Fluidization and attrition of pyroclastic granular solids

Transcription

1 Journal of Volcanology and Geothermal Research 138 (2004) Fluidization and attrition of pyroclastic granular solids T. Gravina a, L. Lirer a, A. Marzocchella b, P. Petrosino a, *, P. Salatino b a Dipartimento di Scienze della Terra-Università degli Studi di Napoli Federico II, Largo S. Marcellino 10, 80143, Napoli, Italy b Dipartimento di Ingegneria Chimica-Università degli Studi di Napoli Federico II, Piazzale Tecchio, 80125, Napoli, Italy Received 24 November 2003; accepted 11 June 2004 Abstract Pyroclastic flows have been frequently addressed in the recent volcanological literature as far as the depositional mechanisms and the field features of the related deposits are concerned. Much less attention has been paid to phenomena that control the dynamic of pyroclastic flow motion. The peculiar feature of pyroclastic flows, hence focused, is the onset of an intense upflow of gas, whose velocity may be so large that a fluidized state may be established in the granular phase. The fluidization behaviour of granular synthetic mixtures has been studied in a broad range of conditions, mostly driven by the widespread use of fluidized beds in the process industry. However, experimental studies focused on fluidization of natural granular mixtures are lacking. The aim of this paper is the characterization of the fluidization behaviour of natural pyroclastic mixtures. In particular, mixing and segregation of granular mixtures of natural origin and fluidization-induced abrasion/fragmentation of pyroclastic particles have been investigated. A pyroclastic mixture extracted from a pumice fall deposit has been used as the starting material, as it is representative of the whole material of a plinian column immediately before collapse and emplacement of the pyroclastic flow deposits. Experimental tests have been also performed using the granular material extracted from pyroclastic flow deposits. Incipient and complete fluidization velocities, onset of particle segregation due to size and density polydispersity of the samples, elutriation of attrition-induced fine particulates and attrition-induced changes of particle size distributions have been characterized. When comparing the segregation propensity of the pumice fall and of the pyroclastic flow starting material, it is concluded that only the former exhibits significant segregation. Results of elutriation and attrition experiments highlight the transient nature of particle attrition phenomena: generation of fines by attrition of relatively coarse granular solids is at a maximum in the early stage of fluidization and decays significantly thereafter. D 2004 Elsevier B.V. All rights reserved. Keywords: pyroclastic density current; fluidization; granular mixtures; fragmentation 1. Introduction * Corresponding author. address: petrosin@unina.it (P. Petrosino). A pyroclastic flow is a two-phase granular mass flow most frequently caused by the gravitational collapse of a plinian column or, to a lesser extent, /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.jvolgeores

2 28 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) by gravitational collapse of lava domes or lateral blasts. It consists of solid particles (juvenile and lithic fragments) dispersed in a gas phase whose main components are water vapour and air. Wilson and Walker (1982) highlighted the existence of three distinct regions within a pyroclastic flow: head, body and tail. The head of the current is characterized by strong recirculation patterns, which induce air entrainment, turbulence and strong shear of the granular phase, resulting in attrition and elutriation of fine solids. The body is a poorly expanded mixture behind the head where solid segregation phenomena mostly occur. The tail is the slowest part of the flow lagging behind the head and the body. A peculiar feature of pyroclastic density currents is the onset of an intense upflow of gas through the current as it runs down the slope of the volcano. This phenomenon is more intense in the frontal region of the current the head where the effects of air entrainment add to the bendogenousq release of gases from pyroclastic solids. The ascent velocity of the gas through the current may be so large that a fluidized state may be established in the granular phase. Accordingly, the weight of the granular solids is partly or entirely supported by the fluidizing gas, the frictional stresses within the sheared granular phase are strongly reduced and the fluidity of the pyroclastic density current moving over the incline is dramatically increased. The effectiveness of gas upflow in promoting fluidization can be significantly enhanced by the presence of very fine particles which contribute to the gas solids momentum exchange. These may either be present in the native pyroclastic solids or be extensively generated in situ by attrition phenomena in the strongly sheared basal regions of the flow. The relevance of fluidization to pyroclastic flows has been frequently addressed in the recent literature both in the volcanological and in the multiphase flow fields (Wilson, 1980, 1984; Marzocchella et al., 1998, 2000; Roche et al., 2002; Olivieri et al., 2004). These studies aimed at understanding: (1) fluidization in binary mixtures of granular synthetic particles; (2) segregation phenomena of the solid fraction inside the flow; (3) the dynamics of pyroclastic flow motion. Wilson (1980, 1984) addressed fluidization of natural granular mixtures, highlighting the role that fluidization plays in both particle transport and deposition, especially in the head and body regions. Wilson (1980) performed fluidization tests on ignimbrite samples, very poorly sorted and containing solid fractions corresponding to all the groups of the Geldart classification of powders, but exhibiting cohesive group-c behaviour, in laboratory rigs at room temperature. Wilson (1980) divided the fluidization curve into three ranges of gas superficial velocities: Range 1 Range 2 Range 3 No expansion of the bed occurs nor does elutriation. The bed is partly fluidized and expanded. Gas flows mostly in channels along the bed. These ranges were used as a basis for classification of pyroclastic flows into three types. Recent experiments by Druitt et al. (2004) showed that the cohesive nature of broadly polydisperse pyroclastic solids may be at least partly overcome under sheared flow conditions. Di Pastena (1997) analyzed the fluidization behaviour of binary mixtures of granular synthetic particles. He concluded that fluidization velocities in a pyroclastic flow should range between a few centimeters per second and values close to the bcomplete fluidization velocity,q or U cf, of the pyroclastic granular mixture. U cf, the velocity at which the weight of the bed is fully supported by the fluidizing gas, is critically dependent on grain size, temperature and humidity of the mixture and takes different values for different kinds of granular mixtures. In particular, it was speculated that complete fluidization can only be achieved for very fines-rich granular mixtures. The light fraction (pumice fragments) floats on the fluidized fine matrix, whereas the heaviest one (lithic fraction) sinks across the current. Similar conclusions were drawn by Roche et al. (2002), who investigated the influence of the initial degree of fluidization and of the particle grain size on flowability. Based on experimental data collected with synthetic granular mixtures, they concluded that the mobility of pyroclastic flows is controlled by the contribution of group A particles [ranging between and Am according to the Geldart s (1973) classification of powders] that form the pyroclastic flow fluidized matrix. The quoted experimental studies were mostly based on the use of fluidized beds of synthetic particle

