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.
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.
Details
| Other ID | JA67ZT94JT |
|---|---|
| Journal Section | Articles |
| Authors | |
| Publication Date | June 1, 2013 |
| Submission Date | June 1, 2013 |
| Published in Issue | Year 2013 Volume: 4 Issue: 1 |