【作者】 郭关柱；

【导师】 范毓润；

【作者基本信息】 浙江大学 ， 机械电子工程， 2008， 博士

【摘要】 空化是液压传动中的一种常见现象,它的发生主要取决于液体的温度和压力,还受到液体的表面张力及抗张强度影响,空化的初生、发展和溃灭是空化的三个重要阶段。剪切空化是两个具有相对滑动壁面间液体破裂的一种重要形式,是液压传动、油膜润滑性能及流变测量等领域中的重要科学和技术问题。关于剪切空化的研究中,前面的研究人员采用的实验介质均为大分子量的高粘度液体,且未考虑液体含气量这一影响因素。对于剪切空化的形成原因,研究者通常采用主应力空化标准、剪切应力促进成核及剪切能成核来加以解释,而我们的研究表明,剪切空化的初生强烈依赖于液体中溶解气体的饱和度。论文分析了强剪切流空化的实验方案,针对外圆筒旋转内圆筒固定在较高雷诺数下的流动仍然是等压的稳定的纯剪切流的优点,提出了一种外圆筒旋转的可调压的可视化同心圆筒强剪切流变仪,解决了流变仪在设计及制造中的三个技术难题,即高转速下均匀小间隙的获得,高转速下的气动密封以及外圆筒内部尺寸、形状及位置精度的控制问题。研制出可调压外筒旋转同心圆筒可视流变装置,利用该实验装置测量了淡水、10cSt及16500cSt二甲基硅油的粘度,基于液体粘温关系及壁面绝热近似,获得液体粘度的理论值,通过理论值与测量结果的对比分析,初步校核了该装置在未安装测温传感器时测量结果的可靠性,实验还获得淡水及两种硅油发生剪切空化的实验证据。增加了设备的测温功能之后,柱坐标系中,利用实验获得的纯剪切力矩,基于均匀的间隙和瞬态传热微分方程,考虑传感器的热惯性,由数值计算方法求出不同时刻强剪切流的瞬态温度,结果表明理论计算值与实验结果较为吻合,校核了强剪切流的剪切力矩、间隙均匀性及瞬态温度测量准确性。对比分析表明,相比前面的初步校核方法,考虑传热后获得的结果精度高且适用范围广。基于现有测量气体在液体中溶解度方法及装置的不足,研制出一种测量气体溶解度的新型精密活塞式装置,测量了温度293K及353K,压力0～350kPa范围内的空气在500cSt二甲基硅油中的溶解度,获得本生溶解度对压力和温度的依赖关系,测量了298K时20cSt二甲基硅油及国产32#抗磨液压油的本生溶解度。精度及可靠性分析表明该测量装置获得的溶解度数据误差范围为6%。本生溶解度与气相压力呈较好的线性关系,以摩尔分数表示的溶解度与压力是非线性关系但可用Krichevsky-Ilinskaya方程拟合。发现分子量差异很大的二甲基硅油对空气的本生溶解度相同,提出基于聚合物链节基团摩尔数来表示摩尔浓度更有利于实验数据的外推和工程应用。对于分子量差别较大或无法确定分子量的溶剂,适宜用本生溶解度确定气体浓度与压力和温度的关系。经典成核理论将空化核分为均相成核和异相成核两类,对于过饱和溶液,通常将气态空化核分为四类。考虑液体表面张力对空化核的影响,基于力学平衡条件,讨论了球状空化核界面的平衡性。分析了气核内气体的热力学行为,基于相平衡条件,还讨论了气体扩散与空化核稳定性的关系。计算出不同浓度比时500cSt二甲基硅油中的临界核半径,开展了过饱和溶液中的成核实验,实验结果及理论分析表明,难于去除壁面凹槽内事先截留气体造成的异相成核,溶液单纯的过饱和状态并不能形成空泡,剪切作用对液体空化起到重要作用。阐述了剪切空化实验中需要解决的微小气泡的排除、照明光线处理、含气量控制及空化初生识别等方面的技术问题,利用强剪切流空化实验装置,开展了含气量对剪切空化初生的影响实验,获得不同饱和比下,液体发生空化时的相关数据,确立了饱和比与剪切空化初生P_eL、C_a及C_i数的经验关系。发现一定压力下空化发生的临界剪切应力与压力成正比,但远低于主应力空化标准的预测值,临界剪切应力与饱和度成反比。剪切空化初生后,剪切力矩下降导致粘性发热减少,是造成剪切流温度下降的原因,力矩变化与温度有一致性,但不敏感,提出温度变化是检测空化初生的有效手段的新观点。应用经典成核理论中关于空化核出现的概率由成核的自由能障碍决定,探索性解释增加剪切速率,可在欠饱和溶液中产生剪切空化的实验现象。更多还原

