Browsing by Author "Tourlidakis, A."
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Item Open Access Analysis of a 115MW, 3 shaft, helium Brayton cycle(Cranfield University, 2002-07) Pradeepkumar, K. N.; Pilidis, Pericles; Tourlidakis, A.This research theme is originated from a development project that is going on in South Africa, for the design and construction of a closed cycle gas turbine plant using gas-cooled reactor as the heat source to generate 115 MW of electricity. South African Power utility company, Eskom, promotes this developmental work through its subsidiary called PBMR (Pebble Bed Modular Reactor). Some of the attractive features of this plant are the inherent and passive safety features, modular geometry, small evacuation area, small infrastructure requirements for the installation and running of the plant, small construction time, quick starting and stopping and also low operational cost. This exercise is looking at the operational aspects of a closed cycle gas turbine, the finding of which will have a direct input towards the successful development and commissioning of the plant. A thorough understanding of the fluid dynamics in this three-shaft system and its transient performance analysis were the two main objectives of this research work. A computer programme called GTSI, developed by a previous Cranfield University research student, has been used in this as a base programme for the performance analysis. Some modifications were done on this programme to improve its control abilities. The areas covered in the performance analysis are Start-up, Shutdown and Load ramping. A detailed literature survey has been conducted to learn from the helium Turbo machinery experiences, though it is very limited. A critical analysis on the design philosophy of the PBMR is also carried out as part of this research work. The performance analysis has shown the advantage, disadvantage and impact of various power modulation methods suggested for the PBMR. It has tracked the effect of the operations of the various valves included in the PBMR design. The start-up using a hot gas injection has been analysed in detail and a successful start region has been mapped. A start-up procedure is also written based on this. The analysis on the normal and emergency load rejection using various power modulation devices has been done and it stress the importance of more control facilities during full load rejection due to generator faults. A computational fluid dynamics (CFD) analysis, using commercial software, has been carried out on some geometry of the PBMR design to find out whether its flow characteristic will have any serious impact on the performance on the cycle during the load control of the plant. The analysis has demonstrated that there will not be much impact on the performance, during load control using pressure level changes, from this geometry. However, some locations in the geometry have been identified as areas where the flow is experiencing comparatively high pressure losses. Recommendations, which include modification in the physical design, were made to improve this. The CFD analysis has extended to a cascade to compare the flow behaviour of Air and Helium with an objective of using air, being inexpensive, to test the helium flow characteristic in a test rig to simulate the behavioural pattern of helium in the PBMR pressure vessel. The specification of a hypothetical test rig and the necessary scaling parameters has been derived from this exercise. This will be useful for designing test rigs during the developmental and operational stage of the PBMR project.Item Open Access Computational analysis of high-speed axial-flow compressors using two deterministic stress models(2002-01) Amaral Teixeira, J. A.; Elder, R. L.; Tourlidakis, A.The employment of CFD methods for the analysis of multistage turbomachinery flows is restricted for the foreseeable future to steady-state methods of which the most widespread one employs the mixing-plane approach. The last method nevertheless is known to introduce jumps in some quantities at the stage interface due to the unphysical pitchwise averaging process it models and when employed in the simulation of multi-stage machines leads to a progressive downstream degradation of the solution accuracy. Adamczk’s average-passage equation system is the steady-state approach which fundamentally overcomes these limitations but the requirement that a set of machine- wide domains be iteratively solved as part of the calculation of each of the rows of interest, entails large computational requirements. A far more economical solution is to combine some of Adamczyk’s equation terms accounting for the cyclic components of the unsteady stage effects, the deterministic stress terms, with a mixing-plane steady-state formulation. This is the approach taken by the two semi- empirical models which were employed in this work to study two high-speed axial- flow compressor configurations, the single-stage Rolls-Royce HP9 and the novel three stage Cranfield University High Speed Research Compressor. The Hall and the Bolger deterministic stress models were combined with the CFD turbomachinery code CFX-TASCflow via a Source Code Interface and results were obtained for the two cases with and without the application of the models. Although the impact of the addition of deterministic stress terms to the solution of HP9 is noticeable but small, these effects are in the Cranfield machine more noticeable and seen to offer some contribution towards the approximation of the computational to the experimental global performance results.Item Open Access Design optimisation of axial flow steam turbines(2004-05) Eyre, C. J.; Tourlidakis, A.This study at Cranfield University was initiated by my sponsoring company’s desire to improve its Steam Turbine design capability. In particular it was thought that emerging