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渣漿泵摩擦阻力損失
摩擦阻力損失(簡稱摩阻損失)指液體流經(jīng)吸入室、葉輪流道、蝸殼和擴(kuò)壓管(或?qū)~)時的沿程摩擦阻力損失以及液流因轉(zhuǎn)彎、突然收縮或擴(kuò)大等所產(chǎn)生的局部阻力損失。
由流體力學(xué)可知，當(dāng)黏性流體沿固體壁面流動時，流體流場可分為兩個區(qū)域，緊靠壁面很薄的一層稱為邊界層，在邊界層中必須考慮流體的黏性力，邊界層中的流動可看成黏性流體的有旋流動。邊界層雖然很薄,但沿其厚度方向流體速度急劇變化，它嚴(yán)重地影響著流體流動過程的能量損失及流體與壁面間的熱交換等物理現(xiàn)象。實(shí)驗(yàn)證明，流體的摩阻損失集中在邊界層中,邊界層以外的中心部分，黏性力很小，可以看作是理想流體的無旋流動。
摩阻損失ht通常用達(dá)西公式計算,即:
式中λ----沿程阻力系數(shù)， 與Re、流道表面相對粗糙度有關(guān)。
由于泵內(nèi)液體流速大，進(jìn)入阻力平方區(qū)以后,可認(rèn)為λ為一常數(shù).因此把全部摩阻損失看成與速度平方，即與流量的平方成正比，表示為:
式中C----與流道表面粗糙度及過流面積有關(guān)的系數(shù)。
將式(1-35)用曲線表示，如圖 1-29曲線6所示，是一 條過坐標(biāo)原點(diǎn)的二次拋物線。
2)沖擊損失
當(dāng)液流進(jìn)入液道(或?qū)~流道)時,液流相對運(yùn)動方向角β1與葉片進(jìn)口角β1A不一致，以及液體離開葉輪進(jìn)入轉(zhuǎn)能裝置的液流角a2與轉(zhuǎn)能裝置中葉片角ax不一致而產(chǎn)生沖擊所引起的能量損失，稱為沖擊損失。
眾所周知，離心泵是在一 定流量下設(shè)計的。葉輪葉片進(jìn)口角β1A是按設(shè)計工況計算的，所以泵在設(shè)計流量Qd下工作時液體進(jìn)入葉輪葉片的液流角β1與葉片角β1A相符，在葉片進(jìn)口速度三角形中，β1=β1A，則液流能平穩(wěn)地進(jìn)入葉輪流道,不產(chǎn)生沖擊。
當(dāng)泵的工作流量Q≠Q(mào)d時，例如Q<Qd; 進(jìn)口液流角β1;<β1A，因而液流便沖向葉片的工作面上，在非工作面上產(chǎn)生旋鍋，造成很大的能量損失。這種損失就是沖擊損失。沖擊損失的大小與葉片角β1A和液流角β1間的差值△β有關(guān)。△β稱為沖角，其定義為 △β=β1A-β1。當(dāng)Q<Qd時，△β>0;當(dāng)Q>Qd時，△β<0,如圖1-28所示。
沖擊損失大小可通過下面公式計算:
hsh=C2(Q-Qd)2
式中hsh----葉輪葉片進(jìn)口和壓液室中液流沖擊所造成的水力損失；
C2-----阻力系數(shù).與過流面積有關(guān)。渣漿泵廠家
將式(1- 36)用曲線表示，如圖1- 29曲線7所示。在設(shè)計流量時沒有沖擊損失，與設(shè)計工況點(diǎn)偏離越多，即工作流量小于或大于設(shè)計流量越多,沖擊損失越大。
由以上分析可知，葉輪給予液體的能量,其中有一部分用于克服從泵入口到排出口間過流部件的摩阻損失和沖擊損失，使得泵的實(shí)際揚(yáng)程H低于有限葉片理論揚(yáng)程Ht,即:
H=Ht-hr-hsh
Friction loss of slurry pump
Friction loss (referred to as friction loss) refers to the friction loss along the way when the liquid flows through the suction chamber, impeller passage, volute and diffuser (or guide vane), as well as the local resistance loss caused by turning, sudden contraction or expansion of the liquid flow.
According to the hydrodynamics, when the viscous fluid flows along the solid wall, the fluid flow field can be divided into two areas. The thin layer close to the wall is called the boundary layer. The viscous force of the fluid must be considered in the boundary layer, and the flow in the boundary layer can be regarded as the swirling flow of the viscous fluid. Although the boundary layer is very thin, the fluid velocity changes sharply along its thickness direction, which seriously affects the physical phenomena such as the energy loss in the process of fluid flow and the heat exchange between the fluid and the wall. The experimental results show that the friction loss of the fluid is concentrated in the boundary layer, and the central part outside the boundary layer has a small viscous force, which can be regarded as the irrotational flow of the ideal fluid.
The friction loss HT is usually calculated by Darcy formula, namely:
Where λ - resistance coefficient along the path, which is related to re and relative roughness of the channel surface.
Since the flow rate of liquid in the pump is large, λ can be considered as a constant after entering the square area of resistance. Therefore, the total friction loss is regarded as being proportional to the square of speed, that is, to the square of flow, which is expressed as:
Where C - coefficient related to surface roughness and flow area of flow passage.
Equation (1-35) is represented by a curve, as shown in Figure 1-29 curve 6, which is a quadratic parabola passing through the coordinate origin.
2) Impact loss
When the liquid flow enters the liquid channel (or guide vane channel), the energy loss caused by the impact is called impact loss because the relative direction angle β 1 of the liquid flow is not consistent with the inlet angle β 1a of the blade, and the liquid flow angle A2 of the liquid leaving the impeller and entering the energy conversion device is not consistent with the blade angle ax of the energy conversion device.
As we all know, centrifugal pump is designed at a certain flow rate. The impeller blade inlet angle β 1a is calculated according to the design working condition, so when the pump works at the design flow QD, the liquid flow angle β 1 entering the impeller blade is consistent with the blade angle β 1a. In the blade inlet speed triangle, β 1 = β 1a, the liquid flow can enter the impeller channel smoothly without impact.
When the working flow of the pump Q ≠ QD, for example, Q < QD; the inlet liquid flow angle β 1; < β 1a, so the liquid flow will rush to the working surface of the blade, which will produce a rotary pot on the non working surface, resulting in great energy loss. This kind of loss is impact loss. The impact loss is related to the difference △ β between blade angle β 1a and liquid flow angle β 1. △ β is called angle of impact, which is defined as △ β = β 1A - β 1. When Q < QD, △ β > 0; when Q > QD, △ β < 0, as shown in Figure 1-28.
The impact loss can be calculated by the following formula:
hsh=C2(Q-Qd)2
In the formula, HSH is the hydraulic loss caused by the impact of liquid flow at the inlet of impeller blade and in the pressure chamber;
C2 is the resistance coefficient, which is related to the flow area. Slurry pump manufacturer
Equation (1-36) is represented by curve, as shown in Figure 1-29, curve 7. There is no impact loss in the design flow, and the more deviation from the design operating point, that is, the more the working flow is less than or greater than the design flow, the greater the impact loss.
It can be seen from the above analysis that part of the energy given by the impeller to the liquid is used to overcome the friction loss and impact loss of the flow passage components from the pump inlet to the discharge port, so that the actual lift h of the pump is lower than the theoretical lift ht of the finite blade, that is:
H=Ht-hr-hsh
