Simulation Of Axial Pumps Using Computational Fluid Dynamics (CFD)

In this assignment need introductions to CFD-Ansys Fluent, Their litrature review and future aspects for the topic.

The Role of Computational Fluid Dynamics in Solving Fluid Mechanical Problems

Axial pumps are a type of centrifugal pumps that generate pump pressure using fluid velocity and momentum. These pumps are known to produce very low heads and high flow rates when pumping a liquid because of the impeller’s axial orientation (Global Spec, (n.d.)). However, the head can be increased significantly by optimizing the design of the impeller (Sung, et al., 2015). There are numerous applications of axial pumps, including: in power plants to facilitate water circulation, in chemical industry for pumping large volumes of fluids in evaporators, flood dewatering where large amounts of water have to be transferred over a short distance (Pump Scout, 2017), and in medical sector especially for heart failure patients, among others (Moazami, et al., 2013); (Singh, et al., 2009). As the application of axial pumps increases across sectors, the need to improve the performance and efficiency of these pumps is rather obvious. One of the commonly used techniques by engineers and scientists to improve the performance, efficiency and sustainability of axial pumps is to perform simulations of various models of these pumps using computational fluid dynamics (CFD). These simulations helps in analyzing and predicting the performance of axial pumps under different operating conditions (Jafarzadeh, et al., 2011).

The role of CFD in solving present-day fluid mechanical problems cannot be overemphasized (Yedidiah, 2008). CFD is a numerical or qualitative tool that is used for analyzing and solving time-dependent and complex 3D problems (Masic, et al., 2013); (Rapp, 2017). This tool is very reliable in the design and optimization of new processes and products (Dhotre, et al., 2013); (Mukesh, et al., 2012). The tool is very useful in modelling and designing fluid mechanical systems by simulating the behaviour of the fluid when flowing through the system (Kethidi, et al., 2011). This makes it easier for engineers and scientists to predict the behaviour of fluids when flowing through mechanical systems hence determining the most appropriate parameters to consider when designing the systems. In other words, CFD simulation allows engineers and scientists to virtually evaluate and test the computer-aided design (CAD) model of mechanical systems during early stages of design process and repeat the design process accordingly until they find the best possible design of the system and also improve it (Crahmaliuc, 2017); (Guo, et al., 2016). The main focus of this paper is on the simulation of axial pumps using CFD technique. The CFD simulations will be done using ANSYS Fluent software. ANSYS Fluent software is widely used in simulating performance of mechanical systems under varied conditions such as temperature and pressure (Singla, et al., 2014). The software applies numerical methods in analysis making it easier to create 3D models, boundary conditions, meshing, fluid domain meshing, etc. (Kumar & Aharwal, 2017). Information obtained from ANSYS Fluent is very useful in predicting performance of 3D models of mechanical systems, such as axial pumps (Manshoor, et al., 2013). The software is suitable for analyzing both laminar and turbulent fluid flows (Ariza, et al., 2018).

The main purpose of the paper is to evaluate the behaviour of fluids in an axial pump so as to establish the best design option that will improve the performance, efficiency and sustainability of axial pumps.

Simulation and CFD Implementation for Improving Axial Pumps

Bellary, et al. (2016) states that performance of centrifugal pumps can be enhanced by applying CFD technique. The researchers conducted a study with an aim of minimizing total time of designing and optimizing. They used numerical analysis on various pump alternatives to investigate how to enhance their efficiency and head at specific flow rates. It requires a lot of data and takes a very long time to analyze the performance and optimize design of multistage pumps such as axial pumps. This consequently increases the design and optimization cost of these pumps. However, experimental and numerical techniques can be used to optimize design parameters of turbo machines thus enhancing their performance. A parameter such as blade angles affects formation of cavitation and hydraulic efficiency of the pump (Ji, et al., 2014); (Sanda & Daniela, 2012); (Yun, 2016), while the number of blades affects formation of fluid jets and pressure in the pump (Houlin, et al., 2010). In this study, the researchers carried out CAD modeling and meshing of the flow in turbo machines using ANSYS software to determine the best design options of impeller, diffuser and rotor of the pump. They found that it is better to create several model simulations of the pump using ANSYS and evaluate them before selecting the most suitable design option. This makes it easier to identify design options with more errors and also determine appropriate design parameters that will improve the performance of pumps (Bellary, et al., 2016).

