Computational Fluid Dynamics 2011 Part 9 doc

Computational Fluid Dynamics 234 Fig. 7. Flow dynamic phenomena around the impeller blade: (a) blade-loading distributions (b) streamtubes for the overall flow, (c) streamline distribution on the pressure side, and (d) streamline distribution on the suction side Fig. 8. Flow dynamic phenomena around the diffuser blade: (a) blade-loading distributions, (b) velocity vectors near the exit hub of the suction side, (c) streamline distribution on the pressure side, and (d) streamline distribution on the suction side Application of Computational Fluid Dynamics to Practical Design and Performance Analysis of Turbomachinery 235 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 η τ ψ Prediction Required design point ( φ = 0.747; ψ = 0.557) The scattered symbols represent test data. ψ , τ and η φ Fig. 9. The performance characteristic curves of a mixed-flow pump 4.4 Cavitation performance characteristics Generally, cavitation occurs in the liquid system when the local absolute static pressure in a flowing liquid is reduced to or below the vapour pressure of the liquid, thereby forming vapour bubbles. The bubbles suddenly collapse as they are convected to a high-pressure region. The consequent high-pressure impact may lead to hardware damage, e.g. local pitting and erosion, and emit noise in the form of sharp crackling sounds. Cavitation may also degrade the performance characteristics of hydraulic machinery. Figure 10 demonstrates the cavitation performance characteristic curves of a mixed-flow pump developed by the present optimal design method, considering the highest pump efficiency. In order to investigate the flow dynamics of cavitation for the near-design flowrate ( φ = 0.738 in Fig. 10) under the design pump speed, the side-view of the cavitating flow along the impeller blade has been photographed through the window of the test facility and compared with the isosurface plots, generated by the CFD code, for the vapour fraction of the fluid. In this figure, the cavitation parameter ( σ ) is defined as the ratio of the net positive suction head (NPSH) to the pump total head. At case (1), a small amount of cavitation, the inception of cavitation, occurs at the tip vortex generated by the tip of the impeller leading edge. With further reduction up to case (4) in the cavitation parameter, the tip-vortex cavitation has been more produced; the generated pump head still remains, however, constant without severe performance degradation. When the NPSH reaches a sufficiently low value over the knee of nearly constant head coefficient (for case (6)), the distortion of the flow pattern, by mixing more tip-vortex and face sheet cavitation with the Computational Fluid Dynamics 236 Fig. 10. Cavitation performance characteristics of a mixed-flow pump: NPSH curves Application of Computational Fluid Dynamics to Practical Design and Performance Analysis of Turbomachinery 237 Fig. 10. (continued) Computational Fluid Dynamics 238 main flow between the impeller blades, extends across the flow channel and consequently leads to a sudden decrease in the total pressure rise. Comparing with the computational results, it is observed that the cavitating region is spread out over the suction surface as well as the leading edge of the impeller blade. By repeating the above procedure for several off- design flowrates under the same rotational speed, the suction performance curves, NPSH versus pump head, have been constructed as shown in Fig. 10. It can be seen that the cavitation performance curves predicted by the CFD code are in good agreement with the measured data. Meanwhile, it is worth noting that the cavitation on the diffuser blade surface has not appeared for the cavitating flow regimes, which means that the diffuser blade design, taking the flow angle leaving the rotating impeller into account, has been successfully carried out in this study. Every pump has a critical NPSH, i.e. the required net positive suction head (NPSH R ), which is defined as the minimum NPSH necessary to avoid cavitation in the pump. Typically, the NPSH R is defined as the situation in which the total head decreases by some arbitrarily selected percentage, usually about 3 to 5%, due to cavitation. Although the pump system operates under the NPSH safety margin, it does not ensure the absence of cavitation, i.e. there might be light cavitation that does not give rise to severe hardware damage. However, further reduction in the NPSH R will lead to a major deterioration in the hydraulic performance. 0.50.60.70.80.91.01.1 0.0 0.4 0.8 1.2 1.6 2.0 2.4 Percentage of head-drop Computation Experiment 5.770 5.324 5.274 5.341 5.513 5.034 5.888 5.609 Constant rotational speed Cavitation parameter, σ Flow coefficient, φ Fig. 11. Cavitation performance characteristics of a mixed-flow pump: NPSH R curve Application of Computational Fluid Dynamics to Practical Design and Performance Analysis of Turbomachinery 239 This article employs an about 5% head-drop criterion to define the NPSH R for a mixed-flow pump. Figure 11 shows the performance characteristic curves for the NPSH R under the operating flowrate conditions. From this figure, it is noted that the NPSH R for a newly designed pump with the highest pump efficiency is minimized near the design flowrate regime. 