Abstract In an aeroengine oil is supplied to the bearings for lubrication and cooling. Subsequently, it creates a two phase flow environment in the bearing chamber that may contain droplets, mist, wall film, ligaments, froth or foam and liquid pools. After the oil has served its purpose, it is scavenged out of the chamber and recycled. Effective oil removal is essential as unnecessary working of the oil can lead to excessive heat generation in the chamber, increased risk of seal leakage, and reduced overall efficiency. However the task of oil removal is not trivial as it is entrained in a highly rotating environment induced by the rotating shafts. Shallow sump variants were investigated using a design of experiments approach. A previous publication discussed the performance of the sump variants using categorization of visualization data relating to the extent of hydraulic uplift in the sump region. Hydraulic uplift was found to be directly related to residence volume in the gravity dominated wall film flow regime. Furthermore, it was found that the flow condition factors such as flow regime and shaft speed are the dominating factors affecting the hydraulic uplift severity. In terms of the geometrical factors, a deeper sump can reduce the hydraulic uplift severity. This reinforces the importance of sump depth, as a deeper sump tends to have lower residence volume but is often not implemented due to space constraint. This paper presents further results from the experimental study on the shallow sump variants in terms of some important flow characteristics such as upstream flow detachment, upstream flow dry-out, and secondary flow. In addition to the geometrical factors, the flow condition factors such as the flow regime defined by how oil enters the chamber, flow rate, shaft speed, and scavenge ratio were investigated. Upstream flow detachment was observed only for cases where airborne droplets were present and at higher shaft speeds, also known as the windage dominated airborne droplets flow regime. It was noted that the geometrical factors are less important than the flow condition factors for upstream flow detachment as well as upstream flow dry-out and secondary flow.

Abstract As modern engine designs target higher efficiencies through increased turbine inlet temperatures, critical turbine components are at increased risk of damage from conditions exceeding material melting temperatures. In particular, improperly designed underplatform hardware components are susceptible to damage when hot main gas path flow is ingested into the stator-rotor cavity. While all turbines inherently experience transients during operation, a majority of turbine tests have been executed using steady operating conditions, and routine transient events are not well understood. To address this need, the present study utilizes a continuous-duration, one-stage test turbine operating with true-scale engine hardware and seal geometries at engine-representative flow conditions. The nature of the continuous-duration facility uniquely supports direct assessment of transient events through its ability to transition between steady-state operating conditions. Specifically, the effects of a transient purge flow were investigated in this study to identify general trends for transient events in a full-scale engine. Results from multiple measurement techniques in the wheelspace region show an interdependence of transient purge flow with a thermal lag of the underplatform hardware. Through experiments conducted at different coolant-to-main gas path temperature ratios, the use of pressure measurements as an indicator of fully-purged behavior was introduced, and a thermally-driven influence on rim seal performance was quantified.

Abstract The present paper focuses on the resilience of 3-hole pressure probes to hot sand fouling in turbomachinery environments. These probes are utilized inside jet engine hot sections for diagnostics and flow characterization. Ingestion of sand and other particulates pose a significant risk to hot section components and measurement devices in gas turbine engines. In this study, wedge, cylindrical, and trapezoidal probes were exposed to hot section turbine aerothermal conditions of 1050°C and 65–70 m/s flow velocity and fouled with 0–5 μm Arizona Road Dust (ARD). Sand accumulated more rapidly on the surface of the trapezoidal and cylindrical probe geometries than on the surface of the wedge probe geometry. Probe calibrations following sand fouling were performed in an ambient temperature, open air, calibration jet at Mach 0.3 and 0.5. Calibration curves using non-dimensional coefficients were used to assess probe error in yaw angle due to sand fouling. Probe error was based on each probe’s ability to accurately measure flow direction over a flow angle range of [−10°, 10°]. On average, the probes displayed greater error at Mach 0.5 than Mach 0.3. The wedge probe performed the best after sand fouling and displayed a maximum error of less than ±2° in yaw angle. In contrast, the cylindrical probe performed the worst after sand fouling and displayed maximum errors of more than ±8° in yaw angle. Transient response did not change notably with sand fouling.