Li, et al. (2015) conducted a study to investigate the complex turbulent flow in an axial pump using CDF numerical simulation. The researchers recognized that design optimization of these pumps requires comprehensive analysis of their complex turbulent flow, which is very difficult to achieve using experimental methods. However, CFD is a numerical simulation technique that makes it easier to obtain fully 3D turbulent flow in axial pumps. They used SST (shear stress transport) model when performing CFD calculation. To achieve their objective, they started by creating a 3D solid model of the axial pump and processed approximate extensions at the inlet and outlet of the pump so as to reduce boundary conditions’ effects and also enhance numerical stability. A structured mesh of the axial pump was then generated to divide the pump into five zones: inlet pipe, outlet pipe, impeller, inducer, guide vane and exit. The mesh system is useful in comprehensive analysis of fluid flow in axial pumps at a localized level. The next step was to specify boundary conditions (mass flow rates and rotating speed) for the simulation. These conditions are useful in determining whether the flow in the pump is laminar or turbulent. In most cases, and for design purposes, it is recommended that the flow in axial pumps be taken to be turbulent. Reynolds Navier-Stokes equations for momentum and continuity were used to generate turbulence model for the axial pump after which numerical methods in ANSYS were used to create CFD simulations. Key findings from this study revealed that numerical simulation using ANSYS is very reliable in analyzing the performance of axial pumps under different conditions. This makes it possible to establish the effect of various design parameters on the overall performance of the pump (Li, et al., 2015). Therefore results obtained from CFD simulations in ANSYS are very useful in optimizing design of axial pumps (Kim, et al., 2010); (Zhang, et al., 2010).

Application of CFD Simulations in Design Optimization of Axial Pumps

Analysis of fluid flow in axial pumps is extremely complex for various reasons, including the 3D flow structure associated with turbulence, unsteadiness, cavitation and secondary flow. Flow of fluids in centrifugal pumps is affected by several factors, especially the design parameters of the pumps, such as geometry. Use of computational resources, such as numerical methods, to analyze the performance of turbo machinery has increased over the past few years. Many researchers are using CFD techniques for this purpose. These techniques are mainly used for predicting performance of centrifugal pumps at different conditions (both design and off-design). A study by Shah, et al. (2013) showed that CFD evaluation of centrifugal pumps can be done best using turbulence model and unsteady Reynolds Navier-Stokes equations. In this study, CFD simulation was performed on centrifugal pump to show how these pumps perform under different conditions. Information obtained from this kind of analysis is very useful in optimizing design of these pumps so as to achieve higher performance and efficiency. Thus considering the complex nature of fluid flow in centrifugal pumps, it is important to use more advanced approaches to analyze this flow. State-of-the-art CFD techniques are the best methods for this purpose because they help in visualizing, predicting and testing the behaviour of the pumps using simulated models (Shah, et al., 2013). Besides improving the performance of pumps, these techniques also reduce the cost of designing and optimizing these pumps (Lopez, et al., 2017).

Performance of centrifugal pumps is largely dependent on the design parameters of these pumps. These pumps are usually used for transporting fluids over medium and short distances where moderate discharge and head are required. According to a study conducted by Chakraborty, et al. (2013), optimal design of centrifugal pumps is facilitated by creation of CFD that are used to predict the complex internal flow of fluids in axial pump impellers. Some of the most critical design parameters that affect the performance of centrifugal pumps include: outlet diameter of impeller, number of blades and blade angle. The performance of the pump is heavily affected by blade number. In their study, the researchers sought to evaluate the performance of a centrifugal pump impellers with different blade numbers but the same outlet diameter. They created a pump model with a rotational speed of 3,300 revolutions per minute (rpp) and varied the number of impeller blades with 4, 5 and 6 blades. A mathematical model of the pump was formulated by developing governing equations, establishing the pump geometry (blade number, inlet and outlet blade angle, shape of blade, and impeller inlet and outlet diameter), and specifying the boundary conditions. ANSYS Fluent software was then used to simulate and predict the characteristics and internal flow fields and performance of the pump. For every impeller (with the specified number of blades), head changes, static and total pressure distribution and efficiencies of the pumps were comprehensively analyzed. The researchers found that an increase in number of blades results to a corresponding increase in static pressure and head of the pump, but there was no clear relationship obtained between the blade number and efficiency. Nevertheless, high efficiency was recorded when the number of blades of the centrifugal pump was 5 (Chakraborty, et al., 2013).

