The design of a tip appendage for the passive control of tip leakage vortices in axial flow fans is investigated for the purpose of fan performance enhancement. The merging and subsequent bursting of the tip leakage vortex further downstream of the blade is associated with having a detrimental effect on fan performance characteristics. Periodic three-dimensional numerical computations are conducted using the open-source CFD library OpenFOAM and a newly implemented open-source meshing strategy.

Both steady and unsteady numerical computations are used to determine the effect of tip clearance on fan performance characteristics. An increase of 15.6 percent in total-to-static pressure rise and a 5.1 percentage point increase in total-to-static efficiency is found with a 66.6 percent reduction in tip clearance at the fan's design point flow rate. Normalized helicity contours are used in the visualization and quantification of the flow near the blade tip. The findings show a reduction in the relative tip leakage flow velocity through the tip gap and a reduction in the tip leakage vortex exit trajectory angle along the blade's chord with a reduction in tip clearance. This paper reports on numerical investigations that were performed on these two flow characteristics with the aim to control them through the implementation of a tip appendage design.

An average reduction of up to 6.8 percent in the tip leakage flow velocity through the tip gap is achieved through the introduction of three constant thickness end-plate designs. Comparing the pressure side constant thickness end-plate design to the fan datum, an increase in the fan's total-to-static pressure rise at lower than design point flow rates is achieved with an increase in the fan's peak efficiency plateau towards the rotor stall margin. However, none of the constant thickness end-plate designs are able to improve the fan's performance characteristics at its design point flow rate.

A novel trailing edge end-plate concept is introduced through modification of the downstream section of the blade tip. The trailing edge end-plate design is found to exhibit control of the tip leakage vortex exit trajectory angle while shifting the blade's peak loading distribution further downstream towards the blade's trailing edge. The introduction of the trailing edge end-plate design is found to increase the datum fan's performance characteristics across the entire evaluated stable operating range, with an indicated increase of 9.7 percent in total-to-static pressure rise and a 1.3 percentage point increase in total-to-static efficiency at the fan's design point flow rate.


Design of a Tip Appendage for the Control of Tip Leakage Vortices in Axial Flow Fans

Nowadays the use of numerical simulations for predicting the performance of turbomachinery is widely accepted and used, also to predict very complex phenomena such as particle/droplet erosion or deposition that occur in several turbomachinery. For instance, some induced draft fans for industrial applications elaborate particle laden flows resulting in a large erosion of the blades and then performance loss. The possibility of predicting such a process would improve the sizing and maintenance scheduling processes. However, particle erosion simulations are usually limited to a fraction of seconds because of computational costs, and from these simulations it is hard to extract some reliable predictions. A main reason is that the relative flow field varies depending on the operating conditions referred to the machine and to the installation environment, therefore, a single numerical experiment cannot include all real-life situation. In some previous publications we developed an algorithm to perform long time predictions, also considering the variations in blade geometry with time. Here, we present a method to obtain, given a fan blade section, its relative velocity triangle and the particle concentration, the time specific erosion damage pattern per unit of span. The resulting pattern is given by an artificial neural network (ANN) trained on a dense dataset of numerical simulations, based on the modelling developed and used by the authors in the last decade.

The synopsis of the method is basically the following. Create a large database of erosion patterns for a specific airfoil, considering as aerodynamic features, the relative angle of attack, the relative flow velocity and Reynolds number, and as particle flow features, the diameter, concentration, hardness and density. Each pattern is obtained simulating the particle erosion on a blade strip, in the condition defined by a specific set of the features defined above. Once the dataset is complete, an ANN is trained to solve the regression problem and estimate the specific erosion damage pattern for any set of features in the dataset bounds.


Integration of Computational Fluid-Particle Dynamics Techniques for the Instantaneous Estimation of Particle Erosion Damage on Axial Fan Blade Sections

Turbine and compressor systems composed of many sub-systems, of which blade-disc assembly forms an important subsystem. All the blade-disc assemblies are designed to have uniform response at high speed rotation. The rotating parts of the system hold cyclic symmetry to provide a single period response or tuned response. On the other hand, the cyclic symmetry significantly simplifies vibration analyses. Non-uniformity in material properties, manufacturing tolerances and wear and tear during operation destroy the cyclic symmetry leading to loss in periodicity. The turbine system then develops non-uniform multi-frequency response. These are known as mistuned turbine system. A mistuned system suffers from localization in modes, subsequently concentration of vibration energy to a few blade-disc assembly. These assemblies undergo huge strain and stress due to high speed rotation. Furthermore, rotation causes centrifugal stiffening and gyroscopic effects. Considering all these parameters and complexities in the mathematical model, finding close form analytical solution is a tedious process. Hence, numerical analysis is carried out using finite element approximation which requires very fine mesh and is computationally exhaustive. Therefore, it is important to find a way to analyse the dynamics of rotating bladed disc systems at the design level with less computational complexities.

Under mistuning, physical parameters of the system vary neither uniformly nor deterministically. Therefore, deterministic analysis will not be able to capture the system response appropriately. This study focusses on probabilistic modelling of the bladed disc system parameters and calculate numerically the distributions of the response quantities of the system. Stochastic finite element model of the system consisting of all the assemblies and the hub is developed and reported. The spatial inhomogeneity of mistuned structures is modelled as non-Gaussian random field. Experimentally, coordinate measurement machine (CMM) is used to get the random geometry of the bladed disk structure. In this analysis, a synthetic data is generated which represent the CMM data. Further, Rosenblatt transformation is implemented to each component of the data set to get the polynomial chaos framework of the system parameters. Since, the random field of the system parameter is approximated as correlated random variables, Spearman’s rank correlation coefficient is used on this manuscript to obtain that correlation among the random parameters across the whole domain. The approximated probability density function through the aforementioned methodology is compared with the target probability density function of the parameter using Kullback – Liebler (KL) entropy.  Also, the same KL entropy is used as a metric to check the convergence of polynomial chaos terms in the expansion. Next, the proposed polynomial chaos method is integrated with commercial finite element software to quantify the propagation of r

