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

Turbulence properties in jets with fractal grid turbulence

We carry out high-resolution particle image velocimetry experiments to characterize the flow field of fractal grids located at the exit section of a turbulent round jet. We comment on the mean flow organization and on the turbulence properties of such jets by comparing the results with those obtained with square grids, a regular grid (having the same effective mesh length) and a jet without turbulator. We find that, different from the case of decaying grid turbulence, a correction must be accounted for to properly scale the turbulence intensity profiles with a length scale based on grid parameters. We perform a low-order reconstruction of the velocity field based on the most energetic proper orthogonal decomposition modes and we compare the flow-field structure produced in the lee of fractal grids with a single square object and the jet without turbulator. The typical turbulence intensity profile detailed in Cafiero et al. (Phys. Fluids, vol. 27, 2015, 115103) for jets with fractal grids is produced by the interaction of small eddies shed by the central grid item. In the single square grid case, the turbulence is built upon the interaction between larger structures. Conversely, the interaction of the outward spreading wake with the external shear layer produces pairs of vortical structures, which we relate to the higher entrainment rate featured by jets with fractal turbulators. The secondary grid iterations have a disruptive effect on the turbulence transport, with a corresponding large correlation between the velocity fluctuations at the jet core with those at the jet shear layer

Blob-enhanced reconstruction technique

A method to enhance the quality of the tomographic reconstruction and, consequently, the 3D velocity measurement accuracy, is presented. The technique is based on integrating information on the objects to be reconstructed within the algebraic reconstruction process. A first guess intensity distribution is produced with a standard algebraic method, then the distribution is rebuilt as a sum of Gaussian blobs, based on location, intensity and size of agglomerates of light intensity surrounding local maxima. The blobs substitution regularizes the particle shape allowing a reduction of the particles discretization errors and of their elongation in the depth direction. The performances of the blob-enhanced reconstruction technique (BERT) are assessed with a 3D synthetic experiment. The results have been compared with those obtained by applying the standard camera simultaneous multiplicative reconstruction technique (CSMART) to the same volume. Several blob-enhanced reconstruction processes, both substituting the blobs at the end of the CSMART algorithm and during the iterations (i.e. using the blob-enhanced reconstruction as predictor for the following iterations), have been tested. The results confirm the enhancement in the velocity measurements accuracy, demonstrating a reduction of the bias error due to the ghost particles. The improvement is more remarkable at the largest tested seeding densities. Additionally, using the blobs distributions as a predictor enables further improvement of the convergence of the reconstruction algorithm, with the improvement being more considerable when substituting the blobs more than once during the process. The BERT process is also applied to multi resolution (MR) CSMART reconstructions, permitting simultaneously to achieve remarkable improvements in the flow field measurements and to benefit from the reduction in computational time due to the MR approach. Finally, BERT is also tested on experimental data, obtaining an increase of the signal-to-noise ratio in the reconstructed flow field and a higher value of the correlation factor in the velocity measurements with respect to the volume to which the particles are not replaced

Effect of the grid geometry on the convective heat transfer of impinging jets

Passive methods are recognized as one of the most efficient means to achieve high heat and mass transfer in impinging jets. In a recent study, Cafiero et al. (2014) demonstrated the effectiveness of square fractal grids (SFGs, obtained repeating the same square pattern at increasingly smaller scales) in terms of heat transfer enhancement when locating the grid in correspondence of the nozzle exit section. Indeed, the capability of producing turbulence at multiple scales and the possibility of tuning the peak in the turbulence intensity profile as a function of the grid geometric parameters are both extremely appealing for heat transfer enhancement purposes. In this study, the effect of the grid geometry on the convective heat transfer rate of impinging jets is assessed and discussed. Three main effects are taken into account: the grid thickness ratio (obtained by varying the thickness of the first iteration of the SFG), the effect of the secondary grid iterations and the choice of the initial pattern. It is demonstrated how a larger thickness ratio, which in the present case corresponds to an anticipated location of the peak in the turbulence intensity profile, is beneficial to get a spotted high convective heat transfer rate at short nozzle to plate distances. Either the use of a single square grid, or the choice of a different initial pattern (for example a circular fractal grid) is instead indicated when it is desirable a uniform distribution of the convective heat transfer rate

Investigation of impinging single and twin circular synthetic jets flow field

Synthetic jets are largely used in the electronic cooling field; indeed their heat transfer performances have been widely investigated. The heat transfer performances have been enhanced through the design of innovative synthetic jet devices, as the twin synthetic jets device. Obviously the heat transfer performances of the classic and innovative synthetic jet devices are strictly related to their impinging flow field. Therefore the behavior of impinging single and twin circular synthetic jets in phase opposition is experimentally investigated by using Particle Image Velocimetry (PIV) at Reynolds and Strouhal numbers equal to 5100 and 0.024, respectively. Several nozzle-to-plate distances (H), ranging between 2 and 10 nozzle diameters (D), have been investigated. The time-averaged behavior of the velocity components has been reported and discussed. Their distributions, near the impinging plate, have been described. For the single jet, at short nozzle-to-plate distances (H/D6), the axial velocity profile is bell-shaped. This is ascribed to the adverse pressure gradient strength and the potential core-like region extension. External oscillations are observed in all the flow field quantities near the impinging plate at 2 diameters from the stagnation point due to a secondary counter rotating vortex ring generation. The presence of such a counter rotating vortex ring decreases as the nozzle-to-plate distance increases. Comparing the two synthetic jet configurations, higher axial velocity and turbulence level but lower axial phase-correlated organized contribution to velocity have been found for the twin case because of the jets interaction. The evolution of the flow field for both configurations has been explained through phase-averaged measurements. High turbulence is observed along the shear layer emanated by the nozzle edge and in the vortex ring core. During the suction phase the saddle point shows a different behavior in the two configurations. In the single case, the saddle point reaches the impinging plate causing injection of air from the plate into the device. Differently the twin configuration generates two saddle points which do not reach the impinging plate because of the presence of the other impinging synthetic jet

Impinging Single and Twin Circular Synthetic Jets Flow Field

The behavior of single and twin circular synthetic jets devices is experimentally investigated by using Particle Image Velocimetry (PIV) at a Reynolds number equal to 5,100 and a Strouhal number equal to 0.024. The twin synthetic jets are in phase opposition and different inter-axes distances (l) have been studied. Moreover, several nozzle-to-plate distances (H/D=2, 4, 6, 8 and 10) have been investigated. The twin synthetic jets show an interaction which causes higher time-averaged axial velocities and fluctuations than the single synthetic jet case and lower jet width. The time-averaged turbulent fluctuations show that both the single synthetic jet and the twin synthetic jets have a region characterized by low values of turbulence (potential core-like region). The evolution of the mean and statistics quantities have been described through phase-averaged measurements. High turbulence is observed along the shear layer emanated by the nozzle edge and in the vortex ring core. Also the saddle point behavior has been investigated