Browsing by Author "Elder, R. L."
Now showing 1 - 20 of 34
Results Per Page
Sort Options
Item Open Access Application of a vane-recessed turbular-passage casing treatment to a multistage axial-flow compressor(Cranfield University, 2001-11) Akhlaghi, M; Elder, R. L.The current study investigates a range of issues relating to the use of a vane-recessed tubular-passage casing treatment as a passive stall control technique in a multistage axial-flow compressor. The focus of the research was to determine whether such a treatment could delay the initiation of stall at lower mass flow rates as well as providing the most beneficial improvement in flow characteristics without sacrificing compressor efficiencies. Specific objectives of this study were to examine possible improvements or deterioration in the flow characteristics including stall margin, peak pressure rise coefficients and maximum efficiency in a multistage axial flow compressor. A casing treatment in addition to several spacer rings was developed from two initial designs and tested on the first stage of a low speed three-stage axial-flow compressor with a (0.7) hub to tip diameter ratio. The treatment configuration consisted of three parts: an outer casing ring, with a tubular shaped passage on the inside diameter, a set of 120 evenly spaced curved vanes, and a shroud or inner ring. The casing treatment was positioned following the inlet guide vanes upstream and partly covering the tip of the rotor blades. The main parts of the casing treatment including the recessed vanes in addition to some of the spacer rings were manufactured from high quality acrylic. Eight additional spacer rings of various shapes and geometry were added. The first ring held and partly covered the IGVs, in front of the casing treatment. The rotor tip exposure ratio was thought to have a significant impact on the effectiveness of the casing treatment. Therefore the other seven rings were used to provide the desired uncovered region of the rotor tip axial chord of about 10% in order to provide a range of exposures of (23.2%, 33.3%, 43.4%, 53.5%, 63.6%, 73.7%, and 83.8%). The results showed significant improvements in stall margin in all treated casing configurations along with insignificant efficiency sacrifices in some compressor builds. About (28.56%) of stall margin improvement in terms of corrected mass flow rate was achieved using a casing treatment with a (33.3%) rotor tip exposure. The compressor build with (0.535) rotor exposure ratios was the best configuration in terms of efficiency gain and loss characteristics. This build was able to provide the highest values of the maximum efficiencies in comparison with the performance achieved from the solid casing. An improvement of (1.81%) in the maximum efficiency in terms of the overall total-total pressure ratio, in association with a (22.54%) stall margin improvement in terms of the corrected mass flow rates were achieved by the application of this treatment configuration. The improvement in the peak pressure rise coefficients in terms of the overall total-total pressure ratio, obtained from this build was (2.33%). The compressor configuration using a casing treatment with a (0.636) rotor exposure ratio was the best build in terms of the pressure rise coefficients. This configuration was able to provide highest value of the peak pressure rise in comparison with the characteristics achieved from the datum build. An improvement of (2.65%) in the peak pressure rise coefficient in terms of the overall total-total pressure ratio, in association with a (22.49%) improvement in stall margin in terms of the corrected mass flow rates was achieved from this casing treatment build. The improvement in maximum efficiency in terms of the overall total-total pressure ratio, obtained from this build was (1.03%). The results suggest that the vane-recessed tubular-passage casing treatment designed as part of this investigation achieved the objectives, which were established for the research. In the majority of instances it not only produced gains in flow range, pressure rise coefficients and efficiencies, but also enabled the rotating stall, which developed at much lower mass flow rates in the compressor, to become progressive rather than abrupt.Item Open Access Application of recess vaned casing treatment to axial flow compressors(Cranfield University, 1987-12) Azimian, A. R.; Elder, R. L.; McKenzie, A. B.In axial flow compressors and fans, the stable working range is restricted by the so called stall line where operation may become unstable and simultaneously a short fall in compressor performance appears. Stall margin improvement has been a major task and experimental observations over the last two decades have shown that modifying the shape of the outer annulus wall, above the tips of rotor blades, is effective in delaying the onset of stall to lower mass flow rates. A wide variety of wall modifications or casing treatments have been tried with a range of stall flow improvement which can amount to about 20% .