Thermal Science 2016 Volume 20, Issue 5, Pages: 1621-1632
https://doi.org/10.2298/TSCI140816138T
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Augmented of turbulent heat transfer in an annular pipe with abrupt expansion
Togun Hussein (University of Malaya, Department of Mechanical Engineering, Kuala Lumpur, Malaysia + University of Thi-Qar, Department of Mechanical Engineering, Nassiriya, Iraq)
Abdulrazzaq Tuqa (Universiti Putra Malaysia (UPM), Department of Mechanical and Manufacturing Engineering, Selangor, Malaysia)
Kazi Salim Nivaz (University of Malaya, Department of Mechanical Engineering, Kuala Lumpur, Malaysia)
Badarudin Ahmad (University of Malaya, Department of Mechanical Engineering, Kuala Lumpur, Malaysia)
This paper presents a study of heat transfer to turbulent air flow in the
abrupt axisymmetric expansion of an annular pipe. The experimental
investigations were performed in the Reynolds number range from 5000 to
30000, the heat flux varied from 1000 to 4000 W/m2, and the expansion ratio
was maintained at D/d=1, 1.25, 1.67 and 2. The sudden expansion was created
by changing the inner diameter of the entrance pipe to an annular passage.
The outer diameter of the inner pipe and the inner diameter of the outer pipe
are 2.5 and 10 cm, respectively, where both of the pipes are subjected to
uniform heat flux. The distribution of the surface temperature of the test
pipe and the local Nusselt number are presented in this investigation. Due to
sudden expansion in the cross section of the annular pipe, a separation flow
was created, which enhanced the heat transfer. The reduction of the surface
temperature on the outer and inner pipes increased with the increase of the
expansion ratio and the Reynolds number, and increased with the decrease of
the heat flux to the annular pipe. The peak of the local Nusselt number was
between 1.64 and 1.7 of the outer and inner pipes for Reynolds numbers varied
from 5000 to 30000, and the increase of the local Nusselt number represented
the augmentation of the heat transfer rate in the sudden expansion of the
annular pipe. This research also showed a maximum heat transfer enhancement
of 63-78% for the outer and inner pipes at an expansion ratio of D/d=2 at a
Re=30000 and a heat flux of 4000W/m2.
Keywords: Sudden expansion, separation flow, Turbulent heat transfer, Annular passage