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渣漿泵流動(dòng)狀態(tài)的分界速度
為了推廣實(shí)現(xiàn)懸浮液從一種狀態(tài)過(guò)設(shè)到另一狀態(tài)的條件，適當(dāng)引用與牛頓流體的雷諾數(shù)相似的無(wú)因次量。具有很大實(shí)際意義的是與過(guò)渡到自模流動(dòng)狀態(tài)相對(duì)應(yīng)的雷諾數(shù)Re.
在液流自模狀志轉(zhuǎn)變的瞬時(shí)，速度足夠大，因而壓力損失足夠大，以便可以利用損失的簡(jiǎn)化公式。
水力摩據(jù)系數(shù)，在流動(dòng)結(jié)構(gòu)狀態(tài)所確定的損失和根據(jù)達(dá)西——威斯巴赫公式計(jì)算的損失相等的條件下可以得到
結(jié)構(gòu)流動(dòng)狀態(tài)過(guò)渡到偽層流狀態(tài)的雷諾數(shù)對(duì)應(yīng)值稱為極限雷諾數(shù)Rep。
根據(jù)各種懸浮液(泥炭漿，磁鐵礦漿和黏土溶液)流動(dòng)試驗(yàn)研究結(jié)果得到:雖然懸浮液參數(shù)(密度，初始切應(yīng)力，結(jié)構(gòu)黏性)變化范圍很大，但雷諾數(shù)Rez值只在1950~3000之間相當(dāng)窄范圍內(nèi)變化(表2-3-1)。對(duì)于煤漿，根據(jù)B. B.特萊尼斯資料，從偽層流狀態(tài)過(guò)渡到紊流狀態(tài)所對(duì)應(yīng)的雷諾數(shù)Re接近4000。
在分界速度以、、加、o"時(shí)根據(jù)雷諾數(shù)Re"值確定流動(dòng)狀態(tài)邊界。目前只能推薦懸浮液從一種流動(dòng)狀態(tài)過(guò)渡到另一種狀態(tài)所對(duì)應(yīng)的雷諾數(shù)Re近似值根據(jù)B.特萊尼斯資料，對(duì)于什維多夫狀態(tài)過(guò)渡到突漢體狀態(tài)(0>01)和賓漢體狀態(tài)過(guò)找到偽層流狀態(tài)(o>o1)所對(duì)應(yīng)的煤漿雷諾數(shù)Re值分別等于10和3000
從嚴(yán)格結(jié)構(gòu)流動(dòng)狀態(tài)過(guò)渡到偽層流狀態(tài)時(shí)的流選稱為極限速度v，此速度與管徑、流速特性和懸浮液密度有關(guān)，并從Re"表達(dá)式確定這個(gè)速度
為了估算極限速度的數(shù)量級(jí)，引用H.莫吉列夫斯基不同密度即有不同就變待性的磁鐵礦在管徑D= 105mm管內(nèi)流動(dòng)資料
引用懸浮液極限流速的概念，對(duì)于分析和描述泵的工作過(guò)程是必需的。
在懸浮液速度超過(guò)速度v(參閱圖2-3-4)時(shí)，流動(dòng)狀態(tài)變?yōu)樗δΣ料禂?shù)入恒定的自模狀態(tài)。這種狀態(tài)具有很大意義，因?yàn)樵?a href="http://www.199mu.cn" target="_blank">渣漿泵的過(guò)流部件流道中，在工作狀態(tài)時(shí)大概會(huì)碰到。根據(jù)試驗(yàn)資料，與過(guò)渡到自模狀態(tài)所對(duì)應(yīng)的雷諾數(shù)Re值，對(duì)于黏土和煤漿，大于40000，對(duì)于磁鐵礦漿，大于11000。
懸浮液偽層流狀態(tài)，甚至在相當(dāng)大的速度時(shí)也相當(dāng)穩(wěn)定，如果θ和η很大。在圖2-3-4上用虛線示出清水在紊流狀態(tài)時(shí)損失曲線。從圖上可知，在具有較小流速的偽層流狀態(tài)時(shí)損失遠(yuǎn)大于它們假定具有同樣速度的紊流狀態(tài)時(shí)的損失。水泵在小于額定流量時(shí)工作，在葉輪流道內(nèi)可能產(chǎn)生懸浮液偽層流狀態(tài)或者甚至結(jié)構(gòu)流動(dòng)狀態(tài)，這將導(dǎo)致葉輪內(nèi)水力損失相對(duì)增大，在水泵特性曲線上稱為所謂的“塌陷”。 這種現(xiàn)象將在第三篇第六章中詳細(xì)研究。
Boundary Speed of Slurry Pump Flow State
In order to popularize the condition of suspension from one state to another, dimensionless quantity similar to Reynolds number of Newtonian fluid is appropriately quoted. The Reynolds number Re corresponding to the transition to the mode flow state is of great practical significance.
In the instantaneous transition of liquid flow self-pattern, the velocity is large enough, so the pressure loss is large enough to make use of the simplified formula of loss.
The hydraulic mooring coefficient can be obtained under the condition that the loss determined by the flow structure state is equal to that calculated by the Darcy-Wiesbach formula.
The Reynolds number corresponding to the transition from structural flow state to pseudo-laminar flow state is called the limit Reynolds number Rep.
According to the experimental results of various suspensions (peat slurry, magnetite slurry and clay solution), although the parameters of suspension (density, initial shear stress, structural viscosity) vary widely, the Rez number varies only in a narrow range from 1950 to 3000 (Table 2-3-1). For coal slurry, according to B. B. Trennis data, the Reynolds number corresponding to the transition from pseudo-laminar state to turbulent state is close to 4000.
The boundary of flow state is determined according to Reynolds number Re when the boundary velocity is -, plus -, o. At present, only the Reynolds number Re approximation corresponding to the transition of suspension from one flow state to another can be recommended. According to B. Trenis data, the Reynolds number Re values of coal slurry corresponding to the transition from Shdov state to Turkish state (0 > 01) and Bingham state (o > o1) are equal to 10 and 3000 respectively.
Limit velocity V is chosen when the flow state transits from a strictly structured state to a pseudo-laminar state. This velocity is related to the diameter of the pipe, the velocity characteristics and the suspension density. The velocity is determined from the Re e­xpression.
In order to estimate the magnitude of the limit velocity, the flow data of magnetite with different densities (i.e., different readiness) in pipe diameter D= 105mm are quoted.
It is necessary to introduce the concept of the limit velocity of suspension for analyzing and describing the working process of the pump.
When the suspension velocity exceeds the velocity v (see Fig. 2-3-4), the flow state becomes a self-model state with constant hydraulic friction coefficient. This state is of great significance, because in the flow passage of the flow components of the pump, it will probably be encountered in the working state. According to the experimental data, the Reynolds number Re corresponding to the transition to the self-model state is greater than 40 000 for clay and coal slurry and 11 000 for magnetite slurry.
The pseudo-laminar flow of suspension is quite stable even at considerable velocities, if theta and_are very large. On Fig. 2-3-4, the loss curve of clear water in turbulent state is shown by dotted lines. It can be seen from the graph that the loss in pseudo-laminar flow with small velocity is much greater than that in turbulent flow with the same velocity. When the pump operates at less than the rated flow rate, the pseudo-laminar flow of suspension or even the structural flow state may occur in the impeller passage, which will lead to the relative increase of hydraulic loss in the impeller, which is called "collapse" in the pump characteristic curve. This phenomenon will be studied in detail in Chapter 6 of Chapter 3.