(of the stalled flow) An alternative technique for stall margin improvement in a single stage axial flow machine has been studied and results will be described in the present thesis. The technique for delaying stall involves a large scale treatment to the outer casing of the compressor which extends partly over and mainly upstream of the rotor. The operating principle appears to be that as the flow is reduced towards the stall point some radially unbalanced cells are transported from the blade tips where they are collected in the treatment , turned by the treatment vanes , and re-introduced to the main flow upstream of the rotor. A rotor with and without outlet stators has been tested ( and also with and without casing treatment) over a range of speeds and flow conditions. Also the flow condition inside the recessed casing has been simulated by means of an existing computer code known as PHOENICS.Item Open Access Cavitation performance of pumped hydrocarbons(Cranfield University, 1997) Lisle-Taylor, S. C.; Elder, R. L.; Rhodes, I. N.; Salisbury, A. G.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 The design and analysis of pipe diffusers for centrifugal compressors(1997-12) Bennett, Ian; Elder, R. L.Diffusers are necessary to convert the high velocity head of centrifugal compressor discharge flow into useful static pressure. One diffuser type that has shown a great deal of promise, particularly at high pressure ratios, are pipe diffusers. These have been successfully incorporated into centrifugal compressor stages in North America since the early 1970’s. With their commercial sensitivity, however, little design or detailed flow information has been published, and most of this literature is old, dating back twenty years to the design’s conception. This thesis is a compilation of pipe diffuser information, formed from the limited literature works and an intensive experimental parametric study. Two themes run through this thesis and are brought about by different reader needs. The first need is for design information. This must show the strengths and weakness of pipe diffusers and be of sufficient accuracy to potentially produce designs at the first attempt. The second need is to understand the flows within the diffuser itself, in order that through this knowledge further improvements can be made. Both of these requirements are addressed in this thesis. The studies are centred on, what was originally, a Rolls-Royce research centrifugal compressor impeller of 6:1 pressure ratio operating at 35,000 rpm with a tip speed of 533ms'1 consuming up to a nominal 1MW of power. Significant modifications to the original compressor assembly were made to allow modular changes of diffuser and permit detailed measurement access. Initially, a vaneless diffuser investigation was carried out with the primary aim of determining the flow entering the diffusers. This was followed by a diffuser throat sizing exercise as it is well recognised that the diffuser throat has a governing control over the complete stage performance. An exercise in changing the number of diffuser pipes was then undertaken, followed by a hybrid diffuser investigation incorporating an oval, rather than circular, cross-section. In all of these cases, detailed flow measurements were carried out using both conventional pneumatic instrumentation and a purposely developed unsteady pressure measurement data acquisition system. This system enabled detailed traverses of the diffuser throats to be undertaken. These unique measurements shed new light on the flow ‘seen’ by the pipe diffuser channel. Computational investigations using computational fluid dynamics (CFD) codes are carried out to compliment the experimental investigations.Item Open Access Design considerations for a three dimensional fiber optic laser Doppler velocimeter for turbomachinery applications(American Institute of Physics (AIP), 1997-12-31) James, Stephen W.; Tatam, Ralph P.; Elder, R. L.Single headed three dimensional (3D) laser Doppler velocimetry (LDV) geometries generally rely upon the use of three Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the nonorthogonal measurement coordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed fiber optic 3D LDV probe based on the use of two Doppler difference channels to directly measure the transverse velocity components, and a reference beam channel to measure the on axis velocity component. The f/4 probe head has a working distance of 200 mm, designed to operate within the constraints of the limited optical access available in turbomachinery applications.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 Dissipation and discretization in time marching CFD calculation(Cranfield University, 1995-07) Alimin, E. K.; Elder, R. L.; Stow, P.; Ho, Y. K.This thesis concentrates on accuracy improvements for an existing software package that solves the three dimensional Reynolds Averaged Navier-Stokes equations in rotating coordinates. It is a cell centred explicit time marching code. Two topics are considered: improvement to the discretization scheme, and reduction of the artificial dissipation. The first topic is the analysis of the straight averaging process which demonstrates that the process can result in inconsistency with a skewed grid. An alternative consistent scheme is proposed which is based upon quadratic interpolation. Improved accuracy can also be obtained by modifying the grid or adopting a cell vertex scheme. The stability of the iterative process is also shown to depend on the time step. The reduction of artificial dissipation (second topic) first considers the role of the so called aspectratio and velocity functions. These are found to be limited in influence and a new function is proposed based upon the local flow gradient. Both two and three dimensional turbomachinery cases are tested