Abstract The Honeywell Uncertified Research Engine (HURE), a research version of a turbofan engine that never entered production, was tested in the NASA Propulsion System Laboratory (PSL), an altitude test facility at the NASA Glenn Research Center. The PSL is a facility that is equipped with water spray bars capable of producing an ice cloud consisting of ice particles, having a controlled particle diameter and concentration in the air flow. To develop the test matrix of the HURE, numerical analysis of flow and ice particle thermodynamics was performed on the compression system of the turbofan engine to predict operating conditions that could potentially result in a risk of ice accretion due to ice crystal ingestion. The goal of the test matrix was to provide operating conditions such that ice would accrete in either the fan-stator through the inlet guide vane region of the compression system or within the first stator of the high pressure compressor. The predictive analyses were performed with the mean line compressor flow modeling code (COMDES-MELT) which includes an ice particle model. The HURE engine was tested in PSL with the ice cloud over the range of operating conditions of altitude, ambient temperature, simulated flight Mach number, and fan speed with guidance from the analytical predictions. The engine was fitted with video cameras at strategic locations within the engine compression system flow path where ice was predicted to accrete, in order to visually confirm ice accretion when it occurred. In addition, traditional compressor instrumentation such as total pressure and temperature probes, static pressure taps, and metal temperature thermocouples were installed in targeted areas where the risk of ice accretion was expected. The current research focuses on the analysis of the data that was obtained after testing the HURE engine in PSL with ice crystal ingestion. The computational method (COMDES-MELT) was enhanced by computing key parameters through the fan-stator at multiple span wise locations, in order to increase the fidelity with the current mean-line method. The Icing Wedge static wet bulb temperature thresholds were applicable for determining the risk of ice accretion in the fan-stator, which is thought to be an adiabatic region. At some operating conditions near the splitter-lip region, other sources of heat (non-adiabatic walls) were suspected to be the cause of accretion, and the Icing Wedge was not applicable to predict accretion at that location. A simple order-of-magnitude heat transfer model was implemented into the COMDES-MELT code to estimate the wall temperature minimum and maximum thresholds that support ice accretion, as observed by video confirmation. The results from this model spanned the range of wall temperatures measured on a previous engine that experienced ice accretion at certain operating conditions. The goal of this study is to show that the computational process developed on earlier engine icing tests can be used to provide an icing risk assessment in adiabatic regions for other engines.

Abstract Being free from carbon content, hydrogen has been considered as a promising candidate to reduce pollutant emissions in Gas Turbine Combustion Systems. Due to hydrogen’s significantly different burning characteristics, its implementation requires adjustments to the design philosophies of traditional combustion chambers. The micromix concept offers an alternative diffusive combustion injection system, improving the mixing characteristics without the risk associated with pre-mixing, thereby reducing the likelihood of hotspots forming. The importance of turbulence-chemistry interaction modelling, particularly for highly diffusive flames such as hydrogen, has been widely addressed. A turbulence-chemistry interaction study on such a micromix injector was performed investigating the coupling between the Flamelet Generated Manifold (FGM) combustion model and different hydrogen reaction mechanisms. This methodology correctly reproduces the typical micromix micro-flame behaviour and the analysed mechanisms are shown to be in good agreement in terms of flow characteristics prediction. A comparative study between two reduced order emissions prediction models was then carried out: a CFD post-processing technique for NO x emissions calculations and a hybrid CFD-CRN approach were explored. Due to the coupling between accurate turbulence-chemistry interaction modelling and the ability to handle detailed chemistry, the hybrid CFD-CRN approach gives valuable results with a modest computational cost and it could be used as an optimising tool during the injector geometry design process.