Stochastic Modelling and Analysis of Rotating Bladed Discs

In this paper we want to present an unsteady simulation of a full rotor of an axial fan with low pressure and high flow rate, developed during the MinWaterCSP EU project, specifically designed and optimized for Concentrating Solar Power (CSP) plants with the aim of reducing water consumption at the condenser stage. An earlier version of the fan, referred as M-Fan, was initially developed to serve the hybrid cooling system, to boost the overall heat transfer performance with dry cooling. In this work, we will evaluate the performance of a redesigned and scaled version of the fan, referred as T-Fan, by performing a Computational Fluid Dynamics (CFD) analysis of the unsteady flow. The T-Fan was designed and optimized to reduce noise emissions and improve efficiency with respect to the M-Fan. We perform the simulation using a in-house built C++ parallel software for CFD analysis using Finite Elements (FE). We model the flow dynamics using the Residual Based Variational MultiScale method (RB-VMS) to obtain a stabilized solution of the Navier-Stokes equation using a Finite Elements (FE) discretization.

We will discuss the simulation results of the RB-VMS model by comparing our the RB-VMS simulation of the T-Fan with LES and experimental results on the same fan.


Unsteady Flow Simulation of an Axial Fan for Dry Cooling in a CSP Plant Using the Variational Multiscale Method

This study investigates the influence of various inducer configurations on the single-phase and two-phase flow behavior. Three pitch values (*P* = 0.151, 0.251, and 0.351 m), as well as three different number of blades (N = 2, 3, and 4 blades), were studied, leading to a total of 9 different inducer geometries. The main objective of the present study is to analyze and compare the corresponding performances and the mixing behavior when pumping a gas/liquid mixture. For this purpose, 3D steady-state simulations using the Moving Reference Frame approach were applied for single-phase flows, while a transient setup using a moving-mesh approach was employed for two-phase simulations. Turbulence was modeled by the Reynolds Stress Model, whereas the standard Volume of Fluid method was applied to model air-water interactions. The results show that an increase in the number of blades leads to a high drop in performance at overload (high-flow) conditions and only a slight performance enhancement at part-load (low-flow) conditions. Additionally, the effective range in which the inducer efficiency is high becomes narrower for a higher number of blades. Concerning now the inducer pitch, at part-load conditions, a lower pitch is slightly beneficial to smoothly suck the flow and damp low-flow vortices. On the contrary, employing a high pitch at these conditions results in intensified flow vortices, reducing the performance. Furthermore, the blade pitch strongly impacts the performance at optimal and overload conditions, where a lower pitch causes flow blockage, leading to significant performance deterioration. Generally, it is found that the inducer configuration affects the performance much more at overload compared to part-load conditions. Concerning two-phase flows, the highest pitch provides the best mixing since the inducer is able to effectively churn the two phases, as compared to low-pitch inducers. Similarly, an increase in the number of blades amplifies the turbulence between the two phases, thus improving mixing. Overall, higher pitches and low to moderate number of blades are favorable to ensure high performance over a wide range of flow conditions and to improve pumping of two-phase flows.


Influence of Blade Pitch and Number of Blades of a Pump Inducer on Single and Two-Phase Flow Performance

This paper describes an investigation into the off-design performance comparison of single and two-shaft gas turbine engines. A question that has been asked for a long time in which gas turbine delivers a better thermal efficiency at part load. The authors, notwithstanding their intensive searches, were unable to find a comprehensive answer to this question. A detailed investigation was carried out using a state of the art performance evaluation method and the answer was found to be: It depends!

 

In this work, the performance of two engine configurations is assessed. In the first one, the single shaft gas turbine operates at constant shaft rotational speed. Thus, the shape of the rotational speed line will have an important influence on the performance of the engine. To explore the implications of the shape of the speed line, two single shaft cases are examined. The first case is when the speed line is curved and as the compressor pressure ratio falls, the non-dimensional mass flow increases. The second case is when the speed line is vertical and as the compressor pressure ratio falls, the non-dimensional mass flow remains constant.

 

In the second configuration, one shaft couples a compressor and a turbine (often called the gas generator shaft). The second shaft couples the power turbine to the driven equipment. The two shafts can be controlled to operate at different rotational speeds and also varying relationships between the rotational speeds. The part-load operation is characterised by a reduction in the gas generator rotational speed.

 

The tool, which was used in this study, is a 0-D whole engine simulation tool, named Turbomatch. It was developed at Cranfield and it is based on mass and energy balance, carried out through an iterative method, which is based on component maps. These generic, experimentally derived maps are scaled to match the design point of a particular engine before an off-design calculation is performed. The code has been validated against experimental data, it has been used extensively for academic purposes and the research activities taken place at Cranfield University.

 

For an ideal cycle, the single shaft engine was found to be a clear winner in terms of part-load thermal efficiency. However, this picture changed when realistic component maps were utilised. The basic cycle and the shape of component maps had a profound influence on the outcome.

 

The authors explored the influence of speed line shapes, levels of component efficiencies and the variation of these component efficiencies within the operating range. This paper describes how each one of these factors, individually, influences the outcome.


Off-Design Performance Comparison Between Single and Two-Shaft Engines: Part 1 \u2014 Fixed Geometry

In modern aviation business, the high bypass ratio turbofan engine is the main power plant of the aircraft. The fan of a high bypass ratio turbo fan engine produces up to 80% of the total thrust of the engine. The aerodynamic performance of low pressure turbine, which drives the fan, plays an important role in the economic benefits of engine. As the diameter of the fan increases, the problem of rotating speed mismatch between the fan and the low pressure turbine becomes more and more severe. The GTF (geared turbofan) engine is a good solution to the problem and has been proved by the Pratt & Whitney Company. For the GTF low pressure turbine, profile losses may account for most of the total losses like traditional LPTs, and these are mainly controlled by the separation, transition, and reattachment of the flow in the suction side of the airfoils. But unlike the traditional LPTs, the GTF LPT works in a higher Mach number environment. The traditional LPTs usually operate at Mach numbers in the range 0.4-0.7 while the GTF LPT could operate at a transonic environment. Generally, the GTF LPT works in a high Mach number and low Reynolds number environment.

