Yüksek devirli rulmanlarda açık yağlama sistemi için sprey enjektör tasarımı
Spray injector design for open lubrication system in high speed bearings
Yazar: ERDEM ÇAĞATAY YALÇIN
Danışman: PROF. DR. FIRAT OĞUZ EDİS
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı / Uçak ve Uzay Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Havacılık ve Uzay Mühendisliği = Aeronautical Engineering ; Makine Mühendisliği = Mechanical Engineering
Dizin: Enjektörler = Injectors ; Rulmanlar = Bearings ; Yağlama = Lubrication

Özet:
Hava araçlarının veya roketlerin itki sistemlerinde kullanılabilen turbo jet motorların kullanıldığı aracın planlanan kullanım ömrüne uygun tasarlanabilmesi mühendislik açısından zorlayıcı bir optimizasyon problemidir.  Motorlarda döner elemanların yataklandığı rulmanlar sistemin en kritik noktasını oluşturmakta dolayısıyla tasarlanan motorların kullanım ömrünü de bu kritik noktaları olan rulmanların kullanım ömrü belirlemektedir. Rulmanların kullanım ömrünün uzatılması ve daha pürüzsüz bir şekilde görevini yerine getirebilmesi için tarihi taş devrine dayanan, günümüze kadar ilk kullanılmaya başlandığı andan itibaren önemini yitirmemiş, uygarlığımızın temeli olan aletlerin ve makinelerin sorunsuz çalışmasında ve uzun ömürlü olmasında en büyük katkıya sahip; yağlama sistemi kullanılmaktadır. 
Yağlama sisteminin kalitesini belirleyen ve sistem gereksinimlerine uygun yağlama yapılabilmesini sağlayan en önemli tasarım noktası doğru enjektörün seçimidir. 
Bu tez çalışmasında yüksek devirli bir rulmanda açık yağlama sistemi gerekliliklerinin belirlenmesi ve bu gerekliliklere uygun yağlama enjektörü tasarımı çalışması anlatılacaktır. Bu tez kapsamında, yağlama enjektörü tasarımına varan ön tasarım çalışmaları 3 aşamada incelenmiş, bu ön tasarım çalışmaları sonucu elde edilen veriler ışığında enjektör tasarımı yapılmış, tasarlanan enjektörün testleri yapılmış ve deneysel sonuçlar elde edilmiş, bu sonuçlar teorik sonuçlar ile kıyaslanmış ve bu kıyaslamaya dayanarak çıkarımlar oluşturulmuştur.
Ön tasarım çalışmasının ilk adımı, yağlama gereksinimlerini karşılamak için rulmanların ürettiği ısının belirlenmesidir. Rulmanlarda üretilen ısı, rulman geometrisine, rulman üzerindeki yüke ve rulmanın dönme hızına bağlıdır. Rulmanlarda ısı oluşumu dönen bilyelerden kaynaklanmaktadır. Bu nedenle hesaplamalarda kullanılacak kritik rulman geometrisi bilgileri bilyelerin bulunduğu bölgede toplanır. Rulmana ait gerekli hız ve boyut bilgileri bu geometri bilgileri kullanılarak hesaplanabilir. Yükler ve bilye hızları hesaplandıktan sonra sürtünmeden kaynaklı kaybedilen enerjinin tamamının ısıya dönüştüğü kabulü ile üretilen ısı hesaplanır. 
Ön tasarım çalışmasının ikinci adımı olarak rulmana gönderilecek olan yağlayıcının debi hesabı yapılmalıdır çünkü rulmanda üretilen ısının sistemden dışarıya atılması gerekmektedir ki rulman sıcaklığının kritik seviyelere ulaşması önlenebilsin. Rulmanın bu kritik seviyelerdeki sıcaklığa ulaşmasını önleyebilmek için rulmana yağlayıcı sıvı gönderilmektedir. Gönderilecek yağlayıcının debisini belirlemek için ısı transferi hesabı yapılabilir. 
Bu çalışmanın başlangıcında, gönderilen yağlayıcı ve havanın rulman çıkışında aynı sıcaklığa ulaşacağı kabulüyle, rulmanda üretilen ısının hava-yağ karışımına aktarıldığı ısıl denge kurulmuştur. Böylece karışımın rulman çıkış sıcaklığı tespit edilmiştir.
Çıkış sıcaklığı belirlendikten sonra rulman sıcaklığını bulmak için iteratif bir çalışma yapılacaktır.
Rulmanlara gönderilecek debi belirlendikten sonra, ön tasarım çalışmasının üçüncü ve son adımı doğru tip enjektör seçimidir. Yağlama için kullanılacak enjektörler sprey ve jet enjektörler olmak üzere iki seçenekte incelenmiştir. Enjektör tipine karar verebilmek için literatür taraması yapılarak sprey ve jet enjektör karşılaştırılmıştır.
