Effects of Size and Temperature-Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air
Effects of Size and Temperature-Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air
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DOI：
10.1029/2018gc007510
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发表时间：
2018-10-01
影响因子：
3.5
通讯作者：
Valentine, Greg A.
中科院分区：
文献类型：
--
作者：
Moitra, Pranabendu;Sonder, Ingo;Valentine, Greg A.
在自由到强制对流的条件下，了解火山碎屑的冷却时间尺度对于微观结构分析、焊接层的形成以及喷发柱和火山碎屑流动的模拟是至关重要的。我们在圆柱形风洞中进行了不同风速范围内加热岩石的冷却实验。为了估算换热系数，我们用激光闪光测量得到的热扩散率和热容量随温度变化的模型模拟了样品中的瞬时温度分布。对于风速高达15m/S、样品温度高达1383K(1110℃)的样品，建立了高达Re=3×10(4)的Nu-Re关系。我们发现，热损失对大小的依赖关系对于大于1 mm的大小的火山碎屑尤为重要。我们的结果表明，火山碎屑的冷却时间尺度可能足够大，从而导致碎屑微结构的碎裂后修饰。我们进一步表明，几厘米或更大的碎屑在几分钟的时间尺度内形成具有熔融核心的固体结壳，因此可能会抑制沉积后的焊接。爆炸性火山喷发通常产生碎裂的岩浆或微米至分米大小的火山碎屑。在环境空气变冷的情况下，这些极热的火山碎屑会随着时间的推移而释放热量。冷却时间尺度决定了火山碎屑的内部结构，它提供了有关岩浆上升和在地球表面侵位的有价值的信息。冷却也决定了火山喷发后的喷发柱或火山碎屑流动力学。采用实验室实验与天然熔岩实验相结合的方法，结合传热学计算，对火山碎屑的冷却进行了研究。我们展示了不同大小的火山碎屑冷却时间尺度的变化，并讨论了它对理解喷发前到喷发后的火山过程的重要性。
Plain Language Summary 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 15m/s and for sample temperatures up to 1383 K (1110 degrees C), a Nusselt-Reynolds (Nu-Re) relationship up to Re=3x 10(4) is established in this study. We find that the dependency of heat loss on size is particularly important for pyroclasts larger than approximate to 1mm. 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.Explosive volcanic eruptions commonly produce fragmented magma or pyroclasts of microns to decimeter in size. These extremely hot pyroclasts looses heat over time in the presence of colder ambient air. The cooling time scale determines the interior texture of pyroclasts, which provide valuable information about magma ascent and emplacement on the Earth's surface. Cooling also determines eruption column or pyroclast flow dynamics following volcanic eruptions. We study the cooling of pyroclast by combining laboratory experiments with natural lava and heat transfer calculations. We show the variation in cooling time scales for pyroclasts of different sizes and discuss its importance for understanding the pre- to post-eruptive volcanic processes.