| 双吸泵空化性能改进 |
| 添加时间：2017/12/2 15:25:30 浏览次数： |
| 1 One 前 言 Preface 空化现象一直以来是离心泵面临的一个难题，空化会使泵的性能下降、腐蚀破坏过流部件、产生振动和噪声等，严重时泵将不能运行[1]。 因此有必要对离心泵汽蚀时的汽-液两相流进行深入研究。 Cavitation has always been a difficult problem for centrifugal pumps. Cavitation will make the pump's performance decline, corrosion and destroy over flow parts, generate vibration and noise. When it is serious, pump will not run [1]. Therefore, it is necessary to study the vapor-liquid two phase flow in the cavitation of the centrifugal pump. 基于经验的传统叶轮设计需要繁琐的模型试验，大大增加了设计成本和设计周期，制约了离心泵高空化性能优化设计的进一步发展[2] 。随着计算流体动力学( CFD) 技术在水力机械内部流场计算中应用日益广泛，CFD成为优化设计的重要工具。长期以来，国内外学者采用数值模拟的方法对水泵内部空化流动进行了很多研究[5,6]，并成功预测了空化临界点[7,8,9]和离心泵叶轮内发生的多区域空化流动现象等[10,11]。如Medvitz等[12]捕捉到了离心泵在偏流量工况的能量和扬程下降特性，Coutier Delgosha等[13]采用正压状态方程对3台离心泵进行定常空化流动计算，成功获取了泵的扬程下降曲线及空泡结构。 Based on experience, traditional impeller design needs complicated model test, which greatly increases design cost and design cycle, and restricts the further development of optimization design of [2]. With the increasing application of computational fluid dynamics (CFD) technology in the calculation of internal flow field in hydraulic machinery, CFD has become an important tool for optimal design. For a long time, domestic and foreign scholars have done many researches on the cavitation flow inside the pump by numerical simulation. [5,6] has successfully predicted the [10,11] of cavitation critical point [7,8,9] and the multi region cavitation flow phenomenon in the impeller of centrifugal pump. For example, Medvitz and [12] captured the energy and lift drop characteristics of centrifugal pump in partial flow condition. [13] and Coutier Delgosha used the barotropic equation of state to calculate the steady cavitation flow of 3 centrifugal pumps, and successfully obtained the pump head drop curve and cavity structure. 不难看出，上述离心泵的空化研究较多集中于对外特性的预测，目前对叶轮空化性能优化改进的相关工作虽然也做了不少，但仍有待于深入研究。本文应用汽液两相流混合模型基本方程，对一台比转速为100的双吸泵叶轮内部空化性能进行数值计算和分析，并通过修改叶轮入口直径、前盖板圆弧半径及叶片进口边形状来提升叶轮空化性能，旨在给出离心泵空化流计算中叶轮改进的合理建议。 It is not difficult to see that the cavitation research of the above centrifugal pump mostly focuses on the prediction of the external characteristics. At present, there are many works related to the optimization and improvement of the impeller cavitation performance, but still need further research. The basic equations of mixed model based on two-phase flow, a specific speed of 100 double suction pump impeller cavitation performance of numerical calculation and analysis, and by modifying the entrance of the impeller diameter, the shroud radius and blade inlet edge shape to improve the cavitation performance of the impeller, aims to give reasonable suggestions of centrifugal pump cavitation flow calculation improved impeller. 2 Two 基本参数及网格划分 Basic parameters and grid partition 本文计算模型为一台双吸泵，流量 QO=7000m3/h，扬程H=55m，叶片数Z=6，转速n =740rpm。计算域由吸水室、叶轮及压水室组成。划分网格时采用非结构化四面体网格，并在叶片头部及尾部，压水室隔舌处进行加密，计算域见图1。进行网格无关性检查时，当扬程的相对误差低于0.5%便可认为网格对计算结果无影响，最后确定计算所采用的网格总数为240万。 The calculation model of this paper is a double suction pump, the flow rate is QO=7000m3/h, the lift is H=55m, the number of blades is Z=6, and the speed is n =740rpm. The computational domain consists of a water suction chamber, a impeller and a water pressure chamber. The unstructured tetrahedral mesh is used when dividing the grid, and in the head and tail of the blade, the pressure water chamber is encrypted at the tongue, and the computational domain is shown in Figure 1. When checking the grid independence, when the relative error of the head is less than 0.5%, we can think that the grid has no influence on the result of calculation. Finally, the total number of grids calculated is 2 million 400 thousand. 1 One 边界条件设定 Boundary condition setting 边界条件在入口设为总压，进口处水的体积分数设为1，气泡的体积分数设为０。出口边界设为质量出口以控制模型的流量。通过调节进口压力改变进口的有效空化余量，从而控制水泵内部空化发生的程度。 The boundary conditions are set at the entrance to the total pressure. The volume fraction of the water at the inlet is set to 1, and the volume fraction of the bubble is set to 0. The exit boundary is set up as the quality exit to control the flow of the model. By adjusting the inlet pressure, the effective cavitation allowance is changed to control the degree of cavitation in the pump. 2 Two 空化模型及湍流模型 Cavitation model and turbulence model 空化计算应用均质多相模型和Zwart-Gerbe-Belamri空化模型来考虑空泡的生长与溃灭，介质的饱和蒸汽压力设置为3574Pa，空泡的平均直径为2*10-6m。湍流模型选用RNG模型，该模型最主要的优点为：考虑到壁面上大尺度分离的影响，能有效地处理高应变率及流线弯曲程度较大的流动，所以在预测流体机械中三维非定常流动，能得出很好的结果。 