Effects of Size and Temperature-Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air

The knowledge of the cooling time scales of pyroclasts, in conditions of free to forced convection, is of paramount importance in microtextural analysis, development of welded deposits, and in eruption column and pyroclast flow modeling. We performed cooling experiments of heated rock inside a cylindrical wind tunnel under a range of air speeds. In order to estimate the heat transfer coefficients, we modeled the transient temperature distribution in the sample with temperature-dependent thermal diffusivity and heat capacity, which were obtained from laser flash measurements. For air speeds up to 15 m/s and for sample temperatures up to 1383 K (1110 ° C), a Nusselt-Reynolds (Nu-Re) relationship up to Re=3× 10⁴ is established in this study. We find that the dependency of heat loss on size is particularly important for pyroclasts larger than ∼1 mm. Our results suggest that the cooling time scales of pyroclasts could be large enough to cause postfragmentation modification of clast microtextures. We further show that clasts of few-centimeter and larger develop solid crusts with molten cores within time scales of a few minutes and therefore may inhibit postdepositional welding.