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Numerical and experimental investigations of the unsteady cavitating flow in a vortex pump
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
319
2010, 22(5), supplement :324-329 DOI: 10.1016/S1001-6058(09)60213-4
Numerical and experimental investigations of the unsteady cavitating flow in a vortex pump Alexander Steinmann1, Hendrik Wurm2, Alfred Otto3 1,2
Group Research and Technology Center – Fluid Mechanics, WILO SE Nortkirchenstr. Germany E-mail: [email protected] , [email protected] 3 WILO EMU GmbH , Germany E-mail: [email protected]
ABSTRACT: In order to develop an understanding of the flow conditions in Vortex Pumps, numerical investigations (commercial URANSE-CFD-Solver ANSYS CFX, Volume-ofFluid Rayleigh-Plesset cavitation model, eight impeller revolutions) and experimental investigations (High-Speed Exposures of the cavitation clouds through acrylic glass window) have been carried out. A further objective was to investigate the URANSE-CFD method using the mentioned cavitation model regarding numerical stability and accuracy. KEY WORDS: vortex pump, centrifugal pump, sewage water pump, cavitation, CFD, URANSE
1 INTRODUCTION Today there is still no detailed knowledge about the flow fields in Vortex Pumps (sewage water pump). The highly unsteady flow conditions and the typical occurrence of cavitation make flow investigations complex and extensive, so that they are very rare. The unsteady flow behavior is caused by the non blade congruent flow in the impeller. In the impeller side room a potential vortex is present. It has to be identified whether the transformation from mechanical power to hydraulic power is basically driven by an exchange of fluid between impeller side room and impeller or by the interaction of turbulence structures with the main flow. In case of fluid exchange this could be simple flow-through from leading to trailing edge or a flow with helical reentries in the impeller blade channels around the blade tips. Generally these
are the same open questions as for Side Channel Pumps. In order to develop an understanding of the flow conditions as well as to maximize the efficiency and to minimize the cavitation occurrence of vortex pumps, numerical and experimental investigations have been carried out. The numerical investigations have been carried out by means of the commercial URANSE-CFD-Solver ANSYS CFX. The transient flow field was captured with eight impeller revolutions (full geometry including casing). The cavitation behavior was estimated using the Volume-of-Fluid Rayleigh-Plesset cavitation model. The experimental investigations consisted of measurements of the characteristic curves and HighSpeed Exposures of the cavitation clouds. These photographs were taken through an acrylic glass window, which was integrated in the casing suction cover and the suction pipe of the Vortex Pump. Beside the determination of the flow conditions in Vortex Pumps another objective of the present work was to investigate the URANSE-CFD method with the mentioned cavitation model regarding numerical stability and accuracy. 2 VORTEX PUMP Vortex Pumps are preferably used for sewage water
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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
transport especially in case of high gas or fiber content in the pumped liquid. The vortex impeller (Fig. 1) consists of a rotating disc (supporting disc) on which curved blades are arranged. In front of the impeller there is a relatively large free space to the casing bottom (Fig. 2), so that a big passage can be realized for the solid matter in the pumped liquid.
Fig. 3 Flow geometry of the investigated Vortex Pump
Fig. 1 Impeller of the investigated Vortex Pump
ANSYS CFX uses the element based finite volume method and an algebraic multigrid approach. The implemented cavitation model uses the Volume-ofFluid model for the formulation of two phase flow. The source/sink term for the vapor phase is estimated via the linearized Rayleigh-Plesset equation. A Tetra/Prism mesh with 5.5 mio nodes was generated. For the advection scheme the hybrid scheme High Resolution was applied. For the transient scheme the Second Order Backward Euler method is used. The convergence criteria were: max. residuals 1.0e-3 and conservation target 0.1%. The timestep for the transient calculation (eight impeller revolutions) represented 1° of rotation. So 2880 timesteps were calculated. Every time step consisted of 8 to 22 inner coefficient loops. With the mentioned timestep the RMS Courant Number followed as 0.4 and the max Courant Number to 3.8.
Fig. 2 Casing of the investigated Vortex Pump
Due to the arrangement of the impeller in the pump casing and the free space below, the impeller acts only on a part of the pumped liquid and consequently guarantees high operation reliability in the station. Despite the lower efficiencies compared with channel impellers, pumps with vortex impellers are an important part in the product range of each sewage pump manufacturer. 3 NUMERICAL INVESTIGATIONS The commercial URANSE-CFD-Solver ANSYS CFX 11.0 was used. The flow geometry of the investigated Vortex Pump is shown in Fig. 3.
