AEIJST – May May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 CFD Analysis of Centrifugal Pump Using Impeller Parameters *P V Senthiil, **V S Mirudhuneka *Director/CE & Head, Department of Mechanical Mechanical Engineering, St. Peters University, Avadi, Chennai-54 **SAP **SAP Consultant, IBM India Ltd, DLF Complex, Porur, Chennai Abstract The impeller of a centrifugal pump is the most important component which increases the kinetic energy of the incoming liquid. It consists of blades which aid in increasing the velocity of the liquid and also a complex asymmetric component which makes it difficult and expensive to modify and experiment physically. Computational Fluid Dynamics (CFD) is a tool which aids in conducting or simulating flow through a pump virtually virtually using computer computer software. software. An attempt has been made to improve the performance performance of a typical centrifugal centrifugal pump obtained from a leading pump manufacturing company. Four parameters namely number of blades, blade inlet angle, and outer diameter of impeller and trim profile of blades are identified to be connected with the performance of the centrifugal pump. These four parameters are set in three levels to form a L 9 orthogonal array to design nine experiments by Taguchi method. Based on the experiment details, the impeller is modeled using CAD software. The modeled impellers are then analyzed using Solid works Flow Simulation, a CFD software package. Discharge is obtained for these models and optimization is performed by Taguchi method to obtain signal-noise ratio. The best possible possible combination of parameters is determined and is implemented implemented in an impeller model and CFD analysis is carried out to verify the obtained results. 1. Introduction Pumping systems are the single largest type of industrial end-user of motordriven electricity, accounting for 25% of industrial motor energy usage. Also, pumping systems account for nearly 20% of the world’s demand for electric energy. While pumps typically operate to serve various chemical process support equipments such as chillers, cooling towers, material transfer, etc., pumping is considered an individual process separate from the processes of the aforementioned aforementioned equipment. A pump is a device used to raise, compress, or transfer fluids. The motors that power most pumps can be the focus of many best practices. It is common to model the operation of pumps by using pump and system curves [4, 5]. Pump curves offer the horsepower, head, and flow rate figures for a specific pump at a constant rpm. System curves describe the capacity and head required by a pump system Pumps are generally grouped into two broad categories, positive displacement pumps and dynamic (centrifugal) pumps. Positive displacement pumps use a mechanical means to vary the size (or move) of the fluid chamber to cause the fluid to flow. Positive displacement pumps have a constant torque characteristic, where centrifugal pumps are variable torque in nature. Centrifugal pumps impart a momentum in the fluid by rotating impellers immersed in the fluid the positive displacement pump is commonly used to feed chemicals into the water or to move heavy suspension, such as sludge. One type of positive displacement pump consists of a piston that moves in a back and forth motion within a cylinder. It is used primarily to move material that has large amounts of suspended material, such as sludges. Another type of positive displacement pump used in the water industry is the diaphragm pump. This pump 1
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 operates the same way as the piston pump except that, in place of a piston that moves in a cylinder, a flexible diaphragm moves back and forth in a close d area. Centrifugal pumps A centrifugal pump is a kinetic device. Liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. Centrifugal pumps impart a momentum in the fluid by rotating impellers immersed in the fluid. The momentum produces an increase in pressure or flow at the pump outlet. It converts driver energy to kinetic energy in a liquid by accelerating it to the outer rim of a revolving device known as an impeller. The faster the impeller revolves or the bigger the impeller, then the higher the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid [1]. The kinetic energy available to the fluid is used to accomplish work. In most cases, the work consists of the liquid moving at some velocity through a system by overcoming resistance to flow due to friction from pipes, and physical restrictions from valves and other in-line devices, as well as elevation changes between the liquid's starting location and final destination. When velocity is reduced due to resistance encountered in the system, pressure increases. As resistance is encountered, the liquid expends some of its energy in the form of heat, noise, and vibration in overcoming that resistance. The result is that the available energy in the liquid decreases as the distance from the pump increases. The actual energy available for work at any point in a system is a combination of the av ailable velocity and pressure energy at that point. