Koruge yüzeyli düz esnek metal hortumlarda basınç düşümü ve ısı transferinin araştırılması
Investigation of pressure drop and heat transfer in straight flexible metal hoses with corrugated surface
Yazar: ZEYNEP MERVE ALPARSLAN
Danışman: PROF. DR. HASAN GÜNEŞ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Makine Mühendisliği = Mechanical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Enerji kaynaklarının hızla tüketiliyor olması birçok sektörü bu hususta önlemler almaya ve iyileştirmeler yapmaya teşvik etmiştir. Isı değiştiricilerinin tasarımının iyileştirilmesi de enerji verimliliğine katkı sağlayacak önemli hususlardan biridir. Isı değiştiricilerinde ısı transferini artırmak amacıyla çeşitli tiplerde borular kullanılmaktadır. Mevcut olan çalışmalarda mikro kanatlı borular, paralel boğumlu borular, spiral boğumlu hortumlar gibi ısı transferini artırmak üzere çeşitli borular kullanılmaktadır. Boyler kazanlarının ısı değiştiricilerinde esnek metal hortumlar kullanılmakta olup ısıl performansının artırılması amaçlanmaktadır. Esnek metal hortumlar paralel boğumlu ve spiral boğumlu olarak imal edilmektedir. Bu hortumların boyler kazanlarındaki ısı değiştiricilerinin performanslarını artırmak için parametrik bir çalışma yapılması gerekmektedir. Bu çalışmanın konusunu oluşturan çok ağızlı spiral boğumlu esnek metal hortumun geometri parametreleri olan boğum hatvesi, boğum derinliği ve hidrolik çap ve spiral boğumların başlangıç sayıları değiştirilerek nümerik analizler yapılarak bu parametrelerin ısı transferine etkisi araştırılmıştır.
Metal hortum özellikleri olarak; 1 m boyunda pürüzsüz yüzeyli düz boru, paralel boğumlu, bir, iki ve üç başlangıç sayılı spiral boğumlu düz hortumların 5.4-19.5 mm hatve aralığında, 24.2 mm, 25 mm ve 26,2 mm hidrolik çaplarda ve 2.1-3.525 mm boğum derinliği değerleri aralığında ele alınarak sayısal analizler gerçekleştirilmiştir. Analizlerden elde edilen basınç düşümü, f sürtünme faktörü ve Nusselt sayıları hesaplanmıştır ve sonuçlar düz boru sonuçlarıyla kıyaslanarak irdelenmiştir.
Yapılan nümerik analizlerde girişe akışın hidrodinamik olarak tam gelişmiş olması için, hidrolik çapın yaklaşık 10 katı uzunluğa sahip 300 mm pürüzsüz düz boru ve yine çıkışa 300 mm pürüzsüz düz boru yerleştirilmiştir. Metal hortum içinden geçen akış tek fazlı olup akışkan sudur, akışkan özellikleri ele alınırken ortalama 25 °C sıcaklık kabul edilmiştir. Sınır şartları olarak giriş ve çıkıştaki pürüzsüz düz borular adyabatik kabul edilip, ısı transferi ve basınç düşümlerinin araştırıldığı 1 m uzunluğunda olan pürüzsüz düz boru, paralel ve spiral boğumlu düz hortumların sabit 70 °C yüzey sıcaklığında olduğu varsayılmıştır. Çıkış basıncı atmosferik olup, giriş hızları türbülanslı akış şartlarını sağlayan 10000, 20000, 30000, 40000, 50000 Reynolds sayıları için hesaplanmıştır. Girişteki akışkan sıcaklığı 15 °C 'dir. Nümerik çalışmalar, sonlu hacimler yöntemini kullanarak sayısal çözümlemeler yapan ve birçok mühendislik uygulamasında tercih edilen ANSYS Fluent yazılım programının 2022 R1 versiyonu ile yapılmıştır.