Literatür taramasından elde edilen bulgular göstermektedir ki, hava akışı hızlandığında jet akış büyük tanecikler halinde parçalandığı için hava akışı ile tam anlamı ile taşınamayıp kendi ağırlığı ile akış içerisinde dağılmaktadır. Bu durum jet akışın istenilen hedefe gitmemesine sebep olmaktadır. Rulmana doğru şekilde ulaşmayan yağlayıcının, rulmandan geçmeden ortamdan atılma olasılığı oldukça yüksektir. Ayrıca kapalı sistemlerde sızdırmazlık elemanları sayesinde rulman dışında gidecek bir yere sahip olmayan yağlayıcı, açık sistemde bu sızdırmazlık elemanlarının bulunmaması sebebi ile doğru taşınma kapasitesine sahip olmaz ise, yağlayıcı rulmana ulaşamayacak ve yağlama görevi başarı ile gerçekleştirilemeyecektir. Dolayısıyla bu tezde söz konusu yüksek devirli rulmanlarda açık yağlama sistemi kullanıldığında yağlayıcının tanecik yapısını küçültmek, dağılımını arttırmak ve ikincil hava akışı ile taşınabilir hale gelmesi için sprey enjektör tercih edilmiştir.
Literatür çalışmalarında görülen, enjektör tasarımında kullanılan denklemlerin çoğunun ampirik yaklaşımlar varsayılarak yapılan testler sonucunda oluşturulan denklemler olduğudur. Bu tezde, tasarlanacak sistemin çalışma aralığındaki ve düşük hata oranlarına sahip denklemler referans alınmıştır.
Bu tezde bahsedilen enjektör için kullanılan tasarım parametreleri; tahliye katsayısı, dağılma uzunluğu, film kalınlığı, yağ çıkış hızı, dağılma rejimleri, sprey dağılma açısı, Sauter ortalama çapı ve yay hesabıdır.
Enjektör tasarımı enjektör gövdesi, döner parça, yay, yay tutucu, filtre ve yüksük olmak üzere 6 parçadan oluşmaktadır. Enjektör boyutları, küçük motorlara uyacak şekilde kasıtlı olarak küçüktür. Her ne kadar bu durum üretimsel bazı sıkıntılara sebep verse de kimi motorlar için küçük boyutlara sahip bir enjektör tasarımı zorunluluk olarak ortaya çıkmaktadır. Bu sebeple küçük boyuttaki enjektörlerin incelenmesi ve bu konuda çalışmalar yapılması bir gereklilik olarak ortadadır.    
Test sisteminde enjektörlerin akış ve basınç ilişkisinin belirlenmesi amaçlanmaktadır. Ayrıca sprey dağılım açısı farklı debilerde gözlemlenmiş ve sprey kalitesinin bir göstergesi olarak kullanılmıştır. Testler kademeli olarak artırılan farklı debilerde gerçekleştirilmiş ve her adımda kamera kayıtları alınarak sprey formları incelenmiştir.
Yapılan testler sonucunda düşük basınçlarda püskürtme kalitesinin kötü olduğu ve istenilen debilerin fazla gönderildiği görülmüştür. Düşük basınçta akış enjektörde girdap şeklinde hızlanamadığı için enjektörden çıkan akışkan saçılarak dışarı çıkamaz. Bu nedenle akış birincil rüzgâr rejiminde gerçekleştiği için yeterli sprey formu elde edilememektedir. Sprey kalitesini artırmak için basıncın artırılması gerektiği açıktır. Ancak bu debiyi artıracağından, debi değeri ile sprey kalitesi arasındaki denge gözetilerek tasarım yapılmalıdır.

Abstract:
Turbo jet engines can be used in aircraft propulsion systems, aircraft engine starting systems or rocket propulsion systems. According to the service life of the platforms on which the engines are mounted, the lifetimes of the engines should also be appropriate. The most important factors that determine the life of the engines are the working life of the turbines and bearings. Lubrication systems are used to increase the working life of the bearings.
The bearings on which the rotating elements are supported in the machines constitute the most critical point of the system. Especially in high load or high speed systems, choosing the right bearing is one of the important engineering problems. In addition to choosing the right bearing, it is extremely important to dissipate the heat produced in the bearing and to keep the operating temperature of the bearing low.
Loads coming from external forces on to the bearing which are centrifugal force caused by rotation and unbalance, cause heat production in bearings. In order to determine these loads, the working loads of the rotary system should be known together with the number of revolutions. 
