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Jianshu Cao - One of the best experts on this subject based on the ideXlab platform.
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a nonequilibrium variational polaron theory to study quantum Heat Transport
arXiv: Chemical Physics, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state (NESS) with an effective temperature, to study quantum Heat Transport at the nanoscale. By combining the variational polaron transformed master equation with the full counting statistics, we have extended the applicability of the polaron-based framework to study nonequilibrium process beyond the super-Ohmic bath models. Previously, the polaron-based framework for quantum Heat Transport reduces exactly to the non-interacting blip approximation (NIBA) formalism for Ohmic bath models due to the issue of the infrared divergence associated with the full polaron transformation. The nonequilibrium variational method allows us to appropriately treat the infrared divergence in the low-frequency bath modes and explicitly include cross-bath correlation effects. These improvements provide more accurate calculation of Heat current than the NIBA formalism for Ohmic bath models. We illustrate the aforementioned improvements with the nonequilibrium spin-boson model in this work and quantitatively demonstrate the cross-bath correlation, current turnover, and rectification effects in quantum Heat transfer.
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a nonequilibrium variational polaron theory to study quantum Heat Transport
Journal of Physical Chemistry C, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state with an effective temperature, to study quantum Heat Transport at the nanoscale. B...
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a nonequilibrium variational polaron theory to study quantum Heat Transport
The Journal of Physical Chemistry, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state with an effective temperature, to study quantum Heat Transport at the nanoscale. By combining the variational polaron transformed master equation with the full counting statistics, we extended the applicability of the polaron-based framework to study nonequilibrium process beyond the super-Ohmic bath models. Previously, the polaron-based framework for quantum Heat Transport reduces exactly to the non-interacting blip approximation (NIBA) formalism for Ohmic bath models due to the issue of the infrared divergence associated with the full polaron transformation. The nonequilibrium variational method allows us to appropriately treat the infrared divergence in the low-frequency bath modes and explicitly include cross-bath correlation effects. These improvements provide more accurate calculation of Heat current than the NIBA formalism for Ohmic bath models. We illustrate the aforementioned improvements with the nonequilibrium spin-boson model in this work and quantitatively demonstrate the cross-bath correlation, current turnover, and rectification effects in quantum Heat transfer.
Chao Sun - One of the best experts on this subject based on the ideXlab platform.
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Experimental investigation of Heat Transport in inhomogeneous bubbly flow
Chemical Engineering Science, 2019Co-Authors: Biljana Gvozdić, Detlef Lohse, On Yu Dung, Elise Alméras, Dennis P. M. Van Gils, Sander G. Huisman, Chao SunAbstract:In this work we study the Heat Transport in inhomogeneous bubbly flow. The experiments were performed in a rectangular bubble column Heated from one side wall and cooled from the other, with millimetric bubbles introduced through one half of the injection section (close to the hot wall or close to the cold wall). We characterise the global Heat Transport while varying two parameters: the gas volume fraction , and the Rayleigh number . As captured by imaging and characterised using Laser Doppler Anemometry (LDA), different flow regimes occur with increasing gas flow rates. In the generated inhomogeneous bubbly flow there are three main contributions to the mixing: (i) Transport by the buoyancy driven recirculation, (ii) bubble induced turbulence (BIT) and (iii) shear-induced turbulence (SIT). The strength of these contributions and their interplay depends on the gas volume fraction which is reflected in the measured Heat Transport enhancement. We compare our results with the findings for Heat Transport in homogeneous bubbly flow from Gvozdić et al. (2018). We find that for the lower gas volume fractions (), inhomogeneous bubbly injection results in better Heat Transport due to induced large-scale circulation. In contrast, for , when the contribution of SIT becomes stronger, but so does the competition between all three contributions, the homogeneous injection is more efficient.
