Dependence Of Heat Transport Research Articles

Effect of vibrations on thermal convection in thick rotating annulus

The effect of transverse vibrations on thermal convection in a rotating thick cylindrical fluid layer is investigated experimentally. The layer rotates about the horizontal axis of symmetry. The temperatures of the layer boundaries are different (the outer boundary is cold) and maintained constant. The study is limited to the case of fast rotation. The centrifugal force of inertia plays a stabilizing role, bringing the fluid into a state of a stable mechanical equilibrium. The vibrations affect the equilibrium of the layer in a narrow frequency range close to the rotation frequency. The structure of the convective flows is studied using PIV-method. It is found that when the frequencies of vibrations and rotation definitely coincide, the convective flow has a form of the couple of symmetric two-dimensional vortices, the position of which is stationary in the cavity reference frame. The convection occurs under the action of an induced inertial force field (superposition of vibrational and centrifugal inertial force fields). With a frequency mismatch, the induced force field rotates in the cavity reference frame. The maximum heat transport corresponds to a resonant excitation of the azimuthal two-dimensional inertial oscillations of the non-isothermal fluid layer. The convective heat transport in this case is much higher than in the case of the frequencies coincidence. The dependence of heat transport, both under resonance conditions and with equal frequencies, on vibration parameters is studied. It is shown that the centrifugal and vibrational mechanisms play a key role in the development of the convection.

Dependence of Heat Transport in Solids on Length-Scale, Pressure, and Temperature: Implications for Mechanisms and Thermodynamics.

Accurate laser-flash measurements of thermal diffusivity (D) of diverse bulk solids at moderate temperature (T), with thickness L of ~0.03 to 10 mm, reveal that D(T) = D∞(T)[1 − exp(−bL)]. When L is several mm, D∞(T) = FT−G + HT, where F is constant, G is ~1 or 0, and H (for insulators) is ~0.001. The attenuation parameter b = 6.19D∞−0.477 at 298 K for electrical insulators, elements, and alloys. Dimensional analysis confirms that D → 0 as L → 0, which is consistent with heat diffusion, requiring a medium. Thermal conductivity (κ) behaves similarly, being proportional to D. Attenuation describing heat conduction signifies that light is the diffusing entity in solids. A radiative transfer model with 1 free parameter that represents a simplified absorption coefficient describes the complex form for κ(T) of solids, including its strong peak at cryogenic temperatures. Three parameters describe κ with a secondary peak and/or a high-T increase. The strong length dependence and experimental difficulties in diamond anvil studies have yielded problematic transport properties. Reliable low-pressure data on diverse thick samples reveal a new thermodynamic formula for specific heat (∂ln(cP)/∂P = −linear compressibility), which leads to ∂ln(κ)/∂P = linear compressibility + ∂lnα/∂P, where α is thermal expansivity. These formulae support that heat conduction in solids equals diffusion of light down the thermal gradient, since changing P alters the space occupied by matter, but not by light.

Convection of a heat-generating fluid in a rotating cylindrical cavity subject to transverse vibrations

abstract The goal of this research is to study experimentally and theoretically the influence of transversal vibrations on the average convection of a non-isothermal fluid in a rotating cavity when the rotational frequency and vibration frequency approach each other. The research involves experimental studies of the convection of a heat-generating fluid in a rotating horizontal cylinder with isothermal boundaries subject to translational vibrations, perpendicular to the rotation axis. In the absence of vibrations, at fast cavity rotation the fluid is in an equilibrium state in the centrifugal force field with the temperature maximum at the cavity axis. The excitation of average convection and increase of heat transport is discovered when the vibration frequency is near to the rotation frequency. At some discrepancy between the frequencies, the resonance excitation of azimuthal two-dimensional inertial oscillations of the non-isothermal fluid column takes place. This results in significant decrease of the temperature at the cavity axis. When the frequencies coincide, the vibrations break the axial symmetry of the centrifugal force field. The resulting force field is stationary in the cavity frame and results in an intensive thermal convection; the theoretical analysis of this phenomenon is presented. The dependence of heat transport on the parameters of vibrations is determined both in the case of resonance liquid oscillations and in the case of equal frequencies. The vibrational and the centrifugal mechanisms play the key role in the convection. The transverse vibrations are demonstrated to be an efficient tool for the control of thermal convection in rotating systems.

Effects of toroidal rotation on electron heat transport via changes in inertial force and impurity density

Two types of JT-60U discharges are studied with an emphasis on toroidal rotation: in one discharge, which is characterized by the existence of an internal transport barrier (ITB), electron heat transport in the core region is affected by the toroidal rotation direction, while in the other discharge, which is a conventional H-mode plasma without an ITB, the clear correlation between the toroidal rotation direction and electron heat transport is not observed. In both discharges, the impurity density is also found to vary together with the rotation velocity profile. With a flux-tube gyrokinetic code, we have found that the effects of the changes in the rotation velocity profile and the impurity density on electron heat transport are different between these discharges. Including the effects explains the tendency observed in the experiments. First, regarding the rotation velocity profile, which influences heat transport through the inertial force, the dependence of heat transport on the rotation direction changes, according to the gradient of the rotation velocity. Next, an increase in the impurity density stabilizes the ion temperature gradient mode, but can destabilize the trapped electron mode. Therefore, it is found that the difference in the impact of the impurity density on electron heat transport in these discharges can be attributed to the difference in the dominant instability.