The lubrication system is critical for the healthy operation of the rotating elements in the machines. In many systems, the working or maintenance life is determined by the rotating elements and therefore the lubrication quality. Since the correct lubrication determines the machine life, establishing the lubrication system that will meet the requirements of the system turns into an optimization problem. For a suitable lubrication system, the lubrication requirement of the system must be determined.
In lubrication systems, the lubrication task is carried out through injectors. Injectors are used for lubricating mechanical parts in engines and gas turbines, and for lubricating and fuel injection in the combustion chamber. The task of injectors used for fuel injection is to increase the surface area of the droplets formed by splitting the fuel and thus to increase the combustion efficiency. For this, spraying should be done in spray form. In lubrication, the purpose is to form a layer that reduces friction of the lubricating fluid in the rotating element and to absorb heat as it passes over the rotating element. Injectors used for lubrication are divided into two as jet injectors and spray injectors.
Jet injectors send the fluid at high speed and in a laminar fashion. Such injectors are generally used in closed-loop lubrication systems or systems where the lubricant must be sent to hard-to-reach areas of the rotating element. An example of such systems is systems that are lubricated under the inner ring of the bearings.
Spray injectors can be used in both open and closed lubrication systems. Lubrication can be provided in cases where lubrication cannot be made with jet injectors, since the lubricant is spread in the form of spray with the spray injectors and can be carried by air.
Spray injectors can be preferred to increase lubricant penetration in open lubrication systems. Spray injectors send the lubricant to the environment by pulverizing at high speeds and by various methods. In this way, the lubricant can be carried by air and can make contact without splashing from the rotating element at high speeds. In order for the lubricant to fulfill its duty, it must be able to absorb heat by contacting the rotating element from as large a surface as possible. Spray injectors can provide this function with pulverized lubricant.
In this thesis, the determination of the open lubrication system requirements of a high speed bearing and the design of the lubrication injector according to these requirements will be explained.
The key to designing a succesfull lubrication system is to starting with preliminary design study, than testing the injectors and comparing the test data to expected results and deciding on the best matching injector.
First step of preliminary design study is to determine the heat generated by the bearings in order to meet the lubrication requirements. The heat produced in the bearings depends on the bearing geometry, the load on the bearing and the rotational speed of the bearing. Heat generation in bearings is due to rotating balls. For this reason, the critical bearing geometry information to be used in the calculations is collected in the area where the balls are located. These informations are, r_dış , r_iç , r_bilye , n , α , m_bilye . Required speed and dimension information of the bearing can be calculated with the inputs given above. Calculated outputs are; r_ort , w_bilye , w_iç , V_bilye ,  V_spin . The CTR in the formula of the angular velocity of the balls is the ratio of the slip velocity of the balls to the rotational velocity (spin to roll ratio). The loads acting on the bearing, which is another input, are divided into four individual loads. F_itki , F_C , F_balanssızlık and F_yerçekimi . After the loads and ball velocities are calculated, the heat produced in the bearing can be calculated.
Second step is to determine the lubricant flow rate. Heat transfer calculation can be made to determine the flow rate of the lubricant to be sent. At the beginning of this study, thermal equilibrium was established in which the heat generated in the bearing is transferred to the air-oil mixture, assuming that the supplied lubricant and air will reach the same temperature at the exit of the bearing. Thus, the bearing outlet temperature of the mixture was determined. To calculate T_(o_ao ) ; Q_b , m ̇_a , c_(p_a ) , T_(i_a ), m ̇_o, c_(p_o ), T_(i_o ) must be used. After the outlet temperature is determined, an iterative study will be made to find the bearing temperature. First of all, the heat transfer coefficients should be determined. Nu numbers and heat transfer coefficients are calculated by calculating the relative velocities of the lubricant and air to the bearing surfaces.
After calculating the nusselt numbers of the inner ring, outer ring and balls according to the air and lubricant, the heat transfer coefficients are calculated.
Since the temperature T_b cannot be separated from the logarithmic mean temperature difference equation, the temperature is found by making an iterative solution. By changing the lubrication flow rate, the bearing temperature is brought to the desired range. In this way, the lubricant flow rate is determined.
After the flow rate to be sent to the bearings is determined, third and final step to preliminary design study is choosing the correct type of injector. Injectors to be used for lubrication were examined in two options as spray and jet injectors. In order to decide on the injector type, a literature search was made and spray and jet injector were compared.
In one of the papers mentioned in the literature search; experiments with various gas weber numbers and fluid weber numbers have been carried out, and the findings show that when the air flow accelerates, the dispersion of the jet stream increases greatly and disperses in a shorter distance, but because the dispersed lubricant is broken into large particles, it cannot be carried by the air flow completely. It leaves the flow at a shorter distance from the desired point with its own weight. This situation causes the jet stream not to go to the desired destination.