5. Conclusions A practical design and performance analysis procedure of a mixed-flow pump, in which the conceptual approach to turbomachinery design using the meanline analysis is followed by the detailed design and analysis based on the verified CFD code, has been presented in this Chapter. Performance curves predicted by a coupled CFD code were compared with the experimental data of a designed, hydrodynamically efficient, mixed-flow pump. The results agree fairly well with the measured performance curves over the entire operating conditions. A study for the cavitation performance characteristic curves of a mixed-flow pump has also been successfully carried out, although further research is definitely needed to suppress the tip-vortex cavitation under the normal condition. The design and predictive procedure, including cavitation, employed throughout this study can serve as a reliable tool for the detailed design optimization and assist in the understanding of the operational characteristics of general purpose hydraulic and compressible flow turbomachinery. 6. Acknowledgements The author would like to thank Dr. E S YOON of the Korea Institute of Machinery and Materials (KIMM) for his advice and support and it is also gratefully acknowledged that Dr. K S KIM and Dr. J W AHN of the Maritime and Ocean Engineering Research Institute (MOERI) provide the experimental data for a mixed-flow pump to publish this Chapter. 7. References Aungier, R. H. (2000). Centrifugal Compressors: A Strategy for Aerodynamic Design and Analysis, American Society of Mechanical Engineers Press, ISBN 0791800938, New York Balje, O. E. (1981). Turbomachines: A Guide to Design, Selection, and Theory, John Wiley, ISBN 0471060364, New York Japikse, D. (1994). Introduction to Turbomachinery, Concepts ETI, ISBN 0933283067, Norwich Neumann, B. (2005). The Interaction between Geometry and Performance of a Centrifugal Pump, John Wiley, ISBN 0852987552, New York Oh, H. W. & Kim, K-Y. (2001). Conceptual design optimization of mixed-flow pump impellers using mean streamline analysis. Proc. IMechE, Part A: J. Power and Energy, 215, A1, 133-138, ISSN 09576509 Computational Fluid Dynamics 240 Stepanoff, A. J. (1993). Centrifugal and Axial Flow Pumps: Theory, Design, and Application, Krieger Publishing Company, ISBN 0894647237, Florida 11 Hydrodynamic Simulation of Cyclone Separators Utikar 1 , R., Darmawan 1 , N., Tade 1 , M., Li 1 , Q, Evans 2 , G., Glenny 3 , M. and Pareek 1 , V. 1 Department of Chemical Engineering, Curtin University of Technology, Perth, WA 6845, 2 Centre for Advanced Particle Processing, University of Newcastle, Callaghan, NSW 2308, 3 BP Kwinana Refinery Pty Ltd, Mason Road, Kwinana, WA 6167, Australia 1. Introduction Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction. Experimental and numerical studies have been carried out in the last few decades to develop a better understanding of the flow field inside the cyclone separators. In the early years, empirical models were built (e.g. Shepherd & Lapple, 1939; Lapple, 1951; Barth, 1956; Tengbergen, 1965; Sproul, 1970; Leith & Licht, 1972; Blachman & Lippmann, 1974; Dietz, 1981 and Saltzmann, 1984) to predict the performance of industrial cyclones. However, these models were built based on the data from much smaller sampling cyclones therefore; they could not achieve desired efficiency on industrial scales as the industrial cyclone operates in the turbulent regime while sampling cyclones operate under the transitional conditions. One of the major drawbacks of these empirical models is the fact that they ignore two critical factors that determine the performance of a cyclone namely the unsteadiness and asymmetry. Many flow phenomena such as high turbulence, flow reversal, high Computational Fluid Dynamics 242 vorticity, circulating zones and downflow also occur. The empirical models do not include these phenomena in their analysis and hence are limited in their application. Computational fluid dynamics (CFD) models on the other hand can accurately capture these aspects and thus can take a significant role in analyzing the hydrodynamics of cyclone separators. A validated CFD model can be a valuable tool in developing optimal design for a given set of operating conditions. However, cyclone separators pose a peculiar fluid flow problem. The flow in cyclone separators is characterized by an inherently unsteady, highly anisotropic turbulent field in a confined, strongly swirling flow. A successful simulation requires proper resolution of these flow features. Time dependent turbulence approaches such as large eddy simulation (LES) or direct numerical simulation (DNS) should be used for such flows. However, these techniques are computationally intensive and although possible, are not practical for many industrial applications. Several attempts have been made to overcome this drawback. Turbulence models based on higher-order closure, like the Reynolds Stress Model, RSM, along with unsteady Reynolds averaged Navier – Stokes (RANS) formulation have shown reasonable prediction capabilities (Jakirlic & Hanjalic, 2002). The presence of solids poses additional complexity and multiphase models need to be used to resolve the flow of both the phases. In this chapter we review the CFD simulations for cyclone separators. Important cyclone characteristics such as the collection efficiency, pressure and velocity fields have been discussed and compared with the experimental data. Several significant parameters such as the effect of geometrical designs, inlet velocity, particle diameter and particle loading, high temperature and pressure have also been analysed. The chapter discusses peculiar features of the cyclone separator and analyses relative performance of various models. Finally an example of how CFD can be used to investigate the erosion in a cyclone separator is presented before outlining general recommendations and future developments in cyclone design. 