Nümerik çalışmaların doğruluğunu göstermek adına bir deney düzeneği kurulmuştur. Pürüzsüz düz boru, paralel boğumlu hortum ve 1 başlangıçlı spiral hortum için kurulan düzenekte, amaçlanan Reynolds değerlerine karşılık gelen hızlar için debiler belirlenmiş ve bu değerler debimetrede ayarlanmıştır. Ayrıca bir smart fark basınç ölçer aracılığıyla incelenen hortumların giriş ve çıkışındaki basınç farkı ±% 0,075 𝐹𝑆 hassasiyetle ölçülmüştür. Veriler anlık olarak veri toplayıcı cihaz kullanılarak bilgisayar ortamına aktarılmıştır. Simülasyon sonuçlarıyla deneysel sonuçlar karşılaştırılmış ve sonuçların uyumlu olduğu görülmüştür.

Abstract:
The rapid consumption of energy resources has encouraged many sectors to take measures and make improvements in this field. Improving the design of heat exchangers is one of the important issues that will contribute to energy efficiency. Various methods are used to increase heat transfer in heat exchangers. Heat transfer enhancement can be done by active and passive methods. Active methods may consist of external effects such as electrostatic fields, magnetic fields, fluid vibration, surface vibration. Passive methods have a wide range of applications. Examples of these are roughened surfaces, extended surfaces of heat transfer, turbulators placed in the flow, systems such as helical tubes. These systems increase the heat transfer surface area and contribute to the formation of turbulent movements in the fluid. However, in addition to these, they also bring pressure losses.
Therefore, when we need to evaluate energy efficiency, i.e. thermal performance, pressure drop should be taken into account just like heat transfer. In order to evaluate these two parameters together, the PEC value, which calculates the improvement in heat transfer at the same fluid pumping power, is widely used in existing studies. The PEC value is expressed as the power of (1/3) of the ratio of Nu and f values for pipes with improved heat transfer surface geometry to Nu and f values in the smooth straight pipe.
In the existing studies, various pipes are used to increase heat transfer such as micro finned pipes, parallel corrugated pipes, spiral corrugated pipes. Flexible metal hoses are used in the heat exchangers of boilers and it is aimed to increase the thermal performance. Flexible metal hoses are manufactured as parallel corrugated and spiral corrugated.
A parametric study is required to improve the thermal performance of these hoses in heat exchangers in boilers. The effect of these parameters on heat transfer was investigated by numerical analysis by varying the geometry parameters of the flexible metal hose with multi-nozzle spiral groove, groove pitch, groove height and hydraulic diameter and the initial number of spiral grooves, which are the subject of this study.
Numerical analyses were carried out for 1 m long smooth surface straight pipe, parallel groove, spiral groove straight hoses with one, two and three initial numbered spiral knuckles at pitch ranges of 5,4 mm, 5,8 mm and 6,5 mm, hydraulic diameters of 24,2 mm, 25 mm and 26,2 mm and groove heights of 2,1 mm, 3,1 mm, 3,225 mm, 3,475 mm and 3,525 mm. Based on the pressure drop and heat convection coefficient h, the friction factor f and Nusselt numbers are calculated and the results are compared with the straight pipe results.
In these numerical analyses, 300 mm pipes with a length of about 10 times the hydraulic diameter were placed at the inlet and outlet to provide full hydraulic development. The flow through the hose is single-phase, the fluid is water, and an average temperature of 25 °C is assumed for the fluid properties. As boundary conditions, the inlet and outlet pipes are assumed to be adiabatic and the smooth straight pipe, parallel and spiral corrugated straight hoses are assumed to have a constant surface temperature of 70 °C. The outlet pressure is atmospheric and the inlet velocities are 10000, 20000, 30000, 40000, 50000 Reynolds numbers which provide turbulent flow conditions. The fluid temperature at the inlet is 15 °C. Numerical studies were carried out with the 2022 R1 version of the ANSYS Fluent package software program, which performs numerical analysis using the finite element method and is preferred in many engineering applications.
In turbulent flow problems, the use of SST k-w turbulence model for internal flows with corrugated surface structure is recommended in the literature. In this study, SST k-w turbulence model is used. In addition, PRESTO (PREssure Staggering Option) model was used as the pressure model suitable for rotational flow.