In present study, a high-lift low pressure turbine cascade designed for GTF engine work environment is numerically investigated. The LES (large eddy simulation) method is used to investigate the influence of Mach number on the development of boundary layer of suction surface in low Reynolds number. Three different isentropic exit Mach numbers, which are 0.14，0.87 and 1.17 respectively, are simulated while the Reynolds number is kept the same value of 1×10^5^, where the Reynolds number is based on the axial chord length of the blade and the exit flow velocity. The condition of exit Mach number of 0.87 and the Reynolds number of 1×10^5^ is at the design point of the turbine. The turbine profile is designed with the after loaded method.

The simulated results show that the development of boundary on the blade suction surface is strongly affected by the Mach number. It is found that the total pressure loss of the turbine cascade is the lowest at the operating condition of the lowest isentropic exit Mach number of 0.14, while the total pressure loss of turbine cascade is the largest at design point condition with exit Mach number of 0.87. At the transonic condition of exit Mach number of 1.17, despite the existence of shock wave, the total pressure of turbine cascade reduced some compared with that at the design point condition.

It is found that the separation, transition, and reattachment process exist in the boundary layer on the suction surface of turbine cascade which is similar as that on traditional low pressure turbine cascade. The separation point is located at about 89% axial chord length and the reattachment point is located at just before the trailing edge. While the turbine cascade works at the condition with the Mach number of 0.87, the boundary layer on suction surface kept laminar and sepa

The Influence of Mach Number on the Boundary Layer Development of High-Lift Low Pressure Turbine in Low Reynolds Number

The supercritical carbon dioxide (sCO~2~) cycle has emerged as a promising power cycle for various types of power conversion systems, based on its high thermal efficiency, (approaching 60%), small-size components and compact footprint. Applications of sCO~2~ power cycle for different heat sources have been investigated. They include nuclear energy, solar energy, fossil fuel, fuel cell and industrial waste heat.

Since the recompression Brayton cycle with sCO~2~ is based on high capacity regenerators processing a large amount of heat (for example, the HTR’s duty is typically about 150% that of the main heater), the effectiveness of these regenerators is critical to attaining high overall cycle efficiency figures. This is proved by basic availability analysis of this cycle.

Effectiveness values up to 99% are currently set as a target for these heat exchangers, which can be achieved by use of the Printed Circuit Heat Exchanger (PCHE) technology. Thus, the design process for these heat exchangers is demanding, especially if one takes into account the peculiarities of variation of CO~2~ density and thermal properties (especially c~p~ which augments by orders of magnitude) near the critical temperature.

A hybrid design methodology for the high-pressure and low-pressure recuperator (HTR and LTR) is formulated and presented in this paper, which employs 3D CFD conjugate heat transfer computation of the performance of a small two-channel module of the PCHE type heat exchanger. Specific design alternatives for channel dimensions, roughness and zig-zag angles are compared and discussed.

The main results of the CFD computation are the heat transfer coefficient and Colburn j-factor for the module, as well as the Fanning friction factor for each typical channel (hot and cold side of the module). The results from the small module computation are employed in a routine segmental method for the performance computation of the full channel length. Thus, the effectiveness and pressure drop characteristics for the full heat exchanger are computed with high accuracy.

Application of the proposed methodology is carried out for the HTR and LTR computation for a recompression sCO~2~ Brayton cycle of a 600 MW~th~ size power plant with turbine entry temperature of 850 K under study. Both regenerators are studied according to the PCHE design approach. Typical hot side inlet temperature to the HTR is 720 K and the respective hot side inlet temperature for the LTR is 430 K. The complexities induced by CO~2~ thermal properties variation, especially for the LTR, are presented and discussed.

Finally, overall design and geometry layout for the full heat exchangers, both HTR and LTR, along with their collectors’ layout are proposed, based on the current state of PCHE technology.


CFD \u2013 Aided Design Methodology for PCHE-Type Recuperators in Supercritical CO2 Recompression Power Cycles

In near future, due to the growing share of variable renewable energy (VRE) in the electricity mix and the lack of large-scale electricity storage, coal power plants will have to gradually shift their role from base-load operation to providing fluctuating back-up power to meet unpredictable and short-noticed load variations, in order to improve the stability of the electrical grid. However, current coal power plants have been designed to operate at base-load and are not optimized for part-load operation, resulting in an intrinsic inadequacy when undergoing power output fluctuations. The founding idea of sCO2-Flex H2020 European funded project is to improve the flexibility of pulverized coal power plants by adopting a sCO~2~ Brayton power cycle instead of common steam Rankine cycles. Despite the large available literature about the design of sCO~2~ systems for different applications (solar, waste heat recovery, nuclear), there is still a little discussion about the part-load strategies that should be implemented in order to maximize system efficiency during real plant operation. This paper aims to provide more insight about the potential of sCO~2~ power plants for small modular coal power plants (25 MW~el~) highlighting the difficulties that must be faced during off-design operation in order to ensure high performance still guaranteeing a safe operation of the components. The selected cycle configuration (recompressed cycle with high temperature recuperator bypass) is implemented in a MATLAB+Refprop numerical tool specifically designed to optimize the plant design, to provide a preliminary sizing of each component and to evaluate and compare different off-design operating strategies. In particular, the off-design behavior and the operational constraints of each component will be implemented on the base of referenced numerical models, adopting reliable correlations and exploiting ad hoc codes for the performance evaluation. After cycle design and optimization in nominal condition the system performance during a reference year will be evaluated allowing to understand the efficiency penalization and the minimum load attainable with sCO~2~ power plants. Different operational strategies will be considered including minimum cycle pressure, maximum cycle temperature and inventory optimization. Results will be compared with the annual average performances of a subcritical steam Rankine cycle (as typical solution for small power output) that will be computed with Thermoflex software. The comparison will show the advantages in adopting sCO~2~ power cycle in terms of coal consumption, CO~2~ emission and minimum achievable load.


