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渣漿泵蝸殼、導(dǎo)葉及吸入室轉(zhuǎn)能裝置
蝸殼和導(dǎo)葉是離心泵的轉(zhuǎn)能裝置，它們的作用是把從葉輪甩出來的液體收集起來，使液體流速降低，把部分速度能頭轉(zhuǎn)變?yōu)閴毫δ茴^后，再均勾地引入下一級成經(jīng)過擴(kuò)散管排出。
1. 蝸殼
蝸殼的形狀通常是按照泵在設(shè)計流量下液體在葉輪中作穩(wěn)定的相對運動，離開葉輪后不受外力作用,按其慣性作自由流動的軌跡而做成。當(dāng)自葉輪流出的液體不受外力(摩擦力等)作用時，則液流對旋轉(zhuǎn)軸中心的動量矩保持不變.故有:
根據(jù)流體連續(xù)性方程，蝸殼任意半徑R處的徑向分速CRr (不計阻塞系數(shù)影響)為:
若蝸殼寬度不變，即bR為定值時，則在定常流動時有:
CRuR=常數(shù)
由此可見，液流在平行板式蝸殼內(nèi)作自由流動時，其流動軌跡的方向角不變，為一條對數(shù)螺旋線，如圖1-59所示。隨著半徑R的增加，對應(yīng)的CR與ce.均減小，故液流速度CR及速度能頭也將減小，而逐漸轉(zhuǎn)換為靜壓能。如果想使速度能充分轉(zhuǎn)換為壓力能，液流必經(jīng)流過較長的路程，徑向尺寸必然過于龐大。因此，限制蝸殼螺旋線的包角不大于360。為了減小徑向尺寸，根據(jù)cRr與蝸殼軸面寬度成反比的關(guān)系，用不斷擴(kuò)大的擴(kuò)張形軸面寬度，如圖1-60所示。這樣，液流方向角a不再是常數(shù),而是隨半徑R和6R增大而減小。但蝸殼截面的擴(kuò)張角0不應(yīng)大于60°，以免因蝸殼通流截面擴(kuò)大太快使液流發(fā)生嚴(yán)重的邊界層分離。蝸殼尺寸縮小后，液體在螺旋線部分只有小部分動能轉(zhuǎn)變?yōu)殪o壓能。為此，在螺旋線末端加一擴(kuò)壓管，其擴(kuò)張角為8°~12°，長度為擴(kuò)壓管進(jìn)口截面直徑的2.5~3.0倍。在擴(kuò)壓管內(nèi)可使80%~85%的動能轉(zhuǎn)變?yōu)殪o壓能。
蝸殼的截面有圓形、矩形和倒梯形幾種,其中，圓形截面用于高比轉(zhuǎn)速泵，倒梯形截面用于中比轉(zhuǎn)速泵,矩形截面用于低比轉(zhuǎn)速泵。
蝸殼結(jié)構(gòu)多用于單級離心泵和水平中開式多級泵中。
2. 導(dǎo)葉
導(dǎo)葉的作用與蝸殼相同，多用于分段式多級泵中，按其結(jié)構(gòu)形式,可分為徑向式導(dǎo)葉和流
道式導(dǎo)葉。徑向式導(dǎo)葉是由正向?qū)~、環(huán)形空間和反向?qū)~組成其結(jié)構(gòu)如圖1-61所示。 正向?qū)~內(nèi)螺旋線AB部分是按照設(shè)計工況下液體的自由流動軌跡得出的，用于收集液體和保證液體在葉道中自由流動;而擴(kuò)散段BC部分則用來把大部分動能轉(zhuǎn)變?yōu)殪o壓能;環(huán)形空間CD則用于改變液流方向。反向?qū)~DE的作用是消除旋轉(zhuǎn)速度，并把液體在無預(yù)選條件下引入下一級葉輪進(jìn)口。 導(dǎo)葉的葉片數(shù)與葉輪葉片數(shù)不應(yīng)相等，一般為4~7片。
流道式導(dǎo)葉如圖1-62所示，其結(jié)構(gòu)與徑向式導(dǎo)葉基本相同，所不同的是徑向式導(dǎo)葉從正向?qū)~出來的液體在環(huán)形空間內(nèi)混合在一起，之后進(jìn)入反向?qū)~。 而流道式導(dǎo)葉的正向?qū)~和反向?qū)~是鑄在一起的 ,中間形成單獨的小流道，各流道的液體不能混合,不易形成死角和突然擴(kuò)散，速度變化比較均勻,水力性能較好,但結(jié)構(gòu)復(fù)雜，制造工藝性差。
與蝸殼相比，導(dǎo)葉具有外形尺寸較小通用性大和制造方便等特點.因為它可以用數(shù)量不同、尺寸相同的導(dǎo)葉組成葉輪尺寸相同的分段式多級泵。但是采用蝸殼作為轉(zhuǎn)能裝置的中開
式多級泵，具有安裝、檢修方便和泵的高效率區(qū)較寬的優(yōu)點；而導(dǎo)葉作為轉(zhuǎn)能裝置的分段式多級泵，安裝、檢修不方便，高效率區(qū)較窄。因為在偏離設(shè)計工況時，液流對每個葉片都會產(chǎn)生沖擊損失，而在蝸殼中只有一個隔舌，所以導(dǎo)葉式泵的H-Q及n-Q性能曲線均比蝸亮泵要陡,平均效率也較低。
3.吸入室
吸入室位于葉輪前,其作用是將液體以最小的損失均勻地引入葉輪。吸入室有錐形管吸入室螺旋形吸入室和圓形吸入室三種形式。
(1)錐形管吸入室用于小型單級單吸懸臂式離心泵中,其結(jié)構(gòu)簡單、 制造方便，如圖1- 63(a)所示。在葉輪入口前使液流造成集流和加速度， 流速均勻，損失較小。
(2)螺旋形吸入室如圖1- 63(b)所示，流動情況較好，速度比較均勻,但液流進(jìn)入葉輪前有預(yù)旋,在一定程度上會降低揚程 ,對低比轉(zhuǎn)速泵,這種影響不明顯。目前，我國懸臂式離心油泵和中開式多級蝸殼泵都采用這種吸入室。
(3)圓形吸入室如圖1- 63(o)所示,結(jié)構(gòu)簡單，軸向尺寸短,但液流進(jìn)入渣漿泵廠家葉輪前有撞擊和旋渦損失，液流也不太均勻，常用于多級分段式離心泵中。
Energy transfer device for volute, guide vane and suction chamber of slurry pump
