  The flow field and the heat transfer around six in-line iso-thermal circular
cylinders has been studied by mean of numerical simulations. Two values of the
center to center spacing ($s=3.6d$ and $4d$, where $d$ is the cylinder
diameter) at Reynolds number of $100$ and Prandtl number of $0.7$ has been
investigated. Similarly to the in-line two cylinder configuration, in this
range a transition in the flow and in the heat transfer occurs. Two different
flow patterns have been identified: the stable shear layer (SSL) mode and the
shear layer secondary vortices (SLSV) mode, at $3.6$ and $4$ spacing ratio
($s/d$), respectively. At $s/d=3.6$ the flow pattern causes the entrainment of
cold fluid on the downstream cylinders enhancing the heat transfer. On the
other hand at $s/d=4$ two stable opposite shear layer prevent the cold fluid
entrainment over the downstream cylinders reducing their heat exchange. The
overall time average heat transfer of the array is enhanced up to 25%
decreasing the spacing ratio from $4$ to $3.6$. Furthermore, it is found that
the increased heat transfer is related to the phase shift between the Nusselt
time series of successive cylinders.
