scholarly journals Temperature fluctuations and heat transport in the edge regions of a tokamak

1986 ◽  
Vol 29 (1) ◽  
pp. 309 ◽  
Author(s):  
P. C. Liewer ◽  
J. M. McChesney ◽  
S. J. Zweben ◽  
R. W. Gould
Keyword(s):  
Heat Transport ◽  
Temperature Fluctuations

An Experimental Study of a Bubble-Driven Heat-Transport Device

2007 ◽  
Author(s):  
Osamu Suzuki
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Heat Transport ◽  
Tube Bundle ◽  
Temperature Fluctuations ◽  
Temperature Gradients ◽  
Heating And Cooling ◽  
Tube Bundles ◽  
Steady State Time ◽  
Transport Device

We experimentally measured the heat-transport characteristics of a bubble-driven heat-transport device. The device consisted of a non-looped copper tube containing water. The tube was either meandered or spiraled to form tube bundles. The inner surface of the tube was smooth and its diameter small enough to enable the formation of vapor and liquid plugs in it. Two copper blocks were attached to the tube bundles, one as a heating block and the other as a cooling block. In the experiment, most of the wall temperatures measured on the tube fluctuated periodically at a quasi-steady state. Time-averaged temperature gradients between the heating and cooling sections of the device were constant. By increasing heater input from 300W to 350W, the amplitude of the temperature fluctuations decreased and the temperature gradients increased significantly. This behavior was regarded as a transition to critical heat transport condition. The effective thermal conductivity of the device was proportional to the heat-transport rate but did not depend on the formation of the tube bundle and the gravity effect. The temperature fluctuations had specific peak frequencies and a positive correlation was found between the frequency and effective thermal conductivity. These experimental results strongly suggest that the main heat-transport mechanism of the investigated device is based on the oscillation-induced transport of sensible heat.

On the Effect of Electron Temperature Fluctuations on Turbulent Heat Transport in the Edge Plasma of Tokamaks

2016 ◽  
Vol 56 (6-8) ◽  
pp. 563-568 ◽  
Author(s):  
C. Baudoin ◽  
P. Tamain ◽  
G. Ciraolo ◽  
R. Futtersack ◽  
A. Gallo ◽  
...  
Keyword(s):  
Electron Temperature ◽  
Heat Transport ◽  
Temperature Fluctuations ◽  
Edge Plasma ◽  
Turbulent Heat

scholarly journals Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing

2019 ◽  
Vol 870 ◽  
pp. 519-542 ◽  
Author(s):  
Zi Li Lim ◽  
Kai Leong Chong ◽  
Guang-Yu Ding ◽  
Ke-Qing Xia
Keyword(s):  
Boundary Layer ◽  
Heat Transport ◽  
Lorentz Force ◽  
Numerical Study ◽  
Temperature Fluctuations ◽  
The Other ◽  
Maximum Heat ◽  
Transport Enhancement ◽  
Viscous Boundary ◽  
Rayleigh Benard

We present a numerical study of quasistatic magnetoconvection in a cubic Rayleigh–Bénard (RB) convection cell subjected to a vertical external magnetic field. For moderate values of the Hartmann number $Ha$ (characterising the strength of the stabilising Lorentz force), we find an enhancement of heat transport (as characterised by the Nusselt number $Nu$). Furthermore, a maximum heat transport enhancement is observed at certain optimal $Ha_{opt}$. The enhanced heat transport may be understood as a result of the increased coherence of the thermal plumes, which are elementary heat carriers of the system. To our knowledge this is the first time that a heat transfer enhancement by the stabilising Lorentz force in quasistatic magnetoconvection has been observed. We further found that the optimal enhancement may be understood in terms of the crossing of the thermal and the momentum boundary layers (BL) and the fact that temperature fluctuations are maximum near the position where the BLs cross. These findings demonstrate that the heat transport enhancement phenomenon in the quasistatic magnetoconvection system belongs to the same universality class of stabilising–destabilising (S–D) turbulent flows as the systems of confined Rayleigh–Bénard (CRB), rotating Rayleigh–Bénard (RRB) and double-diffusive convection (DDC). This is further supported by the findings that the heat transport, boundary layer ratio and temperature fluctuations in magnetoconvection at the boundary layer crossing point are similar to the other three cases. A second type of boundary layer crossing is also observed in this work. In the limit of $Re\gg Ha$, the (traditionally defined) viscous boundary $\unicode[STIX]{x1D6FF}_{v}$ is found to follow a Prandtl–Blasius-type scaling with the Reynolds number $Re$ and is independent of $Ha$. In the other limit of $Re\ll Ha$, $\unicode[STIX]{x1D6FF}_{v}$ exhibits an approximate ${\sim}Ha^{-1}$ dependence, which has been predicted for a Hartmann boundary layer. Assuming the inertial term in the momentum equation is balanced by both the viscous and Lorentz terms, we derived an expression $\unicode[STIX]{x1D6FF}_{v}=H/\sqrt{c_{1}Re^{0.72}+c_{2}Ha^{2}}$ (where $H$ is the height of the cell) for all values of $Re$ and $Ha$, which fits the obtained viscous boundary layer well.