Hydrogen isotope effects on ITG scale length, pedestal and confinement in JT-60 H-mode plasmas

The dependence of heat transport, edge pedestal and confinement on isotopic composition was investigated in conventional H-mode plasmas. Identical profiles for the electron density, electron temperature and ion temperature were obtained for hydrogen and deuterium plasmas, whereas the required power clearly increased for hydrogen, which resulted in a reduction in heat diffusivity for deuterium. The determination of identical temperature profiles, despite the different heating power, suggested that the characteristics of heat conduction essentially differ for hydrogen and deuterium, even at the same scale length of temperature gradient. The self-regulating physics mechanism determining the overall H-mode confinement was also addressed. The inverse of the ion temperature gradient (ITG) scale length, or ∇Ti/Ti, which is required for a given ion heat diffusivity, increased by a factor of approximately 1.2 for deuterium compared with that for hydrogen. The relationship between edge pedestal pressure and global βp holds true consistently regardless of the difference in the isotopic composition. A higher value of βp was obtained for deuterium because of its smaller ITG scale length and because of the additional stored energy in the thermal and fast ion components, the latter due to an increase in the slowing down time with an increase in isotopic mass.

Dependence of heat transport on toroidal rotation in conventional H-modes in JT-60U

The relation between heat transport in the plasma core and the toroidal rotation profile was examined in conventional ELMy H-mode plasmas in JT-60U. Heat transport in the plasma core is not strongly influenced locally by toroidal rotation. The large increase in heat conduction imposes a resilient profile of ion temperature, under which the local effect of the toroidal rotation profile on scale length of the ion temperature gradient is very weak. Energy confinement improvement is observed with toroidal rotation which increases in the co-direction with respect to the plasma current. In this case, the increase in the ion temperature in both the plasma core and the pedestal region is observed. However, heat transport in the plasma core varies while maintaining self-similar temperature profiles as the toroidal rotation profile is varied. Pressure at the H-mode pedestal becomes slightly larger with the toroidal rotation in the co-direction. Thus, energy confinement enhanced with co-toroidal rotation is determined by increased pedestal height and reduced transport brought on by profile resilience. When pedestal temperature was fixed between the cases of co- and counter-NBI by adjusting the plasma density, identical temperature profiles were obtained in spite of totally different toroidal rotation profiles.

Aspect-ratio dependence of heat transport by turbulent Rayleigh–Bénard convection

We have studied the heat transport by steady circulating flows in two-dimensional rectangular cells of different values of the aspect ratio Γ. The cells are heated from below and cooled on top with a fixed temperature difference. These steady circulating flows mimic the mean large-scale circulating wind observed in turbulent Rayleigh–Bénard convection. When the heat transport is dominated by that carried by the mean large-scale wind and when the mean circulating wind is a single roll, we have found the following dependence of the Nusselt number (Nu) and the Reynolds number (Re) on the Rayleigh number (Ra), the Prandtl number (Pr) and the aspect ratio Γ: Nu = B(Pr) N 0 (Γ)Ra 1/4 and Re = C(Pr) R 0 (Γ)Ra 1/2, where N 0(Γ) decreases and R 0(Γ) increases with Γ.

Global-Scale Water Masses, Meridional Circulation, and Heat Transport Simulated with a Global Isopycnal Ocean Model*

Abstract The goal of this study is to fill the gap in equilibrium solutions of the global-scale water masses, meridional circulation, and heat transport with isopycnal ocean models. To this end, a global isopycnal ocean general circulation model is described and used in the present study. A distinguishing feature of the isopycnal model is that it is formulated in the same parameter space, boundary conditions, and model configurations except the vertical coordinates are those of the GFDL model used by Danabasoglu et al. with an isopycnal-depth diffusion parameterization of eddy-induced tracer transport. With the global isopycnal model, the author repeated Danabasoglu et al.’s coarse-resolution simulation for a direct comparison. For the same parameter values (except lateral viscosity), the isopycnal model is able to produce a larger northward heat transport in the Northern Hemisphere, and a better latitudinal dependence of heat transport in the Southern Hemisphere, than the GFDL model. The model is also ab...

A Numerical Study in Three Space Dimensions of Bénard Convection in a Rotating Fluid

Abstract The primitive, nonlinear, Boussinesq equations of motion, continuity and thermodynamic energy are integrated numerically in three space dimensions and time to study convection driven by unstable vertical density gradients and subject to Coriolis forces. Parameter values are chosen to permit quantitative comparison with data from laboratory experiments for rotating Benard convection in water. The model realistically simulates the structure of the convection cells, their horizontal scale, and the mean vertical heat transport. The experimentally observed phenomenon of a non-monotone dependence of heat transport on rotation rate is reproduced and shown to be a consequence of the rotational constraint on the wavelength of the cells.