The volute and guide vane are the energy conversion devices of centrifugal pumps. Their functions are to collect the liquid thrown out from the impeller, reduce the liquid flow rate, change part of the speed energy head into the pressure energy head, and then all lead into the next stage to discharge through the diffusion tube.
1. volute
The shape of the volute is usually made according to the relative movement of the liquid in the impeller under the design flow of the pump. After leaving the impeller, it is not affected by the external force, and it is made according to its inertia as the path of free flow. When the liquid from the impeller is not affected by external forces (friction, etc.), the momentum moment of the liquid flow to the center of the rotating shaft remains unchanged
According to the fluid continuity equation, the radial velocity CRR at any radius r of volute (excluding the influence of blocking coefficient) is as follows:
If the width of the volute is constant, that is to say, when BR is a constant value, there are:
CRuR= constant
It can be seen that when the liquid flows freely in the parallel plate volute, the direction angle of its flow path is unchanged, which is a logarithmic helix, as shown in Figure 1-59. With the increase of radius r, the corresponding Cr and CE decrease, so the liquid velocity Cr and velocity head will also decrease, and gradually convert to static pressure energy. If we want to convert the velocity into the pressure energy, the liquid flow must go through a long distance, and the radial dimension must be too large. Therefore, the wrap angle of spiral is limited to no more than 360. In order to reduce the size of the minor diameter, according to the inverse ratio between the CRR and the width of the volute axial surface, the expanding axial surface width is used, as shown in Figure 1-60. In this way, the flow direction angle a is no longer constant, but decreases with the increase of radii R and 6R. However, the expansion angle 0 of the volute section should not be greater than 60 ° to avoid serious boundary layer separation due to the rapid expansion of the volute flow passage section. When the volute size is reduced, only a small part of the kinetic energy of the liquid in the helix changes into the static energy. For this reason, a diffuser is added at the end of the helix, its expansion angle is 8 ° ~ 12 °, and its length is 2.5 ~ 3.0 times of the diameter of the inlet section of the diffuser. 80% ~ 85% of kinetic energy can be converted into static energy in the diffuser.
