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Abstract

We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat Tb−Ton (Tb is the bottom-plate temperature and Ton is the value of Tb below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying Tb and keeping a fixed difference Tb−Tt≃16 K (Tt is the top-plate temperature). Initially Tb was much larger than Ton (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing Tb in steps until it was below Ton resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, Tb was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing Tb. Varying the cavity density (69, 32, 3.2, 1.2 and 0.3 mm−2) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.
Original languageEnglish
Pages (from-to)331-365
Number of pages36
JournalJournal of fluid mechanics
Volume787
DOIs
StatePublished - 17 Dec 2016

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Two phase flow
Nucleation
Temperature
Heat flux
Vapors
Liquids
Microcavities
Nusselt number
Buoyancy
Bubbles (in fluids)
Thermal gradients
Aspect ratio
Thermal conductivity
Turbulence

Keywords

  • METIS-313706
  • IR-98668

Cite this

Narezo Guzman, Daniela; Xie, Yanbo; Chen, S.; Fernandez Rivas, David; Sun, Chao; Lohse, Detlef; Ahlers, Günter / Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence.

Vol. 787, 17.12.2016, p. 331-365.

Research output: Scientific - peer-reviewArticle

@article{5f7544aa57384bd092c0ce8cd279c9ff,
title = "Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence",
abstract = "We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat Tb−Ton (Tb is the bottom-plate temperature and Ton is the value of Tb below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying Tb and keeping a fixed difference Tb−Tt≃16 K (Tt is the top-plate temperature). Initially Tb was much larger than Ton (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing Tb in steps until it was below Ton resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, Tb was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing Tb. Varying the cavity density (69, 32, 3.2, 1.2 and 0.3 mm−2) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.",
keywords = "METIS-313706, IR-98668",
author = "{Narezo Guzman}, Daniela and Yanbo Xie and S. Chen and {Fernandez Rivas}, David and Chao Sun and Detlef Lohse and Günter Ahlers",
year = "2016",
month = "12",
doi = "10.1017/jfm.2015.701",
volume = "787",
pages = "331--365",

}

Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence. / Narezo Guzman, Daniela; Xie, Yanbo; Chen, S.; Fernandez Rivas, David; Sun, Chao; Lohse, Detlef; Ahlers, Günter.

Vol. 787, 17.12.2016, p. 331-365.

Research output: Scientific - peer-reviewArticle

TY - JOUR

T1 - Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence

AU - Narezo Guzman,Daniela

AU - Xie,Yanbo

AU - Chen,S.

AU - Fernandez Rivas,David

AU - Sun,Chao

AU - Lohse,Detlef

AU - Ahlers,Günter

PY - 2016/12/17

Y1 - 2016/12/17

N2 - We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat Tb−Ton (Tb is the bottom-plate temperature and Ton is the value of Tb below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying Tb and keeping a fixed difference Tb−Tt≃16 K (Tt is the top-plate temperature). Initially Tb was much larger than Ton (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing Tb in steps until it was below Ton resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, Tb was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing Tb. Varying the cavity density (69, 32, 3.2, 1.2 and 0.3 mm−2) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.

AB - We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh–Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat Tb−Ton (Tb is the bottom-plate temperature and Ton is the value of Tb below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying Tb and keeping a fixed difference Tb−Tt≃16 K (Tt is the top-plate temperature). Initially Tb was much larger than Ton (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing Tb in steps until it was below Ton resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, Tb was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing Tb. Varying the cavity density (69, 32, 3.2, 1.2 and 0.3 mm−2) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.

KW - METIS-313706

KW - IR-98668

U2 - 10.1017/jfm.2015.701

DO - 10.1017/jfm.2015.701

M3 - Article

VL - 787

SP - 331

EP - 365

ER -