3 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) mixtures (glass grains, plastic grains, etc.) or natural mixtures of very narrow grain size. The density and shape of particles in synthetic mixtures were accurately controlled and their size fell in a rather narrow grain size range in order to simulate the actual pyroclastic solids. On the other hand, there are several features of real pyroclastic flows that are poorly reproduced in simulation experiments. Pyroclastic solids are characterized by a broad grain size, shape and density particle population. Due to their mechanical strength and angular shape, pyroclastic solids are extremely susceptible to attrition. The present paper reports an investigation of the fluidization behaviour of a natural granular mixture of pyroclastic origin (pumice fall). A pumice fall pyroclastic mixture has been chosen since it represents part of the content of a plinian column. When a pyroclastic flow is generated, the juvenile fraction at first shows morphological shapes similar to those of the sustained column phase (fall deposit). The present study represents one step toward the assessment of the dynamics of gas solids suspensions relevant to dense pyroclastic gravity currents, with specific concern for the occurrence of fluidizationinduced fragmentation and attrition. Consistently with Burgisser and Bergantz (2002) and Neri et al. (2002), it was believed that particles of 500 Am and coarser would predominate in the basal, dense part of a pyroclastic flow, whereas finer (V500 Am) particles originally present in the population would rapidly contribute to the formation of the overlaying ash cloud. Accordingly, the solids population in the sample was limited to particles coarser than 500 Am. Moreover, limiting the original population to solids coarser than 500 Am enabled better assessment of attrition and/or fragmentation phenomena. Characterization of fully polydisperse populations of solids with a significant fraction below 500 Am is currently in progress and will be reported soon. 2. Experimental 2.1. Materials Two types of granular mixtures were used in the experiments, extracted from either a pyroclastic fall deposit or a pyroclastic flow deposit. The pyroclastic fall deposit was sampled at Marigliano (Naples, Italy) from a grey pumice fall layer of the Somma-Vesuvio Avellino eruption (3760F70 BP, Rolandi et al., 1998), one of the most intense Somma-Vesuvio plinian eruption (Rolandi et al., 1993). Avellino eruption pumice fall layers are mainly made up of pumice, lavic and limestone lithic fragments and crystals. White and grey pumices are quite porphyrithic and mainly contain anorthoclase phenocrysts. The pyroclastic flow deposit was sampled at Licola (Naples, Italy) from an unwelded outcrop of Neapolitan Yellow Tuff from Campi Flegrei (12 ky BP, Alessio et al., 1973). This deposit contains subrounded pumices and very rare lithic fragments dispersed in a glass shard matrix whose dusty elements fill pumice fragments cavities. The sampled material was preliminarily sieved in field to eliminate the coarse fraction, then washed in deionised water to eliminate dust. The grain size fraction selected for fluidization experiments ranges in grain size between 1000 and 500 Am (Fig. 1). Lithological component analysis (Table 1) and SEM characterization were performed on this fraction before the fluidization tests. Densities of lava lithic fragments and crystals have been taken from the literature (Deer et al., 1994), whereas density of pumice and limestone lithic fragments was determined by taking the ratio of sample mass over apparent volume. Granular solids were sieved into samples belonging to relatively narrow particle size ranges that were separately tested. Technical-grade air at ambient temperature and pressure was used as the fluidizing gas Apparatus Two fluidization columns were used in the experiments, represented in Fig. 2:! A 40-mm ID PMMA column (Fig. 2a), equipped with a head purposely designed for collection of elutriated material. A feature of the collection head is that it is possible to rapidly switch the exhaust line to either of two filters in parallel. While one filter was in operation, it was possible to substitute the other. This protocol enabled time-resolved collection of elutriated material.