The section of volute includes circle, rectangle and inverted trapezoid. Among them, circle section is used for high specific speed pump, inverted trapezoid section is used for medium specific speed pump and rectangle section is used for low specific speed pump.
Volute structure is mostly used in single-stage centrifugal pump and horizontal split multistage pump.
2. guide vane
The function of the guide vane is the same as that of the volute. It is mostly used in the segmented multistage pump. According to its structure, it can be divided into radial guide vane and flow
Channel guide vane. Radial guide vane is composed of forward guide vane, annular space and reverse guide vane. Its structure is shown in figure 1-61. Part ab of the helix in the forward guide vane is obtained according to the free flow path of the liquid under the design condition, which is used to collect the liquid and ensure the free flow of the liquid in the blade passage; part BC of the diffusion section is used to convert most of the kinetic energy into the static energy; and part CD of the annular space is used to change the direction of the liquid flow. The function of the reverse guide vane De is to eliminate the rotation speed and introduce the liquid into the inlet of the next stage impeller without preselection. The number of guide vane and impeller vane should not be equal, generally 4-7.
The runner guide vane is as shown in figure 1-62. Its structure is basically the same as the radial guide vane. The difference is that the liquid from the radial guide vane is mixed together in the annular space, and then enters the reverse guide vane. The forward guide vane and the reverse guide vane of the runner type guide vane are cast together, and a separate small runner is formed in the middle. The liquid in each runner cannot be mixed, and it is not easy to form dead angle and sudden diffusion. The speed change is relatively uniform, and the hydraulic performance is good, but the structure is complex, and the manufacturing process is poor.
Compared with the volute, the guide vane has the characteristics of small size, large universality and convenient manufacture, because it can be used to form a segmented multistage pump with the same impeller size with different number of guide vanes of the same size. But the spiral case is used as the middle opening of the energy conversion device
The multi-stage pump of type B has the advantages of convenient installation and maintenance as well as wide high efficiency area of the pump, while the segmented multi-stage pump of the guide vane as the energy conversion device is inconvenient for installation and maintenance, and the high efficiency area is narrow. Because of the impact loss of the liquid flow to each blade when it deviates from the design condition, and there is only one tongue in the volute, the H-Q and n-q performance curves of the guide vane pump are steeper than those of the bright volute pump, and the average efficiency is lower.
3. inhalation room
The suction chamber is located in front of the impeller, and its function is to introduce the liquid evenly into the impeller with the minimum loss. There are three types of suction chamber: spiral suction chamber and circular suction chamber.
(1) The conical tube suction chamber is used in a small single-stage single suction cantilever centrifugal pump, with simple structure and convenient manufacture, as shown in Fig. 1-63 (a). In front of the impeller inlet, the liquid flow causes the collection and acceleration, the flow velocity is uniform, and the loss is small.
(2) As shown in Fig. 1-63 (b), the spiral suction chamber has good flow condition and uniform speed, but there is pre rotation before the liquid flows into the impeller, which will reduce the head to a certain extent, and the effect on the low specific speed pump is not obvious. At present, cantilever centrifugal pump and split type multi-stage volute pump all use this kind of suction chamber.
(3) As shown in Fig. 1-63 (o), the circular suction chamber is simple in structure and short in axial dimension, but there is impact and vortex loss before the liquid flows into the impeller of slurry pump manufacturer, and the liquid flow is not uniform, so it is commonly used in multistage segmented centrifugal pump.
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