Wear assessment of heavy-duty centrifugal fans: experimental and numerical analysis of particle erosion

Le turbomacchine elaborano spesso flussi contaminati da particelle solide. Nella maggior parte dei casi, la presenza di particelle micrometriche disperse nel fluido non è desiderata ed è correlata alle condizioni in cui la macchina opera. Questo è il caso di motori aeronautici, che, aspirando grandi quantità di aria ingeriscono significative quantità di particelle solide. In altri casi, le macchine a fluido sono progettate proprio con lo scopo di processare aria contaminata da particelle. Questo è infatti il caso di ventilatori centrifughi di grande taglia impiegati in impianti di produzione, come acciaierie e cementifici, o in sistemi di ricircolo dell'aria, come quelli presenti in miniere o impianti energetici. Indipendentemente dal fatto che la presenza di contaminanti solidi sia voluta oppure no, uno dei problemi che può scaturire in queste condizioni è l'erosione dei componenti fissi o rotanti della macchina. Questo problema concorre a ridurre l'affidabilità e la sicurezza dell'intero impianto, generando anche una riduzione di efficienza della macchina. In base alla severità del processo erosivo, la possibilità che si verifichino rotture premature e improvvise che mettano a rischio la sicurezza dell'uomo può diventare molto alta, soprattutto nei casi in cui le operazioni di manutenzione vengono ritardate o ignorate. Tuttavia, una corretta ed efficiente organizzazione delle operazioni di manutenzione non può essere fatta senza che non si abbia prima una chiara comprensione del fenomeno in gioco. In questo lavoro, l'attenzione è rivolta all'analisi dell'erosione di ventilatori di grossa taglia che processano aria contaminata da particelle solide. La severità del processo erosivo è stata studiata attraverso l'unione di tecniche di analisi numerica e sperimentale. Per prima cosa è stata effettuata un'analisi numerica in modo da comprendere le caratteristiche del fenomeno. In questa fase, sono stati utilizzati modelli e coefficienti di letteratura per studiare il processo erosivo all'interno della macchina. Successivamente, attraverso analisi sperimentali su materiali e contaminanti impiegati in condizioni operative nella macchina oggetto di studio, è stato possibile ottenere coefficienti numerici tarati appositamente sulle condizioni di interesse. In questa parte è stato utilizzato un banco prova appositamente realizzato. I risultati ottenuti dall'analisi sperimentale sono stati utilizzati per completare le analisi numeriche e stimare il danneggiamento dovuto all'erosione. Basandosi sui risultati ottenuti dall'implementazione dei coefficienti sperimentali nel modello numerico, è stato proposto un modello meccanicistico di natura semi empirica per fornire una stima dei tempi di manutenzione, in base a parametri geometrici della macchina, condizioni di impatto dei contaminanti, e resistenza all'erosione dei materiali. Il lavoro presentato in questa tesi è stato arricchito da un'ulteriore analisi sperimentale volta ad investigare l'accumulo del danneggiamento ad erosione su componenti aeronautici realizzata presso l'Institut fur Luftfahrtantriebe (Dipartimento di Sistemi per la Propulsione Aeronautica) dell'Università di Stoccarda.Turbomachinery operate often in conditions where solid particles are suspended into the processed gas flow. In most cases, the presence of micro-metric particles is not desired and it is related to the conditions where the machine operates. This is the case of aero-engines, which, swallowing a huge amount of air, may ingest, as a consequence, significant quantities of solid particles. In other cases, turbomachinery are specifically employed in operations for the transport of solid particles. Such is the case of heavy-duty centrifugal fans employed in production plants, for cement or steel production, or as a part of exhaust systems in mines or energy production plants. Regardless of whether the presence of solid contaminants in the flow stream is desired or not, one of its consequences is the over-time wear of rotating and stationary parts of the machine. This issue contributes to reducing the reliability and the safety of the whole plant and leads to a decrease in the efficiency of the machine. The severity of the erosion, if maintenance operations are postponed or even ignored, can bring the machine to a premature failure, with potentially serious consequences for human safety. However, the proper scheduling of the maintenance operations can not be possible without a deep understanding of the mechanisms involved in the particle erosion phenomenon which affects rotating machines. In the present work, the attention is focused on the investigation of the wear phenomenon in heavy-duty centrifugal fans. A combined experimental and numerical analysis has been carried out to assess the erosion severity on the machine. First, a preliminary numerical analysis has been carried out to investigate the characteristics of the phenomenon. Literature models and coefficients have been employed to assess the erosion behavior of the machine. The need for more reliable coefficients, capable of predicting the erosion behavior, led to testing the particle and substrate materials, actually employed in the machine, on an on-purpose designed test bench. The results have been used in numerical simulations to properly assess the wear severity and erosion-related damages. A mechanistic semi-empirical relation is proposed to assess the time to overhaul based on the results of the present investigation. The present work has been enriched with a further experimental investigation of the erosive damage accumulation of aeronautical components carried out at the Institut fur Luftfahrtantriebe (Institute for Aircraft Propulsion Systems) of the University of Stuttgart