Abstract Hydrogen has been proposed as an alternative fuel to meet long term emissions and sustainability targets, however due to the characteristics of hydrogen significant modifications to the combustion system are required. The micromix concept utilises a large number of miniaturised diffusion flames to improve mixing, removing the potential for local stoichiometric pockets, flash-back and autoignition. No publicly available studies have yet investigated the thermoacoustic stability of these combustion systems, however due to similarities with lean-premixed combustors which have suffered significant thermoacoustic issues, this risk should not be neglected. Two approaches have been investigated for estimating flame response to acoustic excitations of a single hydrogen micromix injector element. The first uses analytical expressions for the flame transfer function with constants obtained from RANS CFD while the second determines the flame transfer function directly using unsteady LES CFD. Results show the typical form of the flame transfer function but suggest micromix combustors may be more susceptible to higher frequency instabilities than conventional combustion systems. Additionally, the flame transfer function estimated using RANS CFD is broadly similar to that of the LES approach, therefore this may be suitable for use as a preliminary design tool due to its relatively low computational expense.

Abstract In the preliminary phase of analysing the thermoacoustic characteristics of a gas turbine combustor, implementing robust design principles is essential to minimize detrimental variations of its thermoacoustic performance under various sources of uncertainties. In the current study, we systematically explore different aspects of robust design in thermoacoustic instability analysis, including risk analysis, control design and inverse tolerance design. We simultaneously take into account multiple thermoacoustic modes and uncertainty sources from both the flame and acoustic boundary parameters. In addition, we introduce the concept of a “risk diagram” based on specific statistical descriptions of the underlying uncertain parameters, which allows practitioners to conveniently visualize the distribution of the modal instability risk over the entire parameter space. Throughout the present study, a machine learning method called “Gaussian Process” (GP) modeling approach is employed to efficiently tackle the challenge posed by the large parameter variational ranges, various statistical descriptions of the parameters as well as the multifaceted nature of robust design analysis. For each of the investigated robust design tasks, we propose an efficient solution strategy and benchmark the accuracy of the results delivered by GP models. We demonstrate that GP models can be flexibly adjusted to various tasks while only requiring one-time training. Their adaptability and efficiency make this modeling approach very appealing for industrial practices.

Abstract The demand for increased efficiencies in modern aeroengines drives designs to higher pressure ratios, temperatures and shaft speeds. Consequently, higher cycle temperatures and parasitic heat loads are likely to become limiting factors in the design of bearing chambers. These chambers are lubricated and cooled with oil and pressurized using air taken from the main gas path. Historically, the temperature and pressure of the sealing air has been low enough to exclude the risk of bearing chamber oil fires caused by oil auto-ignition. However, temperatures are being driven towards the limits set by current design rules. With respect to oil fire risk assessment, current design rules are very conservative as they pessimistically assume the oil is continuously exposed to the maximum temperature expected in operation when determining the minimum residence time required for oil auto-ignition. Improved bearing chamber designs capable of tolerating higher temperatures could therefore be developed by applying a more rational level of conservatism, based on a more physics-based approach (such as considering the effect of an oil droplet being transported through a varying temperature field). In this paper a numerical methodology is developed to provide a pragmatic approach to addressing conservatism in bearing chamber oil fire risk assessment, with respect to oil auto-ignition. An unsteady Eulerian CFD prediction is used to compute the aerothermal flow field within a stylized bearing chamber. A Lagrangian discrete particle method is then used to track the oil droplet trajectories. The time-dependent droplet temperature histories are then used to compute the fractional accumulation of auto-ignition delay time using an empirically derived relationship. Finally, by defining an ‘auto-ignition (AI) energy’ accumulation factor, the methodology assesses whether an individual oil droplet has satisfied the criteria for auto-ignition. The present contribution examines the effects of various modelling parameters on the ‘AI energy’ accumulation factor. These include one/two-way coupling between the air flow and oil droplets, stochastic turbulence modelling of the droplet behavior, and the effect of droplet size and distribution. The work highlights that two-way coupling is required to ensure the thermal effect of the oil is modelled, despite the increased computational demand. Stochastic modelling of interactions between particles and the flow field is also required to capture the spread of particle trajectories and the resulting distribution of particle temperatures. A representative range of droplet sizes must also be simulated as the propensity for oil AI is a function of droplet diameter; the highest risk occurs for the smallest droplets whilst the largest droplets have greater cooling effect on the air flow. Given the extent of model simplification required to allow the work to be completed with a mid-spec desktop computer and the overall scope of the project, a validation of the findings has not been completed. Instead, an experimental validation is proposed as part of future investigation. The authors imagine that with enough investigation and validation, the understanding developed by the work could be applied as part of a computationally efficient industrial design toolset to inform the early stages of product design.