Part Load Strategy Definition and Annual Simulation for Small Size sCO2 Based Pulverized Coal Power Plant

A novel method of parametrization and determining of parameter influence for a large design space optimization of a micro gas turbine compressor stage is presented. A compact, high efficiency, compressor for use in a 600 N thrust UAV engine is developed. The compressor consists of a mixed flow impeller and a continuous vane diffuser that negates the need for dedicated de-swirling vanes. 48 free parameters were used to control the meridional channel, blade camber, and structural geometric features. The optimization focused on determining the optimal impeller meridional discharge (mixed flow) angle, for a predetermined set of constraints. The number of parameters required to achieve a large geometric diversity was greatly reduced by coupling the endwall Bézier control points with user-defined functions that modified multiple control points with a single parameter as input (such as mixed flow angle).

The influence of key geometric features on design performance was assessed using a Pearson correlation coefficient map. From the correlation map, it was observed that stage total-to-static pressure ratio, and efficiency, were strongly influenced by diffuser outlet passage height and diffuser vane wrap angle. This was due these parameters’ control of flow separation magnitude at the diffuser hub in the radial-to-axial bend. The proposed method of assessing parameter influence was found to be very helpful in the free parameter selection process due to the ease with which a large number of parameters can be compared by their performance influence.

A multidisciplinary workflow was scripted to incorporate the CalculiX CrunchiX structural analysis into the NUMECA FINE^TM^/Design3D aerodynamic optimization package. A multipoint and multiobjective optimization was performed using the *design of experiments* method, which was accelerated by using a surrogate model that was constructed using the MINAMO tool to increase design convergence rate. The impeller and diffuser blade rows were simultaneously optimized due to the strong influence of impeller wake on diffuser leading edge design. Structural feasibility constraints were placed on maximum von Mises stress, blade tip displacement, and the resonance frequencies of the impeller. A three-dimensional Pareto front was constructed to assist in the selection of the final design. The final design achieved a total-to-static pressure ratio of 4,15 and efficiency of 86,24%, at a design mass flow rate of 1,089 kg/s. Choke and stall margins of 7,4% and 11,8% were achieved at the design speed of 73 000 RPM. The continuous vane diffuser successfully achieved a high diffusion rate, which allowed for a compact compressor stage of 180 mm maximum diameter and 80 mm axial length, as desirable for aircraft propulsion.


A Large Design Space Multidisciplinary Optimization of a Mixed Flow Micro Gas Turbine Compressor Stage

The formation of water droplets within condensing steam turbines is a complex process that occurs at supersaturated, non-equilibrium conditions and is influenced by, inter alia, the presence of foreign bodies (e.g., dust particles) and other steam impurities, and the unsteady segmentation of blade wakes by successive blade rows.  
The latter is often referred to as \`wake chopping', and its effect on the condensation process is the subject of this paper.  
On-site optical measurements conducted in the late 1980s on operational turbines (e.g. Young et al. (1988)) revealed droplet size distributions that are much broader and with larger average diameters than those observed in (steady flow) condensing nozzle experiments.  
A number of authors  (Bakhtar and Heaton (2005), Petr and Kolovratnik (2000), Guha and Young (1994)) have suggested that wake-induced unsteadiness may be a contributing factor towards this discrepancy.  
The practical significance is that thermodynamic \`wetness losses' (which constitute a major fraction of the overall loss) are strongly affected by droplet size.  
Likewise, droplet deposition and the various ensuing two-phase phenomena (such as film migration and coarse-water formation) also depend on the spectrum of droplet sizes in the primary fog.   

The theory behind wake-chopping models is that the high levels of dissipation within the segmented wakes lead to temperature fluctuations that suppress and postpone the condensation process.  
This results in nucleation occuring in regions with a range of expansion rates which in turn leads to a broader spectrum than would otherwise occur within a steady flow.  
The majority of wake-chopping models presented in the literature (e.g., Petr and Kolovratnik (2000), Guha and Young (1994)) adopt a stochastic approach, whereby large numbers of fluid particles are tracked through (some representation of) the turbine flowfield, assigning a random number at each successive blade row to represent the particle's pitchwise location, and hence its level of dissipation.  
The work described here follows a similar approach but includes  
(a) a comprehensive study of the sensivity to key model parameters (e.g., blade wake shape and wake decay rate);  
(b) an assessment of the impact of circumferential pressure variations;  
(c) a study of the implications for wetness losses and  
(d) a study of the implications for deposition rates.


The Influence of Wake Chopping on Wet-Steam Turbine Modelling

The use of carbon dioxide in supercritical thermodynamic conditions (sCO2) for closed gas cycles is attractive since it provides potential advantages in terms of overall cycle efficiency, size of the components, and dynamics of the system. The combination of the aforementioned features allows adapting the cycle to several alternative energy sources such as nuclear, solar, and waste-heat recovery.

Depending on the specific heat source exploited, several layouts were historically proposed for sCO2 power systems. All of them, however, share the use of at least one compressor operating in close proximity to the critical point, since the thermodynamic optimization of the cycle drives the compression towards conditions featuring the lowest work exchange. For electric power within 500 MW, the recommended machine is a centrifugal compressor.  As a result, the intake conditions of the compressor are close to saturation, making the fluid prone to cavitation or condensation (depending on the actual entropy level of the fluid at the intake, i.e. if lower or larger than the critical value). The intake of a turbo-compressor is indeed characterized by nearly-isentropic suction phenomena, induced by the flow-turning around the leading edge of the blades and by the acceleration due to the blade blockage.

In the present study, we consider a prototype sCO2 centrifugal compressor operating in proximity to the critical point, with intake entropy level of the fluid lower than the critical value. In this condition, the phase change occurs as evaporation/flashing, thus resembling cavitation phenomena observed in liquid pumps, even though with specific issues associated to compressibility effects occurring in both the phases. The flow configuration is therefore highly non-conventional and demands the development of proper tools for fluid and flow modeling, which are instrumental for the compressor design.