Study by Bellary et al. on Performance Enhancement of Centrifugal Pumps

Feng, et al. (2016) emphasizes the wide application of axial pumps in hydraulic engineering works, such as water supply, irrigation and drainage and the negative effects of cavitation on the overall performance and efficiency of axial pumps with inlet guide vanes. In their study, the researchers also investigated the influence of cavitation development on internal flow within the impeller zone. Results obtained from the study showed that it is possible to enhance axial pump’s cavitation performance at off-design flow conditions. This can be achieved by regulating inlet guide vanes angles to positive values when flow rates are low and adjusting inlet guide vanes angles to negative values when flow rates are high. When this is done, it reduces the net positive suction head, which increases vapor fraction slowly then greatly. This increase is a clear representation of cavitation development and can be used for predicting the desired positive suction head thus reducing or eliminating cavitation zones that could otherwise reduce overall performance of the pump (Feng, et al., 2016). These results were similar to the ones obtained from studies conducted by Wang, et al. (2013) and (Tan, et al., 2010), which showed that performance of centrifugal pumps can be improved by optimizing the design of hydraulic components of the pump, such as inlet, blade vanes, impeller and outlet. The study also showed that axial pumps have low heads and there are usually used in irrigation, drainage, water diversion works, power plant projects and water supply projects (Wang, et al., 2013).

Sivakumar, et al. (2016) states that the performance of axial pumps largely depends on geometrical properties of the pump components particularly the impeller. As a result, the performance of these pumps can be improved by optimizing the geometrical design of the impeller. This design can be facilitated by use of CFD. In their study, the researchers discussed various techniques of enhancing performance of centrifugal pump by changing the geometry of impeller. Performance of a centrifugal pump is expressed in form of different characteristics including head, power consumption, discharge and efficiency. The researchers generated the impeller’s 3D model in SOLIDWORKS 2009 and then imported it into ANSYS software for analysis. The mesh of the impeller was then created, boundary conditions specified then various design parameters of the impeller were varied. Some of the key findings from the study are as follows: trimming an impeller has a significant effect on the overall impeller strength; performance of the pump can be improved by changing inlet and outlet angle of impeller blade but the value of the angle should be within a suitable range (when the blade angle values are very high, vacuum gets created in the impeller while when the value are too low, water clogging in the impeller increases); analysis values of inlet and outlet blade angle, efficiency and power consumed were very close to the values predicted using ANSYS; and an increase in blade number results to minimum power consumed, and maximum head and efficiency (Sivakumar, et al., 2016). These findings are very useful in improving overall performance of centrifugal pumps because they can be used as guidelines when determining suitable design parameters of the impeller.

Study by Li et al. on Investigating Complex Turbulent Flow in Axial Pumps using CFD

The need to establish suitable design parameters for improving performance and efficiency of axial pumps cannot be overemphasized. Researchers all over the world are continuing to carry out studies with an aim of helping manufacturers develop better performing and more efficient axial pumps (Hu, et al., 2008). A study conducted by Yang, et al. (2016) presented an investigation on how adjustable outlet guide vanes influences axial pump’s hydraulic performance. ANSYS Fluent software was used to simulate seven different adjustable angles of outlet guide vanes based on Reynolds time-averaged and RNG (renormalization group) k-ε turbulent model equations. These equations are the most preferred and widely used when analyzing performance of turbo machines such as centrifugal pumps. Airfoil flow’s vector graphs were evaluated in varied operating conditions for the seven adjustable outlet guide vanes followed by using numerical results obtained to establish a BP-ANN (back propagation artificial neural networks) prediction model. The prediction model’s effectiveness was verified using numerical simulation and theoretical analysis. Results from the study showed that an increase in adjustable outlet guide vane angle resulted to high efficiency at high flow rate whereas a decrease in adjustable outlet guide vane angle resulted to low efficiency at low flow rate. The guide vane’s internal flow field can also be enhanced by regulating angle. This helps in eliminating or decreasing the flow separation of guide vane inlet and tail thus improving the efficiency of the axial pump. Another crucial finding from the study was that the flexible outlet guide vane can substantially enhance both the head and efficiency of the axial pump when it is operating at off-design conditions (Yang, et al., 2016). These results were similar to those of several other studies discussed above. Basically, the head and efficiency of axial pumps are significantly influenced by the angle of outlet guide vane. These results were similar to the ones obtained from a study by Qian, et al. (2010) which found that adjusting the guide vane angle can significantly improve the head and efficiency of an axial pump (Qian, et al., 2010); (Zhou, et al., 2012). This information is very essential for designers and engineers involved in development of axial pumps because it helps them establish the appropriate angle to meet target head and efficiency depending on the intended application of the pumps.

