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Öğe Ağır Silah Namlusunun Mekanik Otofretaj İşleminde Gerilme Dağılımının Sayısal Olarak İncelenmesi(2023) Baran, Doğan; Bican, Osman; Doğu, YahyaOtofretaj, kalın cidarlı silindirlerin basınç taşıma kapasitesini ve yorulma ömrünü artırmak için silindir et kalınlığında artık kalıcı gerilme oluşturma işlemidir. Bu artık gerilme, çalışma basıncının oluşturduğu gerilmenin bir kısmını nötr ederek, basınçlı kapların basınç taşıma kapasitesini artırır. Pratikte birçok otofretaj yöntemi olmakla beraber, özellikle ağır silah namlularında uygulanan iki otofretaj yöntemi mekanik ve hidrolik otofretaj işlemleridir. Bu çalışmada, ağır silah namlusuna mekanik otofretaj uygulanmasında oluşan gerilmeler sonlu elemanlar metodu (SEM) ile sayısal olarak hesaplanmıştır. SEM modelinde iki boyutlu (2B) aksisimetrik geometri kullanılmıştır. SEM modeli literatürdeki veriler ile doğrulanmıştır. Otofretajsız namluda 670 MPa çalışma basıncı altında namlu iç çapında Von Mises eşdeğer gerilmenin değeri 1350,3 MPa olarak hesaplanmıştır. Otofretaj uygulanmış namluda ise Von Mises eşdeğer gerilmesinin maksimum değeri, namlunun et kalınlığının %63’ne karşılık gelen bölgede 1122,3 MPa olarak hesaplanmıştır. Bu gerilme değerinin namlu akma mukavemeti olan 1195 MPa’ın altında olduğu görülmektedir. Sonuç olarak çalışma basıncı altında otofretaj uygulanmış namluda Von Mises eşdeğer gerilmesi, otofretaj uygulanmayan namluya göre %16,88 oranında azalmıştır. Bu sebeplerden dolayı, ağır silah namlularında otofretaj işleminin uygulanması ve oluşan gerilmelerin hesaplanması namlu ömrü ve basınç taşıma kapasitesi açısından kritik öneme sahiptir.Öğe Brush seal temperature distribution analysis (Conference Paper)(2005) Doğu, Yahya; Aksit, Mahmut F.Brush seals are designed to survive transient rotor rubs. Inherent brush seal flexibility reduces frictional heat generation. However, high surface speeds combined with thin rotor sections may result in local hot spots. Considering large surface area and accelerated oxidation rates, frictional heat at bristles tips is another major concern especially in challenging high temperature applications. This study investigates temperature distribution in a brush seal as a function of frictional heat generation at bristle tips. The two-dimensional axisymmetric CFD analysis includes the permeable bristle pack as a porous medium allowing fluid flow throughout the bristle matrix. In addition to effective flow resistance coefficients, isotropic effective thermal conductivity as a function of temperature is defined for the bristle pack. Employing a fin approach for a single bristle, a theoretical analysis has been developed after outlining the brush seal heat transfer mechanism. Theoretical and CFD analysis results are compared. To ensure coverage for various seal designs and operating conditions, several frictional heat input cases corresponding to different seal stiffness have been studied. Frictional heat generation is outlined to introduce a practical heat flux input into the analysis model. Effect of seal stiffness on nominal bristle tip temperature has been evaluated. Analyses show a steep temperature rise close to bristle tips that diminishes further away. Heat flux conducted through the bristles dissipates into the flow by a strong convection at fence height region. Copyright © 2005 by ASME.Öğe Düz güneş enerjisi toplayıcılarında yutucu plakada oluşan ısı transferinin parametrik analizi(2006) Doğu, Yahya; Çalışkan, Nuriye; Çekel, NurGüneş enerjisi uygulamalarındaki artışa paralel olarak, güneş enerjisinin faydalı ısı enerjisine dönüştürüldüğü düz güneş enerjisi toplayıcılarından ısı transferinin verimli olarak gerçekleştirilmesinin önemi giderek artmaktadır. Bu çalışmada, düz toplayıcılarda güneş ışınım enerjisinin ısı taşıyıcı akışkana aktarıldığı yutucu plakanın ısıl analizi yapıldı. Geliştirilen analitik formülasyonda yutucu plaka tek boyutlu bir kanat olarak incelendi. Yutucu plaka üst yüzeyinde güneşten gelen ışınım akısı ve çevreye olan ısı kaybı tanımlanırken, alt yüzey ideal olarak yalıtılmış kabul edildi. Kanat ısı transfer denklemleri analitik olarak çözülerek, yutucu plaka üzerindeki sıcaklık dağılımını ve plakadan akışkana olan ısı geçişini veren bağıntılar elde edildi. Sıcaklık dağılımı ve ısı geçişi üzerindeki etkili parametreler temel olarak; geometrik boyutlar, akışkan sıcaklığı, yutucu plaka malzemesinin ısı iletim katsayısı, ışınım ısı akısı, çevre sıcaklığı ve çevreye olan toplam ısı kayıp katsayısıdır. Bu parametrelerin etkileri metodik olarak incelendi, inceleme sonucunda; yüksek ısı iletim katsayısına sahip yutucu plakadaki sıcaklık seviyesinin ve akışkana geçen ısı akısının arttığı tespit edildi. Toplayıcıda elde edilen faydalı ısı enerjisinin kullanılabilirliğini belirleyen, pratik çalışma şartlarında ulaşılabilecek maksimum akışkan sıcaklığı, güneş ışınım şiddetinin fonksiyonu olarak elde edildi. Elde edilen tüm sonuçlara bağlı olarak, toplayıcı verimi üzerindeki parametrelerin etkileri tespit edildi.