Iman Ashtiani Abdi

A Comparison Between the Separated Flow Structures Near the Wake of a Bare and a Foam-Covered Circular Cylinder

The flow structures behind bare and aluminum foam-covered single circular cylinders were investigated using particle image velocimetry (PIV). The experiments are conducted for a range of Reynolds numbers from 2000 to 8000, based on the outer cylinders diameter and the air velocity upstream of the cylinder. The analysis of the PIV data shows the important effects of the foam cover and the inlet velocity on the separated structures. The results show a considerable increase in the wake size behind a foam-covered cylinder compared to that of a bare cylinder. Furthermore, the turbulence intensity is found to be around 10% higher in the case of the foam-covered cylinder where the wake size is approximately doubled for the former case compared to the latter. The turbulence kinetic energy, however, is found to be less Reynolds dependent in the case of the foam-covered cylinder. In addition, small scale structures contribute to the formation of the flow structures in the foam-covered cylinder making them a more efficient turbulent generator for the next rows when used in a heat exchanger tube bundle. On the other hand, a higher energy level in such separated structures will translate into increased pressure drop compared to bare cylinders. Finally, the results of this study can be used as an accurate set of boundary conditions for modeling the flow field past such cylinders.

Investigation of Large-Scale Structures Behind a Single Tube (Finned and Foamed Tube) Using Two-Point Correlations

Flow structures downstream of a finned-tube are compared to those of an identical pipe; with the same diameter and length, covered with a foam layer. The standard case of cross-flow over a bare tube, i.e. no surface extension, is also tested as a benchmark. Experiments are conducted in a wind tunnel at Reynolds numbers of 4000 and 16000. Particle image velocimetry (PIV) was used for flow visualization on two different perpendicular planes. To characterize the size of the flow structures downstream of the tube, for each of the aforementioned case, two-point correlation, as a statistical analysis tool, has been used. It has been observed that by decreasing the Reynolds number, the flow structures are further stretched in streamwise direction for both bare and finned-tube cases. This is, however, more pronounced with the former. Interestingly, with a foam-wrapped tube the sizes of the flow structures are found to be independent of the Reynolds number. Finally, the structure sizes are smaller in the case of the foam-wrapped tube compared to those of finned-tube.

Numerical Study of Turbulent Convective in Upward Flows of Supercritical Water in the Triangular Lattice Fuel Rod Bundle

Convection heat transfer in upward flows of supercritical water in triangular tight fuel rod bundles is numerically investigated by using the commercial CFD code, ANSYS Fluent© 14.5. The fuel rod with an inner diameter of 7.6 mm and the pitch-to-diameter ratio (P/D) of 1.14 is studied for mass flux ranging between 550 and 1050 kg/m2s and heat flux of 560 kW/m2 at pressures of 25 MPa. V2F eddy viscosity turbulence model is used and, to isolate the effect of buoyancy, constant values are used for thermo-physical properties with Boussinesq approximation for the density variation with temperature in the momentum equations. The computed Nusselt number normalized by that of the same Reynolds number with no buoyancy against the buoyancy parameter proposed by Jackson and Hall’s criterion. Mentioned results are compared with V2F turbulence model whereas strong nonmonotonic variation of the thermo-physical properties as function of temperature have been applied to the commercial CFD code using user defined function (UDF) technique. A significant decrease in Nusselt number was observed in the range of Display Formula 10 -6 Gr q / Re b 3.425 Pr b 0.8 5 × 10 -6 before entering a serious heat transfer deterioration regime. Based on an analysis of the shear-stress distribution in the turbulent boundary layer and the significant variation of the specific heat across the turbulent boundary layer, it is found that the same mechanism that leads to impairment of turbulence production in concentric annular pipes is present in triangular lattice fuel rod bundles at supercritical pressure.