"Fractal generated turbulence in round jets: flow topology and heat transfer"

Speaker: Tommaso Astarita, University of Naples Federico II, Italy

Numerous studies on fractal generated turbulence in free shear flows have demonstrated outstanding capabilities of such grids in producing a tunable turbulence intensity profile, depending only on the grid geometrical parameters. Indeed, these grids lead to a more elongated turbulence production region, a local peak and a fast decay.
Furthermore, the local Reynolds number value that characterizes the decay region is sensitively larger than that characteristic of regular grids.
This paved the way to a variegate portfolio of possible applications, such as to exploit fractal generated turbulence for convective heat transfer enhancement in impinging jets. As demonstrated via targeted IR Thermography experiments, the introduction of these grids is indeed capable of producing a noticeable increment of the convective heat transfer rate, especially at moderately short nozzle to plate distances, with respect to regular grids and jet without turbulence promoter. The Tomographic PIV investigation of the flow field shows that, in addition to the higher turbulence intensity level, this increment is addressed to the capability of producing streamwise vorticity, which sensitively enhances the jet entrainment rate, thus leading to a more efficient convective heat transfer. On the downside, in the impingement region, the presence of counter-rotating wall vortices is responsible for a local upwash of the wall jet, thus leading to a lower uniformity in the convective heat transfer distribution.