Centrifugal pumps, such as axial pumps, have very complex impeller geometry. The design of these impellers has a significant effect on the performance and efficiency of the pumps. These pumps have become very common in both domestic and industrial applications hence there is great need to improve their performance and overall efficiency. Rajendran & Purushothaman (2012) states that flow in an axial pump is 3D and completely turbulence model thus analysis of these pumps requires use of high-tech computational techniques and tools. In this study, the researchers used ANSYS CFX to simulate flow in a centrifugal pump impeller. The software is very effective and reliable in predicting the complex internal flows present in centrifugal pumps. The typical procedure of completing this kind of study involves identifying specifications of the centrifugal pump being analyzed (such as blade width, pump head, inlet and outlet diameter, angle of outlet blade, impeller rotational speed and flow rate), meshing, specifying boundary conditions, and then creating the simulations. CFD is used to model and solve performance of centrifugal pump impellers. Using CFD, it is possible to predict the value of pressure head at different flow conditions. The researchers also found that high velocities and low pressure are recorded near leading edge as a result of blade thickness (Rajendran & Purushothaman, 2012). This study also affirms the fact that it is possible to analyze and predict performance and efficiency of axial pumps using ANSYS Fluent software. The two elements (performance and efficiency) are largely influenced by the design of the pumps. This makes ANSYS Fluent a very important software in the design and optimization of axial pumps as it enables designers and engineers to predict the performance of these pumps under varied operating conditions. It also helps in determining the best combination of design parameters for various components of the pump.

Conclusion

As aforementioned, axial pumps are also used in the medical sector especially for patients with heart problems. The device is usually implanted in patients with heart problems so as to boost blood circulation. Chimenti, et al. (2008) states that CFD is a very essential tool in optimizing design and development of hydraulic devices, such as axial pumps. The researchers investigated the use of CFD analysis in improving the design and performance of novel axial flow blood pump. The axial pump used in the study had two impellers rotating in an anticlockwise direction. Most axial pumps have a stationary diffuser and an impeller but the one used in the study did not have a stationary diffuser instead a counter-rotating impeller was used. The main function of a stationary diffuser is to decelerate and redirect the flow thus enhancing pump performance. Nevertheless, the diffuser can create stagnation and recirculation sections in the pump thus increasing the likelihood of formation of detrimental blood clot. The main objective of this study was to test the capability of CFD for designing, developing and validating the hydraulic idea of an axial pump with two counter-rotating impellers (Chimenti, et al., 2008). This device could be implanted in patients with heart failure to pump blood and boost blood circulation thus saving lives of many people. An axial blood pump was designed with a head pressure of 100 mmHG, flow rate of 5 litres/min and rotational speed of 8,400 rpm for the inlet impeller and 3,900 rpm for the outlet impeller. A bench test was performed to obtain H-Q curves of turbulence conditions. Numerical methods were used to generate a CFD model for further analysis. The pump model was tested under different conditions and it showed the capacity of the pump to function in varied regimes. From the CFD analysis results, the researchers concluded that development and use of axial pumps with two counter-rotating impellers is practicable. However, there is need to carry out more studies so as to evaluate and minimize the difference between experimental and theoretical or calculated results (Cao, et al., 2013).

