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渣漿泵葉輪流道內(nèi)的脫流
一、葉輪內(nèi)的脫流
在大多數(shù)抽送均質(zhì)液體的葉片泵內(nèi)，液流在遠(yuǎn)離最佳況時(shí)與葉輪葉片流線型表面脫流。如抽送磨蝕性固液混合物的葉輪內(nèi)，由于葉片數(shù)少和在葉片之間流道和軸向斷面流道相當(dāng)寬，不僅在小流量狀態(tài)，而且在最佳狀態(tài)，甚至在大流量狀態(tài)，都能觀察到這種現(xiàn)象。
在全蘇水力機(jī)械科學(xué)研究所對(duì)FpY160/31.5型泵葉輪試驗(yàn)時(shí)，根據(jù)專門確定試樣磨損形貌確認(rèn)了液流脫流區(qū)形成。由非金屬建筑材料科學(xué)研究所利用高速攝影機(jī)對(duì)rpY型泵抽送不同粒徑級(jí)配的固液混合物時(shí)拍照，也確定了這種現(xiàn)象。
I. H.烏達(dá)洛夫在莫斯科礦冶學(xué)院利用高速攝影機(jī)進(jìn)行了一批試驗(yàn)，確定了最佳狀態(tài)和小流量狀態(tài)時(shí)具有不同形狀葉片的試驗(yàn)葉輪中脫流區(qū)邊界。
對(duì)涂色水流拍攝照片得出這樣的結(jié)果，即在最佳狀態(tài)或者接近最佳狀態(tài)時(shí)脫流區(qū)形態(tài)與一般葉片葉輪在小流量狀態(tài)(0. 54Qam和0.375Qaum)時(shí)脫流區(qū)形態(tài)相似。
二、葉輪出口排擠系數(shù)的修正
現(xiàn)在研究渣漿泵葉輪內(nèi)脫流的條件，即根據(jù)葉輪和液流的參數(shù)考慮脫流區(qū)存在來估算葉輪出口排擠系數(shù)V2值。
首先求修正值，即存在脫流區(qū)時(shí)必須將其記在確定液流在葉輪出口處切向分速度的示意圖上。保持斯托多拉和麥捷里提出的基本假設(shè)。并采用下列補(bǔ)充假定：
站出，
(1)脫流從時(shí)片背面開始發(fā)生，在葉輪出口液流被排擠。這時(shí)液流在葉輪出口斷面面積(考慮葉片厚度和可能的脫流)為
（2）在決定葉輪出口葉片間空間內(nèi)流量的流束相對(duì)運(yùn)動(dòng)時(shí)，保持與無排擠時(shí)相同的方向，即限定脫硫區(qū)的表面平行于葉片移動(dòng)（在圖3-2-7上虛線EA）。
由渦漩引起的沿著葉輪出口邊液流的速度環(huán)量
假定表面AB和BC垂直于由渦漩引起的流線，并且是頂角A和C的等分線，點(diǎn)B是液體在葉片間空間范圍內(nèi)旋轉(zhuǎn)中心。用直線三角形面積代替曲線三角形ABC的面積，
式中t2——葉片節(jié)距；
R2——葉輪半徑；
Z——葉輪葉片數(shù)。
于是液體沿著葉輪出口邊平均速度為
式中u2——葉輪出口圓周速度。
用c2m代表不考慮脫流和葉片厚度所引起的排擠時(shí)液體在葉輪出口徑向分速度
當(dāng)存在液流拖流和考慮葉片有限厚度時(shí)，葉輪出口徑向分速度等于。
從葉輪出口速度三角形可以得到考慮有限葉片數(shù)和由脫流和葉片有限厚度引起的排擠時(shí)葉輪出口液流切向分速度值為
當(dāng)轉(zhuǎn)速和流量都恒定時(shí)，等式左邊具有最小值，即在排擠系數(shù)W:一定時(shí)葉輪出口切向分速度具有最大值。假定在給定的條件下，葉輪流道內(nèi)形成不同強(qiáng)度的脫流。作為假定，采用在供給能量最大條件下，即在切向分速度cxa增加最大時(shí)，液流在葉輪內(nèi)可能產(chǎn)生穩(wěn)定脫流。
因而，在葉輪內(nèi)形成穩(wěn)定脫流條件是
對(duì)于流量Q修正值，比較借助高速攝影機(jī)對(duì)3K一6型試驗(yàn)泵試驗(yàn)泵透明葉輪拍照得到的試驗(yàn)資料(HI.H. 烏達(dá)洛夫資料)與式(3-2-9) ~式(3-2-11)計(jì)算結(jié)果。根據(jù)烏達(dá)洛夫資料，在下列特性葉輪內(nèi)，在流量Q= 0.0078m2/s時(shí)觀察不到脫流區(qū): D.-0.26m, b,=0.016m, 4:=19. 8m/s, B-=15，z=4, 9=0.76. 在所研究情況下，根據(jù)式(3-2-10)得到
于是對(duì)應(yīng)無脫流葉片繞流的最小流量為
即采用計(jì)算方法確定Qo的誤差為4%左右。
在低比轉(zhuǎn)速挖泥泵上，在特殊情況下采用流道式葉輪。在這些葉輪出口液流排擠遠(yuǎn)大于葉片式葉輪，即。明顯低于葉片式葉輪。流道輪內(nèi)脫流區(qū)大小取決于非工作流道空間的相對(duì)尺寸;如果其尺寸大于可能脫流區(qū)尺寸，那么就不產(chǎn)生脫流。這種葉輪排擠系數(shù)可以根據(jù)式(3-2-9)確定，但必須預(yù)先根據(jù)葉輪幾何尺寸計(jì)算出o.如果根據(jù)公式得到的w2大于w2那么這就表明在給定工況下葉輪道之間沒有脫流，如果由式（3-2-9）確定的w2小與w0，那么w2小與1，并且在流道式葉輪內(nèi)產(chǎn)生脫流。在w0值如此很小時(shí)，即在幾何排擠很大時(shí)，脫流區(qū)尺寸明顯要小于葉片式葉輪。