and improvements demonstrated. In the second part of the analysis, the eigenvalues of the stability matrix are used to reduce the dissipation in overdamped regions. Again this method is applied to various test cases and improvements demonstrated. The management part of this Total Technology PhD Program discusses topics concerned with collaboration and technology development in the aero engine industry with particular emphasis on the role of an "emerging" partner.Item Open Access Dynamic simulation of centrifugal compressors in a process environment(Cranfield University, 1984-10) Razak, A. M. Y.; Elder, R. L.The recent developments in process plant design have made it desirable that a better understanding of transient compressor performance within the process plant be gained. The model outlined in this thesis is capable of representing most types of process plants. Further the number of degress of freedom, plus the dynamic flexibility of a poly- tropic analysis allow any system transient to be simulated, including compressor surge. A computer program has been developed from the model and validated using a small centrifugal compressor. The results showing successful simulation of compressor transient behaviour (including surge) are given. The model was then applied to study the transient behaviour of a natural Gas Transmission station. The model successfully highlighted compressor surge problems under certain operative conditions. This surge phenomenon is present in the trans- mission station and a possible solution to the problem was suggested due to the better understanding of dynamic response of station.Item Open Access Erosion in centrifugal compressor impellers(Cranfield University, 1996-01) Harris, P. K.; Tan, S. C.; Elder, R. L.An experimental and theoretical study of erosion in centrifugal compressor impellers is presented. An experimental rig using laser anemometry techniques was employed to create a database of particle restitution ratios for a range of materials. This data was unique in that the particle rebound was measured in a quiescent condition where the aerodynamic effects had been minimised, and also parametric factors not previously available were included. These values were incorporated into the existing Particle Trajectory Code developed by Cranfield University and Rolls Royce PLC. The code is used to calculate the trajectories of discrete particles in three dimensional gas turbine geometries, and the ensuing erosion. It was modified to include the effects of the periodic boundary conditions, particle fragmentation, splitter blades, and variations in inlet dust concentration profile. Flowfield calculations were performed on a Rolls Royce GEM-2 and splittered GEM-60 impeller, which both represent the high pressure stage of the axial + centrifugal compression system of GEM engines. A procedure developed by Tourlidakis, for the analysis of steady viscous flow in high speed centrifugal compressors with tip leakage, was used to generate the flowfields. The GEM-2 impeller flowfield was analysed at 1009c speed, and validated with calculations and measurements which had been taken for previous projects. Simulated erosion data under the same conditions was checked using practical results obtained in a Rolls Royce PLC Helicopter Engine Environmental Protection Programme, and good agreement was achieved. In order to provide a qualitative, experimental assessment of erosion, a GEM-60 impeller was coated with four layers of paint of different colours. Two sizes of quartz particle, each at three different vane heights, were then seeded into the impeller while it was run cold at (the maximum) 70% speed. The erosion patterns generated compared well with the results generated by the Particle Trajectory Code.Item Open Access Gas liquid separation within a novel axial flow cyclone separator(Cranfield University, 1998) Dickson, Philip James; Elder, R. L.; McNulty, Gerry; Sarshar, Sacha; Ellix, DavidCyclone separators have been described in detail and, although substantial research has been performed on solid / gas devices, the use of cyclones for gas / liquid separation has been comparatively ignored; this is particularly true for higher concentrations of liquid and for degassing applications. Consequently no generic models are available which will predict separation efficiency or pressure drop for all designs of cyclone. A novel design of axial flow cyclone called WELLSEP was examined for the purpose of degassing. This design was not believed to be optimal and no design criteria or performance prediction models were available for it. An experimental programme was therefore produced and executed to investigate changes in geometry and the affect of fluid dynamics. Changes to the length, vortex finder and swirl generator were examined first and then one design was selected and tested over a number of liquid flow rates, Gas Void Fractions (GVFs) and liquid extractions. Data was collected from the experiments which assisted in the development of semi-empirical models for the prediction of pressure drop and separation efficiency. These models could be used in the design of WELLSEP. Geometric and fluid dynamics changes have both been shown to influence the performance of the tested cyclone. The principal conclusions that have been drawn from this research are: " Of the tested designs, the design based upon a 30mm vortex finder diameter, settling chamber length of three times the diameter of the cyclone and a four start helix gave the optimum separation efficiency over the greatest range of conditions. 