Cavitation calculation application of homogeneous multiphase model and Zwart-Gerbe-Belamri model to consider the growth and collapse of cavitation bubbles, medium saturation vapor pressure is set to 3574Pa, the average diameter of the cavity is 2*10-6m. The turbulence model used RNG model, the main advantage of this model is: considering the effect of wall on large scale separation, can effectively deal with the flow of high strain rate and streamline bending degree is bigger, so in the prediction of 3D fluid machinery in unsteady flow, can obtain very good results. 3 Three 计算结果分析 Analysis of calculation results 首先对双吸泵进行单相流场数值模拟，并以该计算结果为初始值，添加空化模型进行空化两相流计算。 First, the single phase flow field of double suction pump is simulated, and the cavitation model is added to the calculation of the cavitation two phase flow with the result of the calculation as the initial value. 1 One 原始叶轮空化性能预测 Prediction of cavitation performance of the original impeller 图2给出了数值计算所得扬程下降曲线。NPSHa值根据下式计算： Figure 2 gives the head drop curve obtained by numerical calculation. The NPSHa value is calculated according to the following formula: （1）式中:分别为叶轮进口压力，Pa；为工作介质密度，kg/m3；g为重力加速度，m/s2；为计算域进口断面的绝对速度，m/s2；相应温度下水的汽化压力，单位为Pa，文中主要在300K时取3574Pa。 (1) formula: impeller inlet pressure, Pa, working medium density, kg/m3, G and m/s2, the absolute velocity of the inlet section, m/s2, and the vaporization pressure of the water at the corresponding temperature, the unit is Pa. 空化计算收敛的标准通常以进出口流量差（允许的最大流量差≤0.4%）及残差曲线衡量，求解过程中不同空化余量下的流动收敛性都较好。可以看出：NPSH值逐渐减小的过程中，扬程也相应出现下降。 The convergence of the standard cavitation usually in the import and export flow difference (maximum flow rate allowed difference less than 0.4%) and residual curve measure, in the process of solving the flow convergence under different cavitation are better. It can be seen that in the process of gradual decrease of NPSH value, the lift also decreases accordingly. 图3(a)给出了数值计算所得叶轮内空泡的分布（黄色区域为空泡，汽相体积分数等值面取5％），可见空化主要发生在叶片吸力面头部靠前盖板附近，并向流道中心延伸。图3(b)所示现场拍摄的图片中，叶片金属表面几乎在相同位置显现蜂窝状，这主要由空泡溃灭引起局部高速水流打击金属而使其表面疲劳破坏形成，这些蜂窝孔一般与外部相通，且大多数坑槽与都与金属表面垂直。上述分析不难说明计算所采用的空化流数值方法在捕捉空泡现象时具有一定的准确性。 Figure 3 (a) gives the distribution of cavitation in the impeller calculated by numerical calculation (the yellow area is cavitation, and the volume fraction of vapor volume fraction is 5%). The cavitation is mainly located near the front side of the suction surface of the blade and extends to the center of the runner. Figure 3 (b) shows the scene in the picture, the blade metal surface almost in the same position appear honeycomb, which mainly by the bubble collapse caused by local high speed flow against the metal of the surface fatigue failure form, the general honeycomb hole which is communicated with the outside, and most of the pit with a metal surface and vertical. The above analysis is not difficult to explain the accuracy of the cavitation flow numerical method used in the calculation of the cavitation phenomenon. 工程上规定：能量参数下降某一百分数( 1～3％) 时的NPSH值为水力机械的空化余量的临界值，记为 NPSHcr。现取扬程下降3％的点为临界空化余量，得到允许空化余量[12]。 It is stipulated in the project that the NPSH value of a percentage (1 to 3%) of the energy parameter decreases as the critical value of the cavitation allowance of the hydraulic machinery, which is recorded as NPSHcr. The point of 3% of the head drop is the critical cavitation allowance, and the allowable cavitation allowance [12] is obtained. NPSH=NPSHcr+k NPSH=NPSHcr+k （2）式中k取0.3 。该水泵的NPSHcr=6.8m（图2中工况D），因而选用的双吸泵空化余量NPSH=7.1m。综合看来，该水泵的空化性能略差。实际应用中，通常采用修整叶片头部的方法来改善水泵的空化性能，具体做法是将叶片头部背面修薄，原理是减低叶片进口的水流速度和叶轮进口排挤，从而提高泵的抗空化能力。此外，采用耐空化材料，如铜合金、合金铸钢等，也不失为一种提高叶轮空化性能 (2) the K in the formula takes 0.3. The NPSHcr=6.8m of the pump (D in Figure 2), thus the choice of the double suction pump cavitation allowance NPSH=7.1m. In a comprehensive view, the cavitation performance of the pump is slightly poor. In practice, the method of trimming the blade head is usually used to improve the cavitation performance of the pump. The specific way is to trim the back of the blade head, and the principle is to reduce the inlet speed and the impeller inlet, so as to improve the cavitation resistance of the pump. In addition, the cavitation resistance of the impeller is also improved by using the cavitation resistant material, such as copper alloy and alloy cast steel. |
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