The papers discusses the modeling issues from, at first, a thermodynamic perspective and, then, highlighting the implications on the compressor aerodynamics. Then, we propose tailored models to account for the effect of the phase change in 0-D mean-line design tools as well as in fully 3D Computational Fluid Dynamic (CFD) simulations. In this way, a design strategy is build-up as a combination of mean-line tools, industrial design experience, and CFD for detailed flow analysis. The application of the design strategy allowed studying the actual flow configuration within a realistic sCO2 compressor. The study reveals that the onset of the phase change might alter significantly the performance and operation of the compressor, both in design and in off-design conditions.


Implications of Phase Change on the Aerodynamics of Centrifugal Compressors for Supercritical Carbon Dioxide Applications

The main goal of an engineering course is for the students to achieve the defined educational goals, enhance their problem- solving capabilities and develop the essential engineering mindset. The continuous improvement of a course is essential to maintain its challenging nature while improving the course quality. Adapting the teaching methods used to new types of students can provide a significant improvement in student learning.

In that context, a digital tool is employed in an advanced course in Aeroengine Technology. A smartphone application that calculates gas turbine performance is introduced in the course to help students understand some of the key concepts. The purpose of the application is to provide the students with an interactive tool to understand the gas turbine thermodynamic cycle. An exercise regarding this application is assigned to note the performance of different engine technologies used in aircraft propulsion through the years. The assignment with the application is combined with a larger assignment on gas turbine performance. The application is also employed in the final exams of the course.

The purpose of this paper is to present the use of the application in the course and to address any challenges that arise in the implementation of the application in the learning process. The employed teaching methods received positive feedback from the students who indicated that the app assignment helped them understand some of the key concepts in the course. After all, the main aim of novel teaching methods is to make learning more interesting, so that students get more involved in a course.


Introduction of a Smartphone Application in an Aeroengine Technology Course

T800 coatings are commonly applied on damping surfaces of turbine blades to mitigate wear. If severe wear occurs, the effectiveness of the damping effect could decrease. Therefore, vibrations increase and the life of blade could be endanger. These mating surfaces are usually stressed with reciprocating relative displacement in dry condition with low amplitude. In other word, the damping surfaces are usually fretted. This study investigates the wear mechanism at the contact damping interfaces coated by Tribaloy® T-800.

The methodology was based on an experimental tests campaign performed with point contact test rig. A peculiarity of this rig is that relative displacement measurements are not affected by the test rig compliance affects. This is not common in fretting test rig. The reason is that there are test rig components with a compliance between the contact surfaces and displacements measurement points. In other words, there is no difference between the amplitude imposed in the measurement point of relative displacement and effective amplitude of the contact surfaces. Nine sets of experimental parameters was analyzed at three temperatures (room temperature, 600 and 800°C), two normal loads (12 and 32 N), three strokes (30, 60, 150 micrometers) and test durations of 3, 5, 10, 15 M-cycles. Friction coefficient was computed using the hysteresis loops measured during the fretting tests. The worn surfaces were measured by optical instrument based on focus variation. Wear volumes were accurately computed with a procedure that takes into account the roughness of contact surfaces. The wear groove was observed by scanning electron microscopy (SEM) at end of the test.

At room temperature the friction coefficient was substantially independent from the normal load. The wear rates at the room temperature ware higher than at high temperature (600, 800 °C). The scanning electron microscopy (SEM) observation of the worn surfaces reveals a morphology of the wear scar that was completely covered with brittle cracks in each sample. The damage mechanism change from brittle (at room temperature) to ductile (at high temperature). In contrast with relation between wear volume and number of wear cycles, the relation between wear volume and energy dissipated was independent from normal load.


Fretting Wear of T800 Coating in Aero-Engine Applications

The present energy worldwide scenario requires energy plants working far from their on-design conditions: this causes transient operations and partial load working conditions to happen frequently. For dynamic compressors, this may cause them operating close to their instability region, especially during load and velocity transients or emergency shutdowns. Moreover, in recent years the need of improving global efficiency of energy plants has led to the success of advanced cycles, such as fuel cells hybrid systems, based on micro Gas Turbines (mGT) technology. This is an important issue for centrifugal compressors, which are normally employed in this kind of systems, since now they have to face higher volumes in comparison to traditional combined cycle plants.

The present paper shows signal processing techniques applied to experimental data obtained from a T100 microturbine connected with different volume sizes. This experimental activity was carried out with the test rig developed at the University of Genoa for hybrid system emulation. However, these results can be extended to all the advanced cycles in which a microturbine is connected with an additional external component responsible for volume size increase. Since in this case a 100 kW microturbine was used, the volume was located between the recuperator outlet and the combustor inlet like in the typical cases related to small size plants. A modular vessel was used to perform and to compare the tests with different volume sizes.

The results here reported are related to rotating stall and surge operations. This analysis was carried out to extend the knowledge about these risk conditions: the systems equipped with large volume size connected to the machine present critical issues related to surge and stall prevention, especially during transient operations towards low mass flow rate working conditions.  

The main expressions of instability of compression systems are indeed rotating stall and surge, which respectively involve the compressor itself and the whole energy system in which the machine is placed. Both mild and deep surge, indeed, are preceded and accompanied by rotating stall. This phenomenon is characterized by stalled fluid cells that rotate slower than the surrounding medium does; nevertheless, it is still a stable condition in which most centrifugal compressors do work, but it causes a remarkable decrease of performances of the machine together with induced vibrations due to an anomalous pressure distribution along the circumferential direction, which results in an unbalancing force on the rotor, instantaneously varying both in nodulous and direction. For these reasons, the system was equipped with different dynamic probes to measure the vibroacoustic operational response during normal and anomalous operations.

