The effect of free stream turbulence intensity on heat transfer and flow characteristics around four in-line elliptic cylinders in cross flow was experimentally and numerically investigated. The elliptic cylinders examined had an axis ratio (b/c) of (1:3) with a zero angle of attack (where the major axis is horizontal) and were heated under a constant heat flux condition. Three different sizes of free stream turbulence producing grids (screen) inserted at a distance of 300 mm the upstream test cylinders are used to study the effect of the free stream turbulence. The turbulence intensity levels of 5.6%, 8.3% and 10.5% are obtained from the grid sizes. The effects of cylinder spacing ratio (L/c) were examined within Reynolds number (based on free stream velocity and major axis length) ranging from 3640 to 66240. It was observed that the local and average Nusselt numbers
of the tested elliptic cylinders increase with increasing the free stream turbulence intensity. The results showed that the maximum enhancement ratio for the average Nusselt number (for the tested cylinder with grid and for the single elliptic cylinder without grid) was found to be about 1.93 and was obtained for the second elliptic cylinder in four in-line cylinders at L/c=1.5, Re=6150 with free stream turbulence intensity of 10.5%. Also, the flow characteristics varied drastically with both the longitudinal spacing ratio and the inserted grid (screen). Moreover, an empirical correlation for the average Nusselt number of the second elliptic cylinder of the four in-line cylinders was obtained as function of Reynolds number and free stream turbulence intensity.
KEYWORDS: free stream turbulence, turbulence intensity, in-line arrangement, four elliptic cylinders, cross flow
|
