Heat transfer measurements around the centreline circumference of a cylinder in crossflow are
performed in a wind tunnel. The cylinder is placed at several stations downstream of three
turbulence-generating grids with different geometries and different blockage ratios
σ
g
: a reg-
ular grid (RG60) with
σ
g
= 32%
, a fractal-square grid (FSG17) with
σ
g
= 25%
and a single-
square grid (SSG) with
σ
g
= 20%
. Measurements are performed at 20 stations for 3 nominal
Reynolds numbers (based on the diameter
D
of the cylinder)
Re
∞
= 11 100
,
24 500
,
37 900
.
Hot-wire measurements are performed along the centreline, without the cylinder in place,
to characterise the flow downstream of the grids. The extent of the turbulence production
region, where the turbulence intensity
Tu
increases with the streamwise distance
x
from the
grid, is higher for SSG and more so for FSG17 than for RG60. The angular profiles of the
Nusselt number
Nu
are measured in the production regions of these two grids and are com-
pared to those obtained in the decay regions, where
Tu
decreases with
x
. This comparison is
made at locations with approximately same
Tu
. It is found that, for SSG,
Nu/Re
0
.
5
on the
front of the cylinder (boundary layer region) is lower in the production region than in the
decay region. This is explained by the presence of clear and intense vortex shedding in the
production region of SSG which reduces the turbulent fluctuations which are “effective” in
enhancing the heat transfer across a laminar boundary layer. For higher
Re
∞
, the values of
Nu/Re
0
.
5
on the front of the cylinder are higher in the production region of FSG17 than in
that of SSG, despite
Tu
being higher for SSG. This is consistent with a lower intermittency
of the flow for FSG17 caused by the presence of the fractal geometrical iterations. The recov-
ery of
Nu
on the back of the cylinder (wake region) is appreciably higher in the production
region than in the decay region for both FSG17 and for SSG. This can be due to the lower
integral length scale ratio
L
u
/D
in the production region and suggests, for the same
Re
∞
,
a reduction of the vortex formation length downstream of the cylinder, possibly promoted
by the interaction between the wakes of the bars of the grid and the wake of the cylinder.
At a large distance from the grids, the heat transfer enhancement is higher and it is more
efficient for FSG17 and for SSG than for RG60. For high values of
x
in the turbulence decay
region of the grids, the values of
Nu
(circumferential average of
Nu
) are similar for FSG17
and for SSG and they are both appreciably higher than for RG60. This happens despite
both FSG17 and SSG having a lower blockage ratio than RG60. The use of FSG17 has the
practical advantage of combining high heat transfer rates on the cylinder with a weak vortex
shedding from the grid.