Abstract The rotordynamic stability evaluation by measuring the damping ratio or log-dec of the first forward whirl mode in the shop test is of importance to reduce the risk of instability fault in field. This paper focuses on the signal processing of inverse Fourier transform based on the full spectrum technique to reduce the modal overlap of closely-spaced first order modes (in which the amplitudes of the forward and backward responses are comparable) in the time domain. By using this transform method, forward and backward whirl signals are obtained, and blocking excitations in x or y direction can also achieve the same effectiveness as the forward and backward excitation with the active magnetic bearing (AMB) when there is a significant cross-coupled stiffness (CCS). The envelope of the analytical signal obtained by Hilbert transform is used to analyze the component of the excited vibration and show the effectiveness of weakening the overlap of the closely-spaced twin modes. The result from the numerical investigation indicates that for the rotor-bearing system with a significant CCS, the constructed forward and backward whirl signals are clearly dominated by the respective forward and backward modes with no significant overlap. Besides, experiments are also carried out in a nearly industrial centrifugal compressor test rotor with AMB, which is used as an actuator to apply blocking excitations for rotordynamic stability tests, with result that the unidirectional excitation is enough to estimate the log-dec of the rotor system with a slight CCS. Thus, the traditional circular forward/backward whirl excitation and its associated complicated equipment such as AMB is unnecessary and replaceable, providing a sight to simplify the complicated equipment only by using the signal processing techniques when the rotating machinery has an either slight or significant CCS.

Abstract Fluid film bearings for turbomachinery are designed to support the loads applied by the rotor system. Oil-lubricated bearings are widely used in high speed rotating machines. However, environmental issues and risk-averse operations have made water lubricated bearings increasingly popular. Due to different viscosity properties between oil and water, the low viscosity of water decreases film thickness significantly. Crowning and tapers are two main ways to maintain the film thickness requirements in water lubrication, but no studies about the influence of these parameters on the film thickness in water-lubricated bearings have been reported. Therefore, further understanding of the performance associated with optimizing the bearing design with different weighted performance and their relationships to bearing design variables could be invaluable to bearing design engineers. This study explores the impact of three crowning and taper design variables on the performance of one tilting pad thrust bearing using the design of experiments techniques applied to a thermoelastohydrodynamic (TEHD) bearing model. The bearing design variables analyzed in this study include the radius of the ground-in crown, taper circumferential angle offset, and the vertical taper distance at the inner and outer radii. Each of the design variables is first varied over five levels, each in central composite design. The outputs from the TEHD numerical simulations used as performance measures for each bearing design point were the minimum film thickness, the film thickness at the pivot location, maximum film pressure and power loss. Multi-objective optimization was performed. A range of weighting parameters was selected for the optimization function to find a bearing design that maintains the minimum film thickness criterion while minimizing power loss. The resulting optimum design points allowed for a comparison between the design optimization at different weightings. This study demonstrates how designers can use these approaches to view the relationships between design variables and important performance metrics to design better bearing for a wide range of applications.

