Coriolis and rotating buoyancy effect on detailed heat transfer distributions in a two-pass square channel roughened by 45° ribs at high rotation numbers

Heat transfer measurements from a rotating two-pass square channel with two opposite leading and trailing walls roughened by 45° parallel ribs arranged in the staggered manner are performed to examine the effects of Reynolds (Re), rotation (Ro) and buoyancy (Bu) numbers on local and area-averaged Nusselt numbers (Nu and $\overline{\mathit{Nu}}$). Full-field Nu distributions over the two rib-roughened leading and trailing walls are measured at the conditions of 4000 ? Re ? 16,000, 0 ? Ro ? 0.8 and 0.0015 ? Bu ? 0.93 (0.05 ? Δρ/ρ ? 0.1) using the infrared thermography which allows for the detailed examination of the Coriolis and rotating buoyancy effects on Nu distributions over the rotating ribbed surface. Selected heat transfer data in term of Nu ratio between rotating and stationary levels illustrates the influences of rotation on local and area-averaged heat transfer performances. Area-averaged $\overline{\mathit{Nu}}$ for the turn region and the inlet and outlet ribbed legs of the rotating two-pass channel are parametrically analyzed to devise a set of empirical heat transfer correlations that permits the evaluation of the interdependent and individual effects of Re, Ro and Bu on $\overline{\mathit{Nu}}$.     

Flow over and forced convection heat transfer around a semi-circular cylinder at incidence

Wake dynamics and forced convective heat transfer characteristics past a semi-circular cylinder at incidence have been investigated numerically. Utilizing air as an operating fluid computations are carried out for wide ranges of the Reynolds number (80 ? Re ? 180) and angle of incidences (0 ? α ? 180°). Angle of incidence reveals three flow separation zones. Structure properties of shear layer and vortex motions on each flow separation zones are analyzed critically. Functional dependence of drag (C_D), lift (C_L), and moment (C_M) coefficients on the angle of incidence is explored and analyzed in detail. Increase in angle of incidence increases streamline curvature. A structural similarity is observed between the contours of vorticity and the corresponding isotherms. Strouhal number shows a decreasing trend up to certain values of α and thereafter it increases marginally. A new correlation of Strouhal number as a function of Re and α has been established for the present range of Reynolds numbers. At the singularity points a sudden jump in local Nusselt number distribution is observed. The trend of variation of average Nusselt number with α is similar to that of Strouhal number variation. The average Nusselt number is found to vary as ${\mathit{Re}}^{0.529}(1+\alpha {)}^{-0.0476}$.     

Onset of entrainment in transitional round fountains

It is of fundamental interest to understand the behavior of transitional fountains with intermediate Froude and Reynolds numbers, together with the associated entrainment and turbulence. In this work, the transient behavior of axisymmetric fountains with 1 ? Fr ? 8 and 200 ? Re ? 800 is studied by direct numerical simulation. It is found that at Re ? 200, there is little entrainment present at the upflow–downflow interface and at the downflow–ambient interface, even for a value of Fr as high as 8; however, at Re > 200, entrainment is present at these interfaces and the extent increases with Re, which clearly demonstrates that entrainment is strongly dependent on Re whereas the contribution from the Fr effect is relatively much smaller. The DNS results also show that z_m, which is the maximum fountain penetration height, fluctuates, even when the flow reaches full development, due to the entrainment at the upflow–downflow and the downflow–ambient interfaces, and the averaged z_m scales with ${\mathit{Fr}}^{\frac{3}{2}}{\mathit{Re}}^{\frac{1}{4}}$ for 1 < Fr ? 8 and 100 ? Re ? 800.     

Drag force acting on bubbles in a subchannel of triangular array of rods

Forces acting on spherical bubbles in a subchannel of a rod bundle with triangular rod arrangement (the pitch to diameter ratio is $P/D=1.34$) have been studied at low bubble Reynolds numbers O(0.1) ? O(1). The bubble motion has been simulated resolving the interface of the bubble by using the lattice Boltzmann method. Steady drag and virtual mass forces have been determined from the simulation results. Based on the simulation data, the relation $C_D=16.375/{\mathit{Re}}_T$ could be established between the steady drag coefficient $C_D$ and the terminal Reynolds number ${\mathit{Re}}_T$ when the diameter ratio $\lambda =d/D$ of the bubble d and the channel D is less than 0.2. It is found that the virtual mass coefficient can achieve as high value as 7.2, which is a consequence of strong wall effects. Considering interactions between bubbles, cooperation in the axial direction and hindering in the lateral direction could be observed. We demonstrate that the relation between the terminal velocity of a bubble and that of the suspension follows a Richardson–Zaki like correlation, but the exponent is not only a function of the Eotvos and Morton numbers, but it also depends on the particle configuration.     

