研究了圆盘空化器模型的流场。结果表明:随着来流速度增加,绕圆盘流动经过无空化到初生空化的过渡过程后,出现了初生空化、带状空化和团状空化等不同空化阶段;绕圆盘流动阻力系数不断减小,出现了空化后阻力变化梯度降低;绕空化器的流场波动幅度不断加大,但空化器所受阻力主频基本稳定。     

绕水翼初生游离型空化流动结构

为了研究初生空化流动形态及其紊流流场结构,采用高速录像技术观察了绕Clark-Y型水翼初生空化的空化形态,应用LDV分别测量了无空化和初生空化条件下的紊流流场分布.结果表明,绕水翼小攻角无分离流动区域的初生空化形态呈游离发夹涡型空泡团结构,但其具有和单泡相同的发展过程;初生空化和无空化紊流流场的速度和紊流强度没有发现有规律性的差异,初生游离型空穴的形成与发展过程,对雷诺平均流场没有显著的影响.     

绕水翼云状空化流动特性的研究

采用高速全流场显示技术分别观测了绕超空化水翼和Clark-Y型水翼的云状空化.结果表明:绕超空化水翼和Clark-Y型水翼的云状空化具有相同的变化过程,即:生成、成长、膨胀、脱落和消失溃灭五种状态,两种空化云流动都具有明显的脱落周期和脱落轨迹.在翼型尾部存在的反方向射流,致使空化旋涡脱落;尽管模型尺度、来流速度和空化数基本相同,但由于超空化水翼与Clark-Y型水翼断面形状不同,使翼型尾部的反方向射流强度不同,故与来流相互作用强度不同,导致绕两种水翼的空化云脱落周期不同.在本文实验条件下,绕超空化水翼空化云和Clark-Y型水翼空化云的脱落频率分别为13.5 Hz和19 Hz.     

绕Hydronautics水翼空化旋涡的高速录像观察

为了解hydronautics超空化水翼的绕流流场特性,采用高速摄像机等流动显示设备,在空化水洞中针对此种超空化水翼进行了一系列绕流流场显示实验。实验结果表明,绕这种超空化水翼的空化形成与发展,均与产生于翼型前端 (吸力面)和尾端(压力面)的旋涡或二者的结合体:空化涡街有关;空化数的大小,决定了空化涡街形成与溃灭的不同特征;当空化数较低时,涡街中空化旋涡的扩张形成了闭合的超空化区域。     

绕Hydronautics水翼空化云流动的实验研究

采用高速全流场显示技术和DPIV对空化水翼近壁处的空化云流动形态和运动机理进行了讨论.结果表明:绕Hydronautics翼型的空化云运动是一个准周期性过程:稳定空泡团初生在翼型前部,向翼型后部发展布满整个翼面,在翼型后缘出现汽泡团旋涡,伴随反向运动,最终向下游脱落.当前条件下空泡团旋涡脱落周期约为74 ms.空化区与主流区的交界面上存在较大的速度梯度,一组对涡出现在翼型尾部处交界面上.此外,采用以空泡为示踪粒子的DPIV能够对空化流动流速分布进行有效测量.     

绕水翼片状空化流动结构的数值与实验研究

采用数值与实验相结合的方法研究了水翼片状空化流动结构.实验采用高速录像技术观察了片状卒泡形态,应用LDV测量了翼型周围的湍动能和速度分布;采用N-S方程和基于空泡动力学方程的空化模型计算了绕水翼片状空化流场.结果表明:在片状空化阶段,翼型吸力面上附着很薄的一层透明空泡,空泡彤态呈现于指状;随着空化数的减少,空泡尾部水汽交界面相互作用增强,并且空泡尾部出现大的旋涡,影响了空泡尾部区域压力和速度分布,片状空泡尾部的水汽混合区出现不稳定现象,同时存在小的空泡团脱落.数值模拟得到的水翼片状空化流动现象和实验观察到的结果基本一致,验证了计算模型和数值方法的可靠性.     

