自吸喷灌泵自吸过程的非定常流动数值计算与验证

Unsteady numerical calculation and validation on self-priming process of self-priming spray irrigation pump

为了建立一种适合自吸喷灌泵在自吸过程中的气液两相流数值模拟方法,采用ANSYS CFX软件,进行仿真自吸喷灌泵自吸过程的数值计算,研究自吸过程中气液混合、气液分离及气液逸出现象,分析自吸过程中泵内部的速度、压力及含气率的变化规律,了解自吸过程的气液两相流特点。自吸泵的整个自吸过程分为3个阶段:自吸初期由叶轮旋转作用排水产生的吸气阶段(t≤0.5 s)、自吸中期由气水混合及气水分离作用产生排气功能的吸气阶段(0.5 s<t≤4 s)、自吸末期由进口段的水进入泵体内部产生排气功能的吸气阶段(t>4 s)。自吸成功的关键就是叶轮的旋转迫使叶轮进口处少量的水混合着一部分气体沿着叶轮叶片压力面流动至叶轮出口处并进入正反导叶,然后沿着反导叶叶片压力面流出,表明在自吸过程中气水混合物总是沿着较高的压力面流动。通过采用摄影技术得到多级自吸喷灌泵在自吸初期及中期时泵出口段水柱高度的变化规律,发现试验结果与数值计算结果非常相近,不仅两者的变化规律基本相同,而且其结果相差不超过6%。

In order to establish one suitable numerical method of gas-liquid two-phase flow, based on ANSYS CFX software, the numerical calculation on the gas-liquid two-phase flow is conducted to simulate the self-priming process of the self-priming spray irrigation pump. The phenomena of gas-liquid mixing, gas-liquid separation and gas-liquid escape are studied, while the velocity, pressure and gas volume fraction in the pump are analyzed so as to understand the characteristics of gas-liquid two-phase flow. It＇s finally found the whole self-priming process is divided into 3 stages： the gas-suction stage due to the impeller＇s rotating role in the initial self-priming stage, the gas-suction stage due to the gas-water mixing and the gas-water separation role in the middle self-priming stage, and the gas-suction stage due to that the water flows from the pump import into the pump cavity in the last self-priming stage. Moreover, the self-priming time of the initial and last self-priming stages accounts for a small percentage of the whole self-priming process, but the self-priming speed is rather large. The middle self-priming stage is the main stage in the self-priming process, which determines the length of self-priming time. In the initial self-priming stage, the self-priming pump takes the drainage as the principle thing, and much gas mixing with water goes out together from the pump. With the gas rate of impeller increasing, the drainage ability of self-priming pump weakens gradually, and the gas-suction speed is first quick and then slows. After entering the middle self-priming stage, the large negative pressure exists on the impeller inlet, the water column in the import pipe goes up constantly, and its weight increases gradually, so the gas-suction speed slows with it. In the last self-priming stage, the water in the pump import goes into the impeller, whose power capability is enhanced obviously, and the water mixing with the gas goes to the pump export together, so the gas-suction speed of self-priming pump increases obviously and reaches the maximum value in a certain time. After that, the gas-suction speed is slower and slower and finally close to 0 due to that the gas is less and less in the pump. The key of successful self-priming is that the impeller＇s rotation forces a small amount of water mixed with some gas to flow along the pressure side of impeller blade from the impeller inlet to impeller outlet, then go into the positive diffuser, and finally effuse along the pressure side of return diffuser, which shows the gas-water mixture always flows along the high pressure side of the pump in the self-priming process. Finally, the gas-liquid separation of self-priming pump is observed by using the camera, and it is found that the experimental and the numerical results are very similar in the initial and middle self-priming stage; not only their changing law is almost the same, but also the difference of their values is not more than 6%. The result shows that it is rather credible to simulate the self-priming process of self-priming spray irrigation pump by using CFD.