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大型抽沙泵揚(yáng)程怎么計(jì)算
1. 計(jì)算域部分
理論上,離心泵從進(jìn)口到出口的流動(dòng)區(qū)域?yàn)槟Ｐ陀?jì)算域，可分為葉輪進(jìn)口段、葉輪、泵體前腔、泵體后腔、泵體(蝸殼)及出口段。通常的計(jì)算中常常忽略葉輪與蝸殼間軸向間隙，只取進(jìn)口段、葉輪、蝸殼及出口段作為計(jì)算域。計(jì)算域的空間離散也即計(jì)算網(wǎng)格劃分過程，可采用非結(jié)構(gòu)化四面體或結(jié)構(gòu)化六面體網(wǎng)格借助商業(yè)軟件Gambit、ICEM、Pointwise或Turbogrid等進(jìn)行。
2.動(dòng)靜區(qū)域耦合計(jì)算方法
由于計(jì)算區(qū)域中存在動(dòng)靜區(qū)域耦合問題(靜止的進(jìn)口區(qū)與旋轉(zhuǎn)的葉輪區(qū)，旋轉(zhuǎn)的葉輪區(qū)與靜止的蝸殼區(qū)域),通常對(duì)于定常情況下的計(jì)算采用多重參考系(Multiple Reference Frame)方法，假定葉輪不轉(zhuǎn)，僅在葉輪區(qū)域內(nèi)動(dòng)量方程計(jì)算中考慮因旋轉(zhuǎn)而額外產(chǎn)生的項(xiàng)，動(dòng)靜區(qū)域間設(shè)置交界面以傳遞動(dòng)量;對(duì)于非定常情況下的計(jì)算一般采用滑移網(wǎng)格技術(shù)(MovingMesh)來考慮動(dòng)靜區(qū)域的耦合問題。
3.湍流計(jì)算模型
湍流的數(shù)值模擬方法總體上可分為直接數(shù)值模擬和非直接數(shù)值模擬兩大類。直接數(shù)值模擬通過直接求解瞬時(shí)湍流控制方程來求解流場(chǎng)；而非直接數(shù)值模擬則需要將湍流做某種程度的近似和簡(jiǎn)化處理后，通過求解簡(jiǎn)化后的控制方程來求解流場(chǎng)。根據(jù)湍流瞬時(shí)控制方程的近似和簡(jiǎn)化方法不同，非直接數(shù)值模擬又分為大渦模擬與雷諾平均數(shù)值模擬。對(duì)于離心泵而言,其湍流模型的選取還需要考慮高速旋轉(zhuǎn)、流道間存在的較大逆壓梯度及葉片面與流道曲率的影響。
4.邊界條件設(shè)置
(1)對(duì)于泵的進(jìn)口:一般來說給定流量來計(jì)算揚(yáng)程，通常設(shè)置為速度進(jìn)口或質(zhì)量流量進(jìn)口:若需要根據(jù)揚(yáng)程來計(jì)算流量，則要給定進(jìn)口壓力(相應(yīng)出口給定壓力出口條件)，即進(jìn)口壓力條件。
(2)泵的出口:通常給定自由出流或壓力出口條件(計(jì)算中出口回流量較大時(shí)，選用壓力出口較為適宜)。
(3)固壁表面:無滑移壁面條件(旋轉(zhuǎn)葉片區(qū)內(nèi)壁面設(shè)置相對(duì)速度為零，蝸殼壁面設(shè)置絕對(duì)速度為零)。
選取某一單級(jí)單吸離心泵為例進(jìn)行CFD模擬,計(jì)算中選取進(jìn)口段、葉輪、蝸殼及出口段作為計(jì)算域，如圖1- 107所示，其空間離散后隔舌處局部網(wǎng)格如圖1- 108所示。
How to calculate the lift of large sand pump
1. Computing domain
In theory, the flow region from the inlet to the outlet of centrifugal pump is the model calculation domain, which can be divided into impeller inlet section, impeller, pump body front cavity, pump body rear cavity, pump body (volute) and outlet section. The axial clearance between impeller and volute is often ignored in the calculation, and only the inlet section, impeller, volute and outlet section are taken as the calculation domain. Unstructured tetrahedral or structured hexahedral meshes can be used in the spatial discretization of computational domain, and commercial software gambit, ICEM, pointwise or turbogrid can be used.
2. Coupling calculation method of dynamic and static regions
Due to the coupling problem between the static and dynamic regions (the static inlet region and the rotating impeller region, the rotating impeller region and the stationary volute region), the coupling problem between the static and dynamic regions exists, Usually, the method of multiple reference frame is used to calculate the steady state. Assuming that the impeller does not rotate, only the additional terms due to rotation are considered in the momentum equation calculation in the impeller area, and the interface is set between the static and dynamic regions to transfer momentum; In the unsteady case, the moving mesh technique is generally used to consider the coupling problem of the static and dynamic regions.
3. Turbulence calculation model
The numerical simulation methods of turbulence can be divided into direct numerical simulation and indirect numerical simulation. The direct numerical simulation solves the flow field by solving the instantaneous turbulence control equation directly, while the indirect numerical simulation needs to solve the flow field by solving the simplified control equation after the turbulence is approximated and simplified to some extent. According to the different approximation and simplification methods of instantaneous governing equations, indirect numerical simulation can be divided into large eddy simulation and Reynolds average numerical simulation. For the centrifugal pump, the selection of turbulence model also needs to consider the influence of high-speed rotation, large adverse pressure gradient between the flow channels and the curvature of blade surface and channel.
4. Setting of boundary conditions
(1) For the pump inlet: Generally speaking, the head is calculated by the given flow rate, which is usually set as the speed inlet or mass flow inlet: if the flow rate needs to be calculated according to the head, the inlet pressure shall be given (the corresponding outlet pressure is given, and the outlet condition), that is, the inlet pressure condition.
(2) Pump outlet: usually given free flow or pressure outlet conditions (when the outlet return flow is large, the pressure outlet is more suitable).
(3) Fixed wall surface: no sliding wall condition (the relative velocity of the inner wall of the rotating blade area is zero, and the absolute velocity of the spiral case wall is zero).
A single-stage single suction centrifugal pump is selected as an example for CFD simulation. In the calculation, the inlet section, impeller, volute and outlet section are selected as the calculation domain, as shown in Fig. 1-107, and the local grid at the tongue after spatial discretization is shown in Fig. 1-108.
