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

Abstract

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.

Structural and compositional characteristics of Fukushima release particulate material from Units 1 and 3 elucidates release mechanisms, accident chronology and future decommissioning strategy

“…The particulates would have rapidly thermally equilibrated with the surrounding environment due to their small size (each with maximum dimensions of < 500 µm). This is evidenced through the timescale for cooling (τ cool ) by conduction 43 : where L is the characteristic length-scale and D is the thermal diffusivity. For example, τ cool = 0.01 s for D = 10 –6 m 2 /s and a typical particle size with L = 100 mm.…”

Deep-sea eruptions boosted by induced fuel–coolant explosions

“…Our laboratory heat-transfer rates were mediated by vapour almost instantaneously generated when water contacted melt (Leidenfrost effect 21,24 ). Unlike for MFCI 18,21,25 , no stable vapour film is required for IFCI; instead, vapour films inhibit IFCI because they prevent efficient heat transfer from melt to water. Shock waves are generated the moment pressurized gas hits the ceramic barrier and overlying melt, and cause the vapour film to collapse.…”

“…Instead we find fingerprints like those from fuel-coolant (thermohydraulic) ash-forming explosions. Havre is too deep, and rhyolite too viscous, for 'normal' fuel-coolant interactions 6,7,[10][11][12][13][14][15][16][17][18] , and we infer fragmentation by "Induced Fuel Coolant Interaction" (IFCI). Experimental evidence is presented for its role at Havre.…”

Fragmentation Processes During Strombolian Explosions Revealed Using Particle Size Distribution Mapping

“…We see from Figure 4B that very small lapilli-sized fragments landing on the plastic sheets had temperatures high enough to melt the polyethylene plastic, which has a nominal melting point in the range 120-130 • C. However lapilli of 2 mm landing at <50 m from the vent should not be hot enough to melt the tarp (Vanderkluysen et al, 2012;Moitra et al, 2018). Hence, the small holes we found could be due to secondary fragmentation processes, such as the break-up of larger clasts that, upon impacting the sheets, broke into smaller pieces that were still hot and able to melt the plastic.…”