Investigations based on acoustic and vibrational measurements appear to provide an interesting diagnostic and predictive solution by adopting suitable quantifiers calculat

Signal Processing Techniques to Detect Centrifugal Compressors Instabilities in Large Volume Power Plants

The tip leakage flow in turbine and compressor blade rows is responsible for a relevant fraction of the total loss. It contributes to unsteadiness, and have an important impact on the operability range of compressor stages. Experimental investigations and, more recently, scale-resolving CFD approaches have helped in clarifying the flow mechanism determining the dynamics of the tip leakage vortex. Due to their continuing fundamental role in design verifications, it is important to establish whether RANS/URANS approaches are able to reproduce the effects of such a flow feature, in order to correctly drive the design of the next generation of turbomachinery. Base studies are needed in order to accomplish this goal.  
In the present work the tip leakage flow in axial compressor rotor blade cascade have been studied. The cascade was tested experimentally in Virginia Tech Low Speed Cascade Wind Tunnel in both stationary and moving endwall configurations.  
Numerical analyses were performed using the TRAF code, a state-of-the-art in-house-developed 3D RANS/URANS flow solver.  
The impact of the numerical framework was investigated selecting different advection schemes including a central scheme with artificial dissipation and a high-resolution upwind strategy. In addition, two turbulence models have been used, the Wilcox linear \$k-\\omega\$ model and a non-linear eddy viscosity model (Realizable Quadratic Eddy Viscosity Model), which accounts for turbulence anisotropy.  
The numerical results are scrutinized using the available measurements. A detailed discussion of the vortex evolution inside the blade passage and downstream of the blade trailing edge is presented in terms of streamwise velocity, streamwise vorticity, and turbulent kinetic energy contours.  
The purpose is to identify guidelines for obtaining the best representation of the vortex dynamics, with the methodologies usually employed in routine design iterations and, at the same time, evidence their weak aspects that need further modelling efforts.


Prediction of Tip Leakage Vortex Dynamics in an Axial Compressor Cascade Using RANS Analyses

Under new scenarios with high shares of variable renewable electricity, combined cycle gas turbines (CCGT) are required to improve their flexibility, in terms of ramping capabilities and part-load efficiency, to help balance the power system. Simultaneously, the liberalization of electricity markets and the complexity of its hourly price dynamics are affecting the CCGT profitability, which is leading the need of optimizing its operation. Among the different possibilities to enhance the power plant performance, an inlet air conditioning unit (ICU) offers the benefit of power augmentation and “minimum environmental load” reduction by controlling the gas turbine inlet temperature using cold thermal energy storage and a heat pump. Consequently, an evaluation of a CCGT integrated with this inlet conditioning unit including a day-ahead optimized operation strategy is developed in this study.

To establish the hourly dispatch of the power plant and the right operation mode of the inlet conditioning unit  to either cool down or heat up the gas turbine inlet air, a mixed-integer linear optimization (MILP) was formulated using MATLAB, aiming to maximize the operational profit of the plant within a 24 hours horizon. To assess the impact of the proposed unit operating under this dispatch strategy, historical data of electricity and natural gas prices, as well as meteorological data and CO~2~ emission allowances price, have been used to perform annual simulations of a reference power plant located in Turin, Italy. Furthermore, different equipment capacities and parameters have been investigated to identify trends of the power plant performance. Lastly, a sensitivity analysis on market conditions to test the control strategy response was also considered.

Results indicate that the inlet conditioning unit, together with the dispatch optimization, increases the power plant’s operational profit by achieving a wider operational range, particularly important during peak and off-peak periods. For the specific case study, it is estimated that the net present value of the CCGT integrated with the conditioning unit is 0.5% higher than the power plant without the unit. In terms of technical performance, results show that the unit reduces the minimum environmental load by approximately 1.34% and can increase the net power output by 0.17% annually. This study presents the benefits of a novel solution to increase CCGT flexibility while implementing an optimized dispatch strategy.


Short-Term Optimization of a Combined Cycle Power Plant Integrated With an Inlet Air Conditioning Unit

In the frame of a continuous improving of the performance and of the accuracy in experimental tests, the uncertainty analysis on measurements instrumentation and techniques is of paramount importance. For this reason, since the beginning of the experimental activities at the Fluid Machines Laboratory located at Politecnico di Milano (Italy), this issue was addressed and different methodologies were applied.

In the present paper, a review of the different methodologies used is proposed and for each of them a detailed description is provided. The chosen probes for this investigation are a pneumatic 5 hole probe and fast response 2 sensors probe, the latter applied as a virtual 4 hole probe.

During the calibration of multi-hole probes, a set of calibration coefficients has to be defined to correlate the probe pressure taps readings with the static and total pressures, the flow angles and Mach number.

These calibration coefficients, included in calibration matrices, are then used during the probe application in the experimental rigs to recover the unknown flow properties. 

The uncertainty quantification, needed to find out the final uncertainty on the flow properties, starts with the transducers calibration; the second step is the evaluation of the uncertainty in the calibration matrices to end up with the uncertainty quantification in the unknown flow.

Therefore, the purpose of this paper is to show and discuss three different methodologies for the uncertainty quantification of the flow quantities, highlighting their strengths and weakness.

The easiest methodology consists in applying the probe, after having calibrated it, in a known flow and, by making use of the calibration matrices, evaluating the flow properties: the accuracy is calculated by the difference between the imposed flow quantities and the calculated ones.

One of the widespread methodology makes use of the uncertainty propagation techniques: this methodology is applied both in the calibration process, where the calibration coefficients are calculated and their uncertainty quantified. When the unknown flow is measured, the final uncertainty includes the ones of the actual pressure readings and of the calibration coefficients.

The last methodology makes use of the Monte Carlo based approach, applied both in the calibration phase and in the measuring one. In this methodology, different probability functions have been imposed to the different uncertainty contributions.

The uncertainty contributions during the pressure transducer calibration refers to the analog to digital conversion, the reference transducer, the standard deviation of the voltage signals, the repeatability in the measuring, the transducers linearity. In addition to the aforementioned probes values, for the evaluation of probes uncertainty, the angular positioning, calibration coefficients and nozzle static and total pressures errors have to be included.