4 30 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 1. Grain size distribution of Avellino sample (A) and Neapolitan Yellow Tuff (B).! A 120-mm ID PMMA column (Fig. 2b), consisting of five cylindrical segments each 2.5 cm high. A freeboard section 1 m high is assembled on top of the segments. The five segments could be disassembled, and the material contained therein could be separately retrieved. This procedure enabled characterization of the size and nature of bed solids as a function of the axial coordinate along the bed. Gas distribution is accomplished by a porous PTFE plate in the case of the 40-mm ID column by means of a porous sintered plate for the 120-mm ID apparatus. Both fluidization columns are equipped Table 1 Analysis of lithological components of the investigated granular samples (vol.%) Sample / Size range (Am) Mean size (Am) Limestone lithic fragments q=1000 kg/m 3 (%) Lava lithic fragments q=2500 kg/m 3 (%) Crystal grains q=3000 kg/m 3 (%) Avellino sample 1a b a b Neapolitan Yellow Tuff sample 3a b a b Pumice fragments q=1000 kg/m 3 (%)

5 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Experimental procedure Three types of experiments were carried out. In all the tests, granular samples belonging to rather narrow particle size intervals were used (Table 1) Fluidization tests These tests aimed at the characterization of the fluidization parameters of the granular samples. They were performed in the 120-mm ID fluidization column. The granular sample was gently poured into the fluidization column. The gas superficial velocity was quasi-steadily increased, starting from the fixed bed state until the bed became vigorously fluidized. Afterwards, the superficial gas velocity was slowly reduced until the bed returned into the fixed state. During the whole test, the bed level and the time series of the relative pressure along the fluidization column were continuously recorded. At the end of each fluidization test, once the bed returned into the fixed state, bed material was retrieved and analyzed in order to characterize the grain size distribution and the lithological components along the bed. Material present in each of the five segments was retrieved separately and characterized from the standpoint of particle size distribution and lithological features. Bed height was fixed at about 12 cm in order to meet the constraint of operating the bed in the multiple-bubble regime far enough from the onset of slugging. Fig. 2. Scheme of fluidization plant equipped with a 40-mm ID PMMA column (a) and a 120-mm ID Plexiglas column (b). The experimental apparatus consists of: porous sintered plate (A), fluidized bed (B), freeboard (C), head for collection of elutriated fines (D), filters (E), pressure transducer (pt) (not to scale). with electronic pressure transducers providing a time-resolved axial profile of the relative pressure. Gas superficial velocity is measured and controlled by means of rotameters and mass flow meters. Data logging was carried out during the tests by means of a data acquisition unit Attrition tests Attrition tests aimed at the characterization of particle attrition during fluidization, assessed by monitoring changes of particle size distribution along with fluidization. They were carried out in the 40-mm ID fluidization column. During the tests, the bed was kept in the boiling state at a constant value of the gas superficial velocity (U 1 W, as specified below) for specified time intervals. Bed level and time series of the relative pressure along the fluidization column were continuously recorded. At the end of each time interval, the fluidization gas flux was abruptly interrupted, the bed material retrieved and the whole sample analyzed to obtain the particle size distribution. Then, the sample was charged again into the bed layer by layer and

