The role of fluidization in the emplacement of pyroclastic flows, 2: Experimental results and their interpretation

Journal of Volcanology and Geothermal Research, 20 (1984) 55--84

55

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

T H E R O L E O F F L U I D I Z A T I O N IN T H E E M P L A C E M E N T O F P Y R O C L A S T I C F L O W S , 2: E X P E R I M E N T A L R E S U L T S A N D T H E I R INTERPRETATION

C.J.N. WILSON*

Geology Department, Imperial College, London SW7 2BP (England) (Received October 27, 1982; revised and accepted March 2, 1983)

ABSTRACT Wilson, C.J.N., 1984. The role of fluidization in the emplacement of pyroclastic flows, 2: experimental results and their interpretation. J. Volcanol. Geotherm. Res., 20: 55--84. Although often quoted as being important in the emplacement of pyroclastic flows, little relevant experimental fluidization work has been done. Fluidization processes are first reviewed for a simple system, against which ignimbrite samples can be compared. Results from fluidization experiments on ignimbrite samples show that their behaviour differs radically from any simple system, principally because of their extremely poor sorting. At a certain gas velocity, Uie (whose value cannot be predicted), some ignimbrite samples begin to expand, whilst at a higher gas velocity, V m p , the samples begin to show segregation structures. The minimum fluidization velocity, V m f , in simple systems is replaced by U m p ; the value of the latter in pyroclastic materials cannot be easily or reliably predicted from published Umf correlations, and a new empirical method for determining Ump is presented from the experimental data. During fluidization, ignimbrite samples expand much less than more conventional materials due to the bypassing of gas through segregation channels. The total amount of expansion is also limited; e.g. a 100-m-thick pyroclastic flow will, from fairly high gas velocities, deflate to form an ignimbrite which is not less than 70--85 m thick. During fluidization, the poor sorting causes only part of the weight of the ignimbrite samples to be supported by the gas flow, the degree of support increasing with an improvement in sorting. Segregation structures, and grain-size and compositional variations within fluidized ignimbrite samples show varying behaviour, depending on the sorting of the sample. After subjecting a sample to high gas velocities, the net result is a grain-size- and compositionally-zoned bed which becomes richer upwards in pumice and fine material. Once formed, these features cannot be destroyed under laboratory conditions, implying that some segregation structures should be present in any ignimbrite where U m p w a s exceeded.

INTRODUCTION F l u i d i z a t i o n , or processes n o w covered b y t h a t t e r m , has o f t e n b e e n c i t e d as i m p o r t a n t i n t h e e m p l a c e m e n t o f p y r o c l a s t i c f l o w s , t h e e a r l i e s t *Present address: Geology Department, Auckland University, Private Bag, Auckland, New Zealand.

0377-0273/84/$03.00

© 1984 Elsevier Science Publishers B.V.

56 studies in this regard being Anderson and Flett (1903) and Lacroix (1904). Reynolds (1954) reviewed the chemical engineering literature and proposed that several geological processes, including aspects of the emplacement of pyroclastic flows, could be considered in terms of fluidization. Later, McTaggart (1960, 1962) and Brown (1962) considered fluidization specifically with regard to the emplacement of pyroclastic flows and this has become the area of geology in which fluidization is most often invoked. However, little work has been done on determining how pyroclastic flow deposits might react if they were actually fluidized. Bloor (1976) and Sheridan (1979) described fluidization experiments on narrow grain-size cuts from ignimbrites (i.e. pyroclastic flow deposits where the juvenile component is pumiceous), whilst Wilson (1980) presented broad results from more complex particle mixtures, including whole ignimbrite samples. This paper aims to first, provide an introduction to fluidization processes in simple industrial systems, second, present detailed results of the fluidization experiments discussed in Wilson {1980) and third, consider how the fluidization behaviour of ignimbrite materials compares with that of simple systems. For more information on industrial fluidization see the texts by Davidson and Harrison (1963, 1971), Kunii and Levenspiel (1969), Leva (1959) and Zenz and Othmer (1960). FLUIDIZATION IN SIMPLE SYSTEMS In troduction

A fluidized system consists of a bed of particulate, non-cohesive solids, being at their normal loosely packed bulk~lensity, through which a fluid is passed upwards. Fluidization in the strict sense (e.g. Davidson and Harrison, 1963, pp. 1, 3) is the condition attained when, at a certain superficial fluid velocity (the minimum fluidization velocity: Umf), the drag force exerted across the bed by the fluid flow is equal to the buoyant weight of the bed. The superficial fluid velocity {U) is obtained by dividing the mass flow rate of the fluidizing fluid by the fluid density and the cross-sectional area of the empty container. Two points should be noted. First, the term fluidization as used herein specifies that the particulate phase is a solid. The use of the term fluidization to cover liquid/liquid or gas/liquid systems (e.g. Reynolds, 1954; Sheridan and Ragan, 1976; Wohletz and Sheridan, 1979) is considered to extend the use of the term beyond its accepted meaning; in particular, the fluidization correlations discussed below are valid only when particle cohesion is absent. Second, the term fluidization is hereafter used in its looser sense to cover all conditions from very low to very high fluid velocities, i.e. from packedbed conditions (0< U< Umf ) to dilute-phase fluidization (U >> Umf) (cf. Kunii and Levenspiel, 1969, chapter 1; Zenz, 1971). The containers used for fluidization are usually of either the two-dimen-

57

sional (2-D: narrow slot-shape cross-section) or three-dimensional (3-D: circular cross-section) type (e.g. Rowe, 1971); both were used in the experiments described here. With a 2-D rig, direct observations of samples can be made through the sidewalls of the container; however, the close proximity o f the sidewalls can affect measurements and restricts the m a x i m u m particle size. A 3-D rig only allows direct observation of the outside of the bed, but it can be made large enough to minimize wall effects. At room temperatures (T) and pressures (P), two kinds of fluidization behaviour are seen at U'~Umf; which behaviour depends on the density ratio of the solid phase to the fluidizing medium (Fig. 1). Where this ratio is high (e.g. 250 u m ballotini (glass beads) in air), the excess fluid is passed through the bed as bubbles, which resemble those seen in a normal liquid (Davidson et al., 1977); this state is termed aggregative fluidization. Where the density ratio is low (e.g. 250 p m ballotini in water), the bed expands evenly at U~Umf to accommodate the extra fluid flow, giving particulate fluidization. In simple systems, aggregative and particulate fluidization states usually correspond to gas and liquid fluidizing media, respectively. This paper is only concerned with gas fluidized systems and hence aggregative behaviour is to be expected in ignimbrite samples. Packed

Slugging

Dilute

FLUID FLOW RATE

>

Bubbling

AGGREGATIVE

f!:,!!!i' PARTICULATE

,1 --INCREASING

Fig. 1. Schematic diagram (after Zenz, 1971) to show the differences in behaviour between aggregative (most gas--solid systems) and particulate fluidization (most liquid-solid systems).

Gas fluidization o f a simple system

An example o f a simple system investigated is 380 p m ballotini fluidized by air at room T, P. This shows typical aggregative fluidization behaviour, being quiescent at U~< Umf and bubbling as soon as Umf is exceeded. As U

58

is increased further, small particles, whose terminal fall velocity (Ut) is less than U, are elutriated from the system (Leva and Wen, 1971). Changes in the fluidization state of the material are plotted on a graph of U, versus the pressure drop across the bed (AP) divided b y the bed depth (H) (e.g. Zenz and Othmer, 1960, chapter 7). Values of AP/H are drawn as an up curve and d o w n curve as U is increased and decreased during a run, respectively. Three points should be considered from the fluidization plot for this material (Fig. 2). AP/H 1.5o

/

1.0 A

,I

0.5 ii

Umf 1

....

5

....