Uncertainty Evaluation on Multi-Hole Aerodynamic Pressure Probes

Tesla bladeless turbomachines are recently being investigated due to many advantages such as its simple design and ease of manufacturing. It consists of flat, thin, smooth disks mounted parallelly on the shaft with spacers between them to maintain constant gap. Fluid enters tangentially through stator and leaves the rotor axially while imparting torque on the disk. In the past, low efficiency of the Tesla expander is recorded citing main losses occurring in the stator and viscous friction at the end disks due to ventilation. If an efficient design is achieved, this will be a promising machine in the area of small-scale power generation and energy harvesting. This paper focuses on experimental performance investigation of 3 kW (rated electrical power) Tesla bladeless expander along with vibration-acoustic test of the rotor. The Tesla expander and the electric generator are housed in a single casing making it first of its kind being tested with such configuration. The expander is fed with air and operated at high rotational speeds up to 40000 rpm. The expander has two exhausts of which one exhaust passes over generator for cooling purpose. The test is carried out with different number of nozzles to understand its effect on the performance. The real time performance of expander and generator is recorded.

Performance results show that the peak efficiency for two nozzles is better than one nozzle for the same inlet pressure conditions.  Furthermore, one of the most important losses in Tesla turbomachines, ventilation loss, is experimentally evaluated. Ventilation losses, for the first time, are extensively discussed based on experiments.  The ventilation losses together with bearing loss are characterized using experiment data and numerical analysis.

Regarding roto-dynamical features, Tesla turbine rotor assembly is composed of several thin disks which leads to some peculiar vibrational criticalities. Thus, specific vibro-acoustic tests were carried on obtaining a complete machine dynamical characterisation. Data is acquired during machine run up, run down and with some speed regimes. This test was conducted feeding different numbers of turbine nozzles with and without the load due to the generator. Collected data analysis allows to identify, for each turbine speed, the frequency contents in the vibration signals and to associate them with their possible sources. Moreover, vibrational analysis permits the identification of machine critical speeds, rotor and stator resonances. Vibrational measurement provides a base line level related to the machine safety condition which will be used for operational monitoring. The combined analysis of performance and vibro-acoustic tests enables the design of efficient and dynamically stable Tesla expanders.


Experimental Investigation on a 3 kW Air Tesla Expander With High Speed Generator

The use of air-cooled condensers (ACCs) has seen a large uptake in the petrochemical and power generating industry. Typical power plant ACCs utilise axial flow fans approximately 9 m in diameter situated 45 m above ground level. Previous studies have shown that ACCs are adversely affected by environmental conditions, including dry-blub temperature and cross-winds. The complex flow phenomena associated with ACCs under cross-wind conditions require careful consideration in ensuring adequate sizing is attained to meet the heat rejection demands. This has led to an increase in the development of modelling methodologies to adequately predict ACC performance for different environmental and changing heat load conditions. Computational fluid dynamics (CFD) is a toolkit that has been shown to adequately model air-side characteristics, whereas system level modelling software has also been shown to adequately model steam-side characteristics. This study builds on a previous study and aims to determine adequate discretization level of the heat exchanger in order to combine CFD and system-level modelling software to predict the performance of an ACC cell in an ACC bank. This co-simulation approach is applied by using ANSYS Fluent® R1 for the CFD computations and Flownex® 8 for the system-level model.

A single ACC cell situated on the periphery is considered for co-simulation in a 4x3 ACC bank situated 8 m above ground level. The axial flow fan is modelled using the Extended Actuator Disk Model (EADM) and the heat exchanger represented as a porous medium. An axial flow fan currently in use in the power generating industry, designated as the A-fan, is used. In accordance with findings from previous studies, the heat exchanger plenum is modelled using an A-frame configuration. The ACC support struts and fan walkway are also modelled as geometrical structures. Steam characteristics are represented using a 1-D, implicit solver and the steam is represented as a homogeneous two-phase mixture. The heat exchanger is discretised into sections of 3x3, 6x6 and 3x9 and the performance in the form of fan volumetric flow rate, fan power consumption, heat transfer rate and steam quality at the cell outlet is measured. The ACC is subjected to cross-winds of 0, 3, 6 and 9 m/s and a comparative performance analysis is undertaken to ascertain adequate level of discretization. The novelty of this co-simulated approach lies in the correct depiction of steam-side characteristics in the heat exchanger subjected to maldistributed inlet flow conditions present in cross-wind conditions. The model is also able to characterise ACC performance using steam outlet quality as the metric.


A Co-Simulation Approach to Modeling Air-Cooled Condensers in Windy Conditions

High-bypass ratio turbofan engines are commonly employed  
in aircrafts. Their usage is essential to guarantee small specific  
fuel consumption, reduced CO2 emissions and low noise levels.  
Such modern aero-engines benefit from high efficiencies by operating  
at turbine inlet temperatures in excess of the melting point  
of the turbine components. To enable this, compressor air is supplied  
to the turbine for cooling and purging purposes. The reintroduction  
of the cooling air back into the mainstream flow is  
known to alter the flow field and to affect the aerodynamic performance  
of the turbine components. A component especially  
susceptible to the interaction between the mainstream and purge  
flow is the Turbine Center Frame, located between high-pressure  
turbine (HP) and low-pressure (LP) turbine. For ever higher bypass  
ratios, this turbine transition ducts need to be designed with  
axial lengths as short as possible and larger radial offsets to avoid  
engine weight penalties while at the same time maintaining aerodynamic  
performance. More detailed experience in the field of  
intermediate turbine ducts is needed to identify further opportunities  
to improve turbofan engine performance, including an indepth  
understanding of the interaction between mainstream and  
purge flows.  
This paper presents a Computational Fluid Dynamics (CFD)  
study of the effect of the purge flow temperature, and hence density,  
on the aerodynamic performance of an engine representative  
Turbine Center Frame (TCF). Several steady-state Reynoldsaveraged  
Navier–Stokes (RANS) simulations were conducted  
for varying purge flow temperatures using an in-house code  
called LINARS. Time-averaged five-hole-probe measurements  
acquired in the Transonic Test Turbine Facility (TTTF) at Graz  
University of Technology were used as inlet boundary conditions  
to impose an engine-relevant flow field. The results obtained  
from two reduced and two increased purge flow temperature  
conditions were compared to a reference case. The reference  
case results showed agreement with static wall pressure measurements,  
hence validating the simulation. Changing the purge flow  
temperature significantly affected the main flow locally as well  
as overall. The position and size of vortices in the TCF were  
changed under the presence of hotter or cooler purge flows. Additionally,  
a flow separation on the outer duct wall observed in  
the baseline case was suppressed in the cold-purged flow case.  
The cold-purged TCF showed a 40% lower total pressure loss  
than the hot-purged one. This indicates that a more aggressive  
TCF design may be feasible in a cold-purged operation.