表面涂层对绕水翼空化流动影响的实验研究

本文采用高速全流场显示技术观测了两种涂层对绕Clark-Y型水翼空化流动的影响。研究中,随着空化数的降低,主要观测了绕水翼的初生空化、片状空化、云状空化三个阶段σ=1.50时,绕水翼A的空化流动已经发展至片状空化,绕水翼B的空化流动处于初生空化阶段。σ=1.19时,绕水翼A与水翼B空化流动均处于片状空化阶段,围绕两种模型的空穴长度接近,空泡破裂时形成了大量马蹄涡,水翼A表面的附着型锥状空穴产生了大尺度的涡团。σ=0.7时,绕水翼A和水翼B皆为云状空化阶段,绕水翼A的无量纲空化面积小于水翼B;反向射流导致大尺度涡团周期性脱落,绕水翼A的空化涡团脱落较为整齐,绕水翼B则产生分散的马蹄涡。     

混流式水轮机内部流场的三维空化湍流计算

本文通过理论分析,引入三维混合流体完整空化湍流模型,确定了混流式水轮机内部三维空化湍流计算的方法.用数值模拟手段对混流式水轮机的内部空化流场进行了计算,采用了全三维全流道的湍流计算方法,基于RNG的k-ε湍流模型和SIMPLEC算法,从蜗壳进口到尾水管出口,包含所有流道在内的整体一次完成定常.根据得到的三维空化流场数据,对转轮内部流场进行了流场分析,求得了计算临界空化系数,和试验值进行了比较;模拟了特征工况下的尾水管涡带,和试验数据进行了比较.     

非定常粘性空化流动模型及其数值计算

基于气液两相当地均相介质模型,本文给出了一种模拟非定常粘性空化流动的计算模型。认为空化绕流流场中流动介质是一种当地均匀的气液混相物。控制方程采用了应用两相流模型的N-S方程。基于液相和气相的状态方程推导了混合介质密度的表达式。为了保证数值计算的稳定性,控制方程的数值求解采用了TVD-MacCormack格式。为了评价计算模型的可靠性,分别计算了绕台阶和管道水锤的空化流动,所得结果是合理的,说明该方法可以用于空化流动的数值计算。     

Performance evaluation of a novel pilot-scale pinned disc rotating generator of hydrodynamic cavitation

This study investigates hydrodynamic performance of a novel pinned disc rotating generator of hydrodynamic cavitation in comparison with a serrated disc variant on a pilot-scale. Experimental results show that at a given rotational speed and liquid flow rate, the pinned disc generates more intense cavitation (i.e. lower cavitation number, higher volume fraction of vapor and higher amplitude of pressure fluctuations) than the serrated disc, while also consuming less energy per liquid pass (i.e., higher flow rate and pumping pressure difference of water at similar power consumption). Additionally, mechanical and chemical wastewater treatment performance of the novel cavitator was evaluated on an 800 L influent sample from a wastewater treatment plant. Mechanical effects resulted in a reduction of average particle size from 148 to 38 µm and increase of specific surface area, while the oxidation potential was confirmed by reduction of COD, TOC, and BOD up to 27, 23 and 30% in 60 cavitation passes, respectively. At optimal operating conditions and 30 cavitation passes, pinned disc cavitator had a 310% higher COD removal capacity while consuming 65% less energy per kg of COD removed than the serrated disc cavitator. Furthermore, the specific COD-reduction energy consumption of the pinned disc cavitator on the pilot scale is comparable to the best cases of lab-scale orifice and venturi devices operating at much lower wastewater processing capacity.     

Stability of slender axisymmetric ventilated cavities closing on cylindrical hulls

Analytical formulae were proposed to estimate the stability of slender axisymmetric ventilated cavities, which allow calculating the dependences of the critical cavitation number versus vapor cavitation number both for the disc and the slender cavitators. These formulae were applied to calculate the stability curves for different radii of the conical hulls located inside the cavity and different shapes of the cavitator, Froude numbers and directions of the ambient flow.     

Experimental investigation on underwater drag reduction using partial cavitation

For underwater drag reduction, one promising idea is to form a continuous gas or discrete bubbly layer at the submerged surface. Owing to the lower viscosity of gas than of water, this could considerably reduce underwater drag by achieving slippage at the liquid–gas interface. This paper presents an experimental investigation on underwater drag reduction using partial cavitation. Dense hydrophobic micro-grooved structures sustain gas in the valleys, which can be considered as defects that weaken the strength of the water body. Therefore, partial cavities are easily formed at lower flow speeds, and the dense cavities connect to form a lubricating gas layer at the solid–liquid interface. The results indicate that the proposed method achieves drag reduction without any additional energy or gas-providing devices, which should stimulate the development of underwater vehicles.     