Öğe Effects of equivalence ratio and CNG addition on engine performance and emissions in a dual sequential ignition engine(SAGE PUBLICATIONS LTD, 2020) Yontar, Ahmet Alper; Doğu, YahyaCompared to widening usage of CNG in commercial gasoline engines, insufficient but increasing number of studies have appeared in the open literature during last decades, while engine characteristics need to be quantified in exact numbers for each specific fuel and engine. CNG usage in spark-ignition engine offers many advantages such as high specific power outputs, knock resistance, and low CO(2)emission. Engine performance and emissions are strong functions of equivalence ratio. This study focuses on determination of the effects of equivalence ratio on engine performance and emissions for a unique commercial engine for three fuels of gasoline, CNG, and gasoline-CNG mixture (90%-10%: G9C1). For this aim, the tests and the three-dimensional in-cylinder combustion computational fluid dynamics analyses were employed in quantification of engine characteristics at wide open throttle position. Equivalence ratios were defined between 0.7 and 1.4. The engine's maximum torque speed of 2800 r/min was examined. The tested commercial engine is an intelligent dual sequential ignition engine which has unique features such as diagonally positioned two spark-plugs, dual sequential ignition with variable timing and asymmetrical combustion chamber. This gasoline engine was equipped with an independent CNG port-injection system and a specific engine control unit for CNG. In addition, the engine test system has a concomitant dual fuel delivery system that supplies gas fuel into intake airline while liquid gasoline is injected behind the intake valve. Other than testing the engine, the three-dimensional in-cylinder combustion computational fluid dynamics analyses were performed in Star-CD/es-ice software for the three fuels. The CFD model was built by using renormalization group equations, k-epsilon turbulence model, and G-equation combustion model. Computational fluid dynamics analyses were run for the compression ratio of 10.8:1, equivalence ratio of 1.1, and engine's maximum torque speed of 2800 r/min. Test results show that brake torque for all fuels increases rapidly from the lean blend to the rich blend. The brake-specific fuel consumption for all fuels decreases from phi = 0.7 through the stoichiometric region and then slightly increases up to phi = 1.4. The volumetric efficiencies for three fuels have similar decreasing trend with respect to equivalence ratio. Overall, CNG addition decreases the performance values of torque, brake-specific fuel consumption, volumetric efficiency, brake thermal efficiency, while it decreases emissions of CO2, CO, HC, except NOx. Engine model results show that the maximum in-cylinder pressure is 72 bar at 722 crank angle degree (CAD), 68 bar at 730 CAD, and 60 bar at 735 CAD for gasoline, CNG, and G9C1, respectively. The cumulative heat release for gasoline is 9.09% higher than G9C1, while G9C1 is 15.71% higher than CNG. The CO(2)mass fraction for gasoline is about 22.58% lower than G9C1, while it is 40.32% higher than CNG. The maximum mass fraction value of CO is 0.21, 0.17, and 0.08 for gasoline, CNG, and G9C1, respectively. The CO for G9C1 is overall 60.04% lower than CNG and 67.45% lower than gasoline. At maximum point, HC for G9C1 is 31.43% and 71.43% higher than gasoline and CNG, respectively. CNG has the highest level of NO(x)formation. Maximum NO(x)mass fractions are 0.0098, 0.0070, and 0.0043 for CNG, G9C1, and gasoline, respectively. After the ignition, the flame development is completed at 1.07, 1.18, and 1. 28 ms for gasoline, G9C1, and CNG, respectively. Flame velocities are 28.52, 30.93, and 34.11 m/s for CNG, G9C1, and gasoline, respectively, at 2800 r/min and phi = 1.1. When the time between ignition moment and top dead center moment is considered, the increment rate of flame center temperature is 904.19, 884.10, and 861.77 K/s for CNG, gasoline, and G9C1, respectively. The highest temperature increment rate occurs for CNG.Öğe Effects of geometry on brush seal pressure and flow fields - Part I: Front plate configurations(Asme-Amer Soc Mechanical Eng, 2006) Doğu, Yahya; Akşit, Mahmut F.Pressure and flow fields lay at the basis of such common phenomena affecting brush seal performance as bristle flutter, blow-down, hang-up, hysteresis, pressure stiffening, wear and leakage. Over the past two decades of brush seal evolution, manufacturers and researchers have applied many geometric configurations to the front and backing plates of a standard brush seal in order to control the flow field and consequent seal performance. The number of studies evaluating the effect of geometric configitrations on the brush seal flow field remains limited in spite of the high number of filed patent disclosures. This study presents a numerical analysis of brush seal pressure and flow fields with regard to common conceptual front plate