Influence of Purge Temperature Variation on the Performance of Turbine Center Frames

During the last decades there has been a rise of awareness regarding the necessity to increase energy systems efficiency and reduce carbon emissions. These goals could be partially achieved through a greater use of gas turbine - solid oxide fuel cell hybrid systems to generate both electric power and heat. However, this kind of systems are known to be delicate, especially due to the fragility of the cell, which could be permanently damaged if its temperature and pressure levels exceed their operative limits. This could be caused by degradation of a component in the system (e.g. the turbomachinery), but also by some sensor fault which leads to a wrong control action. To be considered commercially competitive, these systems must guarantee high reliability and their maintenance costs must be minimized. Thus, it is necessary to integrate these plants with an automated diagnosis system capable to detect degradation levels of the many components (e.g. turbomachinery and fuel cell stack) in order to plan properly the maintenance operations, and also to recognize a sensor fault. This task can be very challenging due to the high complexity of the system and the interactions between its components. Another difficulty is related to the lack of sensors, which is common on commercial power plants, and makes harder the identification of faults in the system.

This paper aims to develop and test a Bayesian belief network based diagnosis method, which could be used to predict the most likely degradation levels of turbine, compressor and fuel cell in a hybrid system on the basis of different sensors measurements. The capability of the diagnosis system to understand if an abnormal measurement is caused by a component degradation or by a sensor fault will be also investigated. The data used both to train and to test the network will be generated from a hybrid system deterministic model and later modified to consider noise or bias in the sensors. The application of Bayesian belief networks to gas turbine – fuel cell hybrid systems is novel, thus this article could be a significant starting point to understand their potential. The diagnosis system developed for this work is expected to provide essential information regarding levels of degradation and presence of faults. This information could be also integrated with the control system to achieve safer and more efficient operations of these plants.


A Degradation Diagnosis Method for Gas Turbine \u2013 Fuel Cell Hybrid Systems Using Bayesian Networks

Aerodynamic experimental measurements and numerical simulations carried out on a model of a high-pressure valve assembly used for a nozzle governing of a turbine with 135 MW output are described in this paper. Aim of the study is to investigate effects of control valve’s strainers on pressure losses and unsteadiness in the flow field thus its effect to undesirable flow fluctuations which can lead to operation reliability issues. Measurements were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics in Czech Academy of Sciences (CAS) where an aerodynamic tunnel is installed. Numerical simulations were carried out in the Doosan Skoda Power (DSP) Company using the ANSYS software tools. The experimental model consists of one of two identical parts of the real valve assembly. It means it consists of an inlet pipeline, a stop valve, a valve chamber with two independent control valves, its diffusers and outlet pipelines. The numerical model consists of both assembly parts and includes also an A-wheel control stage in order to simulate the real turbine operation points as real as possible. The different lifts of the main cone in each control valve for its useful combinations were investigated. Results were evaluated on the model with control valve´s strainers, which were historically used in order to stabilize the flow, and without them. In order to measure pressure losses, the wall static pressure taps evenly distributed around pipelines and valve diffuser´s circumference were installed at different positions in the model. In order to measure pressure fluctuations, Kulite fast response pressure transducers mounted in control valve´s diffuser throats were used. The results of the experimental measurement were compared with the numerical results in the form of pressure losses prediction. From the pressure fluctuation evaluation, it was found out where and for which conditions a danger of a flow instabilities appearance could appear. It can be concluded that there is a border, in terms of operating conditions, where the flow field starts to be unstable and this border is different dependent of the fact whether the control valve´s strainers are used or not. Therefore, the areas of save and danger operation reliability can be predicted. The influence of the control valve´s strainers on the maximal amplitude of periodic fluctuations appears only for the cases when valves are very overload. For normal operating conditions, there is no difference. As a result, the control valve´s strainers do not have to be used in standard applications of valve assemblies. Furthermore, a loss model for valve pressure loss estimation could be updated. Therefore, a pressure loss should be predicted with a sufficient accuracy for each new turbine bid when a similar valve assembly is going to be used. To sum up, by being able to predict the pressure loss and knowing that the valves are reliable are safe for required operating conditions, increasingly deman

Experimental and Numerical Study on Pressure Losses and Flow Fluctuations in a High-Pressure Valve Assembly of Steam Turbine Governing System

Within the last decade industrial gas turbines (GTs) have withstood numerous changes in both usage as well as in materials selection and design boundaries. The constraints aimed at lowering NO~x~ emissions for improving the footprint, have driven designers to increase the firing temperatures and to look for improved cooling systems of GTs nozzles. These factors led to more severe operating conditions for hot gas path components and to the need for more accurate and comprehensive thermo-mechanical models for life assessment purposes.  Within this context, a Lemaitre-Chaboche viscoplastic model has been coupled with a modified theta projection creep model for MAR-M-247 material to account for steady-state creep effects in a finite element transient analysis (FEA). These material models have been applied on a cooled, first stage nozzle for a recently developed high-efficiency gas turbine. A verification assessment has been performed on a over-fired, off-design transient analysis. The viscoplastic model developed showed the capability of dictating an accurate initiation zone of cracking. The Lemaitre-Chaboche model predicted a stress state in the location where the crack nucleated which led to estimate the number of cycles to initiate with an error of approximately 10%. Furthermore, the model demonstrates the capability of using the Lemaitre-Chaboche plastic model coupled with the theta projection creep model to predict reasonable inelastic strains for the entire lifetime of a gas turbine.


Improved Life Assessment for Ni-Based Gas Turbine First Stage High Pressure Turbine Nozzles

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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