Düz labı̇rent keçelerde oyuk oluşumunun sızdırmazlık performansına etkisi
Düz labı̇rent keçelerde oyuk oluşumunun sızdırmazlık performansına etkisi
Dosyalar
Tarih
2020
Yazarlar
Yalçın, Uğurcan
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Gaz türbinli motorlar birçok uygulama alanı olan, son derece ileri teknoloji içeren akım makinaları çeşitlerinden birisidir. Genel olarak havacılık sektöründe yaygın olarak kullanılsa da, güç üretim sistemleri, denizcilik gibi uygulamalarda da tercih edilmektedir. Havacılıkta kullanılan gaz türbinli motor çeşitleri, farklı amaçlar için yapısı değiştirilerek kullanılabilmektedir. Turbofan, turbojet, turboprop ve turboşaft motor çeşitleri havacılıkta kullanılan gaz türbinli motor çeşitleridir. Bütün bu motor çeşitleri için genel çalışma prensibi benzer olsa da, elde edilen gücü kullanma şekilleri farklılık göstermektedir. Turbofan motorlarda üretilen güç bir fanı çevirmek için kullanılır. Turbojet motorlarda ise türbinden çıkan yüksek enerjili akışkan itki sağlar. Turboprop ve turboşaft motorlarda ise uçak veya helikopter pervanesinin döndürülmesi amaçlanmaktadır. Uzun yıllardır geliştirme sürecine tabi tutulan gaz türbinli motorlar, günümüzde çok yüksek teknolojiye sahip makine çeşitleri olmuşlardır. Bu aşamadan sonra verim üzerinde katkı sağlayacak çok detay çalışmalar yapılmaktadır. Yüksek sıcaklıklara dayanabilen malzemelerin geliştirilmesi, aerodinamik olarak daha iyi tasarlanmış kompresör veya türbinlerin yapılması, sızdırmazlık performansını artıracak elemanların geliştirilmesi büyük motor şirketlerinin amaçları olmuştur. Daha yüksek sıcaklığa dayanabilen malzemeler sayesinde türbinlerin çalışma sıcaklığı artırılabilecek ve motordan elde edilen güç çıktısı artacaktır. Bu sebeple malzeme teknolojisi motor tasarımında kritik bir öneme sahiptir. Motor üzerinde performansa etki eden diğer bir kritik parametre ise ikincil hava sistemlerinin kullanılmasıdır. İkincil hava sistemlerinin temel amacı belirlenmiş bölgeler için sızdırmazlık, soğutma veya sıcak gaz girişinin engellenmesidir. Bu amaçları gerçekleştirebilmek için motorun ana akış yolundan basıncının yükseltilmesi için enerji harcanan hava çekilerek, motorun ilgili bölgelerine gönderilmektedir. Bu basınçlandırılmış hava türbin kanatçıklarının soğutulması, yüksek basınç türbini disklerinin sıcak gaz girişinden korunması, karter bölgesinden yağın dışarı kaçmasını engellemek gibi amaçlar için kullanılmaktadır. Kullanılan bu ikincil akışlar havası, motordan elde edilecek güçten bir kayıba sebep olmaktadır. İkincil hava sistemleri için kullanılan hava miktarı motor performansı üzerinde negatif bir etkiye sahip olduğu için, motor tasarım sürecinde bu hava miktarının kontrol edilmesi ve belirli bir limitte kalması amaçlanır. Bu sebeple akış miktarını kontrol edebilmek için farklı tipte elemanlar kullanılmaktadır. Ancak en yaygın kullanılan eleman çeşitleri labirent keçeler, karbon keçeler ve fırça keçelerdir. Motor performansı üzerindeki etkisi ve çalışma şartlarının güvenli bir şekilde sürdürülmesi için akışı kontrol eden keçe elemanlarının tasarımı kritik bir öneme sahiptir. İyi tasarlanmamış bir keçe sebebiyle, türbin kanatçıkları yeterli soğutulamayabilir ve motorun kullanım ömrü kısalabilir, karter bölgesinden dışarı yağ kaçması sebebiyle motorda yangın görülebilir. İkincil hava sistemleri akışının kontrol edilmesinde yaygın olarak labirent keçeler kullanılmaktadır. Karbon ve fırça keçeye göre sızdırmazlık performansı daha düşük olsa da, geleneksel üretim yöntemleriyle üretildikleri ve tasarımları kolay olduğundan tercih edilmektedir. Labirent keçeler temel olarak ani daralma ve genişleme sonucunda akışkanın enerjisini kaybetmesi prensibine göre çalışmaktadır. Duran ve dönen yüzey arasından geçen akışkan ani daralan-genişleyen kesitlerde enerjisini kaybeder. Bu tez kapsamında labirent keçelerin termal yükler, manevra yükleri gibi sebeplerden dolayı açıklığının kapanması sonucunda karşı yüzeyde oyuk oluşturması durumunun akış üzerindeki etkileri incelenmiştir. Öncelikle HAD metodolojisi literatür ve test sonuçları ile doğrulanmıştır. Doğrulanan modelleme metodolojisi yardımıyla, oyuk oluşması durumunda akış yapısının nasıl değişebileceği ve farklı parametrelerin etkileri gözlenmiştir. Çalışmanın ilk aşamasında, farklı sebeplerden dolayı açıklığın kapanması ve karşı yüzeyde oyuk oluşturduktan sonra labirent keçe dişinin eski konumuna dönmesi durumu incelenmiştir. Bu incelemede keçe dişi oyuk dışında çalışmaktadır. Farklı oyuk derinliklerinin, farklı keçe açıklarında ve basınç oranlarında akış üzerindeki etkisi incelenmiştir. Dar keçe açıklıkları durumunda, oyuk oluşumu ve oyuk derinliğinin artması sızdırmazlık performansı üzerinde negatif bir etkiye sahip olduğu görülmüştür. Akış daralan kesitte oyuk içini doldurmakta ve daha büyük boğaz alanından geçmektedir. Geniş keçe açıklığında ise oyuk oluşması durumunda sızdırmazlık performansı azalmaktadır. Ancak oyuk derinliğinin artması durumunda sızdırmazlık performansı değişmemektedir. Akış derinleşen oyukta benzer boğaz alanından geçmektedir. Daha sonra ki inceleme alanı ise labirent keçe dişinin oyuk içinde çalışması olmuştur. Bu çalışmada bağıl keçe açıklığının farklı değerleri için keçe üzerinden geçen akışın değişimi incelenmiştir. Keçe dişinin oyuk içinde çalışması durumunda akışkan diş çıkışında kinetik enerjisini kaybetmekte ve bir sonraki dişe daha düşük enerjiyle girmektedir. Bundan dolay pozitif keçe açıklığı ve oyuksuz geniş keçe açıklığına göre daha iyi performans sergilemektedir.