Numerical Investigation of a Wood-Chip Downdraft Gasifier

Biomass gasification is regarded as one of the most promising technology in the renewable energy field. The outcome of such operation, i.e. the synfuel, can be exploited in several ways, for example powering engines and turbines, and is considered more flexible than the biomass itself. For this reason, a careful analysis of the gasification performance is of paramount importance for the optimization of the process. One of the techniques that can be used for such a purpose, is the numerical analysis. CFD is indeed a tool that can be of great help in the design and study of the operation of the gasifier, allowing for an accurate prediction of the operating parameters. In this work, a downdraft gasifier is considered, and the biomass is made of wood chip. The present analysis is devoted to build the numerical model and simulate all the reactions that happen inside an actual gasifier, considering the drying of the wood chip, heating, pyrolysis, and combustion. Good match with experimental results is found, making the numerical model here presented a reliable virtual test bench where investigating the effects of variation in the working parameters

Performance Degradation of a Shell-and-Tube Heat Exchanger Due to Tar Deposition

Biomass represents a programmable renewable energy source that is useful for reducing issues related to the transfer from fossil fuels to the renewable energy era. The exploitation of biomass is strongly related to the development of power technologies that are designed to improve efficiency; however, at the same time, they have to be designed to improve the life cycle of the entire installation—especially in relation to maintenance operations. In this paper, a numerical analysis is proposed to assess the performance of a heat exchanger used for separating condensing tar from syngas generated by the gasification of lignocellulosic wood chips and pellets. The analysis included clean, fouled, and clogged conditions. Flow maldistribution characterized the inlet section of shell-and-tube configurations and was responsible for clogging phenomena. Starting from field detection, analyses of fouled and clogged conditions showed a reduction in the effectiveness of the heat exchanger, causing dangerous conditions for the internal combustion engine used to exploit the syngas flow

Experimental Assessment of Fouling Effects in a Multistage Axial Compressor

The study of the adhesion of micro sized particles to gas turbine internal surfaces, commonly known as gas turbine fouling, has gained increasing attention in the last years due to its dramatic effect on machine performance and reliability. On-field fouling analysis is mostly related to visual inspections during overhaul and/or programmed stops, which are performed, in particular, when gas turbine performance degradation falls under predetermined thresholds. However, these analyses, even if performed in the most complete as possible way, are rarely (or never) related to the conditions under which the gas turbine contamination takes place since the affecting parameters are difficult or even impossible to be adequately monitored. In the present work, a small scale multistage axial compressor is used to experimentally simulate the fouling phenomenon. The test rig allows the accurate control of the most relevant operating parameters which influence the fouling phenomenon. The compressor performance loss due to particle contamination has been quantitatively assessed. Soot particles appear stickier, especially in the presence of high humidity, and represent the most harmful operating conditions for the compressor unit. The deposits on the stator vanes and the rotor blades have been detected and post-processed, highlighting the most affected regions of each compressor stage employing an image analysis package tool