In another article, gear wheel lubrication with a jet injector was tested by keeping the speed of the fluid sent from the jet injector constant and only increasing the rotational speed of the gear and was monitored by cameras. As a result of the study; While the rotation speed of the gear wheel is low, the speed of the jet stream can allow the lubricant to enter between the teeth, but the lubricant cannot be fully efficient with the splash effect, when rotation speed of the gear has increased the lubricant has difficulty in reaching the gear surface and the ones that can reach bounce off without getting between the gears. It was observed that the lubrication task failed because the lubricant sent from the jet injector was pushed out of the system by the air flow before it reached the gear, by increasing the speed of the gear wheel considerably.
Since the Weber number mentioned for the rotor examined in this thesis is very high, it was predicted that it would be very difficult for the lubricating fluid to reach the bearing with the air turbulence formed under the rotor rotating at high speed when lubricating with a jet injector. Lubricant that does not reach the bearing correctly has a good chance of returning without passing through the bearing.
In closed lubrication systems, air flow is created to carry the oil from one side of the bearing to the other with a pressure difference although this pressure difference is not very high, the oil cannot be directed in any other direction other than passing through the bearing, since the bearing area is closed with sealing elements. In open lubrication systems, however, this sealing cannot be achieved, so secondary air flows have to undertake this task. However, secondary air flows have low pressure differentials and therefore low flow rates, especially in the first stages of the compressor group. At low air flow, the carrying capacity of the lubricant in the bearing also decreases. If lubrication with a jet injector was preferred for the bearing we examined in this thesis, the lubrication task would not be performed successfully and the bearings would not be able to be cooled as the lubricant did not reach the bearing as a result of the failure of the lubricating oil to be transported to the bearing with the secondary air flow.
In the light of these findings, it was seen that the lubricant should become portable by the airflow under the stator. By changing the lubricant method, it is aimed to reduce the particle structure of the lubricant, to increase its distribution and to make it portable with secondary air flow. Spray injector studies were carried out to increase the dispersion of the lubricant by mixing it with the air flow and to reduce the particle structures.
What is seen in the literature studies is that most of the equations used in injector design are the equations created as a result of tests made by assuming empirical approaches.
From the equations compiled from the literature study, the equations within the operating range of the system to be designed in this thesis and with low error rates were taken as reference.
The design parameters used for the injector mentioned in this thesis are; evacuation coefficient, dispersion length, film thickness, oil output rate, breakup regimes, spray angle, Sauter mean diameter and spring calculation.
The injector design consists of 6 parts as injector body, rotating part, spring, spring holder, filter and ferrule. The diameter of the injector body was chosen with a thread diameter of 6mm. The injector dimensions are deliberately small to suit small engines. The small size of the injector causes the dimensions of the parts inside to be small accordingly. This situation not only makes production difficult, but also it is more difficult for the produced parts to comply with the production tolerance restrictions due to the small production tolerances. However, since the small size of the injector is a necessity in some engines, it is important to examine the injectors of the sizes mentioned in this thesis.
Although only the outer body and the rotating part of the injector affect the flow rate and spray quality, other parts also have functional duties in order for the injector to work. The spring is there to fix the rotator in the axial position inside the injector. Otherwise, if the contact of the rotator with the body is lost, the flow area will change and the injector will not be able to work as it should. As can be understood from the name of the spring holder, it acts as the limiter in the opposite direction of the spring. There is a strainer on the spring holder to prevent the injector from clogging. Finally, the ferrule is responsible for fixing the filter.
Two types of outlet diameter in the injector body were tried, as straight cylindrical and conical. The minimum diameter in the conical hole was kept the same as the flat one and the effect of the conical outlet was investigated.
In the design, the permeability of the spring and the flow area of the hole in the spring holder are larger than the area of the hole in the rotator and injector outlet. In this way, an unwanted throttling effect on the injector is prevented, and the flow control is adjusted from the rotator and injector outlet diameter.
The selection of the filter was chosen not to be larger than the smallest diameter across the injector.
In the test system, it is aimed to determine the flow and pressure relationship of the injectors. In addition, the spray dispersion angle was observed at different flow rates and used as an indicator for spray quality.
The tests were carried out at different flow rates, which were increased gradually, and the spray forms were examined by taking camera recordings at each step.
As a result of the tests, it was seen that the spray quality is poor at low pressures and the flow rates are sent more than desired. Since the flow at low pressure cannot accelerate in the form of a swirl in the injector, the fluid coming out of the injector cannot escape by scattering. For this reason, adequate spray form cannot be obtained since the flow occurs in the primary wind regime. It is clear that the pressure must be increased to improve the spray quality. However, since this will increase the flow rate, the design should be made by considering the balance between the flow value and the spray quality.