Two loads of the pump were calculated BEP and overload (Q/Qopt = 1.9). For both loads NPSH = 10 m were set. For the BEP Fig. 4 to Fig. 10 show pressure distributions, velocity vectors, and a streamline plot for the last timestep (with steady state calculation no convergence was achieved).
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
321
Fig. 4 Pressure distribution in the mid cut plane of the impeller
Fig. 7 Relative velocity vectors in the mid cut plane of the impeller
Fig. 5 Pressure distribution in the mid cut plane of the impeller side room
Fig. 8 Absolute velocity vectors in the mid cut plane of the impeller side room
Fig. 6 Pressure distribution in a meridional plane
Fig. 9 Relative velocity vectors in a meridional plane
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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
Fig. 10 Relative streamlines colored by the relative velocity
It becomes obvious that there is quite a primary flow through the impeller channels from leading to trailing edge. An appearance of helical reentries of the flow in the impeller blade channels around the blade tips cannot be found considerably. There is no evidence for a turbulent momentum exchange.
Fig. 12 Pressure distribution in the mid cut plane of the impeller side room
Beside this for the BEP no cavitation occurs. Pump head and shaft power are determined by averaging of the last impeller revolution. These two calculated values are each approximately 6% higher than the measured integral values (measured shaft power adjusted by hub friction). For the overload conditions Q/Qopt = 1.9, Fig. 11 to Fig. 18 show pressure distributions, velocity vectors, a streamline plot, and the cavitation region.
Fig. 11 Pressure distribution in the mid cut plane of the impeller
Fig. 13 Pressure distribution in a meridional plane
Fig. 14 Relative velocity vectors in the mid cut plane of the impeller
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
Fig. 15 Absolute velocity vectors in the mid cut plane of the impeller side room
323
Fig. 18 Cavitation region
Cavitation occurs at the suction pipe, leading edge and trailing edge. The values of the vapor volume fraction vary between 0 and 10% and therefore the cavitation occurrence is small. The isosurfaces for the cavitation regions in Fig. 17 and Fig. 18 show the 2% border of the vapor volume fraction. The two calculated integral values pump head and shaft power are each approximately 9% lower than the measured integral values. 4 EXPERIMENTAL INVESTIGATIONS The test bench with the acrylic glass window for the experimental investigations is shown in Fig. 19.
Fig. 16 Relative streamlines colored by the relative velocity
Fig. 19 Test bench for the experimental investigations
As camera a video camera LEGRIA HF 200 with a resolution of 1920x1080 was applied. In addition a stroboscope Hofmann SWM-2 was used to provide the adequate light for the high speed exposures. Fig. 17 Cavitation region
For BEP no cavitation occurs. For overload (Q/Qopt = 1.9) conditions Fig. 20 show typical exposures of the cavitation clouds.
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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China exclusively in the impeller channel which is in 3 to 4 o’clock position (pressure pipe at the top). Shape and style of the cavitation are highly fluctuating. On one side there are downstream oriented cavitation vortex filaments and on the other side compact cavitation clouds which extend to the whole channel width. Origins are the leading edges of the blades as well as the edge of the hub. The collapse of the cavitation is located downstream of the origins inside a concentric circle with approx. 1/3 of the impeller diameter. Partly the collapse happens in the flow domain and partly near the impeller wall. 5 CONCLUSIONS Following aspects can be concluded: -
-
-
With transient calculations convergence was achieved for BEP and for overload Q/Qopt = 1.9. At least for BEP there is quite a primary flow through the impeller channels from leading to trailing edge. So the transformation from mechanical power to hydraulic power is not based on the interaction of turbulence structures between impeller and main flow. The CFD as well as the measurements show no cavitation occurrence for BEP. The calculated (CFD) integral values (pump head and shaft power) for BEP are approx. 6% higher than the measured integral values. For overload the calculated values are approx. 9% lower than the measured values. So the steepness of the Q-H curve is not fully captured by the CFD. Generally for the overload conditions the CFD leads to smaller cavitation occurrence than the measurement. The cavitation occurrence in 3 to 4 o’clock position which is shown by the measurements is definitely not captured by the CFD.
To summarize: CFD results turned out to be appropriate to capture the qualitative flow conditions and therefore to increase the knowledge about vortex pumps. Beyond that the CFD results are not fully sufficient to calculate the quantitative values with required accuracy.
