Wake interference effects on the heat transfer enhancement around a row of circular cylinders

The numerical simulations of the heat transfer around an array of isothermal circular cylinders immersed in a stream has been carried out solving the two-dimensional Navier-Stokes equations. The cylinders have been placed in a single row configuration aligned with the free stream velocity at Reynolds number 100 and Prandtl number 0.7. In Fig.1 it is shown the instantaneous temperature distribution for the case of six in-line circular cylinders at spacing ratio (s/d) equal to 4 and 3.6, where s is the center-to-center cylinder spacing and d is the cylinder diameter. In the latter case, a transition in the flow patterns occurs with the flow organized in a vortex shedding responsible for the entrainment of cold fluid in the gaps. This phenomenon makes stronger the thermal gradient close to the cylinders leading the heat transfer enhancement with the Nusselt number 25 % higher respect to the case at s/d=4. Furthermore a frequency analysis of the time dependent Nusselt number, Nui, at the i-th cylinder, shows that the main frequency is the same for all the cylinders. We found evidences that the signature of the heat transfer enhancement could be related to the phase shift between two successive cylinders (i+1- i), where the phase shift (i) is defined as the difference between the phase of each main harmonic component of the Nui respect to the phase of the signal at the first cylinder.