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that transforms the kinetic energy of the liquid into pressure energy. Centrifugal pumps are prevalent for many different applications in the industrial and other sectors. Nevertheless, their design and performance prediction process is still a difficult task, mainly due to the great number of free geometric parameters involved. On the other hand the significant cost and time of the trial-and-error process by constructing and testing physical prototypes reduces the profit margins of the pump manufacturers Although the actual flow patterns within a centrifugal pump are three-dimensional and unsteady in varying degrees, it is fairly easy, on a one-dimensional, steady-flow basis, to make the connection between the basic energy transfer and performance relationships and the geometry or what is commonly termed the “hydraulic design” (more properly the “fluid dynamical design”) of impellers and stators or stationary passageways of these machines. In fact, disciplined one-dimensional thinking and analysis enables one to deduce pump operational characteristics (for example, power and head versus flow rate) at both the optimum or design conditions and off-design conditions. This enables the designer and the user to judge whether a pump and the fluid system in which it is installed will operate smoothly or with instabilities. The user should then be able to understand the offerings of a pump manufacturer, and the designer should be able to provide a machine that optimally fits the us er’s requirements. Open Impeller An open impeller has its vanes exposed on the bottom side, a design that allows the pump to move liquids that contain large solids. Open impellers are used in propeller pumps in which the head is low (usually less than 20 feet) and the 2
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 volume of water pumped is high. The rate of flow can easily be set by adjusting the clearance of the bottom of the impeller to the pump casing. The larger the clearance is, the less will be pumped. Semi-Open Impeller This design contains many of the same characteristics of the open impeller. The semi-open impeller has a shroud, or cover, on one side. It is used to pump liquids that contain medium-size solids. Closed Impeller This is the impeller of choice in most pump designs and is used in cases where the liquid being pumped has few solids since it will pump the liquid with less wasted energy. With this design, there is a cover on both sides of the impeller with the vanes completely enclosed. The eye of the impeller is surrounded by a skirt, which fits into a recess in the pump casing and ensures that the water from the discharge side of the impeller does not re-circulate back to the suction side. The impeller is set in the centre of the pump casing. The skirt of the impeller is surrounded by a wear ring to reduce problems which can seriously affect pump performance and the life of the impeller. The pressure inside the volute of the pump increases when the pump is operating. A zone of high pressure in the volute and low pressure in the suction eye is set up. As the water jets from the high- to the low-pressure area, the material of the volute and impeller will be worn away. Abrasive material in the water can also contribute to the wear. The wear ring on the impeller is designed to be a sacrificial element. It absorbs the wear, saving the impeller from damage. A certain amount of water is re-circulated, causing energy to be wasted, as water jets from the high- to the low-pressure side of the impeller back to the suction eye. Normally the clearance can be adjusted to keep such recirculation to a minimum. Centrifugal pumps can have more than one impeller, often called stage. Each additional stage increases the head that the pump can pump against. If one impeller will pump against 60 feet of head, two will pump against approximately 120 feet, three against 180 feet, etc. The rate of the flow will not be affected by additional impellers since that is dictated by the diameter of the impeller. It will be no greater than what the first impeller can deliver [2]. Velocity triangles A velocity triangle or a velocity diagram is a triangle representing the various components of velocities of the working fluid in a turbo machine. Velocity triangles may be drawn for both the inlet and outlet sections of any turbo machine. The vector nature of velocity is utilized in the triangles, and the most basic form of a velocity triangle consists of the tangential velocity, the absolute velocity and the relative velocity of the fluid making up three sides of the triangle. Each of the three vectors in the triangle of velocities has two properties namely magnitude and direction. This means that there are a total of six components Optimization Optimization is the selection of a best element with regard to some criteria from some set of available alternatives. In the simplest case, an optimization problem consists of maximizing or minimizing a real function by systematically choosing input values from within an allowed set and computing the value of the function. The generalization of optimization theory and techniques to other formulations comprises a large area of applied mathematics. More generally, optimization includes finding "best available" values of some objective function 3