Gas turbine engines are one of the flow machines which have widely usage area and include high technology engineering. Generally, these type of machines are used in aviation application. However, power plants applications, marine and industrial applications are other fields of usage of gas turbine engines. The gas turbines in aviation applications can be used for different purposes with changing architecture of the engine. Turbofan, turbojet, turboshaft and turbopropeller engines are different types of the gas turbines used in aviation applications. Either, all of these engine types have basically same working principle, they are different from each other in terms of power output usage. For the turbofan type of engines, power output is used for rotating inlet fan and generating thrust force. Turbojet engines are similiar to turbofan engines, but high energy fluid flow is used directly for getting thrust with convergent nozzle at the exit of the engine. These type of engines do not include fan part. This is the main difference between turbofan and turbojet engines. Power output of the turboshaft engines provide rotation of the propeller for helicopter platform. Fluid flow energy is converted to the mechanical energy and transmitted to the related parts with gear-box. Gas turbines have been developed for many years. Because of that reason, these machines have become high technology products. For the future, there are many investment and detailed researches about gas turbines to increase their efficiency. High temperature material development, compressor and turbine design evalution for higher aerodynamic efficiency, sealing element design for safety and better leakage performance are the main focuses of the big motor companies. Turbine operating temperature and power output of an engine can be increased with material technology which provides high temperature strength. Because of that reason, material research is the key technology for gas turbine development. Secondary air system is another critial point in terms of engine performance and efficiency. Main purposes of the secondary air system are providing pressurized air to dedicated region for sealing, cooling and preventing hot gas ingestion to the engine inside. Pressurized air is taken from main flow path with dedicated engine component such as orifices, pipes etc. and this pressurized air is generally used for turbine blade cooling to protect blade material from hot gas, preventing hot gas ingestion turbine disc region, and oil leakage from sump cavity. Secondary air system budget causes efficiency penalty for the gas turbine engines. Because of the efficiency penalty, flow rate amount of the secondary air system has to be controlled during the engine development process. For the air flow rate amount control, different types of the sealing elements are used at different section of the engine. The most widely used and popular sealings element types are labyrinth, carbon and brush for aviation application. The design process of the sealing elements which control flow amount of secondary air system are critical due to performance effect and safety issue. Because of poorly designed sealing element, turbine blades might not be cooled enough and life of the engine might decrease. Also, oil leakage might occur from sump cavity, and this causes disk burst which is a very critical damage for engine in terms of safety and realibility. Another critical effect of poor sealing element is increasing axial thrust load on ball bearing. Compressor discharge pressure seals control the flow amount and pressure difference between upstream and downstream of this seal element. Force occuring due to this pressure difference is carried with the ball bearing. Increasing axial load causes decrease in bearing operating hours and engine life cycle. To control the secondary air system budget, labyrinth type seals are the most widely used elements. Sealing performance of the labyrinth seals are lower than carbon or brush seals. However, they are preffered due to convenience for conventional production method and design easiness. Also, brush and carbon seals have some disadvantages. For example, brush seals include many bristle series. In case of rotor dynamic uncertainty, integrity of the bristle could not be provided and leakage air flow rate can increase dramatically. On the other hand, brush seals are not resistive for back flow condition. However, labyrinth seals working mechanism and applications are simplier than other high technology sealing element. Sudden contraction and expansion at seal pockets provide pressure drop or energy loss for labyrinth seal Energy of the fluid which passes through the area between rotor and surface decreases due to contraction and expansion mechanism. In the present study, effects of the rub groove which might occur due to the transient behaviour were investigated for the straight through labyrinth seals. Firstly, CFD calculations were validated with CFD results and test results in the literature separately. Main challenge of the validation with literature was finding fully defined geometry and boundary conditions. None of the cases in the literature includes the analysis geometry. Because of that, correct validation becomes impossible. Only few papers include their analysis conditions and results. One