configurations. A CFD model has been employed to calculate pressure and flow fields in the seal domain. The model incorporates a bulk porous medium approach for the bristle pack.. The effectiveness of various conceptual geometries has been outlined in terms of flow field formation. Results disclose unique effects of geometry on pressure and floss, fields such that a longer front plate drives outward radial flow while playing a protective role against upstream cavity disturbances. Findings also indicate that variations in from plate geometry do not directly affect leakage performance. A long front plate or damper shim considerably changes the flow field while at the same time having limited effect on the pressure field. Moreover; a strong suction towards the clearance enhances inward radial flow in clearance operation.Öğe Effects of geometry on brush seal pressure and flow fields - Part II: Backing plate configurations(Asme-Amer Soc Mechanical Eng, 2006) Doğu, Yahya; Akşit, Mehmet FarukBrush seal dynamic behavior is strongly related to pressure and flow fields. Developments in brush seal design have led to geometric modifications to control flow; field and consequent brush seal issues including blow-down. hang-up, and pressure stiffening. Some of the geometric enhancements have been found to have common use as backing plate modifications. Over the two decades of brush seal evolution, many backing plate configurations have been suggested in numerous parent disclosures. Even so, literature on the effects of geometric modifications oil pressure and flow fields remains limited. This study numerically investigates brush seal pressure and flow fields for such common conceptual backing plate configurations as single and multiple grooves, with and without by-pass passages. The CFD analysis presented employs a bulk porous medium approach for the bristle pack. The effectiveness of various backing plate configurations outlining important flow features is discussed. Results indicate that backing plate configurations have a decisive role in shaping seal pressure fields. In general, it has been found that all cases having bypass configuration leak more. Moreover the major portion of the seal leakage through fence height is fed front the backing plate cavity. The single backing plate groove forms a constant pressure behind the bristle pack. In contrast, multiple grooves form multiple constant pressure regions.Öğe Evaluation of brush seal performance for oil sealing applications(2003) Akşit, Mahmut Faruk; Bhate, Nitin; Bouchard, Charles; Demiroğlu, Mehmet; Doğu, YahyaOil sealing at high speeds is one of the major problems engineers should address in turbomachinery design. High temperatures faced in oil sumps in aircraft engines, and large seal sizes typical in land based turbine applications further complicate the problem. Labyrinth seals can overcome problems faced with carbon seals in high temperature and large size applications. On the other hand, use of labyrinth seals may result in high leakage rates leading to increased oil consumption, unintended oil contamination in some flow cavities, early oil degradation or even fires in some cases. Successful engine secondary flow path applications of brush seals lead to questions of their applicability for oil sealing. Because brush seals are contact seals, oil temperature rise and coking become major issues in addition to leakage performance. This paper presents an investigative study of brush seal leakage and coking performance using common lube oil. Both metallic and non-metallic prototypes have been tested under static and dynamic conditions. It has been concluded that properly designed brush seals can achieve lower leakage rates than labyrinth seals without causing coking problems. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.Öğe Experimental and numerical investigation of effects of CNG and gasoline fuels on engine performance and emissions in a dual sequential spark ignition engine(Taylor & Francis Inc, 2018) Yontar, Ahmet Alper; Doğu, YahyaCompared to widening usage of CNG in commercial gasoline engines, insufficient but increasing number of studies have appeared in open literature during last decades while engine characteristics need to be quantified in exact numbers for each specific fuel converted engine. In this study, a dual sequential spark ignition engine (Honda L13A4 i-DSI) is tested separately either with gasoline or CNG at wide open throttle. This specific engine has unique features of dual sequential ignition with variable timing, asymmetrical combustion chamber, and diagonally positioned dual spark-plug. Thus, the engine led some important engine technologies of VTEC and VVT. Tests are performed by varying the engine speed from 1500rpm to 4000rpm with an increment of 500rpm. The engine's maximum torque speed of 2800rpm is also tested. For gasoline and CNG fuels, engine performance (brake torque, brake power, brake specific fuel consumption, brake mean effective pressure), emissions (O-2, CO2, CO, HC, NOx, and lambda), and the exhaust gas temperature are evaluated. In addition, numerical engine analyses are performed by constructing a 1-D model for the entire test rig and the engine by using Ricardo-Wave software. In the 1-D engine model, same test parameters are analyzed, and same test outputs are calculated. Thus, the test and the 1-D engine model are employed to quantify the effects of gasoline and CNG fuels on the engine performance and emissions for a unique engine. In general, all test and model results show similar and close trends. Results for the tested commercial engine show that CNG operation decreases the brake torque (12.7%), the brake power (12.4%), the brake mean effective pressure (12.8%), the brake specific fuel consumption (16.5%), the CO2 emission (12.1%), the CO emission (89.7%). The HC emission for CNG is much lower than gasoline. The O-2 emission for CNG is approximately 55.4% higher than gasoline. The NOx emission for CNG at high speeds is higher than gasoline. The variation percentages are the averages of the considered speed range from 1500rpm to 4000rpm.Öğe Flame Radius Effects on a Sequential Ignition Engine Characteristics(2018) Yontar, Ahmet Alper; Doğu, YahyaThe effects of the flame radius and flame propagation have been investigated at a sequential ignition engine with numerically. A single cylinder of the sequential ignition engine was modeled in STAR-CD/es-ice software for the gasoline usage taking into account all components related to the combustion chamber. The effect of flame on engine characteristics is the function of flame radius and flame thickness. In the numerical analysis, compression ratio is 10.8:1, air-fuel ratio is 1.2, ignition advance at 30-25 CAD, engine speed is 3000 rpm and the flame thickness is 0.0001 m were kept constant. The analysis, k-? RNG turbulence model, Angelberger wall interaction and G-equation combustion model were used and optimum flame radius value was determined. Three different analysis were carried out to determine the effect of the flame radius and the flame radius was changed to 0.0005 m, 0.0010 m and 0.0020 m, respectively. As a result of the study, images of flame formation and propagation were obtained for the time period up to the top dead center at the time of sequential ignition. The effects of flame radius on CO2 formation and NOx formation were evaluated. The net work area was obtained from the highest engine power and pressure-volume graph when the flame radius was 0.0010 m for the specified operating conditions.Öğe A numerical model to determine temperature distribution in orthogonal metal cutting(Elsevier Science Sa, 2006) Doğu, Yahya; Aslan, Ersan; Camuscu, NecipIn this study, a thermal analysis model is developed to determine temperature distribution in orthogonal metal cutting using finite elements method. The model calculates the temperature distribution as a function of heat generation. The heat generation was introduced in the primary deformation zone, the secondary deformation zone and along the sliding frictional zone at the tool-chip interface, as well. The location and shapes of these zones was determined based on the literature work done so far and the model results. The temperature dependency of material properties was included in the model. A series of thermal simulations have been performed, and the value and location of maximum temperature have been determined for various cutting conditions. The comparison of the simulations with earlier works gave promising trend for the presented model. The thermal aspects of metal cutting as a result of the model findings were discussed. (c) 2005 Elsevier B.V. All rights reserved.Öğe Oil temperature analysis of brush seals(2008) Duran, Ertuğrul Tolga; Aksit, Mahmut Faruk; Doğu, YahyaDue to their superior performance and stable leakage characteristics, brush seals are one of the dynamic seals used in oil and oil mist applications in aero-engines and turbines. The viscous medium between the high speed rotor surface and brush seal bristles generates a hydrodynamic lifting force that determines seal clearance and leakage rate in oil sealing applications. The analytical solution to bristle lifting force can be obtained by using Reynolds formulation. However, there is a strong dependence on oil temperature and viscosity. This work presents a solution to oil temperature using nonlinear pressure distribution. Starting with continuity and Navier Stokes equations, temperature and nonlinear pressure distribution is derived by solving the thermal energy and reduced continuity equations simultaneously. Results of oil temperature estimates using nonlinear pressure analysis are compared with the results of a previous work using linear pressure assumption. Findings indicate that for low rotor surface speeds oil temperature distribution is almost the same for both linear and nonlinear pressure cases. Difference in oil temperature estimates increases with increasing rotor surface speeds. Copyright © 2007 by ASME.