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experimental investigation of Heat Transport in homogeneous bubbly flow
Journal of Fluid Mechanics, 2018Co-Authors: Biljana Gvozdic, Elise Alméras, Varghese Mathai, Dennis P. M. Van Gils, Roberto Verzicco, Sander G. Huisman, Chao Sun, X W Zhu, Detlef LohseAbstract: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 2:5 mm diameter bubbles (corresponding to bubble Reynolds number Reb 600) in a rectangular water column Heated from one side and cooled from the other. The gas volume fraction was varied in the range 0 %–5 %, and the Rayleigh number RaH in the range 4:0109–1:21011. 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 D0:5% onwards, the Nusselt number Nu is nearly independent of RaH, and depends solely on the gas volume fraction . We observe the scaling Nu / 0:45, which is suggestive of a diffusive Transport mechanism, as found by Almeras 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 D 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 (Reb 600) offers a powerful mechanism for Heat Transport enhancement in natural convection systems.
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controlling Heat Transport and flow structures in thermal turbulence using ratchet surfaces
Physical Review Letters, 2018Co-Authors: H Jiang, Detlef Lohse, Varghese Mathai, Roberto Verzicco, Chao Sun, Xiaojue ZhuAbstract:In this combined experimental and numerical study on thermally driven turbulence in a rectangular cell, the global Heat Transport and the coherent flow structures are controlled with an asymmetric ratchetlike roughness on the top and bottom plates. We show that, by means of symmetry breaking due to the presence of the ratchet structures on the conducting plates, the orientation of the large scale circulation roll (LSCR) can be locked to a preferred direction even when the cell is perfectly leveled out. By introducing a small tilt to the system, we show that the LSCR orientation can be tuned and controlled. The two different orientations of LSCR give two quite different Heat Transport efficiencies, indicating that Heat Transport is sensitive to the LSCR direction over the asymmetric roughness structure. Through a quantitative analysis of the dynamics of thermal plume emissions and the orientation of the LSCR over the asymmetric structure, we provide a physical explanation for these findings. The current work has important implications for passive and active flow control in engineering, biofluid dynamics, and geophysical flows.
Changyu Hsieh - One of the best experts on this subject based on the ideXlab platform.
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a nonequilibrium variational polaron theory to study quantum Heat Transport
arXiv: Chemical Physics, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state (NESS) with an effective temperature, to study quantum Heat Transport at the nanoscale. By combining the variational polaron transformed master equation with the full counting statistics, we have extended the applicability of the polaron-based framework to study nonequilibrium process beyond the super-Ohmic bath models. Previously, the polaron-based framework for quantum Heat Transport reduces exactly to the non-interacting blip approximation (NIBA) formalism for Ohmic bath models due to the issue of the infrared divergence associated with the full polaron transformation. The nonequilibrium variational method allows us to appropriately treat the infrared divergence in the low-frequency bath modes and explicitly include cross-bath correlation effects. These improvements provide more accurate calculation of Heat current than the NIBA formalism for Ohmic bath models. We illustrate the aforementioned improvements with the nonequilibrium spin-boson model in this work and quantitatively demonstrate the cross-bath correlation, current turnover, and rectification effects in quantum Heat transfer.
-
a nonequilibrium variational polaron theory to study quantum Heat Transport
Journal of Physical Chemistry C, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state with an effective temperature, to study quantum Heat Transport at the nanoscale. B...
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a nonequilibrium variational polaron theory to study quantum Heat Transport
The Journal of Physical Chemistry, 2019Co-Authors: Changyu Hsieh, Junjie Liu, Chenru Duan, Jianshu CaoAbstract:We propose a nonequilibrium variational polaron transformation, based on an ansatz for nonequilibrium steady state with an effective temperature, to study quantum Heat Transport at the nanoscale. By combining the variational polaron transformed master equation with the full counting statistics, we extended the applicability of the polaron-based framework to study nonequilibrium process beyond the super-Ohmic bath models. Previously, the polaron-based framework for quantum Heat Transport reduces exactly to the non-interacting blip approximation (NIBA) formalism for Ohmic bath models due to the issue of the infrared divergence associated with the full polaron transformation. The nonequilibrium variational method allows us to appropriately treat the infrared divergence in the low-frequency bath modes and explicitly include cross-bath correlation effects. These improvements provide more accurate calculation of Heat current than the NIBA formalism for Ohmic bath models. We illustrate the aforementioned improvements with the nonequilibrium spin-boson model in this work and quantitatively demonstrate the cross-bath correlation, current turnover, and rectification effects in quantum Heat transfer.
