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AF泡沫泵葉輪切割定律與切割拋物線
轉(zhuǎn)速恒定的一臺泵僅有一條H-Q特性曲線。為了擴(kuò)大泵的工作范圍.常采用切制葉輪外徑的方法，使其在一個轉(zhuǎn)遺下的工作范圍由”一條線"變?yōu)?/span>"一個面”。 葉輪切制前后性能參數(shù)的變化關(guān)系近似由切割定律來確定。
1.切割定律
當(dāng)葉輪切割量較小時，可認(rèn)為切割前、后葉片的出口角和通流面積近似不變，泵效率近似相等，即當(dāng)轉(zhuǎn)速恒定n'=n，(帶撇的參數(shù)代表切后的參數(shù))。這樣切割前后葉輪出口通度三角形近制相制。切割前、后的流量間關(guān)系為:
揚(yáng)程間的關(guān)系為:
功率間的關(guān)系為:
式(1-64)、式(1- 65)和式(1-66)為切割定律表達(dá)式。它們表明用減小D2的方法，可在泵轉(zhuǎn)速不變的情況下改變泵的特性曲線。
2.切割拋物線
同一臺泵,在同一個轉(zhuǎn)速下，葉輪逐次切割后，各對應(yīng)工況點的連線稱為切割拋物線。由式(1-64)及式(1- 65)可得:
將上兩式合并得:
式中，k是隨工況而變的常數(shù)。
式(1- 67)說明葉輪切割前、后的揚(yáng)程和流量的比例關(guān)系是一條過原點的二次拋物線方程，這條拋物線稱為切割拋物線。切割拋物線上的點是葉輪切割前、后的對應(yīng)工祝點,但并不是相似工況，因為切割前、后葉輪并不保持幾何相似。當(dāng)葉輪切割量不大時，認(rèn)為對應(yīng)工祝點效率相等，因此切割拋物線又稱為等效率線。
葉輪的切割量不能太大,否則通流面積及葉片出口角改變較大，效率明顯下降,使切割定律失效,故規(guī)定了葉輪的最大切割量。它與葉輪形狀(比轉(zhuǎn)速ns)有關(guān),見表1-8。AF泡沫泵
按表1-8規(guī)定,每臺泵都可以得到一條與最小許可切割直徑(D2m)相對應(yīng)的Q-H特性曲線，如圖1- 38所示。
Impeller cutting law and cutting parabola of AF foam pump
A pump with constant speed has only one H-Q characteristic curve. In order to expand the working range of the pump, the method of cutting the outer diameter of the impeller is often used to change the working range from "one line" to "one surface". The relationship between the performance parameters before and after impeller cutting is approximately determined by the cutting law.
1. Cutting law
When the impeller cutting amount is small, it can be considered that the outlet angle and flow area of the blades before and after cutting are approximately the same, and the pump efficiency is approximately the same, that is, when the rotating speed is constant, n '= n, (the parameter with skimming represents the parameter after cutting). In this way, before and after cutting, the impeller outlet clearance triangle is close to the phase system. The relationship between the flow before and after cutting is as follows:
The relationship between lift is:
The relationship between power is:
Equation (1-64), equation (1-65) and equation (1-66) are expressions of cutting law. They show that the characteristic curve of the pump can be changed under the condition of constant pump speed by reducing D2.
2. Cutting parabola
For the same pump, at the same speed, after the impeller is cut one by one, the line of each corresponding operating point is called cutting parabola. From formula (1-64) and formula (1-65), we can get:
Combine the above two formulas to obtain:
Where k is a constant varying with the operating conditions.
Equation (1-67) shows that the proportional relationship between head and flow before and after impeller cutting is a quadratic parabola equation passing through the origin, which is called cutting parabola. The points on the cutting parabola are the corresponding working points of the impeller before and after cutting, but they are not similar working conditions, because the impeller before and after cutting does not maintain geometric similarity. When the amount of impeller cutting is small, the efficiency of corresponding points is considered to be equal, so the parabola cutting is also called equal efficiency line.
The cutting amount of impeller should not be too large, otherwise the flow area and blade outlet angle will change greatly, the efficiency will decrease obviously, which will make the cutting law invalid, so the maximum cutting amount of impeller is specified. It is related to the impeller shape (specific speed NS), as shown in table 1-8. AF foam pump
According to table 1-8, each pump can get a Q-H characteristic curve corresponding to the minimum allowable cutting diameter (d2m), as shown in Figure 1-38.
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