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An Experimental and Numerical Study of Heat Transfer and Pressure Drop on the Bend Surface of a U-duct

Författare:
  • Tareq Salameh (PhD Student)
Publiceringsår: 2012
Språk: Engelska
Dokumenttyp: Doktorsavhandling

Sammanfattning

This work concerns experimental and numerical studies of pressure drop and heat transfer for turbulent flow inside a U-duct and in particular the bend part. Such duct geometries can be found in many engineering applications where cooling air extracts heat from hot internal walls of the duct, e.g., passage cooling inside gas turbine blades. The details of the duct geometry were as follows: the cross section area of the straight part was 50x50 mm2, the inside length of the bend part 240 mm, the cross section area of the ribs was 5x5 mm2 and the rib height-to-hydraulic diameter ratio, e/Dh, was 0.1.
For the experimental study both friction factors and convective heat transfer coefficients were measured inside a U-duct for three different cases, namely (a) the smooth straight part, (b) the smooth bend (turn) part, and (c) a rough (ribbed) bend (turn) part. The Reynolds number was varied from 8,000 to 20,000. For case (c) different configurations and arrangements of ribs (single rib, two ribs and three ribs as well as continuous and truncated ribs) were tested inside the bend part. A value of Reynolds number of 20,000 was used in comparison between the cases. The test rig has been built in such a way that various experimental setups can be handled as the bend (turn) part of the U-duct can easily be removed and the rib configuration can be changed. Both the U-duct and the ribs were made from acrylic material to allow optical access for measuring the surface temperature by using a high-resolution measurement technique based on narrow band thermochromic liquid crystals (TLC R35C5W) and a CCD camera placed facing the bend (turn) part of the U-duct. The calibration of the TLC is based on the hue-based color decomposition system using an in-house designed calibration box. The ribs were placed transversely to the direction of the main flow at the outer wall of the bend (turn) part where the wall was heated by an electrical heater. It was found that the presence of the rib increased the heat transfer coefficient on the outer wall of the bend part (tip of side U-duct). The uncertainties were 7% and 9% for the Nusselt number and friction factor, respectively.
In the numerical study, two-dimensional numerical simulations of the flow and temperature fields inside the bend (turn) part of a U duct have been performed. Several turbulence model based on two and five equations models were used to solve the smooth bend (turn) part and ribbed bend (turn) part, respectively. For the k-e model, the wall function approach was used at the near wall region where the log-law was assumed to be valid. For the Reynolds stress model two different types of wall boundary were used. Furthermore different inlet and thermal boundary conditions were considered at the inlet and the outer wall of bend part, respectively. The results for k-e model seemed to be more reliable and closer to experimental data.

Disputation

2012-12-14
10:15
Room E:1406, E-building, Ole Römers väg 3, Lund University Faculty of Engineering
  • Je-Chin Han (Professor)

Nyckelord

  • Technology and Engineering

Övriga

  • Bengt Sundén (Professor)
  • ISSN: 0282-1990

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