A Severe Bouyancy Problem Analysis And Solution Using CFD

The 8-Foot High-Temperature Tunnel (8-FT HTT) at the NASA Langley Research Center is a unique world class facility with a wide range of working parameters. New airfoil-shaped fuel injectors and an ignitor are being designed to minimize the effects of the buoyancy in the tunnel. Buoyancy in combustion chambers may seem to be a second order problem, but this is not true if large density gradients or differences are present at high pressure, as happens in the combustion chamber of the 8-FT HTT. The problem arises because not all of the enriched air at 410°R to 470°R is reacted and has a density that is almost 9 times that of the reaction products. The cold, dense surrounding fluid falls to the bottom of the chamber while the hot, low density reaction products tend to segregate to the top of the chamber, resulting in overheating of the liner wall at the top. Fortunately, the problem is solvable with a good computational fluid dynamics (CFD) code like the FLUENT Code (from Fluent, Inc.) and the proper approach, definition, and grid. After validating the code with experimental data, parametric cases were run. A 3D plot of the temperature contours at different sections of the tunnel show how gradually the hot gases tend to rise downstream in the combustor. The temperature profile at the exit of the combustor versus the distance from the bottom for different gaps, which are inlets of cold flow at the border (gaps of 3, 1.5, and 1.0 inch), shows that the smaller the gap, the better the uniformity of the temperature profile. A 3D plot of the temperature contours at different sections of the tunnel, using the 1-inch gap, shows that the buoyancy problem has been minimized, giving uniform temperatures at the top and bottom of the tunnel. A simple solution to a severe buoyancy problem was found by using CFD backed by experimental data. Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X