The first grid cell of the viscous sublayer was assumed to be y+≈1. For this purpose, the first layer thickness calculation was done by choosing correlation. The polyhedral element type was chosen in order to reduce the number of elements in the solution mesh and to reach the solution in a shorter time. In the analyses in this study, the skewness value of the solution mesh is maximum 0,8 and the orthogonal quality is minimum 0,2. The continuity, momentum and energy convergence of the solutions in the analysis were around 10-5.
In order to verify these studies, an experimental setup was established. In the setup for smooth straight pipe, parallel corrugated hose and 1-start spiral corrugated hose, the velocities corresponding to the intended Reynolds numbers were controlled using a flowmeter. In addition, a smart differential pressure gauge was used to measure the pressure difference at the inlet and outlet of the geometries with an accuracy of ± 0,075% FS (Full Scale). The data was instantaneously transferred to a computer using a data logger device. For Reynolds numbers 20000, 30000, 40000 and 50000, a total of 40 measurements were taken in 10 minutes with a frequency of 15 seconds and average results were obtained. The simulation results were compared with the experimental results and it was seen that the f Darcy friction factors were compatible.
Friction factors were obtained experimentally at the same Reynolds numbers to verify the accuracy of the analysis results for hoses with parallel grooves and spiral grooves with 1,2,3 starts. It was observed that there was a maximum difference of 0,04 between the results of numerical analysis and experimental friction factors.
Firstly, the friction factor is calculated from the pressure losses and its variation with Reynolds number is presented in graphs. Taking straight pipe results as reference, friction factors for straight hose with parallel grooves and straight hose with spiral grooves with 1, 2 and 3 starts are compared with each other. Considering only the number of starts, the variation of friction factors with Reynolds number is presented, keeping other parameters constant. For a 1-start spiral hose with groove heights between 2,1 – 3,525 mm, the variation of friction factors with Reynolds number is presented graphically. The variations of the friction factors of 1 and 3 start spiral hoses with the same heat transfer surface areas as the parallel groove hose, the same groove heights, the same pipe lengths, and different pitch intervals due to the number of starts, with the Re number are presented graphically.
In order to show the superiority of 1, 2, 3-start spiral hoses and parallel corrugated hoses over straight pipe in terms of heat transfer, Nusselt numbers were calculated using heat convection coefficients from the analysis results. The variations of Nusselt numbers with Reynolds number are presented as graphs. Taking the straight pipe results as reference, the heat transfer for straight hose with parallel corrugated and straight hose with spiral corrugated with 1, 2, 3 starts are expressed as Nu number and compared with each other. Keeping all geometrical parameters of 1, 2 and 3 start spiral hoses the same, the variation of Nu numbers with Re number is presented. In the 1-start spiral hose, the variation of Nu numbers with Reynolds number is presented graphically for the case where the corrugated heights are in the range of 2,1 – 3,525 mm. The variations of Nu numbers with Re number for the 1 and 3 starter spiral corrugated hoses, which have the same heat transfer surface areas as the parallel corrugated hose, the same corrugated heights, the same pipe lengths, and different pitch intervals due to the number of starters, are presented graphically. In general, Nu numbers increased as the number Re increased. In 2-start spiral hoses, keeping all parameters the same, the change depending on the groove height was examined and the results showed that the friction factor increases with groove height in the same way as the 1-start spiral hose, while the Nu number also increases.
In this study, it is aimed to increase heat transfer by using spiral corrugated hoses and parallel corrugated hoses in boilers. However, due to the increase in pressure losses, the PEC number, which takes into account the pressure drop together with heat transfer in the thermal performance evaluation, was used in this study. In general, the PEC value decreased as the Reynolds number increased. This is because although the Nusselt number increases, this increase is less than the pressure drop. For this reason, at Reynolds number 10000, the PEC value of spiral corrugated hoses with low knuckle height exceeded 1, while the PEC value of spiral corrugated hoses with high groove height remained below 1. When the PEC values of the spiral corrugated hoses with the same number of starts are ranked from the lowest to the highest, the spiral corrugated hose with the highest PEC value is the one with the lowest groove height and the spiral corrugated hose with the highest groove height is the one with the lowest PEC value.