6 32 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) gas superficial velocity (U 2, as specified below) for 45 minutes. The finest particles those whose free-fall velocity is smaller than the gas superficial velocity leave the bed as elutriated material and are collected in the filters. Fifteen filters were used to get the timeresolved elutriation rate from the bed according to the following sequence: one every 1 min for the first 5 min, one every 3 min for the following 15 min and one every 5 min for the following 30 min. Only the Avellino eruption granular material (fall deposit) was tested in this way. 3. Results 3.1. Fluidization tests Fig. 3. Fluidization curve displaying pressure (A) and pressure gradient (B and C) of sample 1b of Avellino eruption pumice fall as functions of gas superficial velocity, obtained in experiments with the 120-mm ID PMMA column. Black symbols refer to measurements carried out at increasing gas velocity; hollow symbols refer to measurements carried out at decreasing gas velocity. z is the height of the pressure taps; Dz is the difference between the pressure taps heights. fluidized for another time interval. The procedure was iterated several times Elutriation tests Elutriation tests aimed at the assessment of fines generation and elutriation during vigorous fluidization of the granular samples. They were carried out in the 40-mm ID fluidization column equipped with the purposely designed head for collection of elutriated fines. During the tests, the bed was kept in a vigorously boiling state at a constant value of the Fig. 3 reports experimental results obtained in a typical fluidization test carried out with sample 1b (Table 1). Results are reported as relative pressure P at different levels along the bed and as pressure gradient (DP/Dz) averaged over the distance between two successive pressure taps. Both variables are expressed as functions of the gas superficial velocity. Profiles corresponding to quasi-steadily increasing (UP) and decreasing (DOWN) gas superficial velocity are compared. It appears that they do not overlap with each other, and hysteresis can be observed upon cyclic change of gas superficial velocity. When focusing the attention on the UP profiles (black symbols in Fig. 3A), consistently with previous observations (Marzocchella et al., 2000; Olivieri et al., 2004), it is possible to recognize the existence of the following phenomenologically distinct regimes: UbU 1 U 1 bubu 1 V The bed is in the fixed state. Relative pressure at the bottom of the bed is smaller than the value corresponding to bed weight per unit cross-sectional area of the column. At U 1, transition from the fixed to the incipiently fluidized state of the bed occurs. The bed is in a homogeneously expanded state. The relative pressure at the bottom of the bed equals the bed weight per unit cross-sectional area of the column. U 1 V is the gas superficial velocity on the verge of bubbling.

7 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) U 1 VbUbU 1 W U 1 WbUbU 2 U 2 bu The bed is in a vigorously bubbling state which enhances particle segregation. U 1 W is the gas superficial velocity at which segregation associated with bed polydispersity is a maximum. The bed is in a vigorously bubbling state. Beyond U 1 W, particle mixing starts overtaking particle segregation. U 2 is the gas superficial velocity beyond which solids mixing fully overtakes segregation. The bed is in a vigorously bubbling state. Solids are well mixed throughout the bed. Analysis of the pressure gradient, measured along the bed as a function of U, is an effective tool to monitor solids mixing/segregation phenomena (Fig. 3B). For U 1 WNUNU 1, the pressure gradient DP/Dz 3 4 (that is, the gradient measured between the fourth and the third pressure taps) decreases as U increases. Correspondingly, DP/Dz 1 2 and (only up to U 1 V) DP/Dz 2 3 increase. When it is considered that the pressure gradient is proportional to the local value of the average bed density, it is concluded that solids segregation is emphasized by an increase of gas superficial velocity in the range U 1 WNUNU 1 : light particles (flotsam) accumulate at, or nearby, the bed surface, whereas denser solids (jetsam) sink toward the bed bottom. At UNU 1 W, pressure gradients measured at different bed levels tend to level off until, at UNU 2, gradients at different levels along the bed tend to a common value. This demonstrates that, beyond U 1 W, bubble-induced solids mixing starts overtaking segregation. Experimental results of fluidization tests carried out with other samples are summarized in Table 2. The qualitative features observed with sample 1b were generally reproduced with other samples of the Avellino pumice fall. Grain-size analysis performed on the Avellino eruption pumice fall at the end of the fluidization tests on sample 1b indicates that a new grain size population, represented by fine particles generated by attrition, can be observed, which was absent in the native particle population. Vertical distribution of particles along the bed, reported in Fig. 4, indicates that, as expected, the finest particles accumulate at the top of the bed, almost entirely represented by pumice and limestone fragments. Mainly crystals and lava lithic fragments can be found at the bottom of the bed (Fig. 5). This lithological distribution is in good agreement with the pressure data recorded at the pressure taps during the fluidization experiments. Fig. 6 reports the lithological fractional distribution measured at various heights along the column for three particle size ranges. It is observed that the lithological fractional distribution is little affected by the particle size range. It is concluded that segregation is controlled by the lithological components and associated particle densities (in particular, density of limestone was found to be much lower than expected, probably because of carbonate decomposition during the eruption) to a larger extent than by particle size polydispersity of the starting material. This is consistent with a wellestablished feature of polydisperse fluidized beds: polydispersity regarding particle density is a much more effective driving force for particle segregation Table 2 Results of fluidization tests performed using the 120-mm ID fluidization column on Avellino eruption and Neapolitan Yellow Tuff samples Sample Size range (Am) Bed mass (kg) Elutriated mass (kg) U 1 (m/s) U 1 V (m/s) U 1 V (m/s) U 2 (m/s) Avellino sample 1a b a b Neapolitan Yellow Tuff sample 3a b a b