Abstract Modern turbomachine designs feature reduced nominal clearances between rotating bladed-disks and their surrounding casings in order to improve the engine efficiency. Unavoidably, clearance reduction increases the risk of contacts between static and rotating components which may yield hazardous interaction phenomena. In this context, the deposition of an abradable coating along the casing inner surface is a common way to enhance operational safety while mitigating interaction phenomena thus allowing for tighter clearances. Nonetheless, interactions leading to unexpected wear removal phenomena between a bladed-disk and a casing with abradable coating have been observed experimentally. Beside of blade damages such as cracks resulting from high amplitudes of vibration, experimental observations included very significant temperatures increase, particularly within the abradable coating, to a point that thermo-mechanical effects may not be neglected anymore. The aim of this work is to investigate the numerical modeling of thermal effects in the abradable coating and the casing due to contact interactions. In particular, the proposed model provides insight on the sensitivity of engines to contact events when the plane had reduced tarmac times between two consecutive flights. A strongly coupled thermo-mechanical model of the casing and its abradable coating is first described. A 3D cylindrical mesh is employed, it may be decomposed in two parts: (1) along the casing contact surface, a cylindrical thermal mesh is constructed to compute the temperature elevation and heat diffusion in the three directions of space within the abradable coating, and (2) the casing itself is represented by a simplified cylindrical thermo-mechanical mesh to compute both temperature elevation and the induced deformations following temperature changes. This 3D hybrid mesh is combined with a mechanical mesh of the abradable layer, dedicated to wear modeling and the computation of normal and tangential contact forces following blade/abradable coating impacts. The heat flux resulting from contact events is related to the friction forces and only heat transfer by conduction is considered in this work. In order to reduce computational times, the time integration procedure is twofold: the explicit time integration scheme featuring reduced time steps required for contact treatment is combined with a larger time step time integration scheme used for the casing thermo-mechanical model. An extensive validation procedure is carried out from a numerical standpoint, it underlines the convergence of the model with respect to time and space parameters.

Abstract Piezoelectric-based vibration reduction has the potential to improve the lifetime and structural integrity of turbomachinery blades by reducing the risk of high-cycle fatigue. Semi-active techniques produce small, self-powered implementations that can meet the strict design requirements for rotating machinery. Most semi-active techniques switch piezoelectric transducers between an open circuit and a shunt circuit in a way that reduces vibration. However, these switches produce a small impulse on the structure due to the electromechanical coupling of the piezoelectric material. Since multiple-mode vibration reduction typically requires distributed sections of piezoelectric material, the switching impulse generated by one transducer may affect others. This study investigates the effect of the switching impulse on the non-switched piezoelectric transducers. Experimental testing shows the switching impulse induces a voltage response at the natural frequencies of the structure, with the strongest responses occurring at modes where both the switched and non-switched piezoelectric transducers have high electromechanical coupling. Furthermore, piezoelectric sections that lack coupling at the excitation frequency of the structure exhibit a more noticeable response to the switching impulse. This response enables remote sensing of switches, which may facilitate wireless coordinated switch timing.

Abstract Gas compressor customers desire to operate gas compressors at higher discharge temperatures. In barrel-style centrifugal compressors, dry gas seals are used to limit the leakage along the shaft from the process fluid to atmosphere. The temperature limit of dynamic O-ring seals in the dry gas seal cartridge is one of the primary limiters to operating gas compressors at higher discharge temperatures. While the present discharge temperature limit is conservative compared to the seal limits, without a detailed understanding of the thermal distribution around the discharge-side end cap, operation at compressor discharge temperatures that exceed seal limits has too much risk. Thus, this paper describes the approach taken to characterize the temperatures around the discharge end dry gas seal of a commercial C335EL centrifugal compressor. A phased test matrix with increasing discharge temperatures was conducted so that the temperature distribution throughout the end cap could be assessed. The full-scale, highly-instrumented compressor was operated at discharge pressures and temperatures ranging from 4.7 to 9.4 MPa (680 to 1370 psi) and 113 to 218 degree Celsius (235 to 425 degrees Fahrenheit), respectively. The experimental test set-up and results are presented herein. Results include demonstration of successful compressor operation at discharge temperatures greater than seal limits as well as the end cap temperature sensitivity to lube oil supply and dry gas supply temperatures.

Abstract Liquefied Natural Gas (LNG) liquefaction plants have become increasingly important as natural gas is exported from the United States of America to markets world-wide. Downtime of any part of the process train (gas turbine, compressors, controls, etc.) due to failure of one or more of its components can result in high costs. The total cost of loss is of great concern to the LNG industry as it moves towards increased LNG exports with required operational efficiency, and downtime reduced to a minimum. This paper reports the application of a methodology of property risk assessment, providing insight into the use of PML (Probable Maximum Loss) and MFL (Maximum Foreseeable Loss) risk measures. Major sources of risk are analyzed, drawing from both technical literature and operational information on typical large LNG liquefaction plants. The outcome of this paper is an estimation of the economic loss associated with property risk for two hypothetical LNG liquefaction plants, based upon sample plants located in North America and characterized by different capacity. These plants represent recently built and commissioned plants and are chosen to take advantage of current technology and plant capacities.