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 given a defined domain, including a variety of different types of objective functions and different types of domains. 2. Taguchi method of optimization Taguchi method is a scientifically disciplined mechanism for evaluating and implementing improvements in products, processes, materials, equipment, and facilities. These improvements are aimed at improving the desired characteristics and simultaneously reducing the number of defects by studying the key variables controlling the process and optimizing the procedures or design to yield the best results. The method is applicable over a wide range of engineering fields that include processes that manufacture raw materials, sub systems, products for professional and consumer markets. In fact, the method can be applied to any process be it engineering fabrication, computer-aided-design, banking and service sectors etc. Taguchi method is useful for 'tuning' a given process for 'best' results. Every experimenter has to plan and conduct experiments to obtain enough and relevant data so that he can infer the science behind the observed phenomenon. [3] (i) Trial and error approach It performs a series of experiments each of which gives some understanding. This requires making measurements after every experiment so that analysis of observed data will allow him to decide what to do next - "Which parameters should be varied and by how much". Many a times such series does not progress much as negative results may discourage or will not allow a selection of parameters which ought to be changed in the next experiment. Therefore, such experimentation usually ends well before the number of experiments reaches a double digit! The data is insufficient to draw any significant conclusions and the main problem (of understanding the science) still remains unsolved. (ii) Design of experiments A well planned set of experiments, in which all parameters of interest are varied over a specified range, is a much better approach to obtain systematic data. Mathematically speaking, such a complete set of experiments ought to give desired results. Usually the number of experiments and resources (materials and time) required are prohibitively large. Often the experimenter decides to perform a subset of the complete set of experiments to save on time and money! However, it does not easily lend itself to understanding of science behind the phenomenon. The analysis is not very easy (though it may be easy for the mathematician/statistician) and thus effects of various parameters on the observed data are not readily apparent. In many cases, particularly those in which some optimization is required, the method does not point to the BEST settings of parameters. A classic example illustrating the drawback of design of experiments is found in the planning of a world cup event, say football [1]. (iii)Taguchi method Dr. Taguchi of Nippon Telephones and Telegraph Company, Japan has developed a method based on “ORTHOGONAL ARRAY” experiments which gives much reduced “variance” for the experiment with “optimum settings “of control parameters. Thus the marriage of Design of Experiments with optimization of control parameters to obtain BEST results is achieved in the Taguchi Method.
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 "Orthogonal Arrays" (OA) provide a set of well balanced (minimum) experiments and Dr. Taguchi's Signal-to-Noise ratios (S/N), which are log functions of desired output, serve as objective functions for optimization, help in data analysis and prediction of optimum results. Centrifugal pump testing In today’s competitive industrial world, the physical testing of various pump models is a laborious process. The trial and error method is time consuming and has many disadvantages. In some instances of the trial and error method, it is impossible to run several experiments all at once. Several different tests have to be conducted to find the best result. When we arrive at the conclusion, a lot of time might be wasted. Trial and error wastes a large quantity of test materials for prolonged tests. In some cases, it will be unable to reuse old test subjects. Blind testing to receive a test result can be risky to an organization and for the test subjects involved. Some of the experiments pay off and result in a groundbreaking breakthrough that can help the entire organization. On the other hand it can also lead to terrible blunders resulting in the downfall of the organization CAD and CFD analysis are useful tools that can boost an organization’s profit and reduce a considerable time that is usually lost in physical testing. CAD aids in constructing the geometrical profile on a computer and CFD analysis provides the necessary virtual simulation without using any physical effort. Benefits of CFD The primary benefits of using CFD are Enhanced Understanding: CFD creates a virtual prototype of the physical, chemical and thermal characteristics of a system. Visualization of the predicted behaviour of the virtual prototype can provide insight into the design, which may be impossible to observe otherwise. This is particularly useful in gaining understanding into systems that are difficult to prototype or obtain detailed internal measurements.