Fig. 20 Typical exposures of the cavitation clouds
Unsteady cavitation occurs. The cavitation exists
319
2010, 22(5), supplement :324-329 DOI: 10.1016/S1001-6058(09)60213-4
Numerical and experimental investigations of the unsteady cavitating flow in a vortex pump Alexander Steinmann1, Hendrik Wurm2, Alfred Otto3 1,2
Group Research and Technology Center – Fluid Mechanics, WILO SE Nortkirchenstr. Germany E-mail: [email protected] , [email protected] 3 WILO EMU GmbH , Germany E-mail: [email protected]
ABSTRACT: In order to develop an understanding of the flow conditions in Vortex Pumps, numerical investigations (commercial URANSE-CFD-Solver ANSYS CFX, Volume-ofFluid Rayleigh-Plesset cavitation model, eight impeller revolutions) and experimental investigations (High-Speed Exposures of the cavitation clouds through acrylic glass window) have been carried out. A further objective was to investigate the URANSE-CFD method using the mentioned cavitation model regarding numerical stability and accuracy. KEY WORDS: vortex pump, centrifugal pump, sewage water pump, cavitation, CFD, URANSE
1 INTRODUCTION Today there is still no detailed knowledge about the flow fields in Vortex Pumps (sewage water pump). The highly unsteady flow conditions and the typical occurrence of cavitation make flow investigations complex and extensive, so that they are very rare. The unsteady flow behavior is caused by the non blade congruent flow in the impeller. In the impeller side room a potential vortex is present. It has to be identified whether the transformation from mechanical power to hydraulic power is basically driven by an exchange of fluid between impeller side room and impeller or by the interaction of turbulence structures with the main flow. In case of fluid exchange this could be simple flow-through from leading to trailing edge or a flow with helical reentries in the impeller blade channels around the blade tips. Generally these
are the same open questions as for Side Channel Pumps. In order to develop an understanding of the flow conditions as well as to maximize the efficiency and to minimize the cavitation occurrence of vortex pumps, numerical and experimental investigations have been carried out. The numerical investigations have been carried out by means of the commercial URANSE-CFD-Solver ANSYS CFX. The transient flow field was captured with eight impeller revolutions (full geometry including casing). The cavitation behavior was estimated using the Volume-of-Fluid Rayleigh-Plesset cavitation model. The experimental investigations consisted of measurements of the characteristic curves and HighSpeed Exposures of the cavitation clouds. These photographs were taken through an acrylic glass window, which was integrated in the casing suction cover and the suction pipe of the Vortex Pump. Beside the determination of the flow conditions in Vortex Pumps another objective of the present work was to investigate the URANSE-CFD method with the mentioned cavitation model regarding numerical stability and accuracy. 2 VORTEX PUMP Vortex Pumps are preferably used for sewage water
320
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
transport especially in case of high gas or fiber content in the pumped liquid. The vortex impeller (Fig. 1) consists of a rotating disc (supporting disc) on which curved blades are arranged. In front of the impeller there is a relatively large free space to the casing bottom (Fig. 2), so that a big passage can be realized for the solid matter in the pumped liquid.
Fig. 3 Flow geometry of the investigated Vortex Pump
Fig. 1 Impeller of the investigated Vortex Pump
ANSYS CFX uses the element based finite volume method and an algebraic multigrid approach. The implemented cavitation model uses the Volume-ofFluid model for the formulation of two phase flow. The source/sink term for the vapor phase is estimated via the linearized Rayleigh-Plesset equation. A Tetra/Prism mesh with 5.5 mio nodes was generated. For the advection scheme the hybrid scheme High Resolution was applied. For the transient scheme the Second Order Backward Euler method is used. The convergence criteria were: max. residuals 1.0e-3 and conservation target 0.1%. The timestep for the transient calculation (eight impeller revolutions) represented 1° of rotation. So 2880 timesteps were calculated. Every time step consisted of 8 to 22 inner coefficient loops. With the mentioned timestep the RMS Courant Number followed as 0.4 and the max Courant Number to 3.8.
Fig. 2 Casing of the investigated Vortex Pump
Due to the arrangement of the impeller in the pump casing and the free space below, the impeller acts only on a part of the pumped liquid and consequently guarantees high operation reliability in the station. Despite the lower efficiencies compared with channel impellers, pumps with vortex impellers are an important part in the product range of each sewage pump manufacturer. 3 NUMERICAL INVESTIGATIONS The commercial URANSE-CFD-Solver ANSYS CFX 11.0 was used. The flow geometry of the investigated Vortex Pump is shown in Fig. 3.
