Gas retention in fine-grained pyroclastic flow materials at high temperatures

Abstract

The ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (t _be ; brief and not considered in this paper), one for hindered settling from the expanded state (t _sett), and one for diffusive release of residual pore pressure from the non-expanded state (t _diff). The relative magnitides of t _sett and t _diff depend on bed thickness, t _sett dominating in thin systems and t _diff in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 μm) from each sample. Careful drying of the materials was necessary to avoid effects of humidity-related cohesion. Two sets of experiments were carried out: (1) expansion in the non-bubbling regime at 50–200°C, (2) bed collapse tests from the initially bubbling state at 50–550°C. Provided that gas channelling was avoided by gentle stirring, all the samples exhibited a regime of uniform expansion prior to the onset of bubbling. Fine particle size (in particular high fines content), low particle density and high temperature all favoured smoother fluidization by increasing the maximum expansion possible in the non-bubbling state. An empirical equation describing the uniform expansion of the materials was determined. High temperature also favoured greater gas partitioning into the dense phase of the bubbling bed, as well (in finer-grained samples) as higher voidage in the settled bed. Large values of t _sett and t _diff were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50–200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (t _sett) or non-expanded (t _diff) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent ‘mobilities’, than coarser-grained ones of comparable volume, as has been observed on Montserrat.

Appendix

Notation

Symbol

A :

surface area of fluidization rig (m⁻²)

ɛ :

hydrodynamic voidage

f :

drag factor (Pa m⁻¹)

g :

gravitational acceleration (9.82 m⁻²)

H :

bed thickness (m)

K :

permeability (m⁻²)

M :

sample mass (kg)

μ :

gas viscosity (Pa s)

n :

exponent in Richardson–Zaki equation

P :

gas pressure (Pa)

P ₀ :

initial gas pressure at base of bed (Pa)

r:

gas constant (296.8 J kg⁻¹ K⁻¹ for nitrogen)

ρ :

density of gas or particles (kg m⁻³)

t :

time (s)

T:

absolute temperature (K)

u :

gas or particle velocity (m s⁻¹)

U :

superficial gas velocity at T of operation (m s⁻¹)

z :

height (m)

Subscript

atm:

atmospheric value

be:

value following bubble evacuation

diff:

pressure diffusion

g:

value for gas phase

mb:

value at minimum bubbling

mf:

value at minimum fluidization

mp:

value at maximum pressure

s:

value for solid phase

sett:

value during or following settling

t :

terminal value