Heat transfer and pressure drop in furrowed channels with transverse and skewed sinusoidal wavy walls

This comparative study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients (f) and thermal performance factors (η) for two furrowed rectangular channels with transverse and skewed sinusoidal wavy walls. Detailed heat transfer measurements over these transverse and skewed sinusoidal wavy walls at the Reynolds numbers (Re) = 1000, 1500, 2000, 5000, 10,000, 15,000, 20,000, 25,000 and 30,000 are performed using the steady-state infrared thermo-graphic method. Impacts of Re on Nu and f for two tested furrowed channels with transverse and skewed waviness are individually examined. In addition to the macroscopic mixing between the near-wall recirculations and core flows due to the shear layer instabilities in each wavy channel, the secondary flows tripped by the skewed wall-waves further elevate heat transfer performances and distinguish their Nu distributions from those over the transverse wavy wall. The area-averaged Nusselt numbers ($\overline{\mathit{Nu}}$) for two tested furrowed channels with transverse and skewed waviness with 5000 < Re < 30000 fall, respectively, in the ranges of 3.45–3.71 and 3.98–4.2 times of the Dittus–Boelter levels. A set of $\overline{\mathit{Nu}}$ and f correlations for each tested furrowed channel is individually derived using Re as the controlling parameter. By way of comparing the thermal performance factors (η) with a selection of rib-roughened channels, the η factors for the present skewed wavy channel are compatible with those in the channel roughened by the compound V-ribs and deepened scales due to the relative low pressure drop penalties with the equivalent heat transfer augmentations to those offered by V-ribs.     

Analysis of entropy generation on mixed convection in square enclosures for various horizontal or vertical moving wall(s)

Entropy generation during the mixed convection process have been studied in a square enclosure for various moving horizontal (cases 1a–1d) or vertical wall(s) (cases 2a–2c) where the bottom wall of the cavity is isothermally hot, side walls are cold, and the top wall is adiabatic. Simulations have been performed for Prandtl number Pr = 0.026 and 7.2, Reynolds number Re = 10 − 100, and Grashof number Gr = 10³ − 10⁵. Results show that, in the case of the horizontally moving wall(s) (cases 1a–1d), the overall heat transfer rate $\left(\overline{N{u}_b}\right)$ and total entropy generation (S_total) are identical for cases 1a–1d and the cup-mixing temperature (θ_cup) is high for case 1b at Pr = 0.026, Re = 100, and Gr = 10⁵. Similarly, in the case of the vertically moving wall(s) (cases 2a–2c), $\overline{N{u}_b}$ and S_total are identical for cases 2a–2c with the maximum θ_cup occurring for the case 2a. At Pr = 7.2, Gr = 10⁵, and Re = 10, case 1a and case 1c are preferable for horizontally moving wall(s) and either of case 2a–2c is preferable for vertically moving wall(s). At Pr = 7.2, Gr = 10⁵, and Re = 100, case 1d may be preferable for the horizontally moving wall(s) and case 2a may be preferable for the vertically moving wall(s).     

Effects of rotation on the trailing vortex and heat transfer from a horizontal rotating cylinder at higher Grashof number

In the present study, effects of rotation on the trailing vortex and heat transfer from a large diameter horizontal rotating cylinder in still air have been experimentally investigated whereas earlier studies considered cylinders with smaller diameters. As the rotational speed n is increased, the trailing vortex would deflect in the rotational direction, the flow state of air near the cylinder would vary from laminar to turbulent, the heat transfer would transit from pure natural convection to mixed convection, and finally to pure forced convection resulted from rotation, the trailing vortex would break down and disappear. The variation of deflection angles ω with the parameter $R{e}_r/\sqrt{Gr}$ can be expressed as $w=32.3R{e}_r/\sqrt{Gr}$ at lower rotational speed. The discriminant to determine the disappearance of trailing vortex can be presented as Re_r²/Gr = 7.51.