of the which was published with ASME was used for validation and comparing mass flow rate results. Another validation challange was about the test results Flow amount of the labyrinth seals is highly dependent on the seal clearances. Test setup does not include any sensitive clearance measuruement. Due to manucfacturing process and other effects, operating seal test clearances can be changed. For the correct data match between test results and CFD analysis, all of the tolerance effects were reflected to the analysis geometry. CFD calculation were run for the maximum and minimum clearance condition. Test mass flow rate result had to be stayed in this range and this requirement was provided during the validation process for different pressure ratios. Additionally, mesh sensitivity analysis was performed for all of the validation process. Similar results were found for different mesh or element number. All of the analysis process were made independent from solution element number. Sphere of influence and inflation around the rotor and stator wall were applied to all of the analysis for validation and parametric study. y+ around the rotor and stator wall were kept around 1. This provided high resolution in capturing the boundary layer effect. After that, flow field behaviour in case of rub groove and different geometrical parameters effects were tried to understand and explained with validated CFD models. First of all, positive seal clearance with rub groove studies were performed with different operating parameters. In this case, rub groove on the static surface forms due to the transient loads such as gyroscopic, maneuver or thermal loads. However, after end of the transient effects, seal teeth go back to their nominal or design location. Labyrinth seal continues working with nominal clearance and additional flow area xxiii which is formed due to permanent deformation of rub groove. Rub groove depth effect for the different seal clearance and pressure ratio was analyzed in this section. Firstly, no grove labyrinth seals with different seal clearances were analyzed for different pressure ratios. It was found that, discharge coefficient of the labyrinth seals increases with increasing pressure ratio. Also, the discharge coefficient of seals which operate with high seal clearance is higher than that of those with the tight seal clearance. Pressure loads of the seals teeth are similar for the different clearance values. This load is actually dependent on the operating pressure ratio. For low pressure ratio between seal inlet and outlet, pressure decrease for each seal teeth is nearly equal to each other. Also velocity distribution has the same behaviour with pressure load for low pressure ratio. However, last tooth creates the most pressure decrease for higher pressure ratio. For high pressure ratio, maximum velocity occurs on the last tooth while first tooth has lowest velocity among all of the teeth. Generating rub grove and increasing depth of the grove have negative effect on the sealing performance for the tight seal clearance. Flow fills into the additional rub grove cavity and uses much more vena contracta area. On ther other hand, generating grove on the stator surface has inverse effect for high seal clearance, too. However, increasing rub grove depth has different effect from tight seal clearance for this condition. Sealing performance does not effect the rub grove depth for high clearance. It was seen that, flow passes same vena contracta area for the different grove depth and does not fill into the grove cavity completely. Very similiar results were found with literature for the positive seal clearance situations. When we compared the CFD results with literature flow vectors, behaviour of the flow field for different conditions such as shallow or depth grove with tight and high clearance, they have same physics. Secondly, seal teeth in the rub grove case were investigated in this study. Flow field behaviour and leakage performance of the seal were investigated for different relative seal clearance or negative clearance. Different negative clearance values with constant grove depth were analyzed with different pressure ratio. In case of negative clearance working condition, %68 sealing performance improvement could be achieved with respect to the same grove depth and positive tight seal clearance.Also, negative clearance condition provides better leakage performance than non-grove and tight clearance condition. While flow leakage performance improvement of negative clearance was found maximum around %40 according to tight clearance and non-grove case, this improvement of negative clearance condition was around %70 according to high clearance and non-grove condition. Fluid loses its own kinetic energy at the exit of the tooth and is transferred to another tooth with lower velocity or energy. Because of that, negative seal clearance provides better leakage performance than high seal clearance configuration without rub grove and positive seal clearance. For the same negative clearance, different pressure ratios were analyzed to understand the Re effects. It shown that, negative seal clearance sealing performance improvement has the same trend for different pressure ratios. Finally, it was found that controlled negative clearance or relative clearance conditions provide the best sealing performance for the straight through labyrinth seals.