Evaluation of the Wear-Resistant Plate Performance on Different Locations over the Flow Path of a Large-Sized Heavy-Duty Centrifugal Fan

In industrial applications such as chemical plants, cement factories, and glassmakers, large-sized centrifugal fans are commonly used for dust-laden flow processing. In many cases, the contamination is due to solid particles responsible for fouling and erosion issues. Erosion induces the reduction of mechanical resistance and, at the same time, the modification of the geometry and the surface characteristics of the internal flow path. The process works according to the characteristics of the erodent particles, such as dimension and hardness, which have to be coupled with the mechanical properties of the substrate, like hardness and roughness level. In addition to this, the intensity of the erosion depends on the dynamic characteristics of particles, especially velocity and impact angle. For these reasons, erosion-related issues are difficult predict and reduce. In an attempt to preserve the structural integrity of the internal walls, wear-resistant plates are positioned where the impacting contaminants are supposed to be more detrimental. In the present work, a combined experimental and numerical approach is proposed to evaluate the proper setup of wear-resistance plates over the flow path of a large-sized centrifugal fan. The results show how different regions (rotating and stationary walls) are subjected to different impact behavior, determining that the design of the position of the wear-resistant plate is not straightforward. Suggestions related to reducing the erosion intensity are reported, highlighting the possibility of designing the best compromise between erosion, performance, and costs

Evaluation of the Wear-Resistant Plate Performance on Different Locations Over the Flow Path of a Large-Sized Heavy-Duty Centrifugal Fan

In industrial applications as chemical plants, cement factories, and glassmakers, large-sized centrifugal fans are commonly used for dust-laden flow processing. In many cases, the contamination is due to solid particles which are responsible for fouling and erosion issues. Erosion induces the reduction of mechanical resistance and at the same time, the modification of the geometry and the surface characteristics of the internal flow path. The process works according to the characteristics of the erodent particles, such as dimension and hardness which have to be coupled with the mechanical characteristics of the substrate, like hardness and roughness level. In addition to this, the intensity of the erosion depends on the dynamic characteristics of particles, especially velocity and impact angle. For these reasons, erosion-related issues are difficult to be predicted and reduced. In the attempt of preserving the structural integrity of the internal walls, wear-resistant plates are positioned where the impacting contaminants are supposed to be more detrimental. In the present work, a combined experimental and numerical approach is proposed to evaluate the proper set up of wear-resistance plates over the flow path of a large-sized centrifugal fan. The results show how different regions (rotating and stationary walls) are subjected to different impact behavior and for this reason, the design of the position of the wear-resistant plate is not straightforward. Suggestions related to the reduction of the erosion intensity are reported, highlighting the possibility to design the best compromise between erosion, performance, and costs

A strategy for the robust forecasting of gas turbine health subjected to fouling

Fouling represents a major problem for Gas Turbines (GTs) in both heavy-duty and aero-propulsion applications. Solid particles entering the engine can stick to the internal surfaces and form deposits. Components' lifetime and performance can dramatically vary as a consequence of this phenomenon. These effects impact the whole engine in terms of residual life, operating stability, and maintenance costs. In the High-Pressure Turbine (HPT), in particular, the high temperatures soft the particles and promote their adhesion, especially in the short term. Unfortunately, predicting the GT response to this detrimental issue is still an open problem for scientists. Furthermore, the stochastic variations of the components operating conditions increase the uncertainty of the forecasting results. In this work, a strategy to predict the effects of turbine fouling on the whole engine is proposed. A stationary Gas Path Analysis (GPA) has been performed for this scope to predict the GT health parameters. Their alteration as a consequence of fouling has been evaluated by scaling the turbine map. The scaling factor has been found by performing Computational Fluid Dynamic (CFD) simulations of a HPT nozzle with particle injection. Being its operating conditions strongly uncertain, a stochastic analysis has been conducted. The uncertainty sources considered are the circumferential hot core location and the turbulence level at the inlet. The study enables to build of confidence intervals on the GT health parameters predictions and represents a step forward towards a robust forecasting tool

