Browsing by Author "Bennett, Ian"

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    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.
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    Reciprocating compressor prognostics of an instantaneous failure mode utilising temperature only measurements
    (Elsevier, 2017-12-14) Loukopoulos, Panagiotis; Zolkiewski, George; Bennett, Ian; Sampath, Suresh; Pilidis, Pericles; Duan, Fang; Sattar, Tariq; Mba, David
    Reciprocating compressors are critical components in the oil and gas sector, though their maintenance cost is known to be relatively high. Compressor valves are the weakest component, being the most frequent failure mode, accounting for almost half the maintenance cost. One of the major targets in industry is minimisation of downtime and cost, while maximising availability and safety of a machine, with maintenance considered a key aspect in achieving this objective. The concept of Condition Based Maintenance and Prognostics and Health Management (CBM/PHM) which is founded on the diagnostics and prognostics principles, is a step towards this direction as it offers a proactive means for scheduling maintenance. Despite the fact that diagnostics is an established area for reciprocating compressors, to date there is limited information in the open literature regarding prognostics, especially given the nature of failures can be instantaneous. This work presents an analysis of prognostic performance of several methods (multiple linear regression, polynomial regression, K-Nearest Neighbours Regression (KNNR)), in relation to their accuracy and variability, using actual temperature only valve failure data, an instantaneous failure mode, from an operating industrial compressor. Furthermore, a variation for Remaining Useful Life (RUL) estimation based on KNNR, along with an ensemble technique merging the results of all aforementioned methods are proposed. Prior to analysis, principal components analysis and statistical process control were employed to create T2 and Q metrics, which were proposed to be used as health indicators reflecting degradation process of the valve failure mode and are proposed to be used for direct RUL estimation for the first time. Results demonstrated that even when RUL is relatively short due to instantaneous nature of failure mode, it is feasible to perform good RUL estimates using the proposed techniques.
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    TERA for Rotating Equipment Selection
    (Cranfield University, 2012-01) Khan, Raja S. R.; Pilidis, Pericles; Ogaji, S.; Bennett, Ian; Nicholls, J. R.
    This thesis looks at creating a multidisciplinary simulation tool for rotating plant equipment selection, specifically gas turbines, for the liquefaction of natural gas (LNG). This is a collaborative project between Shell Global Solutions and Cranfield University in the UK. The TERA LNG tool uses a Techno-economic, Environmental and Risk Analysis (TERA) approach in order to satisfy the multidisciplinary nature of the investigation. The benefits of the tool are to act as an aid to selection, operations and maintenance planning and it also acts as a sensitivity tool for assessing the impact of changes in performance, environmental and financial parameters to the overall economic impact of technology selection. The aim is to not only select technology on the basis of techno-economics but also on the basis of risk analysis. The LNG TERA tool is composed of a number of modules starting with the performance simulation which calculates the thermodynamic conditions in the core of the engine. Next, life estimates of the hot gas path components are made using a mixture of parametric and probabilistic lifing models for the turbine first stage blades, coatings, and combustor liner. This allows for a risk analysis to be conducted before maintenance and economics issues are dealt with. In parallel, emissions estimations are made based on empirical correlations. The modelling exemplifies a methodology which is uniquely applied to this application and there are no studies previous to this which look at so many aspects before making conclusions on plant machinery selection. Comparisons have been done between industrial frame engines based on the General Electric Frame 9E (130 MW) and Frame 7EA (87 MW) engines as well as more complex cycles involving aero-derivation and inter-cooling such as the LM 6000 (42 MW) and LMS 100 (100 MW). Work has also been carried out to integrate the tool to Shell based systems in order to utilise the database of information on failure and maintenance of machinery as well as its performance. The results of the integrated TERA show a clear favour for the aero-derivative engines and the main benefit is the fuel saving, though the life of the hot gas path components is deteriorated much faster. The risk results show that the industrial frame engines have a wider variation in expected life compared to aero-derivatives, though the industrial frames have longer component lives. In the context of maintenance and economics, the aero-derivative engines are better suited to LNG applications. The modular change out design of the aero- derivatives also meant that time to repair was lower, thus reducing lost production. Application of the LNG TERA tool was extended to power generation whereby a series of 6 engines were simulated. The changes required to the modelling were minimal and it shows the flexibility of the TERA philosophy. This study was carried out assuming a given ratio of load split between the engines and hence is sensitive to the way an operator demands power of the engine as opposed to LNG application where the operator tries to drive the engine as hard as possible to get the most production out of the train. The study was limited in the modes of failure which were investigated, a major further work would be to extend the methodology to more components and incorporate fatigue failure. Further, the blade creep and probabilistic coating models were very sensitive to changes in their respective control parameters such as coating thickness allowances and firing temperature. The contribution to the project from the MBA is the statistical techniques used to conduct the risk analysis and data handling as well as financial management techniques such as the Net Present Value (NPV) methodology for project evaluations.
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    Wake affected boundary layer flows within an embedded stage of a multi-stage axial compressor
    (2001-06) Lee, D. Y.; Elder, R. L.; Bennett, Ian
    Boundary Layer Flows in turbomachines have been recognised to crucially influence the stability and performance of gas turbine components particularly the compressor since its function is to provide a pressure rise through diffusion that is accompanied by an adverse pressure gradient. This brings about the danger of separation of flow and thus understanding the behaviour and control of the boundary layer is of great importance. The interaction between stationary and rotating blade rows inevitably make the flow environment within a multi-stage axial compressor unsteady. Dean (1959). Stator blades are subjected to periodic wakes from upstream rotor rows, which initiate transition that is very much unlike those encountered in a steady state (clean) flow conditions. This thesis describes the third stage of the fundamental research on boundary layer flows in turbomachinery at Cranfield University. The feature that makes this experimental work unique is that with constant temperature hot-wire anemometry (CTA) as the principal tool, detailed boundary layer surveys have been carried out within real turbomachinery environment of Cranfield University’s world renowned four-stage low-speed research compressor facility (LSRC). This is a considerable step from the flat plate and wind tunnel arrangement employed in the previous stages of this research programme. Two-dimensional flow field measurements taken downstream of an embedded rotor stage complement the boundary layer survey experiment to contribute toward a better understanding of the unsteady wake affected boundary layer flows that occur in turbomachinery. The current study was conducted with particular reference to a low speed 3 D end bend controlled diffusion blade design. X-array hot-wire measurements of the unsteady flow field have captured the strong wake features exhibiting periodic variations in thickness, which indicates blade loss fluctuations. This is more prominent at Near Stall conditions. Strong variations are present in the end-wall regions where comer stall and comer vortex and tip clearance flows dominate. Radial migration of flow deviation was also detected with each passage of a rotor wake indicating significant radial asymmetries in velocity profiles. Periodic fluctuation of the suction surface boundary layer on a stator blade has been detected starting from the first leading edge measurement location (i.e. 15%) until 37% chord where imminent separation was evident at design flow conditions. Different states of boundary layer flows exist at different heights of a fixed chord position, indicating that the inlet conditions to the stator vary throughout the span. This periodic alternating between laminar and turbulent of the boundary layer appeared to be primarily dependent on the state of the boundary layer itself rather than the wake passing frequency. Results from this first ever attempt of detailed boundary layer survey within turbomachinery environment at Cranfield, though qualitative, has shown that transitional flows occupy up to approximately 40% of the stator blade suction surface. Total separation of the flow, which had been detected at 65% chord in this investigation was not anticipated as the design of the current blading was aimed at delaying if not eliminating any separation until or near the trailing edge.