,

,

10

i

15

'

'

r

,

I

20

U, c m s - '

Fig. 2. Fluidization plot for 380 ~m ballotini sample (Mz = "1.44, a I = 0.20). AP/H is expressed as centimetres of water per centimetre of bed thickness (see Wilson, 1980); filled circles define the up-curve, open circles the down-curve. See text for discussion. First, in this particular example the up- and down-curves only coincide because of the originally loose-packed nature of a very well-sorted, perfectly spherical material. Usually, even in simple systems, some hysteresis is present due to changes in the packing state or sorting of the sample during a run and in ignimbrite samples extreme hysteresis occurs (Wilson, 1980; Fig. 2). Second, Umf is given b y the gas velocity at the intersection of the two portions of the plot. In simple systems Umf is USUally measured using only the down curve (e.g. Richardson, 1971); however, in ignimbrite samples, their p o o r sorting causes such changes in behaviour that Umf cannot be defined and is replaced b y Ump (Wilson, 1980). Third, at U~Umf the b u o y a n t weight of the bed, given by: ( 1 - - 6) ( P s ....

pg) g (bed volume)

(1)

where e is the voidage, Ps the solids clast
59

density (Pb) at U ~> Vmf, which can be compared with that obtained by knowing the sample weight and noting the bed volume. Moreover, with poorly-sorted systems at any gas velocity AP/H can be used to measure how completely fluidized the system is (see p. 71). Three further properties of the simple fluidized system described above, and other published industrial examples, are of relevance to the fluidization of ignimbrite materials. First, the presence or absence of bubbles is crucial in determining the amount and scale of mixing within fluidized systems (Kunii and Levenspiel, 1969, chapters 4--6; Potter, 1971). Unless bubbling occurs, gas-fluidized beds are very poorly mixed because interparticle contacts prevent movement of the material. Also, bubbling must be present for elutriation to occur; when bubbles are absent (i.e. at U ~< Umf in Fig. 2), fine material cannot be moved to the bed surface and so elutriation cannot proceed (Leva and Wen, 1971). The importance of fines elutriation from pyroclastic flows is discussed in Sparks (1976), Sparks and Walker {1977) and Wilson (1980). Second, the resemblance between a fluidized bed and a liquid suggests that a viscosity may be measurable in a fluidized system. Two approaches have been used, one using conventional-liquid viscometers (e.g. Schfigerl et al., 1961; Hagyard and Sacerdote, 1966) and the other utilizing variations in bubble shapes (Grace, 1970). The equivalent newtonian viscosities for gas-fluidized well-sorted materials range from 4 to 13 poises, being higher in beds of coarser and/or denser materials, both methods yielding similar results (Grace, 1970). These viscosities are 4 to 5 orders of magnitude greater than those of the gas phase alone. Frankel and Acrivos (1967) calculate that were the particles actually separated during fluidization (as many have supposed; e.g. Lacroix, 1904; Reynolds, 1954; McTaggart, 1960, 1962; Brown, 1962), then the viscosity of the fluidized bed would be at most about 100 times the viscosity of the gas phase alone. Viscosity measurements on gas-fluidized systems show that interparticle contacts dominate their rheological behaviour and the same is inferred to hold for pyroclastic flows. Third, if large clasts are dropped into a fluidized bed at U > Umf, then they follow Archimedes' principle. Clasts with a density (Ps) less than that of the matrix (Pm) will float, displacing their equivalent weight of bed material, while clasts with Ps > Pm will sink, at rates which are reportedly similar to those in analogous liquids (Gelperin and Einstein, 1971). Estimates of the apparent viscosities of pyroclastic flows have been obtained using the grading behaviour of large clasts in ignimbrites (Sparks, 1976). FLUIDIZATION EXPERIMENTS In troduc tion

A simple fluidized bed presents many useful comparisons which can be used in modelling aspects of pyroclastic flows. As a first step, experiments

60 have been run on several materials to see h o w the fluidization behaviour of ignimbrite samples compares with that of materials such as described above. Experiments were carried out in either 2-1:) (15 runs) or 3-D (45 runs) rigs as shown in Wilson (1980, Fig. 1), using air at room T, P. The materials used and their grain-size parameters are listed in Wilson (1981). The material for each run was thoroughly mixed and placed or poured into the rig, and the bed bulk> Vmf and both up- and downcurves obtained. With poorly-sorted samples only an up curve was obtained. With samples containing any fine material the spent air plus elutriated-fines mix was collected through a vacuum cleaner.

Fluidization performance The results obtained from simple materials (well-sorted ballotini or sand samples) closely resemble those obtained by other workers and give confidence in the techniques used and the consistency of the results (Wilson, 1981). However, fluidization behaviour changes radically as sorting becomes poorer (Wilson, 1980). Well-sorted materials can be completely fluidized to form a normal bubbling system, b u t poorly-sorted materials like ignimbrites can never be fully fluidized because the finest particles are being elutriated long before the coarsest particles are fluidized (Sparks, 1976). Such materials can only be fully fluidized when their sorting is improved, by which time they bear no resemblance to the original sample. This behaviour is n o t recorded in the fluidization literature, for t w o reasons. First, most laboratory experiments use very well-sorted materials (g~ usually <0.5). Second, although some systems with oi as high as 1.0 are used (e.g. Bjerle et al., 1980) and elutriation is often observed, either the gain-size distribution is such that fines losses do n o t alter the fluidization characteristics, or the system is operated so that the lost fines are continually replaced (e.g. Merrick and Highley, 1974). Sparks (1976) proposed that pyroclastic flows are only partly fluidized, consisting of three "phases": (a) Particles with Ut < U (b) Particles with U t f> U~> Umf (c) Particles with Umf > U Thus a pyroclastic flow has a matrix of phases (a) and (b), in which is dispersed phase (c) which tends to float (pumice) or sink (lithic) depending on its density contrast. Phase (a) is lost from this matrix by elutriation to form the dilute ash cloud above the flow. This model is modified b y Wilson (1980) who shows that three fluidization states are possible in pyroclastic flows, giving rise to three flow types: Type 1: No coarse-tail grading, no elutriation ( U < Vie ). Type 2: Slow, matrix-viscosity controlled coarse-tail grading, no bubbling hence no elutriation (Uie ~< U ~< Ump)

61 T y p e 3: R a p i d , b u b b l i n g - c o n t r o l l e d coarse-tail grading a c c o m p a n i e d b y e l u t r i a t i o n ( U > Ump). It is o n l y in t y p e 3 flows t h a t the processes described b y Sparks ( 1 9 7 6 ) can o p e r a t e freely. P a r a m e t e r Ump in p o o r l y s o r t e d systems c o r r e s p o n d s closely t o Umf in well-sorted systems (Wilson, 1980). H o w e v e r , while Vmf is usually d e f i n e d using the d o w n curve (p. 58), Ump has t o be d e f i n e d using the up curve (Fig. 3). F o r this reason, all g m f values r e p o r t e d are also d e f i n e d b y using t h e up curve (N.B. in a well-sorted material, t h e d i f f e r e n c e b e t w e e n Vmf d e f i n e d b y the up- or d o w n ~ u r v e s is small, usually ~< 10--15%). AP/H

/i/1//i/ 7 Z

I

UP CURVE

I

Fig. 3. Schematic diagram to show how parameters Uie and Ump are defined. A typical up-curve for a poorly sorted sample is shown, together with the curve for the bed bulkdensity (Pb) multiplied by the gravitational acceleration (g) (to give Pb the same dimensions as AP/H). Uie marks the gas velocity at which initial expansion of the bed occurs, and Vmp is the gas velocity at the maximum pressure drop that can be tolerated across the bed (i.e. when z~P/His equal to Pb g)- Often, as in the example drawn here, initial expansion or premature channelling cause the up-curve to meet Pb g only at higher gas velocities; in such cases, to give consistent Ump values, the value of Ump (marked on the horizontal axis) represents that which would have been recorded had the expansion and/or channelling not occurred.

Prediction and correlation o f fluidization parameters Uie and Ump

Uie

It appears t h a t Uie f o r a sample c a n n o t be p r e d i c t e d , as e x p e r i e n c e shows t h a t t h e o n s e t o f e x p a n s i o n is sensitive t o the apparatus dimensions (groups o f coarse particles m a y wedge b e t w e e n t h e sidewalls and inhibit e x p a n s i o n ) and t h e state o f t h e material w h e n p o u r e d into t h e a p p a r a t u s (if n o t c o m p l e t e l y m i x e d , p r e m a t u r e channelling o c c u r s at low gas velocities).