Study on mechanism of chitosan degradation with hydrodynamic cavitation

Hydrodynamic cavitation is an effective method for chitosan degradation, of which the mechanism directly determines the molecular weight distribution of degradation products. In this study, based on the Monte Carlo simulation and experimental results, the mechanism of chitosan degradation with hydrodynamic cavitation and molecular weight distribution of products were analyzed. The results showed that the algorithm established in the simulation could effectively analyze degradation mechanism and the factors that influenced degradation mechanism and molecular weight distribution of products. The degradation with hydrodynamic cavitation was caused by chemical and mechanical effects, of which the former dominated the degradation process. The outlet and inlet angles and throat length of the cavitator had major and minor influences on the degradation pattern, respectively. The chemical effect led to random cuts resulting in wide distribution of the products, while the mechanical effect led to central cuts resulting in narrow distribution of the products. With more central cuts, the slide-shaped molecular weight distribution curve of degradation products was gradually transferred into a bell-shaped curve. These results provide instructions for researches on the molecular weight distribution of chitosan products degraded with hydrodynamic cavitation.     

三维小空化数空泡流数值方法

采用均质平衡流空泡模型,引入基于求解液体质量份数输运方程的空泡流模型,提出新的压力密度耦合的压缩性方法和控制气液两相转换的源项,求解汽水混合介质的RANS方程和带低雷诺数修正的k-ε模型,实现了小空化数(σ=0.2~0.01)下水下航行体的空泡流数值模拟.得到清晰的空泡形态特征与内部结构,以及空泡长度和最大直径随空化数的变化规律,给出航行体运动阻力系数与空化数之间的变化关系.将计算结果与解析结果及实验数据进行比较表明,该方法保证了极小空化数下自然空泡流计算的稳定性与收敛性,提高了空泡形态特性的预报精度.     

Experimental study on the characteristics of ventilated cavitation around an underwater navigating body influenced by turbulent drag-reducing additives

In this study, a new control strategy for turbulent drag reduction involving ventilated cavitation is proposed. The configurational and hydrodynamic characteristics of ventilated cavities influenced by turbulent drag-reducing additives were experimentally studied in water tunnel. The test model was fixed in the water tunnel by a strut in the aft-part. Aqueous solutions of CTAC/Na Sal(cetyltrimethyl ammonium chloride/sodium salicylate) with weight concentrations of 100, 200, 400 and 600 ppm(part per million), respectively, were injected into the ventilated air cavity from the edge of the cavitator with accurate control by an injection pump. The cavity configurations were recorded by a high-speed CCD camera. The hydrodynamic characteristics of the test model were measured by a six-component balance. Experimental results show that, within the presently tested cases, the lengths of cavity influenced by drag-reducing solution are smaller than normal condition(ventilated cavity) in water, but the asymmetry of the cavity is improved. The drag resisted by the test model is reduced dramatically(the maximum drag reduction can reach to 80%) and the re-entrant jet is more complex after the CTAC solution is injected into the cavity. Turbulent drag-reducing additives have the potential in enhancement of supercavitating asymmetry and further drag reduction.     

轴流式水力机械转轮中局限空化流动的面元奇点解法

轴流式水力机械转轮中局限空化流动的面元奇点解法刘四清，常近时（北京农业工程大学水利与建筑工程系北京１０００８３）关键词：空化，水力机械，转轮。ＳＯＬＵＴＩＯＮＴＯＰＡＲＴＩＡＬＣＡＶＩＴＡＴＩＯＮＦＬＯＷＩＮＩＭＰＥＬＬＥＲＯＦＡＸＩＡＬ－ＦＬＯＷＨ...     

轴流式水轮机内部空化流动的数值预测

将一种完整空化模型和一种混合流体两相流模型相结合,应用于轴流式水轮机全流道内定常空化流动的数值模拟。根据模拟结果,分别预测了水轮机在正常和较低尾水位情况下运行时,流道内空化发生的部位和程度,重点考察了尾水位的降低对水轮机空化性能的影响,为水轮机在特定运行工况下的空化性能预测提供参考。     

超声清洗装置空化噪声谱分析法空化噪声级测量

空化强度是用以衡量液体介质中空化活动的剧烈程度,同时空化效应在超声清洗中起关键作用,因此,测量超声清洗槽中的空化强度便可了解其中空化活动的情况.当发生空化时,液体介质中会产生成分复杂空化噪声,对空化噪声谱进行分析和计算得到空化噪声级,据此可判断空化强度.实验测得结果表明:超声清洗装置内稳态空化分布广泛、均匀,瞬态空化分...