0 The separation efficiency is affected by the superficial liquid velocity and the liquid extraction but not the GVF. " The dimensionless pressure drop coefficient (Euler number) is a function of liquid extraction and GVF. It may also be a function of the superficial liquid velocity but it is unproven by this research.Item Open Access High temperature particle -to-metal interaction in a simulated gas turbine environment(Cranfield University, 2002-05) Salama, I. M.; Elder, R. L.; Tan, S. C.An experimental study of the particle-to-metal interaction during high temperatures and velocity impact conditions is presented. A novel continuous erosion testing facility have been used to study the effect of particle and metal target temperatures as well as impact particle velocity on the erosion/deposition behaviour of the stainless steel 321, Nimonic 75, and aluminium target materials. The study was carried out to provide database information on the behaviour of those metals under simulated gas turbine conditions. The erosive particles used were quartz sand with diameters ranging from 20-30 μm. The erosion characteristics of stainless steel 321 were recorded at target surface temperature of 285°C, 415°C, 570°C and 715°C. The tests were carried out at two different impingement angles of 30° and 60° and at particle impact velocities of up to 300m/s. The effects of particle temperatures of 550°C, 750°C and 950°C on erosion/deposition rates were examined. The Nimonic 75 target temperatures were slightly modified to give a similar surface to melting point ratio as the stainless steel. The Nimonic 75 was tested at 545°C, 685°C, 825°C and 965°C surface temperatures and at the same particle velocities and temperature used for the stainless steel tests. The Nimonic targets were only tested at one impact angle of 30°. The aluminium targets were only tested at an impact angle of 60° and particle impact velocity of 100 m/s. The surface temperature was modified to give a ratio up to 0.8 of the melting point temperature, where the particle temperature was set to be 350°C, 550°C and 750°C. It was found that particle and target temperatures, impact velocity and angle have a significant effect on the erosion/deposition characteristics. There is a threshold target and particle temperature for which deposition begins, and it depends on impact velocity and angle. The Nimonic 75 targets exhibit a better resistance to particle deposition over the stainless steel 321 at high impact velocity and temperatures. Simple models of the erosion/deposition were established to describe the conditions of particle deposition on the stainless steel and Nimonic targets. The aluminium targets show an increase in the erosion rate as target temperature reaches certain level, which then drops as target temperature continues to increase beyond this point.Item Open Access Impeller diffuser interactions in high speed centrifugal compressors(Cranfield University, 2001-09) He, Ning; Elder, R. L.In the current research work, a computational analysis of a high-speed centrifugal compressor stage for turbocharger applications is presented. A detailed investigation about the interactions between backswept impeller and downstream vaneless and vaned diffusers is carried out. ' A unshrouded backswept impeller with splitters was combined with a vaneless diffuser or a number of different designs of vaned diffusers. The CFD solver CFX-TASCow was used. The three-dimensional Reynolds- Averaged Navier-Stokes equations are solved and a pressure correction method is employed to solve the system of equations. A steady simulation and analysis of the interactions between the impeller and the vaneless diffuser is carried out, emphasis is focused on the comparisons of the different interactions at different conditions regarding the flow structures at different radius ratios, effect of rotational speed, mass flow rate and impeller tip clearance. The predicted results were also compared with the available experimental results in terms of radial Velocity, tangential Velocity and flow angle. In general, the predicted results show a reasonable agreement with the experimental data. A steady state simulation and analysis regarding the interaction between the impeller and various vaned diffusers is carried out. For the interface between the rotational impeller outlet and the stationary vaned diffuser inlet, the stage averaging condition is used. A detailed comparison between the predicted and the available experimental data is performed in terms of static pressure rise, total pressure ratio, choking mass flow and efficiency characteristics, and very good agreement is accomplished. In addition, detailed flow distributions are compared, assessed and critically analysed, regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate. Emphasis is focused on the steady state study of the effect of the number of diffuser vanes on the stage operating range. Further more, unsteady simulation and analysis regarding the interactions between backswept impeller and downstream vaned diffusers is carried out. In the unsteady simulation, a geometry scaling method is used to modify the diffuser geometry to the nearest integer pitch ratio while keeping the throat area, flow direction and area ratio unchanged in order to deal with the unequal pitch ratio