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High amplitude, high frequency harmonic modulation of rotating Rayleigh-Bénard convection can more than quadruple heat transfer

Klein Kranenbarg, L.M. (2020) High amplitude, high frequency harmonic modulation of rotating Rayleigh-Bénard convection can more than quadruple heat transfer.

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Abstract:In harmonically modulated rotating Rayleigh-Bénard convection (MRRB) a time-dependent angular velocity Omega(t)=Omega_0 + DeltaOmega sin(omega t) is added to a RB system. Earlier research suggests possible great enhancement of heat transfer in these systems. We report on Direct Numerical Simulations (DNS) in a standardized system with water as a working fluid (Ra=10^8, Pr=6.4) in a cylindrical cell with height H=1 and diameter D=1. Modulated rotation introduces an Euler force into the governing equations which depends on the frequency and amplitude of the modulation only, as the constant rotation rate is set to zero. Two parameter sweeps for amplitudes were carried out at two different frequencies, complementing earlier simulations performed by Beunen as well as providing an entirely new data set. An impressive increase in Nusselt number of about 335% compared to the non-rotating case was achieved for a modulation with non-dimensional Rossby numbers for amplitude Ro*=0.01 and frequency Ro_omega=0.1. No optimal value of these Rossby numbers was found, but rather an asymptotic region where results suggest a scaling law Nu ~ (Ro*)^{-0.45} for amplitudes Ro* < 0.2 and seemingly independent on frequency. Underlying flow structures include very efficient mixing at the top and bottom plates and a large temperature gradient at half height near the sidewalls. The flow is dominated by fluctuations in the theta-direction due to the extra Euler force. The underlying flow structure in the r-z plane shows a stable double convection roll which might explain the efficiency of heat transfer in these cases. Further research into the influence of the frequency or fluid properties such as the Rayleigh and Prandtl numbers will deepen the understanding of this behaviour, possibly resulting in interesting industrial applications of this extremely efficient heat transfer.
Item Type:Essay (Master)
Faculty:BMS: Behavioural, Management and Social Sciences
Subject:31 mathematics, 33 physics
Programme:Science Education and Communication MSc (60708)
Link to this item:http://purl.utwente.nl/essays/81360
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