Rayleigh-Benard convection in liquid metal layers under the influence of a vertical magnetic field

The influence of a vertical magnetic field on the integral heat transfer and the temporal dynamics of liquid metal Rayleigh-Benard convection is studied in an experiment using a small Prandtl number (Pr approximate to0.02) sodium potassium alloy (NaKr78)-Kr-22 as a test fluid. The test section is a rectangular box of large aspect ratio 20 : 10 : 1 that covers a parameter range of Rayleigh numbers, 10(3)< Ra < 10(5), and Chandrasekhar numbers, 0 <1.44x10(4). The integral heat transfer across the layer is evaluated from the measured temperatures at the upper and the lower boundary and the applied heat flux. Local, time-dependent temperatures are obtained from a four-element temperature probe placed in the middle of the liquid metal layer. The noncoplanar arrangement of the thermocouples enables the evaluation of the time-dependent temperature gradient vector that allows us to estimate the local isotropy properties of the time-dependent flow. From the damping effect of Joule dissipation, the convective heat transport decreases monotonically with increasing Chandrasekhar numbers. Fluctuations of the temperature field are damped significantly by the magnetic field. However, this effect is selective with respect to frequency. Long period fluctuations are strongly damped whereas short period fluctuations are less damped or may even be amplified. The observations show that significant convective heat transport is practically always associated with time-dependent flow. The fluctuating part of the local temperature gradient confirms the horizontal isotropy of the velocity field; no predominant orientation of time-dependent flow structures is established either with or without a magnetic field. (C) 2001 American Institute of Physics.