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 $\overline{\mathit{Nu}}$.     

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.     

Heat transfer of a radially rotating furrowed channel with two opposite skewed sinusoidal wavy walls

An experimental study of heat transfer in a radially rotating furrowed channel with two opposite walls enhanced by skewed sinusoidal waves was performed to generate the full-field Nusselt number (Nu) data over two wavy walls. Although the static wavy channel has been proven as an effective heat transfer enhancement (HTE) measure, no previous study examined its heat transfer performances with rotation. As another first-time attempt for turbine cooling researches, the Nu scans over the entire rotational leading (stable) and trailing (unstable) walls were acquired using the infra-red thermography which proved highly advantageous due to its capability to examine the rotating buoyancy effects in details. A selection of experimental data illustrates the full-field Nu variations responding to the changes of Reynolds (Re), rotation (Ro) and buoyancy (Bu) numbers. Parametric analysis is subsequently followed to disclose the individual and interdependent Re, Ro and Bu effects on Nu in the attempt to derive the heat transfer correlations for the area-averaged Nu over the developed flow region $\left({\overline{Nu}}_{\text{FD}}\right)$ on the rotational leading and trailing wavy walls. Within the parametric ranges tested, the rotational leading and trailing ${\overline{Nu}}_{\text{FD}}$ values respectively fall between 3.4–4.3 and 4.2–6.4 times of the Dittus–Boelter datum, which grant the potential applicability of wavy channel as a HTE measure for cooling of gas turbine rotor blades.     

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 ${\mathit{Re}}^{0.529}(1+\alpha {)}^{-0.0476}$.     

High-pressure burning velocities measurements for centrally-ignited premixed methane/air flames interacting with intense near-isotropic turbulence at constant Reynolds numbers

This paper measures high-pressure turbulent burning velocities (S_T) of lean methane spherical flames at constant turbulent Reynolds numbers (Re_T ≡ u′L_I/ν), where u′ and L_I are the r.m.s. turbulent fluctuation velocity and the integral length scale of turbulence and ν is the kinematic viscosity of reactants. This is achieved by adopting a recently-built double-chamber, fan-stirred cruciform burner with perforated plates that can be used to generate intense near-isotropic turbulence with negligible mean velocities while controlling the product of u′L_I in proportion to the decreasing ν at elevated pressure (p) up to 1.2 MPa. Results show that when Re_T is fixed ranging from 6700 to 14,200, values of S_T decrease similarly as laminar burning velocities (S_L) with increasing p in minus exponential manners, revealing a global response of burning velocities to pressure. In general, the higher Re_T, the higher S_T/S_L at any fixed p. It is found that the curves of S_T/S_L as a function of u′/S_L all exhibit very strong bending under constant Re_T conditions. These results not only reveal that the important effect of Re_T on high-pressure S_T/S_L enhancement, but also suggest that recent findings related with the promotion effect of increasing pressure on S_T primarily due to the enhancement of flame instabilities via the thinner flame without any discussion on the influence of Re_T elevation at elevated pressure should be reconsidered. Moreover, we found that the modified values of S_T at mean progress variable $\overline{c}$ ≈ 0.5 show good agreements between Bunsen-type and spherical flames, suggesting that S_T determined at flame surfaces with $\overline{c}$ = 0.5 may be a better representative of itself regardless of the flame geometries. Finally, various general correlations of $S_{\text{T,}\overline{c}=0.5}$ are compared and discussed. It is found that the present scattering data under different p and Re_T conditions can be merged onto a single curve of ($S_{\text{T,}\overline{c}=0.5}$ ? S_L)/u′ = 0.14Da^0.47, where Da is the turbulent Damköhler number.