62 Ump Three approaches to predicting Ump for a given sample were tried.

(a) Using complex published equations. The relationship between Umf and Ump suggests that one of the many published equations predicting Umf could also be used to predict Ump (useful compilations are in Babu et al., 1978, and Grewal and Saxena, 1980). Most Umf equations relate the pressure drop for fluid flow through a bed of particulate solids to the buoyant bed weight per unit area of bed. One often used is that based on the Ergun (1952) equation for the pressure drop across a packed bed (e.g. Kunii and Levenspiel, 1969, p. 73): 1.75 fdp Umf P g ] 2 1 5d0 ( ~1 - - pe m fg) [ d( p Up m fsp g-_ ] p g- ) g +

(2)

where ~ is the particle sphericity, emf the voidage at Umf, dp the particle diameter and pg the gas viscosity. If the particle Reynolds number, defined by:

dp UpK Rep - - -

(3)

#g

is less than 20, eq. 2 simplifies to: Umf

(~ alp) ~ p~ -pg -

-

-

150

pg

g

~f

(4)

1 -- emf

whilst at Rep > 1000: U2mf_ a d p 1.75

Ps--Pg g e~nf pg

(5)

In cases where emf and/or a are unknown, the correlations presented by Wen and Yu (1966) can be used. Generally: dp UmfPg~g -

[(33.7) 2 +0.0408 d~pg(Ps--Pg)g] 1/'p~g

-- 33.7

(6)

whilst, at Rep < 20: Umf = d~ (Ps -- Pg) g 1650 ~g

(7)

and at Rep > 1000: U2mf = dp (Ps --Pg) g 24.5 pg

(8)

In fluidizing ignimbrite samples, it is uncertain how equations 2 to 5 apply to materials which may have a 1st percentile over 10 4 times the size of the

63 99th percentile, cover a range of particle densities from 0.2 < Ps ~< 3.0 g cm -3 and contain particles of very different sphericities from ~ 0.75--0.65 for pumice clasts {Sparks, 1974) to irregular tricuspate shards. Can any of the relevant variables be quantified for an ignimbrite sample? First, because of the great range in sphericities, numbers and sizes of particles, it is d o u b t f u l if a meaningful value of ~ can ever be calculated. Second, values of emf might be estimated. Sheridan and Ragan (1976) present two equations:

Pb =~Ps

(9)

and =

1

--/3 = (Ps --PD)/Ps

(10)

where /3 is the solids volume fraction and ~ the pore space volume fraction o f the sample. However, they then make the incorrect inference that: = e

(11)

To show this is false, consider two assemblages of smooth spheres of the same diameter and composed of the same material, one assemblage being of solid and the other of hollow spheres, respectively. Equation I0 states that the two assemblages have differing values of ~ (because of the differing densities of the individual spheres), yet the voidages (e) of the two assemblages, as measured by the flow of gas around the particles, are identical. Thus for mixtures such as ignimbrite samples where some clasts are porous (i.e. hollow) V; should replace e in eq. 1, but the substitution of ~; for e in eqs. 2, 4 and 5 will lead to errors in Umf estimates. Vesicles within a pumice clast act merely to reduce its density and have no influence on the flow of gas through the mixture of which that clast is part. Sheridan and Ragan (1976) use eqs. 9--11 to infer that e in pyroclastic flows lies between 0.6 and > 0 . 9 (also q u o t e d by Sheridan, 1979, and Wohletz and Sheridan, 1979). However, m a n y pyroclastic flows and all the ignimbrite mixtures used in the experiments described here contain large amounts of pumice and hence @ derived by eqs. 9 and 10 is not equivalent to e in these materials. A lower limit for e is given by the closest packing state of a wide grain-size range assemblage of spheres which can reach e = 0.04 (Wakeman, 1975). One-phi interval grain-size cuts of ~> 2 mm pumices and lithics from New Zealand ignimbrites (author's unpublished data) have e = 0.4. Filling the interstices of these clasts with a fine matrix will give e < 0.4 and it is inferred that e in a typical pyroclastic flow will lie between 0.4 and <0.1. An upper limit for e is given by the sub-1/16-mm fractions of the same samples, composed o f irregularly shaped but non-porous glass shards, which when loosely packed have 0.56 ~< e -~ emf ~< 0.71. Thus emf in pyroclastic flows m a y vary greatly, depending on the grain-size distribution of the material, from ~ 0 . 7 in very fine grained flows to <0.1. Third, Umf estimates are very dependent on the value taken for the

64 particle diameter, dp, and problems arise when the sample contains a range of particle sizes. For such mixtures, a weighed mean grain size, dp, given by: dp -

1

n i= 1

(12)

xi (dp)i

where xi is the mass fraction of grain size (dp)i in the sample, can be substituted for dp in eqs. 2--8. However, calculation of dp is rather inaccurate unless closely spaced grain-size data are available, especially in the finer fractions, and in addition the dependence of gmf on ff~ for fine materials (eqs. 4 and 7) will exaggerate any errors in calculating dp. To sum up, a rigorous approach to estimating Ump for an ignimbrite sample appears unworkable for two reasons; first, the sample data are inadequate to use the full Umf eqs. 2, 4 and 5, and second, the complexities of the samples may invalidate the assumptions made by Wen and Yu (1966) in deriving the simplified eqs. 6--8.

(b) Using an empirical published equation. A less rigorous approach tried used one of the reduced Umf correlations for fine materials; (Rep < 20), of the general form (e.g. Davidson and Harrison, 1963; Richardson, 1971; BotteriU, 1975):

Umf =

k 3~ ( P s - - P g ) g #g

(13)

where k is an empirical constant. Data for those fluidization runs where dp could be calculated are plotted in Fig. 4. The resulting values of k range from ~0.0004 to 0.004, the variation being greatest for the finer samples where eq. 13 should be most accurate. The great variations in k for a given value o f (~p mean that this approach to predicting Ump is not useable.

(c) Using available experimental data. A third, simpler, approach developed by the author is presented here, specifically for use with ignimbrite samples, where the only data required are the grain-size distribution and the loose packed bulk
Ump or Umf U*p = loose-packed bed bulk-densitY

(14)

Next, the Folk and Ward (1957) grain-size parameters Mz and oi are calculated for the samples. These parameters were found to provide the best compromise between the relatively insensitive Inman (1952) parameters and other oversensitive measures such as the method of moments (Krumbein

65 I

I

/

4000-

/"

I

/Q

//

2800

,i

/

/ /'

2000-

//

/ •

/J

/

1400

....

.,

1000-

//

///

// / /

J

•

,/

/~ ,

355

//

/,

//

j

'

/

710 500"

/

/

/

/

/

///

/

/'

/

/

J

/

~

250-

,/

• // /

•/

180-

/

125-

/f //•

/

•

90634531-

/

•

0:4

Ump ,cm4g -ls -1

2 ,i

lb. 2'o 4o 16o

1 oo

Fig. 4. Plot o f V*mp values (eq. 14) versus dp (eq. 12) for all fluidization runs, bar four (where insufficient grain~size data were available to calculate dp). Squares and circles represent data from the 2-D and 3-D rigs, respectively. T h e fitted lines are values o f the c o n s t a n t , k, in eq. 13, calculated for a material w i t h Ps - - Pg = 1.67 g c m -3 (resulting in o v e r e s t i m a t e s o f k for sand and ballotini samples where 2.6 ~< (Ps - - Pg) ~< 2.9 g cm-3), fluidized b y air at r o o m T,P. N.B. values o f k > 0 . 0 0 4 relate to s o m e coarse, p o o r l y sorted ignimbrite samples where premature channelling occurred at U < Vmp.

and Pettijohn, 1938). These data for all the fluidization runs are plotted in Fig. 5, whilst Fig. 6 compares U*p data obtained by the author from relatively well-sorted samples with U*p values calculated from data in the literature. To calculate Ump for any unknown material, simply estimate its U*p value from Fig. 5 and then multiply by its loose-packed bulk-density. Although in both Figs. 5 and 6 there is much scatter (up to + ~50%),

66 4.0-

0--I

/'1.0 2.6•

0.~5

0.5

,,"

}.73

QSO

,'

3.0

2 21 •

\8o5 ~i 7 , 0.48 \~11 •1.7 L °O 47

",,

'"\

97. 7

,, 1.

' • ~ 10.84

\

0.49•0.47 •0.55 '1'Q.38

0.59 ..... .

2.0

1£' ~',

"~\ 4 3 ,

9:1

\\,,

.0 "94

2.3 2.5

• 0.70 " " ......

~

0.37 e

1.0

~- - '

• 247 •562

m166

•

261

149•

i

*

1

J /

16 22 m l

o

,

\.