在推導(dǎo)式(3-2-9)時(shí)，沒有引人工作狀態(tài)方面的限制，因此公式可以在所有狀態(tài)下下應(yīng)用。根據(jù)流量變化一倍以上的很大范圍內(nèi)系數(shù)w2計(jì)算值(3-2-9)和試驗(yàn)值(烏達(dá)落夫資料)的比較，可以得到滿意的一致性，因此在確定理論揚(yáng)程時(shí)，可以利用式(3-2-9)估算對(duì)應(yīng)泵特性曲線工作部分的所有狀態(tài)下葉輪流道內(nèi)脫流區(qū)尺寸。渣漿泵
De-flow in impeller runner of slurry pump
I. De-flow in impeller
In most vane pumps pumping homogeneous liquids, the flow is separated from the streamlined surface of the impeller blade when it is far from the optimum condition. For example, in the impeller pumping abrasive solid-liquid mixture, this phenomenon can be observed not only in the small flow state, but also in the optimum state, even in the large flow state, because of the small number of blades and the relatively wide flow passage between blades and the axial section.
When the impeller of FpY160/31.5 pump was tested by Quansu Institute of Hydraulic and Mechanical Sciences, the formation of liquid flow stripping zone was confirmed according to the wear morphology of specimens. The phenomenon was also determined by the use of high-speed camera by the Institute of Nonmetallic Building Materials Science when pumping solid-liquid mixtures of different particle sizes by rpY pump.
I. H. Udalov carried out a series of experiments at Moscow Institute of Mining and Metallurgy using high-speed cameras to determine the boundary of the detachment zone in the test impeller with different shape blades under the optimal and small flow conditions.
Photographs taken of the color-coated flow show that the shape of the stripping zone is similar to that of the general impeller blade in the small flow state (0.54Qam and 0.375Qaum).
2. Correction of extrusion coefficient at impeller outlet
Now, the condition of flow separation in the impeller of slurry pump is studied, that is, the extrusion coefficient V2 of the impeller outlet is estimated by considering the existence of flow separation zone according to the parameters of impeller and fluid flow.