Computational Fluid Dynamics software might provide an alternative and improved approach to existing turbomachinery design techniques in use. My course of study has enabled me to understand the fundamentals of the numerical approaches and methods employed by the commercially available software codes. The brief was to review and if appropriate select and implement a CFD solution into Peter Brotherhood Ltd. The current range of steam turbines have power outputs ranging from 500 kW to 30 MW. These machines are primarily Rateau impulse turbines operating under a pressure compounding arrangement. More recent developments particularly on condensing machines incorporating twisted and tapered blades have led to partial reaction stages being used. Most of the machines produced today are impulse combined with partial reaction. After a review of software vendors AEA TASCflow software was purchased and was used throughout this study. This work concentrates on the technical design o f steam turbine nozzles and blading. It proposes alternative blade and nozzle geometry along with new methods of construction and manufacture. It is recognised however that in order to evaluate nozzle performance it is necessary to consider the downstream blade and thus the performance of the complete turbine stage. Throughout comparisons are made with existing fortran PITCH software described in chapter 1. A literature survey investigated many approaches and factors that can improve turbine efficiency and power output. A selection of these more applicable to the smaller power output designs of turbine produced by Peter Brotherhood Ltd have been investigated to evaluate their merits. These are outlined in chapters 2, 3 and 4. Results from these studies indicate that new complex geometry nozzles when matched to improved blading with improved flow incidence angles, correct axial spacing and casing shroud flaring can lead to stage power increases of over 15%. CFD has provided a much improved insight into the three dimensional aspects and flow phenomena. The introduction of CFD has provided a boost to the design capability and it is used regularly with confidence as a development tool within turbomachinery research.Item Open Access Design optimisation of centrifugal pump impellers using parallel genetic algorithm(Cranfield University, 2001-09) Wahba, Waleed; Elder, R. L.; Tourlidakis, A.Computational Fluid Dynamics (CFD) techniques have settled to a stage, where it is possible to gain significant insight into fluid flow processes of turbomachinery. However, the purpose of fluid dynamics naturally goes beyond improved understanding to the aim of improving the performance of the engineering systems. Consequently, the present thesis investigates the use of a automated design optimisation method using CFD. This presents a new design method for a important turbomachinery part, blade profiles of centrifugal pump impellers, based on a shape optimisation algorithm in combination with CFD. The use of genetic algorithms in shape optimisation dose not allow the design engineer to use any derivative information on the evolution of the shape, but only simple evaluation techniques. A optimisation library (GAlib), based on a genetic algorithm (GA), was used. GA controls the evolution of a population of profiles towards an optimum design. The optimisation process can handle simple objectives as well as conicting ones. The fitness value of each population element is evaluated using a CFD flow solver (Mac_LNS) based on nite-difference discretisation of the incompressible, Navier-Stokes (N-S) equations on structured polar-coordinate meshes. A number of design examples have been developed and the behaviour of the genetic algorithm has been tested using different kinds of objective functions. I addition, the algorithm was tested with a multi-objective mction. Bézier curves were selected to represent the impeller profile. A symmetric profile, identical profile for the pressure side (PS) and suction side (SS), was used as a basic shape to generate the population elements. GAlib was modified to run as a parallel algorithm using Message Passing Interface (MPI). It is indicated that parallelisation using MPI is good technique to overcome the time taken by GA and CFD, and quite good optimisation convergence criteria was obtained by using parallelisation. The obtained results show that the genetic algorithm is capable of achieving satisfactory designs of centrifugal impeller blade profiles effectively and with a minimum amount of user expertise.Item Open Access Influence of deterministic stresses on flow prediction of a low-speed axial flow compressor rear stage.(Professional Engineering Publishing, 2001-10-02T00:00:00Z) Li, Y. G.; Tourlidakis, A.Deterministic stresses are used to describe the unsteady effects on the flow field in multistage axial flow compressors. In this paper, a circumferentially non-uniform mixing plane method implemented with deterministic stresses is used for the flow prediction inside the rear stage of an axial flow compressor in a multistage environment. The deterministic stress field is calculated with an overlapped solution approach for the same flow passage and implemented with the Navier-Stokes equations in order to provide continuous interfaces. The aerodynamic boundary condition is simplified with the application of a repeating stage model where only the mass flowrate and stage exit average static pressure are required. Two computational cases for the third stage of the Cranfield low-speed research compressor, one with the mixing plane model and the second with the mixing plane model in combination with deterministic stresses, are examined and compared with each other and also with corresponding experimental data. It is shown that the implementation of the deterministic stresses is beneficial for the flow prediction but the improvement in accuracy for