",,, 4.6 , .... 47 7 ~ ', " • l "~)

7•

40•

l

,

"

1

"

,

2.3~ \ .... 72~8 "wl/ 2"7~L "i 1.8 / - -

r

2

,

3

f

I

P

I

I

100

10

5

2

1.0

"4 Theoreticcli

Mz ULp

f o r O- I = 0 . 0

Fig. 5. Plot o f M z versus o I for all samples used in fluidization runs. Squares and circles represent data from the 2-D and 3-D rigs, respectively. The figures against each s y m b o l represent its U*mp value in cm 4 g-1 s 1 obtained from the fluidization runs. Isopleths o f equal U * p values are marked; the values are in cm 4 g-1 s ~ for air at r o o m T,P. The figures b e l o w the M z axis are theoretical U*mp values for a perfectly-sorted spherical material w i t h ns = 1.67 and emf = 0 . 4 , fluidized b y air at r o o m T and P, calculated using eqs. 2--5.

most values obtained are consistent between the different equipment and materials used as well as with theoretical U * n values calculated for a perfectly sorted material. In comparison, Wen a n d Yu (1966) report that their correlation (eq. 6) predicted Vmf values for well-sorted systems with a standard deviation of 34% over a similar range of Rep values. Thus Fig. 5 is a useable method of estimating Vmp for ignimbrite samples, and even for well-sorted systems it is no less accurate than the more rigorous approaches tried earlier. Note also that the iso-U*~ lines in Fig. 5 curve such that samples with the same Mz values, but differing sorting characteristics, can have U*mp values differing by nearly two orders of magnitude. The position of the 10 cm 4 g- 1 s -1 iso-U*mp curve in the coarser poorly sorted systems is uncertain; samples in this grain-size range contain much coarse material and segregation usually occurs even when pouting the sample into the rig. For example, in t w o runs at least, the VSmpvalues (21 and 50 cm 4 g-1 s -]) are too high because of premature channelling. Values of Ump calculated from Fig. 5 for typical ignimbrites (cf. Sparks, 1976, fig. 2) range from ~ 1 0 to < 0 . 5 cm s -1, air at room T,P equivalent. Such gas velocities would remove particles of up to only ~ 2 0 / ~ m diameter (Kunii and Levenspiel, 1969, p. 76) and the particle size removed is, for the

o:1

67

o-,

S47 261 145

"562 .~66

Mz

h^

mm•

mn

-

I

6

-4 IO0

200

%

•

ulI A

b.37

.2.7 1.0 56

55

.4.6

.1.5

• 17

o

4Q .i

•

o o

0.5

• • 23

22.16

4.7 o o

•

•

8"5

•

•

2.7

".73

o %28 "2.4

•

!.0"

o

°o

••A~

°~3'2 %•o

o

•

•

•

000(30 G g O ~ I L • • ~ •

•

l

0

*1

A

I

I 10

2

i3 1.0

4

I

I

Mz

+5

0.1

o -I

1.0-

u

m

[]

u

D u

u

0.5 ¸

~,

•

Mz

% 01

+1o 001

0.001 Theoretico

U Om p

f o r CTI = 0 0

Fig. 6. Plots of M z versus a I for moderately- to well-sorted material, to compare the author's U*mp data with examples calculated from the literature. The dots with U* values attached represent the author's data (from Fig. 5). The svmbols renresent Ump values calculated from data m the references hsted below, taking Ump to be equal to the reported Umf values, using the presented grain-size data to calculate Mz and a I and using reported values of Pb {or Ps plus e). The symbols denote U*m, values in c m ' g-1 s-1 which are grouped as follows: 200--100, open triangles; 100--10, ];illed squares; 10--1.0, open circles; 1 . 0 - 0 . 1 , filled triangles; 0.1--0.01, open squares; 0.01--0.001, crosses; 0.001-0.0001, plus signs. The figures below the M z axis are theoretical U*mp values, calculated as in Fig. 5. The literature data refer to materials as diverse as copper shot, cracking catalyst, sugar balls, washing powder, ballotini and peas, and are taken from the following references: Baerns (1966); Becket (1961); Bjerle et al. (1980); Botterill and Desai {1972); Canada et al. (1978); Chang and Wen (1966); Chiba et al. (1979); De Jong and Nomden (1974); Gbordzoe et al. (1981); Geldart (1972, 1973); Geldart and Cranfield (1972); Godard and Richardson (1968); Hsiung and Thodos (1977); Kono (1981); Kunz (1971); Loew et al. (1979); Makishima and Shirai (1969); Mii et al. (1973); Nguyen et al. (1973); Nienow et al. (1978); Partridge et al. (1969); Rowe et al. (1972b) and Saxena and Vogel (1977)• •

.

.

~

~

m p

68 grain sizes considered, a relatively insensitive function of temperature and the gas species (Kunii and Levenspiel, 1969). Thus most layer 3 deposits, which have average grain sizes appreciably coarser than 20 ~m (cf. Sparks, 1976, fig. 5), must represent the elutriated fines from flows at U > Ump. This conclusion agrees with that reached from the laboratory studies, that significant elutriation will only occur from type 3 flows (Wilson, 1980).

Bed expansion during fluidization By knowing the volume, depth and bulk~lensity of the bed at the start of a run, the bed expansion and, if no material is lost during runs, the bed bulk 10, at which point the systems are becoming gas-sorted (see p. 74, below). Most ignimbrite samples fail to reach 10% expansion. The differences in expansion between the better sorted and ignimbrite samples reflect the differing bed behaviour at U > Ump (Umf). In the former, the bed expansion is simply a function of the n u m b e r and size of bubbles in the bed. In the latter, excess gas is concentrated into segregation channels and pipes (Wilson, 1980, fig. 4), only accompanied at high gas velocities by general bubbling near the bed surface; as the channels and pipes are coarse and better sorted, they can bypass large quantities of gas and overall bed expansion is restricted. At high gas velocities, the channels and pipes coalesce and the bed becomes size- and density-sorted. Then the upper part, which is better-sorted than the bed as a whole and enriched in fine/light material, forms a bubbling layer overlying a coarse/dense-material enriched quiescent base. While channelling dominates, bed expansion is rather irregular, whereas once layering dominates, expansion is more even though no more rapid (Fig. 8).

69 5040-

®

30x+ +

2015-

~x

z

Q ~oU3 Z Q

x w w D ._1 © >

543o

2-

1.5-

Ump

2'0

5'0

Fig. 7. V o l u m e percent e x p a n s i o n over the l o o s e - p a c k e d state o f samples, versus the V/Vmp ratio for the m a x i m u m gas v e l o c i t y r e a c h e d . T h e s y m b o l s are as follows: wells o r t e d s a m p l e s (a I < 0.4) - - plus signs; m o d e r a t e l y - s o r t e d samples (0.4 ~< o I <~ 1.5) in w h i c h no s u b - l / 1 6 m m fines are p r e s e n t - - crosses; m o d e r a t e l y - s o r t e d s a m p l e s w h e r e s u b - l / 1 6 m m fines are p r e s e n t , plus all p o o r l y - s o r t e d samples (a I > 1.5) - - filled circles. F o r the first t w o s y m b o l s , if t h e y are e n c l o s e d b y squares t h e n this d e n o t e s data from the 2-D rig; all o t h e r data are from the 3-D rig.

In some samples, the total bed deflation is measured from Umax to U=O and the apparatus thumped vigorously (to imitate the m a x i m u m compaction available in a non-welded ignimbrite). The data show (Fig. 9) that in ignimbrite samples the total compaction available from U/Vmp > 10 is <40% o f the final bed thickness; e.g. a 100-m-thick pyroclastic flow at 10 ~< V/Ump ~ 15 will deflate to form a (non-welded) ignimbrite n o t less than 70--85 m thick. The samples usually compact in two stages, the first from Umax to U=O when the bed deflates gradually as the gas supply is cut off and the second as the apparatus is thumped and the bed compacts suddenly,

70 EXl )onsion, °Io /. /

30

/ \

/

z

z

\ 20

Layering dominant

~ \ 1

Channelling dominant

" //"

\

,/.

/

.... /J~

/

Io

%

0

mp

g

10

1'5

Fig. 8. Bed expansion (volume p e r c e n t ) versus the relative gas velocity (U/Ump) for three p o o r l y - s o r t e d ignimbrite samples, showing t h e transition from a bed which is d o m i n a t e d b y channelling (irregular expansion) to one which is d o m i n a t e d by layering (more-regular expansion).

