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泡沫泵葉片類型對理論揚程的影響
葉片類型對理論揚程的影響主要是分析葉片出口的葉片角B2A對理論揚程Hτo的影響。根據(jù)B2A的大小,將葉輪分為三種:B2A<90°時，葉片彎曲方向與葉輪旋轉(zhuǎn)方向相反，,稱為后彎葉片型葉輪[圖1- 11(a)];B2A = 90°時,葉片出口為徑向,稱為徑向葉片型葉輪[圖1- 11(b)];BA>90°時,葉片彎曲方向與葉輪旋轉(zhuǎn)方向相同，稱為前彎葉片型葉輪[圖1-11(c)]。
從式(1- 16)可以看出，對于一個給定的葉輪(D2和Bs為定值)，當(dāng)轉(zhuǎn)速n一定時，H與QT呈線性關(guān)系,其斜率與，有關(guān)，如圖12所示。
當(dāng)B=90”時H。與Q無關(guān)，在H-. H↑與圖上為一條水平線。
當(dāng)A<90°時，Hx-Q1是一條斜率為負(fù)值的向下的直線。
當(dāng)B2s>90°時，Hro-Qr是一條斜率為正值 β=90°的向上的直線。
由式(1-16)和圖1-12可以分析出，當(dāng)D2、n和Qr確定時，B2A越大,Hr。越大。是否這種葉片就越好呢?還必須進(jìn)一步分析HT。中的靜揚程0 QrHp與B2A的關(guān)系。
2. B2A與反作用度的關(guān)系
流體通過葉輪時使其能量增加,增加的能量分為動能和勢能(位置勢能和壓力勢能),相應(yīng)
的揚程也可分為動揚程和靜揚程。
由圖1-7的速度三角形得:
將上面兩式代入式(1-9)得到:
上式中的第一項為動揚程Ham,第二項和第三項之和為靜揚程Hpot。靜揚程指液體從葉輪葉片進(jìn)口流至葉片出口時靜壓能的增加，動揚程就是速度能的增加，由此有:
在考慮流體黏性的情況下，動揚程越大，流體速度越大,由于流動阻力與速度平方成正比，導(dǎo)致阻力損失很大,因此，工程中希望靜揚程所占比例高些。
靜揚程Hpor在理論揚程HTo中所占的比例稱為反作用度,用PRo表示:
根據(jù)速度三角形幾何關(guān)系可得:
在一般離心泵中，由于葉輪進(jìn)出口過流面積相差不大，即nD.b.T≈rD2brT2,C.變化不大，可認(rèn)為cIi,≈C2,且葉輪進(jìn)口無預(yù)旋時存在Gin =0，則有：
將式(1- 10)、式(1- 12)和式(1- 19)代入式(1-18),得：泡沫泵
分析式(1- 20)可以看出，隨B:n的增大反作用度PR減小,葉輪使液體獲得的靜揚程Hw在理論揚程HT中的比例減小，這是不希望的。因此，離心泵葉輪大多采用后彎葉片型葉輪，通常B,n=15°~40°。在以后的討論中,僅對后彎葉片型葉輪進(jìn)行分析。
Influence of blade type of foam pump on theoretical lift
The influence of blade type on the theoretical lift is mainly to analyze the influence of blade angle B2A at the blade outlet on the theoretical lift h τ o. According to the size of B2A, the impeller is divided into three types: when B2A is less than 90 °, the bending direction of blade is opposite to the rotation direction of impeller, which is called backward curved blade impeller [figure 1-11 (a)]; when B2A is 90 °, the outlet of blade is radial, which is called radial blade impeller [figure 1-11 (b)]; when Ba is more than 90 °, the bending direction of blade is the same as the rotation direction of impeller, which is called forward curved blade impeller [figure 1-11 (c)].
From equation (1-16), it can be seen that for a given impeller (D2 and BS are constant values), when the rotating speed n is fixed, h and QT show a linear relationship, and its slope is related to, as shown in Figure 12.
H when B = 90 ". It is independent of Q. it is a horizontal line on H -. H ↑ and graph.
When a < 90 °, hx-q1 is a downward straight line with a negative slope.
When b2s > 90 °, HRO QR is an upward line with a positive slope of β = 90 °.
According to formula (1-16) and figure 1-12, when D2, N and QR are determined, the larger B2A is, the HR is. The bigger. Is this blade the better? Further analysis of HT is necessary. The relationship between static head 0 qrhp and B2A in.
2. Relationship between B2A and reaction degree
When the fluid passes through the impeller, its energy increases, and the increased energy is divided into kinetic energy and potential energy (position potential energy and pressure potential energy), corresponding to
The head can also be divided into dynamic head and static head.
From the velocity triangle of figure 1-7, we can get:
Substituting the above two formulas into formula (1-9), we can get:
The first item in the above formula is dynamic head ham, and the sum of the second and third items is static head hpot. Static head refers to the increase of static pressure energy when the liquid flows from the impeller blade inlet to the blade outlet. Dynamic head is the increase of speed energy, thus:
Considering the viscosity of the fluid, the greater the dynamic head is, the greater the fluid speed is. Because the flow resistance is proportional to the square of the velocity, the resistance loss is very large. Therefore, it is hoped that the static head will account for a higher proportion in the project.
The proportion of the static head hpor in the theoretical head HTO is called the degree of reaction, which is expressed by PRA:
According to the geometric relationship of the velocity triangle, we can get:
In general centrifugal pump, because the difference of flow area between the inlet and outlet of impeller is not big, that is, nd.b.t ≈ rd2brt2, C. has little change, it can be considered that CI, ≈ C2, and there is gin = 0 when there is no pre rotation at the inlet of impeller, then there are:
The formula (1-10), type (1-12) and type (1-19) are substituted (1-18): foam pump.
It can be seen from formula (1-20) that with the increase of B: n, the reaction degree PR decreases, and the proportion of static head HW obtained by the impeller in the theoretical head HT decreases, which is not desirable. Therefore, the impeller of centrifugal pump mostly adopts backward curved blade impeller, usually B, n = 15 ° ~ 40 °. In the later discussion, only the backward curved blade impeller is analyzed.
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