Off-line washing effectiveness on a multistage axial compressor

The interaction between gas turbines and airborne particles determines detrimental effects on the performance, efficiency, and reliability of the power unit. When it is possible, the interaction is reduced by the use of inlet separators and filtration systems. In an aero engine, these barriers are difficult to implement, and only bigger particles (usually greater than 10 μm) are separated from the airflow. Small units, especially those equips helicopters, are usually affected by fouling issues, especially when the aircraft is employed in harsh environments such as firefighting and rescue activities. To recover this contamination, the unit is washed after the mission by ground operations to restore the unit performance by removing the deposits. This operation occurs during a sub-idle unit operation, and the washing process has to be effective when the engine operates in this off-design condition. In this paper, the evaluation of the washing performance during a sub-idle unit operation is carried out. The compressor unit is a multistage axial compressor that equips the Allison 250-C18 engine. The washing operation was performed by water, and a sensitivity analysis is carried out to discover the capability of water droplets to remove the contaminants. The experimental analysis involves the contamination of the unit by micro-sized soot particles and a washing operation by micro-sized water droplets. These experimental results are compared to numerical simulations to discover the effects of the washing operation on a small power unit during sub-idle operating conditions. The off-design regime imposes a specific evaluation of the proper setup of the washing strategy: flow separations involve wider regions in the compressor unit, and the removal capability is strongly related to the droplet path through the stages. The results show how in the off-design washing operation, the droplet diameter has greater importance than the water flow rate for reducing the deposits over the compressor stages

Haemodialysis treatment in uremic patients: investigation of the correlation between depression, reward system and chronic fatigue

Haemodialysis is considered a life-saving treatment, allowing a good level of survival for uremic patients waiting for organ transplant. Uraemia, if not addressed in an adequate manner, can lead to death. However, treatment greatly influences the patient\u2019s life. Eighty percent of patients has chronic fatigue, affecting the quality of life (QoL). Fatigue is a multidimensional symptom. The \u201cphysical\u201d dimension is linked to obvious bodily limitations, but recovery mechanisms save all of their effectiveness, allowing to get the strength back as a result of an adequate rest period. The \u201cmotivational\u201d dimension, however, is more complex and refers to a central fatigue, often associated with pain, disturbed sleep, affective and cognitive disorders. In spite of studies carried out in groups of patients with chronic disease, suggest that psychological interventions to decrease the fatigue is necessary, these interventions are not available for haemodialysis patients. The aim of this study is to evaluate the level of fatigue, assuming a comorbidity with a past of anxiety and depression, and how the motivation mechanisms are affected with a significant impact on QoL. Evaluative scales have been administered to a sample of haemodialysis patients (N=94) older than 18 years, with no neurocognitive disease. Fatigue Severity Scale (FSS) has been applied to measure the impact of fatigue on motivation and social functionality. BIS/BAS Scale has been administered to investigate the mechanisms of reward: Behavioural Activation System (BAS) and Behavioural Inhibition System (BIS) result in behavioural inhibition/activation to rewards/punishments. Finally, the State-Trait Anxiety Inventory (STAI-Y) and the Back Depression Inventory (BDI-II) have been used to evaluate the potential presence of anxiety and depression disorders. For a qualitative analysis, a semi-structured interview has been performed. The results show a linear correlation between the fatigue and depression level: a. Fatigue is directly proportional to symptoms such as sadness, frustration, irritability, difficulty concentrating and loss of interest. b. The inclination to act, led by the BAS system, is inversely proportional to depression. Therefore, its indirect correlation with chronic fatigue is conceivable. c. The BIS system, on the other hand, is directly related to the sense of fatigue, especially in men. This study shows that the sense of fatigue is not exclusively linked to pathophysiologic factors of uraemia and haemodialysis but to a central fatigue on a psychological level. A psychosocial intervention would be useful to improve the QoL of the haemodialysis patients, lessening the \u201cfatigue\u201d symptom