【Abstract】 Cavitation often appears in hydraulic pressure system. The influence of liquid temperature and pressure and surface tension and tensile strength is obvious. Bubble inception and grow and collapse is a particularly important process. As for the film rupture in the sliding solid wall, the influence of shearing cavitation is obvious. Of course, shearing cavitation has highly important effects on the hydraulic pressure system efficiency and the lubricating performance of film oil and rheology measurement. The former researcher had only used highly viscous liquid. Furthermore, they had not considered carefully the influence of the solubility of air in the solution. The researcher had explained the shearing cavitation by the principal normal stress cavitation criterion or the shear stress nucleation or the shear energy nucleation. The research by us indicated that the influence of the solubilities of air in the solution and the film stability is obvious.The experimental system principle and its structure and its function are indicated in this paper. Base on the techniques of high-precision designing and machining, a new Couette apparatus where its outer cylinder rotating with flow visualization and pressure control has been made. Utilizing the well-known viscosity-temperature formula and approximate adiabatic surroundings, the viscosities of water and 10cSt silicone oil and 16500cSt silicone oil at high shear rates measured by this apparatus agree well with that obtained the computed viscosities at various temperatures. At a certain shear stress, cavitation occurred for fresh water and two silicone oils with low and medium viscosities. After the thermocouple is located in the stationary cylindrical layer, in a Cylindrical coordinate system, with the experimental shearing torque and transient heat transfer differential equation, the transient computed temperature of high shearing flow are calculated by numerical equation. The transient computed temperature was consistent with the experimental temperature. It shows that the shearing torque and the symmetrical gap and the transient temperature of pressure fitting Couette flow apparatus where its outer cylinder rotating is transparent are precise. The elevated transient temperature of high shearing flow is calculated by the computed methods in the simple calculation. It indicated that the transient computed temperature by the transient heat transfer differential equation is precise and the accuracy verification way can be well fitted by many applications.Overcoming some drawbacks of previous methods measuring gas-solubility, a new piston apparatus was established in which the state equation of ideal gas is used to determine the amount of gas-dissolution. An expression of the Bunsen solubility of air in 500cSt silicone at temperature of 293.2 and 353.2 K and in the gas-pressure range of 0-350 kPa has been determined. Also measured is the Bunsen solubility of air in 20cSt silicone and Chinese 32# hydraulic oil at 298.2 K under various gas-pressures. Reliability and accuracy analyses indicate that the error range of the experimental data is about 6%. The Bunsen solubility and gas-pressure exhibit good linearity while the relationship between the molar fraction and the pressure in the experimental range is nonlinear but can be well fitted by the Krichevsky-Ilinskaya equation. It was discovered that the Bunsen solubilities of air in silicone with quite different molecular mass are closed with each other. We suggest that the molar fraction based on the monomer mass is more appropriate than that based on the molecular weight for engineering extrapolation of the solubility data of small and non-polar solutes in polymer solvents. At last, for the solvent containing much different or uncertain molecular weight, the commonplace in engineering applications, the Bunsen solubility is appropriate to give the dissolved gas concentration.There are two categories of nucleation about classical nucleation, i.e. homogeneous nucleation and heterogeneous nucleation. Four possible types of nucleation are described from gas cavities. Considering influence of liquid surface tension, equilibrium of a spherical nucleus has been discussed based on mechanical equilibrium of the interface. We analyzed the thermal behavior of the gas content. Gas diffusion and nucleus stability has been discussed based on diffusive equilibrium of the interface. The critical nucleus radii where the ratio of the dissolved concentration of air in 500cSt silicone is different have been calculated. The experiment about heterogeneous nucleation has been carried by us in supersaturated still solution. It is clear that the small crevices in solid alls or in solid particles carried by the liquid flow can shelter stable heterogeneous nuclei. Of course, gas cavities can not be formed in still liquid unless there is shearing flow.It is highly important to remove visible small air bubble and to get lighting and to control air in the solution and to select appropriate distinguishing cavitation inception ways in shearing cavitation experiment. This apparatus has been used in the cavitation inception experiment after the saturation ratio of air has been changed. We had given an empirical relationship between the saturation ratio and Peclet number and the Capillary number and the cavitation index. We found that critical shear stress of cavitation inception is roughly proportional to the applied pressure, but it is less than results of PNSCC. The critical shear stress of cavitation inception is roughly inversely proportional to the saturation. The transient temperature of shearing flow will decrease when the cavitation is appearing because quantity of heat is decreasing by shearing torque. The decrease of temperature is consistent with the shearing torque. But the torque is not sensitive. We think that the change of temperature is one of the effective ways about identification shearing cavitaion. According to the classical nucleation theory, the probability of occurrence of nuclei is determined from the free energy required for the formation of a critical bubble formation, the more high the saturation ratio of air in solution is, the more easily the shearing cavitation has been formed. In the gas-undersaturated liquid, shearing cavitation can be formed by increasing the shear rate.更多还原