Detlef Lohse - One of the best experts on this subject based on the ideXlab platform.
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Experimental investigation of Heat Transport in inhomogeneous bubbly flow
Chemical Engineering Science, 2019Co-Authors: Biljana Gvozdić, Detlef Lohse, On Yu Dung, Elise Alméras, Dennis P. M. Van Gils, Sander G. Huisman, Chao SunAbstract:In this work we study the Heat Transport in inhomogeneous bubbly flow. The experiments were performed in a rectangular bubble column Heated from one side wall and cooled from the other, with millimetric bubbles introduced through one half of the injection section (close to the hot wall or close to the cold wall). We characterise the global Heat Transport while varying two parameters: the gas volume fraction , and the Rayleigh number . As captured by imaging and characterised using Laser Doppler Anemometry (LDA), different flow regimes occur with increasing gas flow rates. In the generated inhomogeneous bubbly flow there are three main contributions to the mixing: (i) Transport by the buoyancy driven recirculation, (ii) bubble induced turbulence (BIT) and (iii) shear-induced turbulence (SIT). The strength of these contributions and their interplay depends on the gas volume fraction which is reflected in the measured Heat Transport enhancement. We compare our results with the findings for Heat Transport in homogeneous bubbly flow from Gvozdić et al. (2018). We find that for the lower gas volume fractions (), inhomogeneous bubbly injection results in better Heat Transport due to induced large-scale circulation. In contrast, for , when the contribution of SIT becomes stronger, but so does the competition between all three contributions, the homogeneous injection is more efficient.
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experimental investigation of Heat Transport in homogeneous bubbly flow
Journal of Fluid Mechanics, 2018Co-Authors: Biljana Gvozdic, Elise Alméras, Varghese Mathai, Dennis P. M. Van Gils, Roberto Verzicco, Sander G. Huisman, Chao Sun, X W Zhu, Detlef LohseAbstract: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 2:5 mm diameter bubbles (corresponding to bubble Reynolds number Reb 600) in a rectangular water column Heated from one side and cooled from the other. The gas volume fraction was varied in the range 0 %–5 %, and the Rayleigh number RaH in the range 4:0109–1:21011. 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 D0:5% onwards, the Nusselt number Nu is nearly independent of RaH, and depends solely on the gas volume fraction . We observe the scaling Nu / 0:45, which is suggestive of a diffusive Transport mechanism, as found by Almeras 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 D 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 (Reb 600) offers a powerful mechanism for Heat Transport enhancement in natural convection systems.
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controlling Heat Transport and flow structures in thermal turbulence using ratchet surfaces
Physical Review Letters, 2018Co-Authors: H Jiang, Detlef Lohse, Varghese Mathai, Roberto Verzicco, Chao Sun, Xiaojue ZhuAbstract:In this combined experimental and numerical study on thermally driven turbulence in a rectangular cell, the global Heat Transport and the coherent flow structures are controlled with an asymmetric ratchetlike roughness on the top and bottom plates. We show that, by means of symmetry breaking due to the presence of the ratchet structures on the conducting plates, the orientation of the large scale circulation roll (LSCR) can be locked to a preferred direction even when the cell is perfectly leveled out. By introducing a small tilt to the system, we show that the LSCR orientation can be tuned and controlled. The two different orientations of LSCR give two quite different Heat Transport efficiencies, indicating that Heat Transport is sensitive to the LSCR direction over the asymmetric roughness structure. Through a quantitative analysis of the dynamics of thermal plume emissions and the orientation of the LSCR over the asymmetric structure, we provide a physical explanation for these findings. The current work has important implications for passive and active flow control in engineering, biofluid dynamics, and geophysical flows.
Olga Shishkina - One of the best experts on this subject based on the ideXlab platform.
