Silindir etrafındaki etilen glikol akışının sayısal olarak incelenmesi

Abstract

Bu çalışmada silindir yüzey üzerinden etilen glikol akışı nümerik olarak incelenmiştir. Çalışma Reynolds sayısının 40000 ve 70000 durumları için laminar, 200000 ve 400000 durumları için türbülanslı olarak ele alınmıştır. Etilen glikol için Prandtl sayısı 103 olarak alınmıştır. Sayısal çalışma ANSYS CFX 11.0 programı kullanılarak gerçekleştirilmiştir. Zaman ve eleman sayısı tasarrufu için çalışma simetri olarak gerçekleştirilmiştir. Türbülans model olarak SST türbülans model seçilmiştir. Türbülans etkilerinin iyi gözlemlenebilmesi için silindir cidarındaki y+ mesafesi 0.5 alınmıştır. Silindir yüzeyde doğru sonuç elde etmek için sıralı 50 tabaka ağ kullanılmıştır. Modelde toplam 27166 eleman, 41350 düğüm kullanılmıştır. Kullanılan silindir yüzeyin çapı 0.1 m alınmıştır. Dikey mesafe 0.75 m, yatay mesafe 2.5 m, blokaj oranı 10 olarak alınmıştır. Etilen glikol 25 oC sıcaklıkta alınmış, silindir yüzeye 50 oC sabit sıcaklık sınır şartı verilmiştir. Üst ve sağ kısma serbest sınır şartı uygulanmıştır. Yapılan çalışma literatürde yapılmış benzer çalışmalarla kıyaslanmış ve uyumlu sonuçlar elde edilmiştir. Çalışma sonunda Prandtl ve Reynolds sayılarının Nusselt sayısı üzerindeki etkisini gösteren bağıntılar elde edilmiştir. Ayrıca Reynolds sayısının 40000 < Re < 400000 aralığı için sürüklenme katsayıları elde edilmiştir. Silindir yüzey üzerindeki yerel Nusselt sayılarının ortalama Nusselt sayılarıyla değişimi 0° < θ < 180° aralığında açısal olarak incelenmiştir. İnceleme sonucunda yerel Nusselt sayılarının literatürde belirtildiği gibi laminar akış durumunda bir minimum noktadan geçtiği, türbülanslı akış durumunda iki ayrı minimum noktadan geçtiği bulgusuna ulaşılmıştır. Ayrıca Reynolds ve Prandtl sayılarının Nusselt sayısı üzerindeki etkisini içeren bağıntılar, farklı Reynolds sayıları için direnç katsayıları elde edilmiştir.

Keywords

• silindir, Reynolds sayısı, Nusselt sayısı, türbülans, direnç katsayısı,

Numerical analysis of ethylene glycol flow around a circular cylinder

Abstract

The concept of the external flow is noticeable in many engineering applications. Many topics such as vehicles, power lines, lift force created by the wings of aircraft and blood flows are covered by outer flow. Therefore, detailed studies of the external flow such as planes, cars, buildings, ships, submarines and turbines play an important role in many engineering system designs. The effect of heat transfer on cross flow around the various geometries was experimentally and numerically investigated for many years. Studies on various geometries like circle, square and triangle are discussed. Constant heat flux or constant temperature boundary conditions of heat transfer between the fluid and the wall has been studied extensively. Studies are based on determination of the structures of heat transfer, Nusselt, Prandtl and Reynolds numbers and the relationships between each other. In particular, their effects on the Nusselt number, Prandtl and Reynolds number of the external flows are examined in detail. A lot of research has been done in this field with the circular cylinder especially. The reason for this is that circular cylinder is used in many areas like heat exchangers, thermal, and mechanical systems, and electrical systems. Despite extensive research on air flow over a circular cylinder, in the different fluids has not been investigated in detail adequately. In the last few years, lots of experimental and numerical studies of heat transfer around a circular cylinder have been done. In this study, laminar and turbulent flow were examined numerically on the cylinder surface. Effects of Prandtl and Reynolds numbers on Nusselt number for cylinder surface were investigated. A wide range of Reynolds number between 4x104 and 4x105 was chosen for both laminar and turbulent flow. Ethylene glycol Prandtl numbers 103 was taken as working fluid for the analysis. Total horizontal distance was selected as 2.5 m, total vertical distance was as 0.75 m and the blockage rate as 10. Inlet temperature was 25 oC. Cylinder surface was 50 oC. Upper and lower portion were open boundary condition. Symmetry boundary condition to the bottom of the model was given. For numerical analysis ANSYS CFX 11.0 software program was used. Geometry and mesh structure of the models were obtained in the Workbench package program. Shear Stress Transport (SST) was chosen as turbulent model. Turbulent flows in case of y+ < 5 were higher viscous damping. To obtain more precise results, the wall distance y+ was chosen as 0.5. Total number of elements of model 27166 and 41350 total number of nodes was taken as. To obtain the optimum number of nodes, the upper and lower values of the node numbers were tested. To save time and number of element, work was carried out in symmetry. To obtain more sensitive results, meshes passed cylinder surface, has been determined as the layer 50. In order to verify numerical results fluid flow and heat transfer around a cylindrical surface that was widely studied in the literature was investigated for different Reynolds and Prandtl numbers, and obtained results were compared with values of literature studies. Change of the local Nusselt numbers for turbulent flow in the literature as follows: Initially, local Nusselt numbers flow over the cylinder surface, valued at the highest. Then it began to decrease rapidly due to cooling cylinder surface by the fluid. It passed through the minimum point of the range of 80o -100o . Due to the transition to turbulence, local Nusselt numbers began to increase rapidly after this point. Due to boundary layer separation and vortex consist of second minimum point approximately at 140o . A similar situation was observed in this study. Local Nusselt numbers in laminar flow had a minimum point. Similarly, the turbulent flow consisted of two minimum points. As a result was obtained local Nusselt number and drag coefficients. In addition, average Nusselt number correlations were obtained for cylinder surface depending on Reynolds and Prandtl numbers.

Keywords

• Cylinder, Reynolds number, Nusselt Number, turbulence, drag coefficient