(a) 5-

10D

x

+

20-

z

×

Q 3o<~ 4 0 _.1 i, Is/ w _A © >

(b) 510x

+

2030-

%,

Ump

IO

2'0

71

often with the ejection of gas and dust through old segregation channels. If a ratio, W, is defined as: W=

deflation from U = Umax to U=O

(15)

compaction induced b y thumping the rig at U=O

and plotted v e r s u s U/Ump at U m a x (Fig. 10), then W is in all b u t three cases less than 1.0. The exceptions (not plotted in Fig. 10) are quartz sand mixes with W values of 4.9, 6.8 and 10.4. The great difference in W values is because the ignimbrite samples contain rough-surfaced particles (pumices and shards) and can stay very loosely packed due to particle interlocking, whereas the sand grains are smooth surfaced and can densify readily. A high proportion of the total deflation from pyroclastic flow to ignimbrite may thus be induced, after fluidization has entirely ceased, b y purely mechanical compaction, and estimates of the former fluidization state of deposits using compaction structures (e.g. Sheridan, 1979, fig. 5) must be treated with caution. W 1.0x

li

x+

•

0.5

Ump 0

.

0

.

.

.

i

5

.

.

.

.

i

.

10

.

.

.

i

15

Fig. 10. Parameter W, as defined in eq. 15, reached. Symbols are as in Fig. 7.

versus

V/Umpat

the maximum gas velocity

Degree of bed support during fluidization As mentioned above (p. 58), AP/H reflects the proportion of the bed weight being supported b y the gas through-flow which can then be compared with Pb. A factor, Y, is defined as: Y=

~P/H (Pb derived from the bed weight) g

at V = Vr,~

(16)

At U = Umax, if the bed weight is fully supported then Y =1, b u t if full bed-support is n o t reached then 0 < Y < 1. Values of Y are plotted versus Fig. 9. Volume-percent deflation, from the fluidized state at the maximum gas velocity reached to the non-fluidized, thumped-bed state, v e r s u s U/Ump at the maximum gas velocity reached. The deflation data are expressed relative to the thumped-bed state in part (a) and the expanded-bed state in part (b). Symbols are as in Fig. 7.

72

oi in Fig. 11, which shows that better~orted samples (ol ~< 1.0--1.5) have most or all of their bed weight supported at high gas velocities (Y ~> 0.85), whereas poorly sorted samples have only part of their bed weight supported (0.15 ~ Y ~ 0.70), even at gas velocities sufficient to cause gas-sorting of the bed. Y

1.0 --Ill,IF ~ t

i

0.8 /

o

• UmGx

o

. ~

•

izL_/1 z Ump

o

o

0.6-

,

•

© ©

0 0

0.4-

0.2

o

o

do

2:0

1

3'.0

4.0 O- z

Fig. 11. Parameter Y, as defined in eq. 16, versus a! for 47 runs from both rigs. The open and filled symbols refer to the mode of calculation o f Y (see inset). Because the value of AP/H may happen to be increasing at the particular maximum gas velocity (Umax) reached, parameter Y is calculated from either the lowest Ap/H value obtained at U > Vmp (open symbols) or the value of AP/H at U = Umax, whichever gives the lower value of Y.

Structures and grain-size variations within the fluidized bed Qualitative observations can be made through the container walls of bed structures during runs. Invariably, grain-size variations and segregation in the samples are always accompanied and caused by bubbling, and their nature changes with gas velocity and the sorting of the sample.

Well-sorted samples (oi < 0.4) Ballotini samples show behaviour similar to their counterparts in the literature. Immediately U m f is exceeded, bubbles appear and induce mixing and circulation within the bed. Sand samples show variable behaviour. With the coarsest of four samples, bubbling starts as soon as U m f is exceeded, but the finer samples show a gap between V m f and the gas velocity at which bubbles are first s e e n ( V m b ) . This

73 change in behaviour with grain size forms the basis of a fluidization behaviour model (Geldart, 1973), which distinguishes (Fig. 12) between materials that have Vmb ~- Vmf (group B) and those with Umb > Vmf (group A). The variable behaviour of the sand samples at U > Umf is thus explained; the four samples straddle the group A--group B boundary {Fig. 12). With finer samples, there is an interval Umf <: U < Grab where the bed is expanded but not bubbling, i.e. it is behaving as if it is particulately fluidized. In samples consisting of narrow grain-size cuts from normal ignimbrite, expansion sometimes occurs at U < Vmf (see p. 75). Bubbling always commences at U > Umf ' but mixing within the bed, except in the topmost few centimetres, is restricted by the particle surface roughness to m o v e m e n t immediately around rising bubbles. In these samples, bubbles often follow preferential tracks (cf. Whitehead, 1971; Werther, 1978) even though the bed is fully fluidized. Where the bubbles reach the bed surface a low cone forms from material ejected by the bubbles; these cones are persistent features and show that the bed still possesses a yield strength at depth. /Ds

~g,gcm -3

10-

\\

8 6

\~

'"

4

o

°

\\

\

\%\

C

0.4 0.2

~

\

2 1,00.8 0.6

D

",,

B

\\. ~p, jJm

0.1 ' ' ' 6o ' ' obo ' 10 Fig. 12. Plot of Ps -- Pg versus dp, showing the four groups of powder behaviour delineated by Geldart (1973). The groups are: A: powders which fluidize easily and have Umb > Umf; B: powders which fluidize easily and which bubble immediately Umf is exceeded; C: powders which fluidize only with difficulty (due to the dominance of interparticle surface-forces), showing a strong tendency to channel; and D: powders which require very high gas velocities to fluidize; the bubble rise velocity in the fluidized material is less than Umf. The four data points represented four well-sorted sand samples, whose

behaviour is discussed in the text.

Moderately-sorted samples (0.4 <~ oi <~ 1.0--1.5) In a few cases, the bed expands at U < Ump (p. 75). In ballotini samples with 0.4 ~ a I ~ 0.8--1.0, bubbling commences at gas velocities just above Ump ; the bubbles rapidly coalesce into a single

74 track and generate a non-bubbling segregation channel that propagates upwards. Bubbles are generated at the top of this channel and initiate circulation in the overlying material. After a few tens of seconds the channel reaches the bed surface, AP/H drops and bubbling and circulation cease, though fine material carried to the bed surface forms a vigorously bubbling layer on the otherwise quiescent bed. Each time U is increased, the existing channel is broken up by renewed bubbling into crescentic pods (cf. Wilson, 1980, fig. 4c) which either sediment downwards or are remixed into the bed, and a new segregation channel is generated. Eventually the bed becomes gas-sorted into layers, with coarser better-sorted material forming a basal packed bed, intermediate sized moderately-sorted material circulating above it and a finer-grained, better-sorted, vigorously fluidized layer at the top. As U approaches Umf for the basal material, circulation in the middle of the bed remixes the basal layer and the whole bed becomes fluidized and vigorously bubbling. As U is decreased from its maximum, the same processes operate in reverse to produce a bed which has a visible normal grading. The bed appears similar whether U is decreased rapidly or slowly, except that in the former case the lower part of the bed includes some fine material, due to the lack of time available for its removal. Sand mixtures with 0.81 ~< oi ~< 1.38 show broadly similar behaviour to the ballotini mixtures described above. However, the elutriation of trace amounts of fines occurs at gas velocities below those required to thoroughly mix the bed and these mixtures can only just be fully fluidized before their sorting is significantly improved by the loss of fines. This degree of sorting, samples more poorly-sorted than which cannot be fully fluidized, coincides with values of Y ~ 0.85 (Fig. 11). These sorting limits ( o i ~ 1.5) are those towards which more poorly-sorted samples will trend under sufficiently high gas velocities, and those which the better-sorted channels, pipes and layers within poorly-sorted fluidized samples will try to reach. Samples blended from narrow grain-size cuts of ignimbrite behave similarly to the sand mixes except that the varying particle densities cause compositional variations within the bed (p. 77) and any circulation within the middle of the bed is restricted by the surface roughness of the particles. Observations from fluidizing these moderately-sorted samples leads to the following conclusions: (a) That once gas velocities are increased beyond a certain range of U/Ump values (Fig. 13), the bed becomes layered and size-sorted, and (with ignimbrite samples) compositionally layered. (b) Once formed, any grain-size and/or compositional layering cannot be destroyed. If U is increased further to remix the bed, the layering re-appears as U is decreased. (c) If U is slowly reduced from high values, the bed becomes graded such that the sorting at any given level within the bed is better than that for the bed as a whole.