low-speed axial flow compressor rear stages is not significant.Item Open Access Numerical investigation of recess casing treatments in axial flow fans(Cranfield University, 2003-02) Ghila, Abdurazag M.; Tourlidakis, A.; Elder, R. L.T he casing treatment technique for the axial fan has never been more significant since its potential applications were recognized in gas turbines, tunnel ventilation and many other industrial applications where the axial fan would benefit from the casing treatment. In the last two decades experimental investigations were carried out at Cranfield University to examine the influence of recess casing treatment on stall margin, operating efficiency and flow field of a low-speed axial flow fan. They showed more than 50% improvement in the stall margin with a negligible loss in the efficiency. However, a little work has been done on the numerical simulation of casing treatments due to its complexities, even though in recent years computational fluid dynamics [CFD] analysis has been very active in the prediction of various phenomena in turbomachinery. This work presents numerical investigation of flow in a single axial-flow fan with and without recess casing treatment. It involves the detailed effect of the recess casing on stall margin improvement as well as its influence on global performance parameters. The project offers a contribution to the understanding of the physical processes occurring when approaching stall and the working mechanism by which recess casing treatments improve stall margin. A Reynolds-Averaged Navier-Stokes CFD code was used for the analysis using steady and unsteady simulations. The numerical investigation of the overall performance, efficiency and work-input characteristics of the fan were found to agree very well with the previously reported experimental results. The effect of casing treatment was investigated using two types of configurations, vaneless and vaned casing. The vaneless casing treatment produced a sizeable stall margin improvement with a measurable loss in both pressure rise and efficiency. The recess was fitted later with vanes and was shown to offer both a further stall margin improvement and an increase in the pressure rise coefficient without any significant drop in efficiency at design conditions. The effect of number of vanes inside the recess was also investigated by doubling and halving the number of vanes originally adopted. The predicted results highlighted the importance of the vane inside the casing. Unsteady simulations for the fan with solid and treated casing were carried out. The solid casing simulated for a single blade passage as well as for the entire fan containing all 27 blades highlighted the flow physics of the tip stall growth process, as a large amount of radial flow injected from the hub at the blade suction side near the trailing edge towards the outer casing and occupy this through a mechanism of radial low momentum flow transport. This transport process is the main contributor to the very large separation observed in the shroud region in addition to the locally induced separation due to high blade loading and tip clearance. Although the examination of the unsteady simulation of the recess treatment cavities does not offer an image of large scale unsteady activity at the flow condition investigated, this is on itself quite significant and enables the drawing of an important conclusion namely that large casing treatments rely primarily on a steady-state flow process. The corollary of this conclusion is of course that a steady-state simulation should then be sufficient to capture the essential features of the recess treatment.Item Open Access Numerical modelling of viscous turbomachinery flows with a pressure correction method(Cranfield University, 1992-03) Tourlidakis, A.; Elder, R. L.; Lobo, M.A fully elliptic computational method for the analysis of steady viscous flow in high speed subsonic centrifugal compressor impellers with tip leakage, is presented. A generalised curvilinear, non-orthogonal grid is utilised and the timeaveraged Navier-Stokes equations are transformed and expressed in a fully conservative form. The discretisation of the governing equations is performed through finite volume integration. The solution procedure employs a non-staggered variable arrangement and a SIMPLE based method for coupling the velocity and pressure fields. The turbulence effects are simulated with the use of the k-e model, modified to account for rotation and streamline curvature, and the near-wall viscous phenomena are modelled through the wall function method. The numerical model is implemented for the flow prediction in a series of two and three dimensional test cases. Incompressible flow predictions in twodimensional cascades and three-dimensional ducting systems with different geometrical features and inlet conditions are initially performed and the numerical results are compared against available experimental data. The final objective of the present study is achieved through the comparative study of the predictions obtained against the results of Eckardt's experimental investigation of the viscous compressible flow in a high speed radial impeller operating at design condition and in a backswept impeller at design and off-design conditions. In addition, the flow is simulated in the passages of the Rolls Royce GEM impeller which was tested at Cranfield at design and off-design flow rates. A jet/wake pattern was discerned in all the simulated centrifugal compressor cases and a good overall agreement was achieved with the measured wake formation and development; and, encouraging results were obtained on the evolution of the secondary flows. The tip leakage effects influenced the loss distribution, the size and the location of the wake flow pattern at the rotor exit. The effects of the flow mass rate on the detailed flow pattern and on