A recent study by Sankar (2018) has provided evidence that ANSYS Fluent software is very efficient in simulating and predicting characteristics and inner flow fields of centrifugal pumps. In his study, the researcher investigate the performance of a centrifugal pump with the following specifications: 6 blades, 28° outlet angle and a rotational speed of 250 rpm. The study focused on the head, flow separation and overall efficiency of the pump. He developed a 3D model of the impeller using ANSYS Fluent software. The 3D model was then discriticized into elements – a process known as meshing in fluid dynamics analysis. Boundary conditions were then applied on the meshed model to complete fluid dynamics. When using ANSYS Fluent software to perform dynamic analysis calculations, the following parameters must be specified: the motor’s rotating axis, the centrifugal impeller’s rotating speed, fluid flowing through the pump, and inlet boundary conditions. Findings from this study showed that flow of fluid through a centrifugal pump impeller is fully turbulent and three dimensional. Other key findings were as follows: an increase in the rotor’s rotating speed resulted to an increase in pressure head; an increase in the rotor’s rotating speed resulted to a decrease in efficiency of the impeller; an increase in number of blades resulted to a corresponding increase in pressure head; and in increase in blade outlet angle resulted to an increase in pressure head (Sankar, 2018). The software was also used to show flow separation in the velocity profile. Occurrence of flow separation is as a result of the fluid getting disconnected from the boundary layer. This usually happens because of sudden change in the velocity profile, abrupt deceleration and change in pressure in the boundary layer.

ANSYS Fluent is a very accurate and reliable software for analyzing and predicting performance of complex 3D turbulent models of axial pumps (Manshoor, et al., 2013); (Yang & Liu, 2013). The software package makes it possible to evaluate the effect of a wide range of design parameters such as type/shape of impeller blades, number of impeller blades, inlet and outlet blade angle, and impeller inlet and outlet diameter, on the performance and efficiency of the pump (Panday, et al., 2012); (Zhang, et al., 2013). The emergent and increasing usage of axial pumps in different sectors certainly necessitates improving the performance and efficiency of these pumps (Bartzanas, et al., 2012). This affirms the importance of CFD simulation in the design and optimization of axial pumps. One of the future trends of CFD simulation is real-time computations. This implies that besides using ANSYS Fluent software to optimize the performance of various components of the pump during design phase, the optimization will also be done concurrently during operation stage. This means that during operating stage, sensors will be installed in the axial pump to detect any changes to the operating conditions. When a change is detected, a signal will be send to an integrated computer system that uses technological tools such as ANSYS Fluent to perform CFD analysis and establish the best action to improve performance and efficiency of the pump. For instance, from CFD simulation, the best decision may be to adjust the angle of inlet or outlet blade, or increase/reduce the number of blades. Having said that, this can only be made possible through other interventions such as design the axial pumps in such a way that it is possible to adjust the number of blades or angle of inlet/outlet blades.

Another important aspect of CFD simulation of axial pumps is integration of direct numerical simulation (DNS) and graphics processing unit (GPU) in the ANSYS Fluent software. DNS is a CFD simulation tools that generates numerical solutions of Navier-Stokes equations without using a turbulence model. Instead, the turbulence model’s temporal and spatial scales are resolved in the mesh created. The main benefits of DNS are cost and time savings. On the other hand, GPU makes it easier for the designers and engineers to create and control performance of axial pumps using videos and 3D graphics. This involves use of virtual reality in generating 3D models of the complex geometries of axial pumps. The ANSYS Fluent can be designed such that both GPUs and computer processing units (CPUs) perform their designated functions concurrently. It is rather obvious that graphics and videos provide a better understanding of the behaviour of turbo machines than texts and numerical values. The engineers, scientists and designers will be able to see the effect of changing or removing a particular design parameter from the pump and make necessary changes that will improve its performance and efficiency. Last but not least is inclusion of sustainability in the design and optimization of axial pumps. In the future, it is expected that as CFD simulations are performed, they will be done to ensure that the axial pump models created are sustainable. This will help in increasing the longevity of these pumps, improve their resource efficiency, reduce their lifecycle costs and minimize the resources they use at different stages of their lifecycle.

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