75

mp

10.

o

Layering dominant

ul

o •

0 •

oo

•

Channelling dominant

0 0

u3

(3-I t

L

2.o

3'.o

Fig. 13. Plot o f the bed state at the given VlUmp ratio, versus o I. If the bed is still dominated by channelling at the m a x i m u m gas velocity, then a filled circle is plotted. If the bed b e c o m e s d o m i n a t e d by layering, then the U/Urn p ratio at which this occurs is marked by a filled triangle. The stippled area marks the b o u n d a r y b e t w e e n the two bed states. All but five o f the data points are f r o m ignimbrite samples.

Poorly-sorted samples (o I > 1.0--1.5) All samples in this sorting category are ignimbrite mixtures or natural ignimbrite samples. Several samples, together with some better-sorted materials, start to expand at a gas velocity Uie which is less than Ump (Wilson, 1980). The expansion from Uie ~< U ~< Vmp is always small (Fig. 14) and shows no correlation with Mz or o~ for the sample. At U > Ump, poorly-sorted samples initially behave similarly to the moderately-sorted samples described above. Fine material is carried to the bed surface to form a vigorously bubbling layer at all U > Ump and this surface layer propagates downwards as U increases until a layered structure becomes the dominant feature of the bed (Fig. 13). Simultaneously, as U is increased, portions of the segregation channels within the bed sink to build up a basal fines-depleted layer (Wilson, 1980, fig. 4). Unlike in the moderately-sorted systems, this basal layer cannot be remixed, but tends to become better sorted as U is increased. At higher gas velocities, poorly~sorted samples differ in two respects from the moderately-sorted samples. First, elutriation is observed at all U > Ump as Ut < U for part of the grain-size distribution and second, the transition

76 ,°/o E x ~ansion f r o m U. to U le mp 4-

×

+

•

°o

U.

o:5

le

1:o Ump

Fig. 14. A m o u n t of bed expansion from Uie to U m p , expressed as a percentage of the original loose-packed bed volume, versus the relative gas velocity at which the expansion is first noted (Vie/Ump). Symbols are as in Fig. 7.

from a channelling- to a layering-dominated bed occurs at higher U/Ump ratios {Fig. 13). The upper fines-rich layer at U > Ump is always vigorously fluidized. Bubbles emanating from segregation channels in the underlying material agitate the layer and induce strong circulation which is such that the channels are usually prevented from propagating to the bed surface. Thus, although strongly fluidized, the upper part of the bed may show little or no sign of segregation pipes. Despite having a range o f dp values that should cause them to show some variable behaviour in the fluidization behaviour model of Geldart (1973; Fig. 12), all poorly-sorted samples show bubbling at U > U m p . Thus it appears that Geldart's scheme is invalid for poorly-sorted materials whose grain-size variations dominate their fluidization behaviour. However, these materials share some characteristics in c o m m o n with all four of Geldart's p o w d e r groups, showing some expansion w i t h o u t bubbling at Uie ~< U ~< Ump (cf. group A}, invariably bubble at U > U m p ( c f . g r o u p S ) , show strong channelling tendencies (cf. group C) and contain portions -- the coarse/ dense-material enriched segregation bodies and layers -- which should show group D behaviour. As in moderately-sorted systems, any layering and channelling structures formed are preserved as U is decreased, and observations from these systems lead to two conclusions in addition to those on p. 74: (d) The remixing and destruction of any segregation channels is impossible, unless gas velocities are increased to levels where the bed becomes layered.

77 (e) The remixing and destruction of any layering in the bed is impossible without resorting to gas velocities at which the grain-size distribution of the sample is so altered by elutriation that oi < 1.0--1.5.

Compositional variations within fluidized ignimbrite samples All the ignimbrite samples contain three kinds of clast; pumices (0.2 Ps ~ 1.5), lithics (2.0 ~ Ps ~ 2.6) and crystals (Ps ~ 2.6), each of which behaves differently during fluidization. In runs on well- to moderately-sorted samples at U > Ump, lithics and crystals are observed to concentrate at the base, and pumice at the top, of the bed. This overall zonation is not destroyed by bed circulation at higher gas velocities. In poorly-sorted samples, bed expansion at Uie ~< U ~< Ump may permit coarse-tail grading (Sparks, 1976; Wilson, 1980), but none was observed during these experiments for four reasons. First, the time during which Vie ~ V ~ Vmp was at most a few minutes, a much shorter time than would be available during the emplacement and degassing history of a pyroclastic flow (cf. Sparks, 1978). Second, no pumices or lithics more than a few centimetres across were present in the samples, and such small clasts tend to grade at very slow rates, if at all (Sparks, 1976). Third, the apparatus dimensions are such that wall effects would inhibit grading. Fourth, only the outside of the bed could be viewed, so that movement of clasts inside the bed would escape notice. The lack of coarse-tail grading at Uie ~< U ~< Ump during these experiments should not be taken to suggest that such grading does not occur at these gas velocities in pyroclastic flows. At U > Ump , when channels develop they tend to be composed of coarser pumices and finer lithics and crystals whereas at higher gas velocities the channels are more uniformly rich in lithics and crystals. The accompanying upper segregation layer always appears to be purely pumiceous. In some cases, channels and pipes are visibly richer in pumice towards their tops and in lithics and crystals towards their bases, reflecting in part the composition of the bed surrounding each level in the pipe; the lack of bubbling within the pipes themselves prevents internal re-sorting of the components. At gas velocities sufficient to induce layering (Fig. 13), coarser pumices and finer lithics and crystals tend to go together to the base, and finer pumiceous material to the top of the bed. The resulting strong stable density-stratification is one of the reasons why remixing of the bed at higher gas velocities is impossible. At moderately high gas velocities, coarse pumices can occasionally be seen circulating in the upper segregation layer; it is suggested that the degree of coarse-tail grading is also greatly accelerated by the presence of bubbling at U ) Vmp (Wilson, 1980). Other workers, using bimodal mixtures of particles which differ in size and/or density (Rowe et al., 1972a, b; Rowe and Nienow, 1976; Nienow et al., 1978) conclude that the degree of segregation of two kinds of particle

78

is proportional to their size ratio to the p o w e r 0.2 and their density ratio to the p o w e r 2.5. Using these criteria, the ratio of "degrees of segregation" in a typical ignimbrite sample, due to the grain-size variation as opposed to the density variations is about (10000) 0.2 : ( 2 . 6 / 0 . 2 ) 2"s -~ 1 : 100. This implies that density-induced segregation will dominate over size-induced segregation, b u t such is not the case. Segregation pipes and channels in ignimbrite samples are distinguished from their host material by their contrasting grain-size characteristics and only as gas velocities are increased further do the segregation bodies or layers become better density-sorted. Thus size segregation dominates in poorly-sorted materials, whilst density segregation becomes increasingly important as the sorting improves, though at no stage in the runs is segregation of lithics from crystals of the same size observed. From this, it is predicted that segregation bodies in ignimbrites will stand o u t because of their grain-size distribution contrasting with the host material and, although fines-poor, will contain a mixture of pumices, lithics and crystals. Because the lost fines are dominantly pumiceous (cf. Sparks, 1976; Sparks and Walker, 1977), lithics and crystals will be enriched in the segregation b o d y , b u t the crystal:lithic ratios in the segregation b o d y and host material should be the same. Elutriation

In well- or moderately-sorted ballotini and sand samples, elutriation is only seen at the highest gas velocities reached. In well- or moderately-sorted ignimbrite samples, trace amounts of fines generated during handling are lost as soon as bubbling commences. Poorly-sorted ignimbrite samples show no elutriation at U ~< Ump , unless accompanying premature channelling. At U ~ Ump, elutriation is always present, though in greater amounts at higher gas velocities when the bed is layered. When channelling occurs, the only material lost is that which is carried up within the channels, together with that carried off by the overlying bubbles. In all the runs, their duration was such that the loss of fines was incomplete (i.e. material with Ut < U was left in the bed). CONCLUSIONS

We k n o w little about the fluidization behaviour of pyroclastic flows and the work reported here represents only a beginning of our understanding. Seven conclusions are made from the present data. (a) The grain-size distribution of a typical ignimbrite is so wide that it dominates the fluidization characteristics of the ignimbrite samples and makes them behave in a radically different fashion to any described industrial material. {b) The crucial parameter to be determined for an ignimbrite sample is