Gas turbine engines are one of the flow machines which have widely usage area and include high technology engineering. Generally, these type of machines are used in aviation application. However, power plants applications, marine and industrial applications are other fields of usage of gas turbine engines. The gas turbines in aviation applications can be used for different purposes with changing architecture of the engine. Turbofan, turbojet, turboshaft and turbopropeller engines are different types of the gas turbines used in aviation applications. Either, all of these engine types have basically same working principle, they are different from each other in terms of power output usage. For the turbofan type of engines, power output is used for rotating inlet fan and generating thrust force. Turbojet engines are similiar to turbofan engines, but high energy fluid flow is used directly for getting thrust with convergent nozzle at the exit of the engine. These type of engines do not include fan part. This is the main difference between turbofan and turbojet engines. Power output of the turboshaft engines provide rotation of the propeller for helicopter platform. Fluid flow energy is converted to the mechanical energy and transmitted to the related parts with gear-box. Gas turbines have been developed for many years. Because of that reason, these machines have become high technology products. For the future, there are many investment and detailed researches about gas turbines to increase their efficiency. High temperature material development, compressor and turbine design evalution for higher aerodynamic efficiency, sealing element design for safety and better leakage performance are the main focuses of the big motor companies. Turbine operating temperature and power output of an engine can be increased with material technology which provides high temperature strength. Because of that reason, material research is the key technology for gas turbine development. Secondary air system is another critial point in terms of engine performance and efficiency. Main purposes of the secondary air system are providing pressurized air to dedicated region for sealing, cooling and preventing hot gas ingestion to the engine inside. Pressurized air is taken from main flow path with dedicated engine component such as orifices, pipes etc. and this pressurized air is generally used for turbine blade cooling to protect blade material from hot gas, preventing hot gas ingestion turbine disc region, and oil leakage from sump cavity. Secondary air system budget causes efficiency penalty for the gas turbine engines. Because of the efficiency penalty, flow rate amount of the secondary air system has to be controlled during the engine development process. For the air flow rate amount control, different types of the sealing elements are used at different section of the engine. The most widely used and popular sealings element types are labyrinth, carbon and brush for aviation application. The design process of the sealing elements which control flow amount of secondary air system are critical due to performance effect and safety issue. Because of poorly designed sealing element, turbine blades might not be cooled enough and life of the engine might decrease. Also, oil leakage might occur from sump cavity, and this causes disk burst which is a very critical damage for engine in terms of safety and realibility. Another critical effect of poor sealing element is increasing axial thrust load on ball bearing. Compressor discharge pressure seals control the flow amount and pressure difference between upstream and downstream of this seal element. Force occuring due to this pressure difference is carried with the ball bearing. Increasing axial load causes decrease in bearing operating hours and engine life cycle. To control the secondary air system budget, labyrinth type seals are the most widely used elements. Sealing performance of the labyrinth seals are lower than carbon or brush seals. However, they are preffered due to convenience for conventional production method and design easiness. Also, brush and carbon seals have some disadvantages. For example, brush seals include many bristle series. In case of rotor dynamic uncertainty, integrity of the bristle could not be provided and leakage air flow rate can increase dramatically. On the other hand, brush seals are not resistive for back flow condition. However, labyrinth seals working mechanism and applications are simplier than other high technology sealing element. Sudden contraction and expansion at seal pockets provide pressure drop or energy loss for labyrinth seal Energy of the fluid which passes through the area between rotor and surface decreases due to contraction and expansion mechanism. In the present study, effects of the rub groove which might occur due to the transient behaviour were investigated for the straight through labyrinth seals. Firstly, CFD calculations were validated with CFD results and test results in the literature separately. Main challenge of the validation with literature was finding fully defined geometry and boundary conditions. None of the cases in the literature includes the analysis geometry. Because of that, correct validation becomes impossible. Only few papers include their analysis conditions and results. One of the which was published with ASME was used for validation and comparing mass flow rate results. Another