Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing

Lim, Zi Li1; Chong, Kai Leong1; Ding, Guang-Yu1; Xia, Ke-Qing1,2,3

2019-07-10

10.1017/jfm.2019.232

JOURNAL OF FLUID MECHANICS   影响因子和分区

0022-1120

1469-7645

We present a numerical study of quasistatic magnetoconvection in a cubic Rayleigh-Benard (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 Haopt. 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-Benard (CRB), rotating Rayleigh-Benard (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 Ha, the (traditionally defined) viscous boundary ffiv 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 Ha, ffiv exhibits an approximate 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 ffiv D H = p c1Re0 : 72 C c2Ha2 (where H is the height of the cell) for all values of Re and Ha, which fits the obtained viscous boundary layer well.

Benard convection magneto convection plumes thermals

SCI ; EI

英语

通讯

Research Grants Council (RGC) of HKSAR[CUHK14301115] ; Research Grants Council (RGC) of HKSAR[CUHK14302317] ; Research Grants Council (RGC) of HKSAR[N_CUHK437/15]

Mechanics ; Physics

Mechanics ; Physics, Fluids & Plasmas

WOS:000467920500001

CAMBRIDGE UNIV PRESS

20192106956738

Lorentz force ; Natural convection ; Reynolds number ; Temperature distribution

Fluid Flow, General:631.1 ; Thermodynamics:641.1 ; Heat Transfer:641.2 ; Electricity and Magnetism:701

ENGINEERING

Web of Science

1.Chinese Univ Hong Kong, Dept Phys, Shatin, Hong Kong, Peoples R China
2.Southern Univ Sci & Technol, Ctr Complex Flows & Soft Matter Res, Shenzhen 518055, Peoples R China
3.Southern Univ Sci & Technol, Dept Mech & Aerosp Engn, Shenzhen 518055, Peoples R China

Lim, Zi Li,Chong, Kai Leong,Ding, Guang-Yu,et al. Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing[J]. JOURNAL OF FLUID MECHANICS,2019,870:519-542.

Lim, Zi Li,Chong, Kai Leong,Ding, Guang-Yu,&Xia, Ke-Qing.(2019).Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing.JOURNAL OF FLUID MECHANICS,870,519-542.

Lim, Zi Li,et al."Quasistatic magnetoconvection: heat transport enhancement and boundary layer crossing".JOURNAL OF FLUID MECHANICS 870(2019):519-542.