Structure of the thermal boundary layer for turbulent Rayleigh-Bénard convection of air in a long rectangular enclosure

Influence of Vacuum Level on Heat Transfer Characteristics of Maglev Levitation Electromagnet Module

The vacuum tube transportation (VTT) system has been a promising direction of future transportation. Within this system, a high-speed maglev travels in a low-vacuum environment to reduce aerodynamic drag. However, the heat dissipation of on-board heating devices will be compromised under low-vacuum conditions, and the device performance may thus be lowered. This study investigates the low-vacuum conjugate heat transfer characteristic of a levitation electromagnet module of a maglev using an experimentally verified numerical method. During the heating process, the surface temperature distribution of the levitation electromagnet, and the temperature and velocity characteristics of the flow field are examined. It is found that, as the vacuum level increases from 1.0 atm to 0.1 atm, the total heat dissipating from the levitation electromagnet module is decreased by 49% at 60 min, the contribution of convection heat flux over the total heat flux is decreased from 49% to 17%, and the convection heat transfer coefficient of the levitation electromagnet is decreased by 89%. This study can provide an efficient numerical model for low-vacuum heat transfer study on a VTT system as well as help the evaluation and optimization of low-vacuum maglev thermal management systems.

Temperature Fluctuation Profiles in Turbulent Thermal Convection: A Logarithmic Dependence versus a Power-Law Dependence

We report an experimental measurement of the rms temperature (σ_{T}) profiles in two regions inside a large aspect ratio (Γ=4.2) rectangular convection cell. It is found that, in the region where the boundary layer is sheared by a large-scale wind, σ_{T} has a power-law dependence on the vertical distance (z) from the plate, whereas in the region where plumes are abundant, σ_{T} has a logarithmic dependence on z. The power-law profile may be understood by balancing the inertia force and the viscous force in the equations of motion, and the logarithmic profile may be understood in terms of the balance between the buoyancy and the inertia forces. When normalized by a convective temperature scale, θ_{*}, the profiles of σ_{T} collapse onto a single curve for different values of the Rayleigh number. This shows that the convective temperature first proposed by Deardorff is the suitable temperature scale outside the thermal boundary layer for both logarithmic and power-law profiles. Our finding suggests a strong connection between plumes and the logarithmic rms temperature profile. The present Letter reveals that multiple force balance mechanisms can coexist in the bulk of highly turbulent flows.

Applicability of Taylor's hypothesis in thermally driven turbulence

In this paper, we show that, in the presence of large-scale circulation (LSC), Taylor's hypothesis can be invoked to deduce the energy spectrum in thermal convection using real-space probes, a popular experimental tool. We perform numerical simulation of turbulent convection in a cube and observe that the velocity field follows Kolmogorov's spectrum (k^-5/3). We also record the velocity time series using real-space probes near the lateral walls. The corresponding frequency spectrum exhibits Kolmogorov's spectrum (f^-5/3), thus validating Taylor's hypothesis with the steady LSC playing the role of a mean velocity field. The aforementioned findings based on real-space probes provide valuable inputs for experimental measurements used for studying the spectrum of convective turbulence.

Thickness of the diffusive sublayer in turbulent convection

In this paper we address the following question: how thick is the diffusive fraction of the thermal boundary layer in highly turbulent thermal convection? We have studied this problem in a large-scale Rayleigh-Bénard experiment at fixed aspect ratio Gamma=1.13 and variable Rayleigh number 5.2x10(10)<Ra<9.6x10(11) using air as the working fluid. By measuring profiles of the mean temperature in the vicinity of the heated bottom plate with a higher spatial resolution than in any other previous experiment and by complementing them with simultaneous independent measurements of the local heat flux, we have determined the shares of diffusive and convective vertical heat fluxes inside the thermal boundary layer. Our measurements show that the thickness of the sublayer where heat is exclusively transported by diffusion is only about 1/25 of the nominal thickness of the thermal boundary layer and depends only weakly on Ra in the parameter domain investigated here. This result implies that phenomenological theories which assume a diffusive heat transport in the whole thermal boundary layer are unlikely to be correct at very high Rayleigh numbers.

Structure of viscous boundary layers in turbulent Rayleigh-Bénard convection

Highly resolved profiles of the mean velocity in turbulent Rayleigh-Bénard convection in air are presented and discussed. The present work extends our recently performed experiments at constant aspect ratio [Phys. Rev. Lett. 99, 234504 (2007)] to variable aspect ratios. The experiments cover a range of Rayleigh numbers 10(9)<Ra<10(12) and aspect ratios 1.13<Gamma<11.3 whereas the Prandtl number is fixed at Pr=0.7. The major finding of the present work is that the profiles of the mean horizontal velocity and its fluctuations are virtually invariant against the variation in Ra or Gamma if the wall distance is scaled by the displacement thickness of the boundary layer. Furthermore we have studied typical length scales of the boundary layer and their scaling with Ra and Gamma. Regarding a potential transition of the heat transport toward the ultimate regime we found that the boundary layer Reynolds number remains below Redelta=250 which is significantly lower than the predicted limit.