Abstract In many supercritical CO 2 cycle implementations, compressor or pump inlet conditions are relatively near the two-phase region. Fluid acceleration near the compressor inlet can result in the potential for condensation or cavitation at the inlet. Despite potential mitigating effects or evidence in the literature, potential two-phase operation is a high-risk condition and may not be recommended for high-reliability system design. This paper presents a summary of the existing literature documenting inlet phase change in sCO 2 , and presents an analysis of required conditions to avoid phase change as a function of inlet pressure, temperature, and Mach number. Static conditions at the inlet are calculated based on the real gas approach documented in ASME PTC-10, Appendix G. In addition, various total-to-static iteration challenges are discussed and avoided through solution of the inverse problem to convert limiting static conditions at saturation to the full range of limiting total conditions for various Mach numbers up to 1.0. The results show that a threshold total temperature exists above which phase change cannot occur, ranging from 31.1 to 66.95 °C and increasing with Mach number. Lower temperatures below this threshold may also avoid phase change depending on the total pressure. The documented results are useful as a reference for use by cycle designers to impose design limits that minimize risks associated with two-phase flow in the compressor.

Abstract An experimental facility is currently operating at Cranfield University in the UK and it is being used to explore supercritical carbon dioxide as a working fluid for future bottoming power cycle applications. The initial objective of this experimental programme is to de-risk and demonstrate the robustness of a closed-loop system, whilst proving the function and performance of individual components and various measurement and control modules. This paper describes the first operational experience gained whilst operating the test facility. More specifically, it summarizes the lessons learned from the commissioning phase and first test campaigns carried out in 2018.

Oil-lubricated bearings are widely used in high speed rotating machines such as those found in the aerospace and automotive industries. However, environmental issues and risk-averse operations are resulting in the removal of oil and the replacement of all sealed oil bearings with reliable water-lubricated bearings. Due to the different fluid properties between oil and water, the low viscosity of water increases Reynolds numbers drastically and therefore makes water-lubricated bearings prone to turbulence effects. This requires finer meshes when compared to oil-lubricated bearings as the low-viscosity fluid produces a very thin lubricant film. Analyzing water-lubricated bearings can also produce convergence and accuracy issues in traditional oil-based analysis codes. Thermal deformation largely affects oil-lubricated bearings, while having limited effects on water lubrication; mechanical deformation largely affects water lubrication, while its effects are typically lower than thermal deformation with oil. One common turbulence model used in these analysis tools is the eddy-viscosity model. Eddy-viscosity depends on the wall shear stress, therefore effective wall shear stress modeling is necessary in determining an appropriate turbulence model. Improving the accuracy and efficiency of modeling approaches for eddy-viscosity in turbulence models is of great importance. Therefore, the purpose of this study is to perform mesh refinement for water-lubricated bearings based on methodologies of eddy-viscosity modeling to improve their accuracy. According to Szeri [1], ε m /v for the Boussinesq hypothesis is given by Reichardt’s formula. Fitting the velocity profile with experiments having a y + in the range of 0–1,000 results in Ng-optimized Reichardt’s constants k = 0.4 and δ + = 10.7. He clearly states that for y + > 1000 theoretical predictions and experiments have a greater variance. Armentrout and others [2] developed an equation for δ + as a function of the pivot Reynolds number, which they validated with CFD simulations. The definition of y + can be used to approximate the first layer thickness calculated for a uniform mesh. Together with Armentrout’s equation, the number of required elements across the film thickness can be obtained. For typical turbulence models, the y + must be within a certain range to be accurate. On the condition that the y + is fixed to that of a standard oil bearing for which an oil bearing code was validated, the number of elements across the film thickness and coefficients used in the eddy-viscosity equation can be adjusted to allow for convergence with other fluids other than that which the traditional oil bearing code was designed for. In this study, the number of required elements across the film for improved prediction quality was calculated based on the proposed eddy-viscosity model mesh correction from the known literature. A comparison between water lubrication using the parameter correction and oil lubrication was also made. The results of this study could aid in improving future designs and models of water-lubricated bearings.

