The finite element simulations suggest that thermal gradients are an unavoidable consequence of non-steady state heating of homogeneous particles irrespective of size due to the thermal lag in the equilibration of the core of the particle relative to the surface. The rate of increase of temperature of the core of the particle reaches that applied to surface only after a specific equilibration time which is dependent on particle size. Equilibration times are ~5 ms for a 100 |xm diameter and -0.1 s for a 500 p.m diameter particle and are independent of the heating rate. The temperature difference maintained across a particle is thus determined by particle size, which controls equilibration time, and the heating rate with smaller temperature differences at higher surface heating rates.

The temperature differences calculated for 100 p.m and 500 p.m diameter particles are much lower than observed in micrometeorites recovered from the Earth's surface. The calculations indicate that although non-steady state heating does maintain temperature gradients across micrometeoroids these are only -30 K for particles 500 p.m in diameter and -3 K for particles 100 p.m in diameter at heating rates of 500 K s"1.

Simulations were also performed to model the equilibration of thermal gradients at peak temperature using the temperature profiles generated in the heating calculations and a constant surface temperature. The results of these simulations indicate that the small temperature differences generated during heating disappear rapidly (i.e. -0.1 s for a 500 |xm particle).

Was this article helpful?

0 0

Post a comment