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TECHNICAL PAPERS

Large Eddy Simulation of Flow and Heat Transfer in the 180Deg Bend Region of a Stationary Gas Turbine Blade Ribbed Internal Cooling Duct

[+] Author and Article Information
Evan A. Sewall

Mechanical Engineering Department,  Virginia Tech, Blacksburg, VA 24061esewall@vt.edu

Danesh K. Tafti

Mechanical Engineering Department,  Virginia Tech, Blacksburg, VA 24061

J. Turbomach 128(4), 763-771 (Feb 01, 2005) (9 pages) doi:10.1115/1.2098769 History: Received October 01, 2004; Revised February 01, 2005

Large eddy simulation of the 180 deg bend in a stationary ribbed duct is presented. The domain studied includes three ribs upstream of the bend region and three ribs downstream of the bend with an outflow extension added to the end, using a total of 8.4 million cells. Two cases are compared to each other: one includes a rib in the bend and the other does not. The friction factor, mean flow, turbulence, and heat transfer are compared in the two cases to help explain the benefits and disadvantages of the wide number of flow effects seen in the bend, including flow separation at the tip of the dividing wall, counter-rotating Dean vortices, high heat transfer at areas of flow impingement, and flow separation at the upstream and downstream corners of the bend. Mean flow results show a region of separated flow at the tip of the dividing region in the case with no rib in the bend, but no separation region is observed in the case with a rib. A pair of counter-rotating Dean vortices in the middle of the bend is observed in both cases. Turbulent kinetic energy profiles show a 30% increase in the midplane of the bend when the rib is added. High gradients of heat transfer augmentation are observed on the back wall and downstream outside wall, where mean flow impingement occurs. This heat transfer is increased with the presence of a rib. Including a rib in the bend increases the friction factor in the bend by 80%, and it increases the heat transfer augmentation by approximately 20%, resulting in a trade-off between pressure drop and heat transfer.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

The 180deg bend domain consists of three ribs upstream and downstream of the bend. An extension region is included downstream to negate effects from the outflow boundary condition. The dividing wall is 12Dh in width, and the duct cross-sectional area is 1 Dh2 throughout the bend. A comparison is made between having no rib in the bend (Case 1) and having a rib in the bend (Case 2).

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Figure 2

The prominent features of the bend for (a) Case 1 and (b) Case 2 include separation at the end of the dividing wall and recirculation at the upstream and downstream corners, resulting in impingement of highly accelerated flow on the back wall and outside wall of the downstream leg.

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Figure 3

A pair of counter-rotating Dean vortices at the midplane of (a) Case 1 and (b) Case 2 shows impingement on the back wall. Case 1 shows a low velocity in the recirculation bubble near the dividing wall.

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Figure 4

The (a) streamwise velocity and (b) streamwise rms quantity are compared between Case 1 (solid line) and Case 2 (dashed line). The LDV measurements correspond to the calculations in Case 1.

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Figure 5

Turbulent kinetic energy in the bend in (a) Case 1 and (b) Case 2 shows the combination of shear layers as well as transport of turbulence up along the inside wall from the motion of the Dean vortices.

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Figure 6

Heat transfer augmentation on the back wall and tip of the dividing wall with (a) Case 1 and (b) Case 2 shows the difference in heat transfer augmentation near the top and bottom walls and the higher heat transfer augmentation on the tip of the dividing wall due to the presence of the rib.

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Figure 7

Heat transfer comparisons with Han (20) along an inner line, center line, and outer line in the (a) upstream leg and (b) downstream leg of the ribbed wall show good agreement between the mass transfer experiments and heat transfer results. The measurements of Han did not have a rib in the bend (Case 1), and the dashed lines denote the LES calculations of Case 2.

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Figure 8

The heat transfer augmentation on the ribbed wall in (a) Case 1 and (b) Case 2 shows the high heat transfer in the immediate vicinity of the rib and a slightly increased heat transfer augmentation downstream.

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Figure 9

Average heat transfer augmentation on (a) the ribbed walls and (b) the inside and outside walls shows the increase in heat transfer due to the rib.

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