2. Basic design of cyclone separators A cyclone separator uses inertial and gravitational forces to separate particulate matter from gas. Accordingly various designs have been proposed in literature (Dirgo & Leith, 1986). Figure 1 shows a schematic of widely used inverse flow cyclone and depicts main parts and dimensions. The particle laden gas enters the cyclone separator with a high rotational velocity. Different inlet configurations like tangential, scroll, helicoidal and axial exist to provide high rotational velocity. Of these, the tangential and scroll configurations are most frequent. The rotational flow then descends near the wall through the cyclone body and conical part until a reversal in the axial velocity making the gas flow in the upwards direction. Where this occurs is called as the vortex end position. The upward rotating flow continues along the cyclone axis forming a double vortex structure. The inner vortex finally leads the flow to exit through a central duct, called the vortex finder. The vortex finder protrudes within the cyclone body. It serves both in shielding the inner vortex from the high inlet velocity and stabilizing its swirling motion. The solids are separated due to the centrifugal force and descend helicoidally along the cyclone walls and leave the equipment through the exit duct. [...]... et al 199 2) At increased particle concentration, the tangential velocities will be lower and accordingly will yield a lower pressure drop Gimbun et al (2005) studied the effect of the inlet velocity and particle loading on pressure drop They compared experimental values by Bohnet ( 199 5) with empirical models by 254 Computational Fluid Dynamics Shepperd and Lapple ( 193 9), Casal and Martinez ( 198 3), Dirgo... Duty D a/D b/D De/D S/D h/D H/D B/D Stairmand ( 195 1) High efficiency 1 0.5 0.2 0.5 0.5 1.5 4 0.375 Stairmand ( 195 1) High throughput 1 0.75 0.375 0.75 0.875 1.5 4 0.375 Lapple ( 195 1) General purpose 1 0.5 0.25 0.5 0.625 1 4 0.25 Swift ( 196 9) High efficiency 1 0.44 0.21 0.4 0.5 1.4 3 .9 0.4 Swift ( 196 9) General purpose 1 0.5 0.25 0.5 0.6 1.75 3.75 0.4 Swift ( 196 9) High throughput 1 0.8 0.35 0.75 0.85 1.7... cyclone using (a) uniform particle size of 80µm at the inlet and (b) Rosin – Ramler particle size distribution at the inlet with average particle size of 80µm The results show that the particles follow a distinct path rather than 260 Computational Fluid Dynamics cluttering on the cyclone walls while swirling down For the uniform particle size, most severely eroded sites were the top part of the cyclone cylinder... and particle flow rate 262 Computational Fluid Dynamics At lower gas velocities, lower momentum is imparted by the gas on the particle, which sometimes prevents a second rebound to happen and the particle is forced to stay near the cyclone wall Consequently, the rate of erosion is lesser at lower velocities As the gas velocity is increased, the particle rebound is more likely to happen Since the particle... 247 Fig 3 Comparison of tangential velocity profiles (Adapted from Hoekstra et al., 199 9) form when the extra cost of the calculation is affordable (Hogg & Leschziner, 198 9) Within the differential RSMs, the difference between a basic and an advanced differential RSM is also of relevance For example, Grotjans et al ( 199 9) compared the predictions of various turbulence models with LDA measurements for... the particle loading The result is consistent with most of the previous studies (like Stern et al 195 5) Different mass loading for various particle group-sizes affect the grade efficiency differently Smaller particle group sizes show a higher efficiency increase compared to the larger particle size groups These findings are also confirmed by the simulation and experimental study by Luo et al ( 199 9)... et al (20 09) The increase in cyclone efficiency with solid loading is more pronounced at lower gas velocities (Hoffmann et al 199 1, 199 2) Fig 10 Separation efficiency simulation result for various inlet particle concentration with constant inlet velocity (Adapted from Qian et al., 2006) Mass loading effect is usually coupled with the particle diameter At lower mass loadings, the smaller particles ( . Shepherd & Lapple, 193 9; Lapple, 195 1; Barth, 195 6; Tengbergen, 196 5; Sproul, 197 0; Leith & Licht, 197 2; Blachman & Lippmann, 197 4; Dietz, 198 1 and Saltzmann, 198 4) to predict the performance. Typical design of cyclone separator Source Stairmand ( 195 1) Stairmand ( 195 1) Lapple ( 195 1) Swift ( 196 9) Swift ( 196 9) Swift ( 196 9) Duty High efficiency High throughput General purpose. streamline analysis. Proc. IMechE, Part A: J. Power and Energy, 215, A1, 133-138, ISSN 095 765 09 Computational Fluid Dynamics 240 Stepanoff, A. J. ( 199 3). Centrifugal and Axial Flow Pumps: