İçten yanmalı motor egzoz portu etrafındaki kafes yapılarının termomekanik davranışının incelenmesi ve efektif modelleme yaklaşımının geliştirilmesi
Thermomechanical investigation of lattice structures around exhaust port for iternal combustion engine and development of a effective modelling approach
Yazar: CEMRE OĞUZ ERŞAHİN
Danışman: DR. ÖĞR. ÜYESİ HİKMET ARSLAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Otomotiv Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Mühendislik Bilimleri = Engineering Sciences

Özet:
Teknolojinin hızla ilerlediği bir dünyada hiçbir şey stabil değil. Ekonomik, çevresel ve toplum ihtiyaçlarını karşılamak için rekabet seviyesi hep yukarı taşınmaktadır. Bununla birlikte gelen devamlı performans artırımı ihtiyacı, araştırma ve geliştirmeye yatırım yapmayı şirketler için zorunlu hale getirmiştir. Bu noktada AM ile geleneksel üretim metotları ile üretilemeyecek parça içi boşluklu kafes yapıları kullanılarak yüksek mukavemetli düşük kütleli parçalar üretmek mümkündür. Ancak AM ile üretilecek bu komponentler için yeni tasarım yaklaşımı ve analiz stratejileri gerekir. Bu tez çalışmasında öncelikle bir içten yanmalı motor baz silindir kafası tasarımı belirlenerek termal ve yapısal olarak egzoz portu ve çevresi incelenmiştir. Ardından 3 boyutlu metal yazıcıların avantajları kullanılarak egzoz portu çevresine bütünleşmiş edilecek boşluklu bir yapı ile bu bölgede egzoz portunun yüzey sıcaklığının artırılması ve bu şekilde soğuk başlangıç koşullarında ve yanma sonrası egzoz gazının kullanıldığı emisyon düşürücü işlemlerde verimlilik artırılmasına yardımcı olmak hedeflenmiştir. Yanma sonrası egzoz gazının kullanıldığı bu işlemler tezin kapsamında olmayıp, portun yüzey sıcaklığındaki değişim ve bu esnada kafes yapılarının termomekanik yükler altındaki davranışı çalışmanın asıl konusunu oluşturmuştur. Termal ve yapısal açıdan geometrik değişimler ile yapılan parametre çalışmasının ardından bilgisayar yükü açısından efektif bir modelleme tekniği geliştirme üzerine çalışılmıştır. Bu geliştirilen teknik ile literatür çalışmaları incelendiğinde daha önce karşılaşılmamıştır ve çalışmaya sıra dışı bir değer katan önemli bir kısımdır. Sürecin ilk aşamasında egzoz portunun üzerinde konumlandırılacak boşluklu hacmin belirlenmesi üzerine çalışılmıştır. Ardından bu baz hacme entegre edilmiş boşluklu kafes yapısının farklı kafes yapı elemanı ölçüleri ile termal ve yapısal davranışı incelenmiştir. Bu iterasyonlar sonucunda ne tür tasarımsal değişikliklerin sisteme nasıl etkileri olduğu hakkında çıkarımlarda bulunulmuştur. Termomekanik yükler altında kafes yapılarının davranışının incelenmesinin yanısıra, bu yapıların bilgisayar yükü açısından daha efektif bir FEA modelleme yaklaşımı ile uygulanabilirliğini artırmak üzere eşdeğer indirgenmiş modeller ve modelleme metodolojisi üzerine çalışılmıştır. Eşdeğer model ile detaylı modele yakın ve daha kısa çözüm süresinde güvenilir sonuçlar elde edilmeye çalışılmıştır ve başarılı olunmuştur.

Abstract:
In a World, where technology is advancing rapidly, nothing is stable. The level of competition is always raising to meet economic, environmental and social needs. However, the need for continuous performance improvement has made it compulsory for companies to invest in research and development. At this point, designing lightweight parts with topology optimization, achieving thermal and mechanical improvements can be challenging with traditional manufacturing methods. With AM, it is possible to produce a high strength, low mass internal components with or without internal lattice structures.  For example, while parts of a jet engine can be optimized and lightened using additive manufacturing technology, better thermomechanical properties can also be achieved with the inside cooling channels, which is formed inside the blades of the jet engine and results in better cooling performance compared to a fin produced by conventional methods.
However, these components to be produced with AM require a new design approach and analysis strategies. Analysis methods for solid structures, such as FEM or traditional rigid body mechanics and material strength theories, are still investigating the applicability for parts with microstructures like lattices, which are manufactured by AM technology. Experimental curves such as Hooke's law, Poisson ratio, Wöhler curves for LCF and HCF are constructed with experimental "pure" material samples and standard loads. Although these methods are widely used today, their results in practice may give unrealistic results for parts containing lattice structures.
The main purpose of this thesis is to examine the thermomechanical behavior of a lattice structure within a cavity that can be applied around the exhaust port of an internal combustion engine, which is going to be produced using additive manufacturing technology. In addition to thermomechanical investigation of these lattice structures, effective FEM approach in terms of computer load has been developed to increase the applicability of lattice structures in FEA.
Firstly, an internal combustion engine cylinder head design was determined as base and the exhaust port and its surroundings were examined thermally and structurally. Later, by using the advantages of 3D metal printers, it is aimed to increase the surface temperature of the exhaust port in this area with a cavity between exhaust port and water jacket filled with lattice structures, thereby helping to increase efficiency in cold start conditions and in emission reduction processes where exhaust gas is used after combustion. These processes in which exhaust gas is used after combustion are not within the scope of the thesis, instead the change in the surface temperature of the port and the behavior of the lattice structures under thermomechanical loads constitute the main subject of the study.
After the parameter study for geometric changes on cavity volume and lattice structures in thermal and structural aspects, an effective modeling technique was developed in terms of computer load. This technique has not been encountered before during literature review, and it is an important part that adds an extraordinary value to the study.
In the first stage of the process, it was studied on determining the cavity volume to be located between the exhaust port and water jacket. The importance of this volume is that it can act as insulation and reduce the cooling of the exhaust port by the water jacket. Although this helps maintain the surface temperature of the exhaust port, it can cause the material to reach a temperature above its creep temperature and is therefore a critical point. Since this point is critical and in order to see its effect on the temperature of the exhaust port, 3 different-cavity volume have been determined. By changing cavity space temperature results have been also changed drastically and determined as main parameter for exhaust port temperature results.
After determining the cavity volume, the package volume of the lattice element to be placed in is determined. A cubic unit lattice element package volume has been decided inside the cavity volume, and the lattice element is placed inside this package. Three variations of hourglass shaped lattice elements with changing throat radius have been determined, and the effects to the system in terms of thermal and structural aspects have been investigated. Changing minimum diameter of hourglass shaped lattice structures had not much effect on temperature in comparison to cavity volume but it affected the temperature distribution and resulted in more homogenous temperature distribution on exhaust port surface.
After the first thermomechanical analysis, it is observed that, higher temperatures compared to the base version force the exhaust port and lattice structures to expand drastically. Expansions have led to stress accumulation in the region. At this point, by using the design freedom we have for an engine to be manufactured with 3D printers, a rough voiding process to reduce stiffness has been applied for the upper part of the water jacket. Thus, it was expected to see an easier expansion around the exhaust port area and less stress accumulation on lattice structures. The fact of having stresses under yield stress in thermomechanical analysis for location, where voiding process has been done, supported the idea of stiffness reduction by voiding process. Although a basic stiffness reduction trial has been worked on and a moderate impact has been seen, being able to create internal shapes and to reduce or increase the stiffness of a cylinder head with a freedom according to needs brings an huge advantage to design a tailor made cylinder head.
After thermomechanical investigation of lattice structures equivalent FEM concept has been developed. Lattice structures with complex geometries are modeled with a high number of nodes and these nodes increase the time to solve the model. Especially in cases where FEM models such as internal combustion engine are complex, with the addition of lattice structures, serious prolongations in solution times can be seen. The main purpose of the equivalent FEM concept is to achieve realistic results in a shorter time by reflecting the mechanical and thermal properties of the equivalent structural elements and lattice structures.
Firstly, thermal equivalent FEM concept has been created with single hex mesh instead of detailed tetra lattice model, which reduces number of nodes in a model drastically by only doing this for lattice structures around exhaust port. Relative density is an important parameter for the equivalent structure to behave thermally similar to lattice structures. It is possible to make an correlation between the relative density and the average effective heat transmission area. In this way, the conduction coefficient of the material to be assigned for the equivalent structure with bigger conduction area but with a simpler geometry can be reduced according to relative density and so that results are close to detailed tetra lattice model can be obtained. Although thermal analysis can be done first order and does not require much time to solve, this equivalent FEM model is pretty usefull for large models, where many areas with lattice structures exist.
Although relative densitiy can be used for equivalent thermal FEA, when elasticity module of the equivalent structure to be used for structural analysis is changed according to the relative density, it is possible to obtain realistic results in the displacement values, but the stress values do not give real stress results due to the change of the area. This causes an additional submodelling step to calculate stress values on lattice structures.
In this work, spring elements are used to create a structurally equivalent model. These spring elements were placed in the model in a way to reflect the stiffness of the lattice structure, so that displacement results can be got close to the reality. These displacement results were used with real lattice structure geometry in a sub-model to get stress values. The stiffness values of the lattice structures were determined by the displacement reactions they give to the forces applied on them in tensile, compression and bending directions in a test environment created with FEM.
As a result of these iterations, inferences are made about design changes and their effect on the system. In addition to investigating the behavior of lattice structures under thermomechanical loads, equivalent FEA models and modeling methodologies were studied in order to increase the applicability of lattice structures with a more effective FEA modeling approach in terms of computer load. With the equivalent model, it has been achieved to obtain reliable results close to the detailed lattice model and in a shorter solution time.