M. Hort et al., Convection and crystallization in a liquid cooled from above: an experimental and theoretical study, J PETROLOGY, 40(8), 1999, pp. 1271-1300
Evidence from experiments and theoretical modeling suggests that systems cr
ystallizing exclusively from the top down and rejecting a buoyant liquid ef
fectively cease convecting once the initial superheat has been lost. We rep
ort here on a combined experimental and theoretical study designed to inves
tigate in some detail the interaction of convection and crystallization in
a fluid cooled from above. The experiments are carried out in a small tank
where temperature, composition, mush thickness, and convective velocities h
ave been monitored. After an initial period of turbulent convection removin
g the superheat, the bulk fluid temperature holds steady at the liquidus te
mperature. Further convection at Ra similar to 10(6) is characterized by a
gentle, broad stirring of the entire tank through upward boundary layer flo
ws hugging the tank walls, which are inferred to be sustained by a small bu
t steady leakage of heat into the tank through the sidewalls. The thickness
of the overlying mush zone increases linearly with root t and is found to
be very sensitive to leakage of heat through the sidewalls. Within the unce
rtainty of the liquidus determination, thee is no measurable undercooling a
nd no crystallization is observed within the bulk fluid. The experimental r
esults are investigated with a comprehensive analytical model employing sid
ewall heating and, among other things, either equilibrium or disequilibrium
crystallization. Either crystallization model gives satisfactory agreement
with the experiments. More importantly, however, once the superheat is los
t all the convective motion is well explained by the unwanted sidewall heat
ing, and if this heat source is then 'analytically' turned off, convection
ceases upon loss of the superheat. In sum, this combined study supports the
conclusion that convection is similar binary phase systems crystallizing f
rom the top down and rejecting a buoyant liquid becomes non-turbulent or ev
en ceases upon loss of the superheat. Transferring these results to magmati
c systems, we suggest that the dynamics inside intrusive bodies upon coolin
g are very sensitive to the actual phase diagram, the kinetics of crystalli
zation and the density relation between crystals and melt.