The Wake of Two Staggered Circular Cylinders

2006 ◽  
Author(s):  
J. C. Hu ◽  
Y. Zhou ◽  
H. F. Wang
Keyword(s):  
Heat Transport ◽  
Flow Structure ◽  
Initial Conditions ◽  
Reference Signal ◽  
Temperature Fluctuations ◽  
Passive Scalar ◽  
Circular Cylinders ◽  
Incident Flow ◽  
Wire Probe ◽  
Longitudinal Distance

The work aims to study experimentally the flow structure, heat and momentum transport in the wake of two staggered circular cylinders. The cylinder center-to-center pitch ranged from 1.2 to 4.0, and the angle between the incident flow and the direction through the two cylinder axes was between 0° and 90°. In order to characterize the heat transport of this flow, both cylinders were slightly heated so that heat generated could be considered to be a passive scalar. The velocity and temperature fluctuations were measured using a movable three-wire (one X-wire plus a cold wire) probe across the wake and a fixed X-wire, which acted to provide a reference signal. Measurements were conducted at x/d = 10 and 20 and a Reynolds number of 7000, where d is the cylinder diameter and x is the longitudinal distance from the mid point between the cylinders. Based on phase-averaged vorticity contours and sectional streamlines, the flow structure behind two staggered cylinders is classified into five typical patterns. The flow behaviours for each pattern are examined in detail, including its topology, vortex strength, stability and decay rate. The different behaviours between the patterns are further linked to the different initial conditions. The momentum and heat transport are not presented due to limited space.

A Study on Heat Transport Phenomena in a Developed Thermal Boundary Layer of Drag Reducing Channel Flow

2016 ◽  
Author(s):  
K. Watanabe ◽  
Y. Kaiho ◽  
S. Hara ◽  
T. Tsukahara ◽  
Y. Kawaguchi
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Transport ◽  
Thermal Boundary Layer ◽  
Transport Phenomena ◽  
Surfactant Solution ◽  
Temperature Fluctuations ◽  
Large Temperature ◽  
Wall Region ◽  
Solution Flow

The heat transport phenomena in a developed thermal boundary layer of surfactant solution flow were investigated experimentally. The experiment was conducted under different surfactant additive concentrations. The temperature fluctuations in a thermal boundary layer in a smooth channel flow were measured by fine-wire thermocouple probe. Heat transfer reducing rate and temperature fluctuation characteristics including mean temperature distribution, intensity, wave form, spectral density function, and skewness factor were studied. The results showed that the turbulent transport is obstructed by additives, and the temperature field shows dramatic changes. High frequency component of temperature fluctuation of surfactant solution flow was decreased due to suppression of turbulence and viscoelasticity. Large temperature fluctuations occur in the thermal boundary layer because the development of the thermal boundary layer is obstructed, and large temperature fluctuations appear to concentrate the temperature gradient in the near-wall region (10 < y+ < 60). In contrast, viscous sublayer expands due to viscoelasticity, and the temperature gradient and turbulence fluctuation are small in the near-wall region of y+ < 10. As a result, two layers having significantly different characteristics seem to coexist. The heat transfer reduction is constant with variation of additive concentration condition, but heat transport phenomena were microscopically influenced by viscoelasticity.

scholarly journals Experimental investigation of heat transport in homogeneous bubbly flow