validation challange was about the test results Flow amount of the labyrinth seals is highly dependent on the seal clearances. Test setup does not include any sensitive clearance measuruement. Due to manucfacturing process and other effects, operating seal test clearances can be changed. For the correct data match between test results and CFD analysis, all of the tolerance effects were reflected to the analysis geometry. CFD calculation were run for the maximum and minimum clearance condition. Test mass flow rate result had to be stayed in this range and this requirement was provided during the validation process for different pressure ratios. Additionally, mesh sensitivity analysis was performed for all of the validation process. Similar results were found for different mesh or element number. All of the analysis process were made independent from solution element number. Sphere of influence and inflation around the rotor and stator wall were applied to all of the analysis for validation and parametric study. y+ around the rotor and stator wall were kept around 1. This provided high resolution in capturing the boundary layer effect. After that, flow field behaviour in case of rub groove and different geometrical parameters effects were tried to understand and explained with validated CFD models. First of all, positive seal clearance with rub groove studies were performed with different operating parameters. In this case, rub groove on the static surface forms due to the transient loads such as gyroscopic, maneuver or thermal loads. However, after end of the transient effects, seal teeth go back to their nominal or design location. Labyrinth seal continues working with nominal clearance and additional flow area xxiii which is formed due to permanent deformation of rub groove. Rub groove depth effect for the different seal clearance and pressure ratio was analyzed in this section. Firstly, no grove labyrinth seals with different seal clearances were analyzed for different pressure ratios. It was found that, discharge coefficient of the labyrinth seals increases with increasing pressure ratio. Also, the discharge coefficient of seals which operate with high seal clearance is higher than that of those with the tight seal clearance. Pressure loads of the seals teeth are similar for the different clearance values. This load is actually dependent on the operating pressure ratio. For low pressure ratio between seal inlet and outlet, pressure decrease for each seal teeth is nearly equal to each other. Also velocity distribution has the same behaviour with pressure load for low pressure ratio. However, last tooth creates the most pressure decrease for higher pressure ratio. For high pressure ratio, maximum velocity occurs on the last tooth while first tooth has lowest velocity among all of the teeth. Generating rub grove and increasing depth of the grove have negative effect on the sealing performance for the tight seal clearance. Flow fills into the additional rub grove cavity and uses much more vena contracta area. On ther other hand, generating grove on the stator surface has inverse effect for high seal clearance, too. However, increasing rub grove depth has different effect from tight seal clearance for this condition. Sealing performance does not effect the rub grove depth for high clearance. It was seen that, flow passes same vena contracta area for the different grove depth and does not fill into the grove cavity completely. Very similiar results were found with literature for the positive seal clearance situations. When we compared the CFD results with literature flow vectors, behaviour of the flow field for different conditions such as shallow or depth grove with tight and high clearance, they have same physics. Secondly, seal teeth in the rub grove case were investigated in this study. Flow field behaviour and leakage performance of the seal were investigated for different relative seal clearance or negative clearance. Different negative clearance values with constant grove depth were analyzed with different pressure ratio. In case of negative clearance working condition, %68 sealing performance improvement could be achieved with respect to the same grove depth and positive tight seal clearance.Also, negative clearance condition provides better leakage performance than non-grove and tight clearance condition. While flow leakage performance improvement of negative clearance was found maximum around %40 according to tight clearance and non-grove case, this improvement of negative clearance condition was around %70 according to high clearance and non-grove condition. Fluid loses its own kinetic energy at the exit of the tooth and is transferred to another tooth with lower velocity or energy. Because of that, negative seal clearance provides better leakage performance than high seal clearance configuration without rub grove and positive seal clearance. For the same negative clearance, different pressure ratios were analyzed to understand the Re effects. It shown that, negative seal clearance sealing performance improvement has the same trend for different pressure ratios. Finally, it was found that controlled negative clearance or relative clearance conditions provide the best sealing performance for the straight through labyrinth seals.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2020
Anahtar kelimeler
gaz türbini,
gas turbine,
gaz türbinli motorlar,
gas turbine engines