Numerical study of solution structure and nonlinear behavior of Dean flow with vortex structure in a bending square duct

The convection heat transfer of a viscoelastic Giesekus fluid between a square cavity and a heated inner elliptical column is considered by the open source software OpenFOAM. When the Prandtl number is 7, physical parameters, such as Rayleigh number (103≤Ra≤107), the long semiaxis of the inner elliptical column (0.2H≤a≤0.4H), Weissenberg number (1≤Wi≤100), mobility factor (0≤α≤0.5), and the polymer viscosity ratio (0.1≤β≤0.9), are investigated. These combined effects are discussed using streamlines, thermal field distributions, and average Nusselt numbers. The numerical results are validated by the comparison with other numerical and experimental ones. The high Rayleigh number destroys the symmetry of the streamlines of the Giesekus fluid, which is a phenomenon not present in Newtonian fluids. At the same time, the high Rayleigh number enhances the flow and heat transfer while exacerbating the instability, leading to periodic oscillations in both flow and heat transfer. The variations of multi-physics fields are analyzed for the cases such as strong elasticity without shear thinning, medium elasticity with medium shear thinning, and weak elasticity with strong shear thinning effects. Stronger elasticity favors the enhancement of flow and heat transfer in viscoelastic fluids, and this effect is more significant when the Weissenberg number is small.

Numerical simulation of thermal convection of Giesekus fluid in a square cavity with the elliptical column

Zhang,

Guo,

2024

Enhancing thermal-hydraulic performance in curved pipes through optimal radial fin placement: A numerical investigation

Ghaemian,

Maghrebi

2024

Increasing heat transfer in straight pipes, particularly in applications like heat exchangers, can be achieved by incorporating fins into the pipe wall. However, in curved pipes, the presence of more intricate flows resulting from centrifugal forces can alter this effect. The current study investigates how both the height and angular position of radial fins simultaneously influence the flow patterns within curved pipes. Adjusting the placement of radial fins is identified as a cost-effective and strategic approach to improve both the hydrodynamic and thermal efficiency in curved pipe systems. The numerical analysis focuses on studying laminar, incompressible flow in curved tubes with radial fins. The mass, momentum, and energy conservation equations in toroidal coordinates were discretized with the second-order finite difference method on a staggered grid, followed by their solution through the projection algorithm. The results indicate that adapting the angular position of the fins improves the thermal-hydraulic performance by 51.8%, 48.4%, 36.3%, and 20.6% for one to four fins, respectively. These changes are closely related to the behavior of the secondary flows. Furthermore, altering the height of the fins reveals that for three fins within the tube, the most optimal fin height is half of the tube radius. In other cases, a fin height equal to 0.7 multiplied by the tube radius provides the highest performance. From the numerical results, it is found that the primary factor affecting the heat transfer rate in curved pipes is the strength of secondary motions, while the generation of friction is influenced only by the axial velocity.

Study on secondary motions in supersonic boundary layers of a bent pipe

Chen,

Liang,

Yang

et al. 2024

The present study employed direct numerical simulation to investigate the supersonic flow of Mach 3 in a bent pipe with a curvature of 0.0825, elucidating the dynamic mechanism of secondary motions within the turbulent boundary layer. The findings indicate that the compressible flow, affected by the wall curvature, is differentiated into several motion patterns as the bending angle increases: a portion of the outer fluid close to the wall, driven by the circumferential pressure gradient, moves inward through the lateral wall, causing an increase in the mass rate toward the lateral boundary layer and promoting the circumferential transport of energy and vorticity; other outer fluids at the start of the bent section, due to the centrifugal force, approach the wall to form a thinner boundary layer downstream; meanwhile, the fluid near the inner wall experiences the expansion, followed by the flow separation and reattachment at a bending angle of 14.6° and 22.0°, respectively, which induce a shear layer that develops from the inner end point toward the mainstream center, gradually reshaping the high-speed flow area within the pipe cross section into a U-shape, and enhancing the vorticity and temperature field of the inner region. Additionally, this study reveals a remarkable phenomenon that the separated flow in a localized inner region forms a rotating field, inducing vortices distinct from the mainstream Dean vortices in the low-speed flow region enclosed by the shear layer.