79

Ump. At U ~< gmp , the material is quiescent, with only slow, viscosity dominated coarse-tail grading possibly occurring. At U > Ump, bubbling commences and the processes o f segregation and grading within and elutriation from the bed occur as a direct result. Published Umf correlations predict the value of Ump for an ignimbrite sample with less accuracy than an empirical m e t h o d developed for this paper (Fig. 5), which only requires the grain-size distribution and the loose-packed bulk-density of the sample. (c) Measurements of the bed volume before, during and after a run show that even at high U/Ump values, representative ignimbrite samples rarely exceed 20% expansion over their non-fluidized loosely packed volumes. At any U/Ump ratio, representative ignimbrite samples show only one quarter to one half the expansion seen in well- or moderately-sorted systems. (d) The pressure drop across the bed can be used to measure the proportion of the bed weight which is supported by the gas flow. Well- to moderately-sorted samples have ~ 85% o f their weight supported at all U > Ump whereas poorly-sorted ignimbrite samples have only 15--70% of their weight supported, even at very high gas velocities. (e) A characteristic set of features develops within fluidized representative ignimbrite samples. At U ~< Ump, the sample may show a limited amount o f expansion, whilst at U > Ump, segregation structures are formed. At moderate gas velocities, these consist of pods and sub-vertical pipes and channels which are depleted in pumice and fine material and enriched in lithics and crystals, together with a fines- and pumice-rich layer at the top o f the bed. At higher gas velocities (Fig. 13), the material becomes grain-size and Ump elutriation is obviously present. During each run, the overall loss of material is relatively small due to the limited duration of the experiments and the magnitudes of the gas velocities reached. (h) From data on the proportion of the bed weight supported by the gas flow, and observations of structures within the samples, the boundary between materials that can and cannot be fully fluidized lies at o I -~ 1.5, which is comparable to a theoretically inferred figure of o¢ = 1.0 (Wilson, 1980). During fluidization, materials with o I > 1.5 will generate segregation bodies or layers which have oi ~ 1.5, and at sufficiently high gas velocities the

80

whole sample will lose fine material by elutriation so that its overall sorting is improved to this level. A value of ~i ~ 1.5 thus represents a level of "ideal" sorting which ignimbrites, or portions thereof, will try to attain when fluidized. Representative ignimbrite samples thus have fluidization characteristics which are markedly different from those of any industrial material. The presence of indestructible segregation structures in all samples where U Ump implies that in any pyroclastic flow deposit where Ump was exceeded some segregation structures should be present. The complexities involved in extrapolating the laboratory behaviour of ignimbrite samples to conditions within a pyroclastic flow will be considered in another paper in this series. ACKNOWLEDGEMENTS

The fluidization experiments were carried out in the Chemical Engineering Department at Imperial College. I am indebted to Dr D.L. Pyle for his advice and help in undertaking this work. Assistance in setting up the rigs was provided by W. Meneer and C. Boyle. The manuscript was greatly improved by the comments of Professor Janet Watson, Dr S. Self, Dr R.S.J. Sparks and an anonymous reviewer. The receipt of a NERC research studentship and continuing support from a Royal Society Mr and Mrs John Jaff~ Donation Research Fellowship are gratefully acknowledged. NOMENCLATURE

dp dp g H

k Mz

P

Rep T U

Vie Vm~,, Umb Umf Ump

ut V*mp W Y

Particle diameter Weighed mean grain-size (eq. 12) Acceleration due to gravity Thickness of a fluidized bed (strictly, that part o f the bed thickness across which h p is measured -- see Wilson, 1980, fig. 1). Constant in simplified Umf correlation (eq. 13) Folk and Ward (1957) mean grain-size parameter, given by ( ~ , + $50 + ¢a4)/3, where ~ = --log2 (grain size in mm) Pressure Particle Reynolds number (eq. 3) Temperature Superficial gas velocity Superficial gas velocity at which bed expansion first noted Maximum superficial gas velocity reached during a run Superficial gas velocity at which bubbling first noted in bed Superficial gas velocity at m i n i m u m fluidization Superficial gas velocity at the m a x i m u m pressure-drop that can be sustained across the bed Superficial gas velocity which is equal to the terminal fall-velocity of a singleparticle Ump divided by the bed bulk-density (eq. 14) Parameter equal to the ratio o f the fluidization-induced to mechanically-induced compaction of a material (eq. 15) Parameter expressing the degree of bed support which is derived from gas-drag effects (eq. 16)

81 c~

E

emf

ttg Pb Pg Pm P$ (~I

¢;

Particle sphericity Volume fraction of solids in a particulate material Pressure drop, measured across thickness H of a fluidized bed Void fraction in a particulate solid Void fraction in a particulate solid at U = Umf Gas viscosity Bulk density of a particulate solid Gas density Bulk density of the matrix surrounding a large particle immersed in a fluidized bed Particle density Folk and Ward (1957) sorting parameter, (~gs -- ~ ) / 6 . 6 + (~s, -- ~ , ) / 4 Porosity of a particulate solid

REFERENCES Anderson, T. and Flett, J.S., 1903. Report on the eruptions of the Soufri~re in St Vincent, and on a visit to Montagne Pel6e in Martinique, Part I. Philos. Trans. R. Soc. London, Ser. A, 200: 353--553. Babu, S.P., Shah, B. and Talwalkar, A., 1978. Fluidization correlations for coal gasification materials -- minimum fluidization velocity and fluidized bed expansion ratio. AIChE Syrup. Ser., 74 (176): 176--186. Baerns, M., 1966. Effect of interparticle adhesive forces on the fluidization of fine particles. Ind. Eng. Chem. Fundam., 5: 508--516. Becker, H.A., 1961. An investigation of the laws governing the spouting of coarse particles. Chem. Eng. Sci., 13: 245--262. Bjerle, I., Eklund, H. and Svensson, O., 1980. Gasification of Swedish black shale in the fluidized bed. Reactivity in steam and carbon dioxide atmosphere. Ind. Eng. Chem., Process Des. Dev., 19: 345--351. Bloor, C.A., 1976. Pyroclastic Flows. B.Sc. Thesis, University of Lancaster, 38 pp. (unpubl.). Botterill, J.S.M., 1975. Fluid-Bed Heat Transfer. Academic Press, London, 299 pp. Botterill, J.S.M. and Desai, M., 1972. Limiting factors in gas fluidized bed heat transfer. Powder Technol., 6 : 231--236. Brown, M.C., 1962. Nu6es ardentes and fluidization: discussion. Am. J. Sci., 260: 467-470. Canada, G.S., McLaughlin, M.H. and Staub, F.N., 1978. Flow regimes and void fraction distribution in gas fluidization of large particles in beds without tube banks. AIChE Symp. Ser., 74 (176): 14--26. Chang, T.M. and Wen, C.Y., 1966. Fluid-to-particle heat transfer in air-fluidized beds. AIChE Symp. Ser., 62 (67): 111--117. Chiba, S., Chiba, T., Nienow, A.W. and Kobayashi, H., 1979. The minimum fluidization velocity, bed expansion and pressure-drop profile of binary particle mixtures. Powder Technol., 22 : 255--269. Davidson, J.F. and Harrison, D., 1963. Fluidised Particles. Cambridge University Press, Cambridge, 155 pp. Davidson, J.F. and Harrison, D. (Editors), 1971. Fluidization. Academic Press, London, 847 pp. Davidson, J.F., Harrison, D. and Guedes de Carvalho, J.R.F., 1977. On the liquidlike behaviour of fluidized beds. Annu. Rev. Fluid Mech., 9: 55--86. De Jong, J.A.H. and Nomden, J.F., 1974. Homogeneous gas-solid fluidization. Powder Technol., 9: 91--97. Ergun, S., 1952. Fluid flow through packed columns. Chem. Eng. Prog., 48: 89--94.