problems which exist in the unsteady simulation. The unsteady investigation was undertaken regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate and impeller tip clearance. The detailed interactions at different conditions are compared, assessed and analysed. The studies focus on the analyses of the effect of the different interactions on the stage operating range, peak efficiency, total pressure ratio, level of unsteadiness, flow structures, flow angle or incidence angle, etc. In addition, the' predicted results are compared with available experimental data and a quite good agreement is achieved although the geometry is scaled. On the other hand, a detailed investigation on the differences between the time averaged unsteady simulation results and steady simulation results was performed at different conditions. The comparisons were carried out regarding static pressure, total pressure, speed, flow angle (or incidence angle) and isentropic efficiency. The investigation confirms that unsteady simulation is still quite important, since some of the steady state simulation results are still not similar to the time averaged ones. Designers should take into account the influence of the unsteadiness on the flow fields when they employ the steady state model in the design process.Item Open Access Low speed axial compressor design and evaluation : High speed representation and endwall flow control studies(Cranfield University, 1999-01-28) Lyes, Peter A.; Elder, R. L.; Ivey, Paul C.; Ginder, R. B.This Thesis reports the design, build and test of two sets of blading for the Cranfield University low speed research compressor. The first of these was a datum low speed design based on the fourth stage of the DERA high speed research compressor C 147. The emphasis of this datum design was on the high-to-low speed transformation process and the evaluation of such a process through comparing detailed flow measurements from both compressors. Area traverse measurements in both the stationary and rotating frame of reference were taken at Cranfield along with overall performance, blade surface static pressure and flow visualisation measurements. These compare favourably with traverse and performance measurements taken on C147 before commencement of the PhD work. They show that despite the compromises made during the transformation process, due to both geometric and aerodynamic considerations, both the primary and secondary flow features can be successfully reproduced in the low speed environment. The aim of the second design was to improve on the performance of the datum blading through the use of advanced '3D' design concepts such as lean and sweep. The blading used nominally the same blade sections as the datum, and parametric studies were conducted into various lean/sweep configurations to try to optimise the blade performance. The final blade geometry also incorporated leading edge recambering towards the fixed endwalls of both the rotor and stator. The '3D' blading demonstrated a 1.5% increase in efficiency (over the datum blading) at design flow rising to around 3% at near stall along with an improvement in stall margin and pressure rise characteristic. The design work was completed using the TRANSCode flow solver for both the blade-to-blade solutions (used in the SI-S2 datum design calculation) and the fully 3D solutions (for the advanced design and post datum design appraisal). The 3D solutions gave a reasonable representation of the mid-span and main 3D flow features but failed to model the corner and tip clearance flow accurately. An interesting feature of the low speed flowfield was the circumferential variation in total pressure observed at exit from all rotors for both designs. This was not present at high speed and represents one of the main differences between the high and low speed flow. Unsteady modelling of mid- height sections from the first stage indicate that part of this variation is due to the potential interaction of the rotor with the downstream stator while the remainder is due to the wake structure from the upstream stator convecting through the rotor passage. Finally, the implications for a high speed design based on the success of the 3D low speed design are considered.Item Open Access The modelling of three-dimensional transonic flows in turbomachines using time-marching techniques(Cranfield University, 1992-01) Cheng, C. P.; Elder, R. L.For the efficient design of transonic turbomachinery systems, understanding of the complex flow phenomena inherent in the flow passages is essential. In the present study, a computational technique is adopted to meet this formidable goal. A code using a time-marching technique has been developed first for quasi three-dimensional cascades (that is two-dimensional computation with the varying streamtube height in the third dimension taken into consideration) and then extended to fully three-dimensional flows within the rotating flow passages. Each code has a built-in switch for in viscid and viscous flows. The basis of the codes is the conservative form of the Reynolds- averaged Navier-Stokes equations in a rotating framework. This is supplemented by either the Baldwin-Lomax (algebraic) or the k-e (two-equation) turbulence model. For solving the hyperbolic type governing equations, spatial derivatives are first discretized on the easily-constructed H-type grid system using a central-difference finite-volume approximation