The emission laws for internal combustion engines become more and more strict. Therefore, new concepts have to be implemented. In the so called Miller approach the intake valve is closed before the intake stroke is finished, thus resulting in a lower combustion end temperature and pressure. As a negative result, the specific power of the engine is reduced. This disadvantage has to be compensated by an increased boost pressure delivered by a turbocharger compressor. For the turbomachinery this means for low end torque engine operation a compressor operating point at high pressure ratio and low mass flow. Thus an increased risk for surge results. A cost-effective measure to establish an utilizable compressor map in this regime is a ported shroud casing treatment. Here, a circumferential cavity connects the low channel at the inducer with the compressor housing inflow, allowing fluid to recirculate at low mass flows. Thereby, the gross inducer mass flow is increased, the flow stabilized and hence the surge line improved. In this paper, a ported shroud casing treatment is developed employing CFD. The aim is to improve the surge line as well as the stability of the compressor characteristics and to minimize the impact on compressor efficiencies at high flows as well as the acoustic behaviour at the same time. In order to validate the performance of the design, standard hot gas measurements as well as acoustic measurements are conducted and analyzed. Furthermore, the impact of a commonly applied 90° inflow bend on the performance of the ported shroud cavity is investigated by experimental data.

The need to mitigate the climate change has brought in the last years to a fast rise of renewable technologies. The inherent fluctuations of the solar resource make concentrating solar power technologies an application that demands full flexibility of the steam turbine component. A key aspect of this sought steam turbine flexibility is the capability for fast starts, in order to harvest the solar energy as soon as it is available. However, turbine start-up time is constrained by the risk of low cycle fatigue damage due to thermal stress, which may bring the machine to failure. Given that the thermal limitations related to fatigue are temperature dependent, a transient thermal analysis of the steam turbine during start process is thus necessary in order to improve the start-up operation. This work focuses on the calculation of turbine thermo-mechanical properties and the optimization of different start-up cases in order to identify the best solution in terms of guaranteeing reliable and fast start-ups. In order to achieve this, a finite element thermal model of a turbine installed in a concentrating solar power plant was developed and validated against measured data. Results showed relative errors of temperature evolutions below 2%, making valid the assumptions and simplifications made. Since there is trade-off between start-up speed and turbine lifetime consumption, the model was then implemented within a multi-objective optimization scheme in order to test and design faster start-ups while ensuring safe operation of the machine. Significant improvements came up in terms of start-up time reduction up to 30% less than the standard start-up process.

Stall followed by surge in a high speed compressor can lead to violent disruption of flow, damage to the blade structures and, eventually, engine shutdown. A knowledge of unsteady blade loading during such events is crucial in determining the aeroelastic stability of blade structures, experimental test of such events is however significantly limited by the potential risk and cost associated. Numerical modelling, such as unsteady CFD simulations, can provide a more informative understanding of the flow field and blade forcing during post-stall events, however very limited publications, particularly concerning multi-stage high speed compressors, can be found. The aim of this paper is to demonstrate the possibility of using CFD for modelling full-span rotating stall and surge in a multi-stage high speed compressor, and, where possible, validate the results against experimental measurements. The paper presents an investigation into the onset and transient behaviour of rotating stall and surge in an 8-stage high speed axial compressor at off-design conditions, based on 3D URANS computations, with the ultimate future goal being aeroelastic modelling of blade forcing and response during such events. By assembling the compressor with a small and a large exit plenum volume respectively, a full-span rotating stall and a deep surge were modelled. Transient flow solutions obtained from numerical simulations showed trends matching with experimental measurements. Some insights are gained as to the onset, propagation and merging of stall cells during the development of compressor stall and surge. It is shown that surge is initiated as a result of an increase in the size of the rotating stall disturbance, which grows circumferentially to occupy the full circumference resulting in an axisymmetric flow reversal.