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the influence of the cell inclination on the Heat Transport and large scale circulation in liquid metal convection
Journal of Fluid Mechanics, 2020Co-Authors: Lukas Zwirner, Ruslan Khalilov, Ilya Kolesnichenko, Andrey Mamykin, Sergei Mandrykin, A Pavlinov, Alexander Shestakov, Andrei Teimurazov, Peter Frick, Olga ShishkinaAbstract:Inclined turbulent thermal convection in liquid sodium is studied at large Rayleigh numbers based on the results of both experimental measurements and high-resolution numerical simulations. For a direct comparison, the considered system parameters are set to be similar: in the direct numerical simulations (DNS), in the large-eddy simulations and in the experiments, while the Prandtl number of liquid sodium is very small ( ). The cylindrical convection cell has an aspect ratio of one; one circular surface is Heated, while the other one is cooled. Additionally, the cylinder is inclined with respect to gravity and the inclination angle varies from , which corresponds to Rayleigh–Benard convection (RBC), to , as in a vertical convection (VC) set-up. Our study demonstrates quantitative agreement of the experimental and numerical results, in particular with respect to the global Heat and momentum Transport, temperature and velocity profiles, as well as the dynamics of the large-scale circulation (LSC). The DNS reveal that the twisting and sloshing of the LSC at small inclination angles periodically affects the instantaneous Heat Transport (up to of the mean Heat Transport). The twisted LSC is associated with a weak Heat Transport, while the sloshing mode that brings together the hot and cold streams of the LSC is associated with a strong Heat Transport. The experiments show that the Heat Transport scales as in both limiting cases (RBC and VC) for Rayleigh numbers around , while any inclination of the cell, , leads to an increase of .
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the influence of the cell inclination on the Heat Transport and large scale circulation in liquid metal convection
Journal of Fluid Mechanics, 2020Co-Authors: Lukas Zwirner, Ruslan Khalilov, Ilya Kolesnichenko, Andrey Mamykin, Sergei Mandrykin, A Pavlinov, Alexander Shestakov, Andrei Teimurazov, Peter Frick, Olga ShishkinaAbstract:Inclined turbulent thermal convection in liquid sodium is studied at large Rayleigh numbers $Ra\gtrsim 10^{7}$ based on the results of both experimental measurements and high-resolution numerical simulations. For a direct comparison, the considered system parameters are set to be similar: $Ra=1.67\times 10^{7}$ in the direct numerical simulations (DNS), $Ra=1.5\times 10^{7}$ in the large-eddy simulations and $Ra=1.42\times 10^{7}$ in the experiments, while the Prandtl number of liquid sodium is very small ( $Pr\approx 0.009$ ). The cylindrical convection cell has an aspect ratio of one; one circular surface is Heated, while the other one is cooled. Additionally, the cylinder is inclined with respect to gravity and the inclination angle varies from $\unicode[STIX]{x1D6FD}=0^{\circ }$ , which corresponds to Rayleigh–Benard convection (RBC), to $\unicode[STIX]{x1D6FD}=90^{\circ }$ , as in a vertical convection (VC) set-up. Our study demonstrates quantitative agreement of the experimental and numerical results, in particular with respect to the global Heat and momentum Transport, temperature and velocity profiles, as well as the dynamics of the large-scale circulation (LSC). The DNS reveal that the twisting and sloshing of the LSC at small inclination angles periodically affects the instantaneous Heat Transport (up to $\pm 44\,\%$ of the mean Heat Transport). The twisted LSC is associated with a weak Heat Transport, while the sloshing mode that brings together the hot and cold streams of the LSC is associated with a strong Heat Transport. The experiments show that the Heat Transport scales as $Nu\sim Ra^{0.22}$ in both limiting cases (RBC and VC) for Rayleigh numbers around $Ra\approx 10^{7}$ , while any inclination of the cell, $0<\unicode[STIX]{x1D6FD}\leqslant 90^{\circ }$ , leads to an increase of $Nu$ .
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effect of prandtl number on Heat Transport enhancement in rayleigh benard convection under geometrical confinement
Physical Review Fluids, 2018Co-Authors: Kai Leong Chong, Olga Shishkina, S R Wagner, Matthias Kaczorowski, Keqing XiaAbstract:Heat Transport enhancement and flow topology of turbulent Rayleigh-B\'enard convection under geometrical confinement are studied numerically for over two decades of Prandtl numbers, with Ra fixed at ${10}^{8}$.