with the flow variables stored at the cell centre. An explicit multistage Runge-Kutta scheme is then employed for the time integration o f the resulting ordinary differential equations. The accuracy of the quasi three-dimensional code is initially evaluated by predicting the flows through cascades with simple geometry. Its robustness is then confirmed by two realistic configurations with a wide range o f operating conditions. Finally the fully three-dimensional code is applied to two highly loaded transonic rotors with complicated geometry at peak efficiency and near stall operating conditions. An extensive comparison in terms of detailed flowfield and overall performance between the predictions and experiments with laser anemometry and conventional probes shows the accuracy o f the codes and also indicates that the present study has great potential to be a viable aerodynamic design and analysis tool in the development of transonic turbomachinery systems.Item Open Access Modelling variable stator vane setting in multistage axial flow compressors(Cranfield University, 1998-08) Sun, J; Elder, R. L.A numerical approach for modelling variable stator stagger in multistage stage axial flow compressors is presented. The development of such an approach has been motivated by the requirements of an optimisation methodology for stator vane setting and active control of instability using controlled stator vane setting. The optimisation methodology has been further developed but active control approaches are discussed as future considerations. Varying upstream stator vane stagger . changes the incident flow angle on . the downstream rotor thus affecting the entire flow distribution within the compression systems. The approach therefore begins by investigating the effect of a change in stator stagger setting on stage performance. A meanline method was used for nu- merical prediction of stage characteristics as it can simulate the effect of a change in stagger settings and ( or) in rotational speeds. Overall compressor performance was obtained by stacking the (experimental or predicted) stage characteristics and the surge conditions predicted using a stage-by-stage dynamic compression model where the compressibility was considered explicitly. This approach for variable stagger set- ting was incorporated into a FORTRAN code and validated using the data from the 12-stage HP SPEY jTAY variable geometry compressor. To optimise the setting, a direct search method incorporating a Sequential Weight Increasing Factor Technique (SWIFT) algorithm was incorporated into the variable stagger model. The objective function in this optimisation is penalised externally 11 with an updated factor which helped to accelerate convergence. The methodology has been incorporated into a FORTRAN program and its validations were conducted using the data from the 7-stage LP OLYMPUS and the 12-stage HP SPEY /TAY compressors. Results have demonstrated that variable stagger setting is a powerful method to rematch stages and which can be used to improve the desired overall performance, and that the potential benefits of introducing additional rows of variable setting vanes can be achieved. Future work arising from the present study has been discussed and highlighted, which involves the enhancement of the model capacity and development of active control approaches. In addition the thesis involves several reviews focusing on different topics. Most reviews contain considerable information and it is expected that the information can be of help for the interested readers to trace more relevant references. These reviews consist of a general review in chapter 1; a brief review on stage characteristics modelling in chapter 2; a comparative review on incompressible and compressible surge models in chapter 3; a review of various optimisation methods for practical problems, especially for constrained non-smooth problems, in chapter 4; and a review of the state-of-the-art active approaches in chapter 7. The suitability of various approaches has been highlighted. Steinke's meanline method is suitable for investigating the in- influence of stagger resetting on stage performance. To predict the surge conditions for a (high-speed) multistage environment, the stage-by-stage compressible models are III more promising. For constrained non-smoothed optimisation, the SWIFT algorithm can be an alternative. The controlled stator vane regulated through nonlinear control law will permit the robust control of compressor instabilities.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 Optimising stator vane settings in multistage axial flow compressors(Cranfield University, 2002-04) White, Nicholas M.; Elder, R. L.There is a common requirement in the process, oil and gas turbine industries for high performance axial flow compressors operating over a wide range of mass flow rate and rotational speed at high efficiency. The trends have been for higher blade loadings (greater pressure rise per stage) and higher efficiency which are increasingly achieved through sophisticated Computational Fluid Dynamics designs. These trends, however, tend to mitigate against stable operating flow range (reduced surge margin), which can often lead to performance compromises. The objective of this work is to investigate the possibility of using alternative means to gain ow range by better use of variable geometry, which may permit design objectives to be better