Two loads of the pump were calculated BEP and overload (Q/Qopt = 1.9). For both loads NPSH = 10 m were set. For the BEP Fig. 4 to Fig. 10 show pressure distributions, velocity vectors, and a streamline plot for the last timestep (with steady state calculation no convergence was achieved).
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
321
Fig. 4 Pressure distribution in the mid cut plane of the impeller
Fig. 7 Relative velocity vectors in the mid cut plane of the impeller
Fig. 5 Pressure distribution in the mid cut plane of the impeller side room
Fig. 8 Absolute velocity vectors in the mid cut plane of the impeller side room
Fig. 6 Pressure distribution in a meridional plane
Fig. 9 Relative velocity vectors in a meridional plane
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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
Fig. 10 Relative streamlines colored by the relative velocity
It becomes obvious that there is quite a primary flow through the impeller channels from leading to trailing edge. An appearance of helical reentries of the flow in the impeller blade channels around the blade tips cannot be found considerably. There is no evidence for a turbulent momentum exchange.
Fig. 12 Pressure distribution in the mid cut plane of the impeller side room
Beside this for the BEP no cavitation occurs. Pump head and shaft power are determined by averaging of the last impeller revolution. These two calculated values are each approximately 6% higher than the measured integral values (measured shaft power adjusted by hub friction). For the overload conditions Q/Qopt = 1.9, Fig. 11 to Fig. 18 show pressure distributions, velocity vectors, a streamline plot, and the cavitation region.
Fig. 11 Pressure distribution in the mid cut plane of the impeller
Fig. 13 Pressure distribution in a meridional plane
Fig. 14 Relative velocity vectors in the mid cut plane of the impeller
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China
Fig. 15 Absolute velocity vectors in the mid cut plane of the impeller side room
323
Fig. 18 Cavitation region
Cavitation occurs at the suction pipe, leading edge and trailing edge. The values of the vapor volume fraction vary between 0 and 10% and therefore the cavitation occurrence is small. The isosurfaces for the cavitation regions in Fig. 17 and Fig. 18 show the 2% border of the vapor volume fraction. The two calculated integral values pump head and shaft power are each approximately 9% lower than the measured integral values. 4 EXPERIMENTAL INVESTIGATIONS The test bench with the acrylic glass window for the experimental investigations is shown in Fig. 19.
Fig. 16 Relative streamlines colored by the relative velocity
Fig. 19 Test bench for the experimental investigations
As camera a video camera LEGRIA HF 200 with a resolution of 1920x1080 was applied. In addition a stroboscope Hofmann SWM-2 was used to provide the adequate light for the high speed exposures. Fig. 17 Cavitation region
For BEP no cavitation occurs. For overload (Q/Qopt = 1.9) conditions Fig. 20 show typical exposures of the cavitation clouds.
324
9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China exclusively in the impeller channel which is in 3 to 4 o’clock position (pressure pipe at the top). Shape and style of the cavitation are highly fluctuating. On one side there are downstream oriented cavitation vortex filaments and on the other side compact cavitation clouds which extend to the whole channel width. Origins are the leading edges of the blades as well as the edge of the hub. The collapse of the cavitation is located downstream of the origins inside a concentric circle with approx. 1/3 of the impeller diameter. Partly the collapse happens in the flow domain and partly near the impeller wall. 5 CONCLUSIONS Following aspects can be concluded: -
-
-
With transient calculations convergence was achieved for BEP and for overload Q/Qopt = 1.9. At least for BEP there is quite a primary flow through the impeller channels from leading to trailing edge. So the transformation from mechanical power to hydraulic power is not based on the interaction of turbulence structures between impeller and main flow. The CFD as well as the measurements show no cavitation occurrence for BEP. The calculated (CFD) integral values (pump head and shaft power) for BEP are approx. 6% higher than the measured integral values. For overload the calculated values are approx. 9% lower than the measured values. So the steepness of the Q-H curve is not fully captured by the CFD. Generally for the overload conditions the CFD leads to smaller cavitation occurrence than the measurement. The cavitation occurrence in 3 to 4 o’clock position which is shown by the measurements is definitely not captured by the CFD.
To summarize: CFD results turned out to be appropriate to capture the qualitative flow conditions and therefore to increase the knowledge about vortex pumps. Beyond that the CFD results are not fully sufficient to calculate the quantitative values with required accuracy.
Fig. 20 Typical exposures of the cavitation clouds
Unsteady cavitation occurs. The cavitation exists