Firstly, the corrected value must be recorded in the diagram of determining the tangential velocity of the liquid flow at the outlet of the impeller when there is a detachment zone. Keep the basic assumptions put forward by Stodora and McGerry. The following supplementary assumptions are adopted:
Stand out.
(1) De-flow begins at the back of the time sheet and is squeezed out at the outlet of the impeller. At this time, the area of the liquid flow at the outlet section of the impeller (considering the thickness of the blade and possible de-flow) is as follows
(2) When determining the relative motion of the flow beam in the space between the blades at the impeller outlet, the direction is the same as that without squeezing, i.e., the surface of the desulfurization zone is parallel to the blade movement (dashed EA in Fig. 3-2-7).
Velocity circulation of liquid flow along the outlet of impeller caused by vortices
Assuming that the surface AB and BC are perpendicular to the streamline caused by the eddy and are equal to the apex angle A and C, point B is the center of rotation of the liquid in the space between the blades. The area of curve triangle ABC is replaced by the area of straight triangle.
T2 - blade pitch;
R2 - impeller radius;
Z - Number of impeller blades.
So the average velocity of the liquid along the outlet of the impeller is zero.
U2 - the circumferential velocity of impeller outlet.
C2m is used to represent the radial partial velocity of liquid at the outlet of impeller when the extrusion caused by no consideration of the deflux and blade thickness is taken into account.
The radial partial velocity of the impeller outlet is equal to that of the impeller outlet when there is a liquid drag and the finite thickness of the blade is taken into account.
From the triangle of impeller outlet velocity, the tangential velocity values of fluid flow at impeller outlet can be obtained considering the number of finite blades and the displacement caused by the defLOW and the finite thickness of blades.
When the rotational speed and flow rate are constant, the left side of the equation has the minimum value, that is, the tangential velocity of impeller outlet has the maximum value when the extrusion coefficient W: is constant. It is assumed that under given conditions, different strength of the flow passage of the impeller will be formed. As a hypothesis, under the condition of maximum energy supply, that is, when the tangential velocity CXA increases to the maximum, the steady flow may occur in the impeller.
Therefore, the conditions for the formation of stable flow separation in the impeller are as follows:
For the flow Q correction value, the experimental data (HI. H. Udalov data) and the calculation results of formula (3-2-9) ~formula (3-2-11) obtained by high-speed camera photographing the transparent impeller of the 3K-6 test pump are compared. According to Udalov's data, no bleeding zone can be observed in the following characteristic impellers at flow Q= 0.0078 m2/s: D. -0.26m, b, = 0.016m, 4:= 19.8m/s, B-= 15, z = 4, 9= 0.76. Under the studied conditions, the bleeding zone can be obtained by formula (3-2-10).
So the minimum flow rate corresponding to the flow around the blade without detachment is as follows
That is to say, the error of Qo determined by calculation method is about 4%.
In the low specific speed dredging pump, the flow passage impeller is used under special circumstances. At the outlet of these impellers, the flow displacement is much larger than that of vane impellers. It is obviously lower than the vane impeller. The size of the stripping zone in the runner wheel depends on the relative size of the Non-Runner space; if the size of the stripping zone is larger than the size of the possible stripping zone, no stripping will occur. The extrusion coefficient of this impeller can be determined by formula (3-2-9), but it must be calculated in advance according to the geometric size of impeller. If the w_2 obtained by formula is larger than w_2, this indicates that there is no flow separation between the impeller passages under given working conditions. If w_2 determined by formula (3-2-9) is smaller than w_0, w_2 is smaller than 1, and in the flow passage impeller. Desulfurization occurs. When the W0 value is so small, that is to say, when the geometrical extrusion is very large, the size of the exit zone is obviously smaller than that of the vane impeller.
When deriving formula (3-2-9), there is no restriction on the working state of the introducer, so the formula can be applied in all states. Comparing the calculated value of coefficient W2 (3-2-9) with the experimental value (Udalov data) in a wide range with more than twice the flow rate variation, the satisfactory consistency can be obtained. Therefore, when determining the theoretical head, the size of the detachment zone in the impeller runner under all conditions corresponding to the working part of the pump characteristic curve can be estimated by formula (3-2-9).