2018 ◽  
Vol 845 ◽  
pp. 226-244 ◽  
Author(s):  
Biljana Gvozdić ◽  
Elise Alméras ◽  
Varghese Mathai ◽  
Xiaojue Zhu ◽  
Dennis P. M. van Gils ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Heat Transport ◽  
Single Phase ◽  
Volume Fraction ◽  
Spectral Power ◽  
Bubbly Flow ◽  
Temperature Fluctuations ◽  
Wide Range ◽  
Gas Volume Fraction ◽  
Gas Volume

We present results on the global and local characterisation of heat transport in homogeneous bubbly flow. Experimental measurements were performed with and without the injection of ${\sim}2.5~\text{mm}$ diameter bubbles (corresponding to bubble Reynolds number $Re_{b}\approx 600$) in a rectangular water column heated from one side and cooled from the other. The gas volume fraction $\unicode[STIX]{x1D6FC}$ was varied in the range 0 %–5 %, and the Rayleigh number $Ra_{H}$ in the range $4.0\times 10^{9}{-}1.2\times 10^{11}$. We find that the global heat transfer is enhanced up to 20 times due to bubble injection. Interestingly, for bubbly flow, for our lowest concentration $\unicode[STIX]{x1D6FC}=0.5\,\%$ onwards, the Nusselt number $\overline{Nu}$ is nearly independent of $Ra_{H}$, and depends solely on the gas volume fraction $\unicode[STIX]{x1D6FC}$. We observe the scaling $\overline{Nu}\,\propto \,\unicode[STIX]{x1D6FC}^{0.45}$, which is suggestive of a diffusive transport mechanism, as found by Alméras et al. (J. Fluid Mech., vol. 776, 2015, pp. 458–474). Through local temperature measurements, we show that the bubbles induce a huge increase in the strength of liquid temperature fluctuations, e.g. by a factor of 200 for $\unicode[STIX]{x1D6FC}=0.9\,\%$. Further, we compare the power spectra of the temperature fluctuations for the single- and two-phase cases. In the single-phase cases, most of the spectral power of the temperature fluctuations is concentrated in the large-scale rolls/motions. However, with the injection of bubbles, we observe intense fluctuations over a wide range of scales, extending up to very high frequencies. Thus, while in the single-phase flow the thermal boundary layers control the heat transport, once the bubbles are injected, the bubble-induced liquid agitation governs the process from a very small bubble concentration onwards. Our findings demonstrate that the mixing induced by high Reynolds number bubbles ($Re_{b}\approx 600$) offers a powerful mechanism for heat transport enhancement in natural convection systems.

Core temperature fluctuations and related heat transport in the Texas Experimental Tokamak‐Upgrade

1995 ◽  
Vol 2 (3) ◽  
pp. 720-726 ◽  
Author(s):  
G. Cima ◽  
R. V. Bravenec ◽  
A. J. Wootton ◽  
T. D. Rempel ◽  
R. F. Gandy ◽  
...  
Keyword(s):  
Heat Transport ◽  
Core Temperature ◽  
Temperature Fluctuations

Convectively driven superfluid turbulence in dilute solutions of 3He in superfluid 4He

1987 ◽  
Vol 65 (11) ◽  
pp. 1322-1327 ◽  
Author(s):  
Robert E. Ecke ◽  
Hans Haucke ◽  
John Wheatley
Keyword(s):  
Heat Transport ◽  
Quantitative Estimate ◽  
Temperature Fluctuations ◽  
Superfluid 4He ◽  
Dilute Solution ◽  
Superfluid Turbulence ◽  
Normal Fluid ◽  
Vortex Lines ◽  
Critical Velocities ◽  
Good Agreement

A dilute solution of 3He in superfluid 4He usually behaves as a single-component classical fluid in the context of thermal convection. However, certain convective states can be excited that do not seem to exist in classical convection. These states are characterized by noisy temperature fluctuations and a pronounced decrease in heat transport relative to the classical convecting states. Critical convective-flow fields are observed analogous to critical velocities for superfluid turbulence in pipes. The magnitudes of the average critical velocities for these two types of superfluid turbulence are in good agreement. Also, a quantitative estimate of energy dissipation due to the interaction of normal fluid and quantized vortex lines is consistent with the large decrease in heat transport for the turbulent states. These states are identified as states of convectively driven superfluid turbulence.

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