CFD can be used to test "what if" questions quickly even before physical prototyping or testing can be conducted. By changing the input variables to the numerical model, predictions on the system performance can be obtained for any range of operating conditions to facilitate optimization of the design[6]. Shorter Lead Times: Fast accurate analysis and a thorough understanding of the design leads to shorter design cycles. CFD can be used for compressing the development cycle while managing technical risk. CFD is most effectively applied to:
Design evaluation
Optimization
Problem solving in existing equipment
Performance evaluation
Scale-up evaluation
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732
3. Problem definition An impeller of a 5 hp centrifugal pump is studied to predict the pressure, velocity distributions and efficiency by altering certain key parameters in the impeller. Initially the standard impeller was modelled by CATIA V5 and analyzed using Flow Simulation software to identify the deviation of CFD results from the standard practical value. The impeller is then studied by modifying certain parameters. Design of experiments The techniques for laying out experiments when multiple factors are involved, has been known for a long time and is popularly known as the Factorial design of experiments. This method helps the researcher to determine the possible combinations of factors and to identify the best combination. However in industrial settings it is extremely costly to run a number of experiments to test all combinations [7]. The Taguchi approach developed rules to carry out experiments, which further simplified and standardized the design of the experiment, along with minimizing the number of factor combinations that would be required to test for the factor. Orthogonal arrays The Taguchi design of experiment makes use of Orthogonal Arrays (OA) to help design the experiment. By combining the orthogonal Latin squares in a unique manner Taguchi prepared a set of common OA’s to be used for a number of experimental situations. In this work, four independent variables namely blade number, inlet blade angle, outer diameter of impeller and trim profile having three 6
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 factor level values are considered. By using the Taguchi method, L9 orthogonal array is selected and the experiments are designed to conduct the experiments virtually using computer software. Experiments In Solid works Flow Simulation software, results are obtained in the Goal Plot dialog box. All added goals together with their current values are listed at the top part of the box, as well as the current progress towards completion given as a percentage. The progress value is only an estimate and generally (but not necessarily) increases with time.
Conclusion In this work, an impeller model of a centrifugal pump was created using CATIA V5 software from the data provided by a leading pump manufacturer and then analyzed in Solidworks Flow Simulation software. The relative error percentage was determined to find the deviation of the CFD results from the actual standard results. An attempt was made to improve the performance of the impeller by modifying certain key parameters like blade number, outer diameter of impeller, inlet blade angle and trim profile. The experiments were designed based on L9 orthogonal array with three levels of four parameters. The experiment models were constructed using CATIA V5 software and the models were analyzed in Solidworks Flow Simulation, CFD software. The results obtained from the experiments showed a reasonable increase in volume flow rate output. The results obtained were optimized by Taguchi method to identify a suitable combination of parameters. Another model was constructed in CATIA V5 software based on the best 7
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AEIJST – May 2014 -Vol 2 Issue 5 ISSN - 2348- 6732 combination of optimized parameters. The model was then analyzed virtually to validate the results obtained by optimization and they showed good conformity. References [1] Khalid.S.Rababa (2011), “The Effect of Blades number and Shape on the Operating characteristics of Groundwater Centrifugal Pumps”, European Journal of scientific research, vol 2, No.6, pp 243-251. [2] Ll Gang Wen (2011), “inverse design of impeller blade of centrifugal Pump with a singularity method”, Jordan Journal of Mechanical and Industrial Engineeri ng, vol 9, No. 6, pp 119-128. [3] BPMA, “The European System Guide”, First Edition 2011. [4] Jacobsen Brix Christian, “The Centrifugal Pump”, Grundfos, First Edition. [5] Bureau of Energy Efficiency, “Pumps and Pumping System”, First edition 2002. [6] Toru Shigemitsu, Fukitomi Junichiro, Kensuke Kaji and Wada Takashi (2012), “Unsteady Internal Flow Conditions of Mini Centrifugal Pump with Splitter Blades”, Proceedings of 4th Asian Joint workshop on Thermophysics and Fluid Science. [7] Subramaniam.L and Sendilvelan.S, (2012), “Modal Analysis of a Centrifugal Pump Impeller”, International journal of engineering, Science and technology, vol 9, No, 2, pp 5-14.
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