the compressor performance have been well represented. In certain cases, the quality of the present predictions is an improvement over that obtained by other "state-of-the-art" Navier-Stokes solvers. In conclusion, the developed finite volume flow model has captured a large number of complex flow phenomena encountered in the tested impellers and is expected to provide a useful aerodynamic analysis tool for stationary or rotating, axial or radial turbomachinery components.Item Open Access A robust and accurate Navier-Stokes algorithm for three-dimensional applications adopting arbitrary modelling of the Reynolds stresses(Cranfield University, 1999-09) Botte, Vincenzo; Tourlidakis, A.In this thesis a new Navier-Stokes solver for complex three-dimensional geometries, adopting arbitrary modelling of the Reynolds stresses, is presented. Ak-s model, adopting a modelling of the turbulent transport not based on the eddy viscosity, has been written in generalised coordinates and solved with a finite volume approach, using both a GMRES solver and a direct solver for the solution of the linear systems of equations. The results presented show that the modification adopted for the modelling of'the turbulent transport also provides a more accurate value of the physical diffusion and, as a consequence, improves the increase in accuracy when using higher-order convection schemes. A simple non-linear modelling of the Reynolds stresses has been designed introducing an additional term, quadratic in the main strain rate, to the basic Boussinesq's form; the corresponding constant has been evaluated through comparison with experimental data. The computational procedure is implemented for the flow analysis in a 90 ° square section bend and the obtained results show that with the non- linear modelling a much better agreement with the measured data is obtained, both for the velocity and the pressure. The importance of the convection scheme is also discussed, showing how the effect of the non-linear correction added to the Reynolds stresses is effectively hidden by the additional numerical diffusion introduced by a low- order convection scheme as the first-order Upwind, thus making necessary the use of higher-order schemes. Some results for centrifugal turbo machinery are also presented, giving some initial indications on the effects of the proposed modification in the modelling of the turbulent diffusion on the prediction of the flow in rotating passages.Item Open Access Three-Dimensional Flow and Performance Simulation of Multistage Axial Flow Compressors(Cranfield University, 2000-03) Li, Yiguang; Elder, R. L.; Tourlidakis, A.\Yith the current develop111ent in computer technology and Computational Fluid D)"n<'tlllics techniques, t.he si11utlation within axial flow compressors becomes 1110re and 1110re pract.ical and beneficial to the compressor designs. Due to the insufficient capabilit)" of today's COll1put.ers for three-dimensional unsteady flow 1110delling of 111Ult i~Llg(' axial flow compressors, sophisticated models of steady state flow and perfor111ance 1110delling of the C0111prcssors deserve to be thoroughly investigated. In l1utltistage C0111pressor sinlulations with steady state methods, frame of reference is fixed on blades and the c0111putational domains for rotors and stators haye relati\"e rotation. One of the difficulties in such simulations is how to pass information across the interfaces between blade rows without losing continuity. Two 111ajor stead)" state modelling approaches, a mixing plane approach based on Denton's circu111ferentially non-uniform mixing plane model and a deterministic stress approach based on Adamczyk's average passage model, are investigated and compared with each other through the flow predictions of the third stage of Cranfield Low Speed Research Compressor at peak efficiency operating condition. In the deterministic stress approach, overlapped solution domains are introduced to calculate deterministic stresses in order to "close" the time-averaged governing equation system and the influence of the downstream blade row of the blade row under investigation has to be imposed through the simulation of bodyforce and blade blockage effect of the downstream blade row. An effective method of simulating bodyforce and blade blockage effect has been developed and proven to be simple in programming. ConYentionally, boundary conditions are specified in CFD calculations based on experimental data or other empirical calculations. By taking advantage of the special flow features in rear stages of multistage axial flow compressors where each rear stage behaves like a repeating stage of its neighbouring stages in terms of flow pattern at the inlet and the exit of these stages, a repeating stage model has been developed aiming at significantly simplifying the boundary conditions when simulating rear stages of a multistage axial flow compressor with only mass flow rate and stage exit average static pressure required as global input. A computer simulation system 1'/ STurbo3D has been developed to investigate a11d assess different steady state simulation models within multistage compressor environment. It has been proven that with the mixing plane model M STurbo3D is able to predict flows in multistage low speed axial flow compressors with acceptable accuracy. Application of the repeating stage model to the third stage of LS RC shows that the prediction with this model has equivalent accuracy to the prediction with the conventional boundary setting, and proves that the repeating stage model is an effective alternative to the expensive complete compressor simulation. The deterministic stress model provides more information of rotor-stator interaction and slightly better performance prediction than the mixing plane model, but the benefits of the model is not significant when applied to low speed axial flow compressors.