achieved. Variable geometry of the type envisaged is already often employed to overcome part-speed operating problems, but it proposed here that there may be additional benefits from their more intelligent control. The operation of axial compressors with a wide operating range is limited by instabilities, which cause a full breakdown of the flow, which is surge. These instabilities, which are caused by high incidence and subsequent stalling of stages occur due to different phenomena at part and full speed operation. The problem at part-speed is that the front stages are often heavily stalled and rear stages choked, whereas at high speeds, the front stages are operating close to choke and the rear stages tend to be stalling. Optimisation of the design to full load conditions can often provide part-speed problems and to achieve the acceptable performance, variable geometry over the front region of the compressor is sometimes used to modify the flow angles and avoid stage stall and subsequent surge. To-date, such variable settings follow some schedule established by analysis and experiment whereas this work presents a methodology of setting blade rows using an optimisation procedure and investigates the likelihood of performance benefits being obtained by a control technique which reacts° to these changing conditions. The construction of the numerical method presented in this thesis was done with an emphasis upon its intended contribution towards a eventual online control application. Therefore, a practical approach has been employed in the development of the compressor modelling techniques used in the work. Specifically, a highly empirical one-dimensional performance prediction code was constructed, employing successful correlations taken from the literature. This was coupled to a surge prediction method that has been shown in the past to function more than satisfactorily in a multistage environment. Finally, the predicted stage and overall performance (including the surge point) characteristics were passed to a optimisation program, which allowed these simulated conditions to be investigated. It is hoped that the work presented has illustrated the potential (from a aerodynamic performance point of view) of such a control technique to offer additional freedom in the operation of a multistage axial flow compressor. Moreover, the numerical modelling techniques have been developed enough to envisage (at least in part) their simple integration within a practical system. Clearly, some further investigations are required to take this work forward and the next logical step would be to improve the empirical rules with which the blade performance is predicted. A experimental programme would also be of great advantage, for example in the study of how the deviation angle for a given blade row varies over time (operating hours) in a real machine due to ageing and fouling. This would allow better estimates of the stage work during long term operation so that the optimiser could adapt to the slowly degrading performance of the blades. Finally, it is important to verify the simulated results with measured data, taken at the same optimal stator vane settings as given by the program. This must be carried out before it can be applied to a real application, although a limited study of this nature is presented in chapter 6.Item Open Access The prediction of transonic flows using a potential method(1986-09-26) Toolsie, K. O.; Goulas, A.; Elder, R. L.Transonic flows are simulated within convergent divergent nozzles and within turbomachinery blade rows. The flow is represented by the conservative full potential equation approximated by a nine-node central-difference scheme, which is third order accurate. Artificial viscosity is included into the central-difference approximation of the potential equation, in regions where the flow is locally supersonic. The approximation of the potential equation by central-differences, with an artificial viscosity term included, is equivalent to the approximation by upwind differences and ensures that the upwind nature of the domain of dependence of supersonic flows is correctly modelled. The exact form of this artificial viscosity term is derived and contains third order derivatives of velocity-potential. The inclusion of artificial-viscosity allows the potential equation to be approximated everywhere by central-differences and the flow equation is everywhere elliptic. The Neuman boundary-condition is applied, along solid surfaces, if an inviscid solution is desired. Viscous effects are incorporated by the modification of this condition so as to allow a transpiration flow through the solid surfaces. A standard Successive-Line-Over-Relaxation technique, developed for the solution of simultaneous elliptic equations, is used to solve the discretized potential flow equations. Predictions are presented for both the inviscid and the viscous-corrected potential codes applied to the simulation of transonic flow through nozzles and cascade blade-rows. Comparisons are made with other theoretical models and with experimental data. The problem of non-uniqueness is considered and an estimate of numerical error is made by the application of the inviscid code with two computational grids of different levels of refinement. The stability of this potential code is examined and is found to depend on the level of smearing of the shock discontinuity predicted by the theoretical model.