Effects of the secondary iterations of square fractal grids on continuous and synthetic jets

The Nile river, the Italian coastline, the snowflakes, the tree branches, mountains, clouds, seashells all belong to a class of objects known as fractals. Fractals are neverending patterns, widely diffused in nature, which repeat themselves over and over, at different length scales. Abstract fractals can be also generated by a computer calculating a simple equation over and over. Fractals have more and more applications in science modelling, as they very often describe the real world better than traditional mathematics and physics. Moreover, fractal-shaped elements have been successfully applied to many industrial fields such as the mixing, aeronautical, automotive, power generation and wind energy industries. Among the other, Cafiero et al. (2014, 2015, 2016, 2017) proposed the use of fractal grids to increase the mixing and heat transfer features of turbulent jets, typically used for cooling of turbine blades and of electronic components, for paper and film drying, for glass annealing and tempering, etc. Their results showed that fractal turbulators significantly increase the heat transfer properties of round turbulent jets or jet equipped with regular grids having the same blockage ratio of the fractal ones (Cafiero et al.,2014). Subsequent 2D and 3D flow fields measurements (Cafiero et al., 2014; Cafiero et al., 2015) showed that the main factors which are responsible for this increase are the higher turbulence intensity levels due to the fractal pattern and the ability of the fractal turbulator in producing streamwise vorticity. Following these works, this thesis focuses on the effects of the secondary iterations bars on turbulent jets equipped with square-fractal inserts. The obtained results are organised in four different chapters. In Chapter 4, the effects due to the introduction of the secondary iterations on continuous turbulent jets equipped with square-fractal inserts are analysed by introducing either a single square grid or a 3-iterations square-fractal grid at the exit section of short pipe nozzle. Measurements are carried out at fixed Reynolds number, Re≈6,700, by means of planar Particle Image Velocimetry. A double-camera configuration is used to simultaneously analyse the effects of the grids’ bars and the interaction between the grid-generated turbulence and the jet shear layer. The flow fields are investigated both in terms of first and second order statistics. In order to assess the effects of the smallest bars of the fractal insert on the wake generated by the largest bars, the Proper Orthogonal Decomposition snapshot method is applied to a small measurement volume past the 1st iteration bars. In Chapter 5, the same single square grid and fractal square grid analysed in Chapter 4 are inserted at the exit section of a synthetic jet device. Measurements are carried out, for three different device actuation frequencies, at a fixed Reynolds number Re≈6,700. The results are investigated both in terms of time-averaged and phase-averaged flow fields. The generation and the evolution of the coherent vortical structures are analysed by means of the Q-criterion. The effects of the secondary iterations introduction and of their thickness in turbulent continuous jets equipped with fractal-square grids are discussed both in terms of heat transfer properties (Chapter 6) and flow field measurements (Chapter 7). In particular, in Chapter 6, IR Thermography measurements, combined with the heated-thin-foil heat flux sensor, are performed to compare, under the same power input, several fractal and single square grids in terms of both spatial averaged and local convective heat transfer. Moreover, the effect of the grid geometry onto the convective heat transfer uniformity is investigated. Finally, the jet flow developed downstream of each of the grids analysed in Chapter 6 is investigated, at a fixed Reynolds number Re≈16,000. The jet flow due to the introduction of a regular grid characterised by the same blockage ratio of the fractal one is also investigated. Finally, the axisymmetric turbulent jet evaluated at the same Reynolds number is reported as reference. Results are analysed both in terms of first and second order statistics. Moreover, the effects of the grid geometry on the large-scale isotropy and on the spatial-averaged velocity power spectra are discussed.