82 Folk, R.L. and Ward, W.C., 1957. Brazos River bar, a study in the significance of grainsize parameters. J. Sediment. Petrol., 27: 3--27. Fouda, A.E. and Capes, C.E., 1977. Hydrodynamic particle volume and fluidized bed expansion. Can. J. Chem. Eng., 55: 386--391. Frankel, N.A. and Acrivos, A., 1967. On the viscosity of a concentrated suspension of solid spheres. Chem. Eng. Sci., 22: 847--853. Gbordzoe, E.A.M., Bulani, W. and Bergougnou, M.A., 1981. Hydrodynamic study of floating contactors (bubble breakers) in a fluidized bed. AIChE Symp. Ser., 77 (205): 1--7. Geldart, D., 1972. The effect of particle size and size distribution on the behaviour of gas-fluidized beds. Powder Technol., 6: 201--215. Geldart, D., 1973. Types of gas fluidization. Powder Technol., 7 : 282--292. Geldart, D. and Cranfield, R.R., 1972. The gas fluidization of large particles. Chem. Eng. J., 3: 211--231. Gelperin, N.I. and Einstein, V.G., 1971. The analogy between fluidized beds and liquids. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 541--568. Godard, K. and Richardson, J.F., 1968. The behaviour of bubble-free fluidized beds. Inst. Chem. Eng., Symp. Ser., 30: 126--135. Grace, J.R., 1970. The viscosity of fluidized beds. Can. J. Chem. Eng., 48: 30--33. Grewal, N.S. and Saxena, S.C., 1980. Comparison of commonly used correlations for minimum fluidization velocity of small solid particles. Powder Technol., 26: 229-234. Hagyard, T. and Sacerdote, A.M., 1966. Viscosity of suspensions of gas-fluidized spheres. Ind. Eng. Chem., Fundam., 5: 500--508. Hsiung, T.H. and Thodos, G., 1977. Expansion characteristics of gas fluidized beds. Can. J. Chem. Eng., 55: 221--224. Inman, D.L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., 22: 125--145. Kono, H., 1981. Attrition rates of relatively coarse solid particles in various types of fluidized beds. AIChE Symp. Ser., 77 (205): 96--106. Krumbein, W.C. and Pettijohn, F.J., 1938. Manual of Sedimentary Petrology. AppletonCentury-Crofts, New York, N.Y., 549 pp. Kunii, D. and Levenspiel, O., 1968. Bubbling bed model. Ind. Eng. Chem., Fundam., 7: 446--452. Kunii, D. and Levenspiel, O., 1969. Fluidization Engineering. John Wiley & Sons, New York, N.Y., 534 pp. Kunz, R.G., 1971. Minimum fluidization velocity: fluid cracking catalysts and spherical glass beads. Powder Technol., 4: 156--162. Lacroix, A., 1904. La Montagne Pelde et ses Eruptions. Masson et Cie., Paris, 662 pp. Leva, M., 1959. Fluidization. McGraw Hill, New York, N.Y., 327 pp. Leva, M. and Wen, C.Y., 1971. Elutriation. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 627--650. Loew, O., Shmutter, B. and Resnick, W., 1979. Particle and bubble behaviour and velocities in a large-particle fluidized bed with immersed obstacles. Powder Technol., 22: 45--57. Makishima, S. and Shirai, T., 1969. Dead zone piled on the plate immersed in a fluidized bed. J. Chem. Eng. Jpn., 2: 12&-125. McTaggart, K.C., 1960. The mobility of nudes ardentes. Am. J. Sci., 258: 369--382. McTaggart, K.C., 1962. Nudes ardentes and fluidization: a reply. Am. J. Sci., 260: 4 7 0 476. Merrick, D. and Highley, J., 1974. Particle size reduction and elutriation in a fluidized bed process. AIChE Symp. Ser., 70 (137): 366--378.

83 Mii, T., Yoshida, K. and Kunii, D., 1973. Temperature-effects on the characteristics of fluidized beds. J. Chem. Eng. Jpn., 6: 100--102. Nguyen, X.T., Leung, L.S. and Weiland, R.H., 1973. On void fractions around a bubble in a two-dimensional fluidized bed. Proc. Int. Symp. "La Fluidization et ses Applications". Soci4t~ de Chimie Industrielle, Toulouse, pp. 230--239. Nienow, A.W., Rowe, P.N. and Cheung, L.Y.-L., 1978. A quantitative analysis of the mixing of two segregating powders of different density in a gas fluidized bed. Powder Technol., 20: 89--97. Partridge, B.A., Lyall, E. and Crooks, H.E., 1969. Particle slip surfaces in bubbling gas fluidized beds. Powder Technol., 2: 301--305. Potter, O.E., 1971. Mixing. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 293--381. Reynolds, D.L., 1954. Fluidization as a geological process, and its bearing on the problem of intrusive granites. Am. J. Sci., 252: 577--614. Richardson, J.F., 1971. Incipient fluidization and particulate systems. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 26--64. Richardson, J.F. and Zaki, W.N., 1954. Sedimentation and fluidization: part I. Trans. Inst. Chem. Eng., 32:35--53. Rowe, P.N., 1971. Experimental properties of bubbles. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 121--191. Rowe, P.N. and Nienow, A.W., 1976. Particle mixing and segregation in gas fluidized beds. A review. Powder Technol., 15: 141--147. Rowe, P.N., Nienow, A.W. and Agbim, A.J., 1972a. The mechanisms by which particles segregate in gas fluidized beds -- binary systems of near spherical particles. Trans. Inst. Chem. Eng., 50: 310--323. Rowe, P.N., Nienow, A.W. and Agbim, A.J., 1972b. A preliminary quantitative study of particle segregation in gas fluidized beds -- binary systems of near spherical particles. Trans. Inst. Chem. Eng., 50: 324--333. Saxena, S.C. and Vogel, G.J., 1977. The measurement of incipient fluidization velocities in a bed of coarse dolomite at temperature and pressure. Trans. Inst. Chem. Eng., 55: 184--189. Schfigerl, K., Merz, M. and Fetting, F., 1961. Rheologische Eigenschaften yon gasdurchstrSmten Fliessbettsystemen. Chem. Eng. Sci., 15: 1--38. Sheridan, M.F., 1979. Emplacement of pyroclastic flows: a review. In: C.E. Chapin and W.E. Elston (Editors), Ash-Flow Tuffs. Geol. Soc. Am., Spec. Pap., 180: 125--136. Sheridan, M.F. and Ragan, D.M., 1976. Compaction of ash-flow tuffs. In: G.V. Chilingarian and K.H. Wolf (Editors), Compaction of Coarse-Grained Sediments, Vol. II; Developments in Sedimentology 18B. Elsevier, Amsterdam, pp. 677--717. Sparks, R.S.J., 1974. The Products of Ignimbrite Eruptions. Ph.D. Thesis, University of London, 289 pp. (unpubl.). Sparks, R.S.J., 1976. Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology, 23: 147--188. Sparks, R.S.J., 1978. Gas release rates from pyroclastic flows: an assessment of the role of fluidization in their emplacement. Bull. Volcanol., 41 : 1--9. Sparks, R.S.J. and Walker, G.P.L., 1977. The significance of vitric-enriched airfall ashes associated with crystal-enriched ignimbrites. J. Volcanol. Geotherm. Res., 2: 329-341. Wakeman, R.J., 1975. Packing densities of particles with log-normal size distributions. Powder Technol., 11: 297--299. Wen, C.Y. and Yu, Y.H., 1966. Mechanics of fluidization. AIChE Symp. Ser., 62 (62): 100--111. Werther, J., 1978. Effect of gas distributor on the hydrodynamics of gas fluidized beds. Ger. Chem. Eng., 1: 166--174.

84 Whitehead, A.B., 1971. Some problems in large-scale fluidized beds. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 781--814. Wilson, C.J.N., 1980. The role of fluidization in the emplacement of pyroclastic flows: an experimental approach. J. Volcanol. Geotherm. Res., 8: 231--249. Wilson, C.J.N., 1981. Studies on the Origins and Emplacement of Pyroclastic Flows. Unpubl. Ph.D. Thesis, University of London, 402 pp. (unpubl.). Wohletz, K.H. and Sheridan, M.F., 1979. A model of pyroclastic surge. In: C.E. Chapin and W.E. Elston (Editors), Ash-Flow Tuffs. Geol. Soc. Am., Spec. Pap., 180: 177--194. Zenz, F.A., 1971. Regimes of fluidization behaviour. In: J.F. Davidson and D. Harrison (Editors), Fluidization. Academic Press, London, pp. 1--23. Zenz, F.A. and Othmer, D.F., 1960. Fluidization and Fluid--Particle Systems. Reinhold, New York, N.Y., 513 pp.