8 34 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 4. Grain size analysis of sample 1b at the end of fluidization test, performed on the plant equipped with the 120-mm ID PMMA column. The particles b500 Am in grain size were generated by abrasion during the fluidization test. than size differences among particles (Nienow and Chiba, 1985). The analysis of fluidization curves obtained on Neapolitan Yellow Tuff samples (Fig. 7) (flow deposit) shows little variation in pressure gradients trend and, as a consequence, negligible segregation of lithological components along the bed. This is consistent with the observation that the Neapolitan Yellow Tuff is much more homogeneous as regards lithological distribution than the Avellino pyroclastic fall sample. Grain size analysis performed on the Neapolitan Yellow Tuff at the end of the fluidization tests indicates that a new population of fine-grained material appears, as a consequence of attrition during the fluidization experiment. Finally, the density of the bed is the highest at the bottom, where pumice fragments containing crystals sank, and lowest at the top, where only glass shards occur, whereas in the intermediate part, the values are constant (Fig. 8). be some 30% larger than the corresponding value in the 120-mm ID apparatus Elutriation tests Elutriation tests were carried out as described in Section 2 at a constant gas superficial velocity U equal to U 2 for any given sample. It is worth noting that the values of U=U 2 adopted in the elutriation tests were those assessed in experiments carried out in the 40-mm ID fluidization column. Due to the different size of the column, and consistently with the known influence of the bed size and height on solids mixing/ segregation, U 2 in the 40-mm ID column turned out to Fig. 5. Fluidization test performed on sample 2b in the plant equipped with the 120-mm ID column. On the top of the bed, the segregation of the lightest material (pumice and limestone lithic fragments, light in colour), on the bottom of the bed the segregation of the heaviest material (lava lithic fragments and crystals, dark in colour) can be observed.

9 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 6. Lithological distribution of particles in sample 1b at the end of fluidization experiments.

10 36 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 7. Fluidization curve displaying pressure (A) and pressure gradient (B and C) of sample 3b of Neapolitan Yellow Tuff eruption pumice flow, as functions of gas superficial velocity, obtained in experiments with the 120-mm ID PMMA column. Black symbols refer to measurements carried out at increasing gas velocity; hollow symbols refer to measurements carried out at decreasing gas velocity. z is the height of the pressure taps; Dz is the difference between the pressure taps heights. Elutriated dust was collected on a filter at pre-set times from the beginning of the test. Only the Avellino eruption granular material (fall deposit) was tested in this respect. Fig. 9 reports the results of a typical elutriation test. It can be noted that elutriation rate is very large at the beginning of the test, approaching a much smaller ultimate value after about 30 min. It must be recalled that the bed material initially consisted of nonelutriable material under the fluidization conditions tested. Therefore, elutriated material was only that generated by attrition during fluidization of bed solids.

11 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 8. Axial profile of density of bed material after fluidization tests. Sample 3b of Neapolitan Yellow Tuff. The pronounced peak of the elutriation rate in the early stage of the experiments is explained as a consequence of the removal of surface asperities from the rather angular byoungq material, which emphasizes attrition at the beginning of the tests. The subsequent decay of the attrition rate is related to the progressive brounding offq of bed material which reduces its propensity to attrition (see Section 3.4). On the average, elutriation rates were in the order of kg of elutriated material/(kg of bed material per second). At the end of each elutriation test, the bed material was tested as regards the distribution of grain sizes and of lithological components. Grain-size analysis performed on the Avellino eruption pumice fall (sample 1b) at the end of the elutriation tests indicates that the vertical distribution of particles is not affected by particle size, but rather by density. Pumice and limestone fragments are more abundant at the top of the bed, crystals and lava lithic fragments at the bottom. Experimental results obtained with all the samples tested are summarized in Table 3. Elutriation rates reported in Table 3 are directly related to attrition rates of the granular material due to surface wear. According to Ray et al. (1987), attrition rate scales as the mechanical power dissipated in the granular bed. It is worth noting that the power per unit mass of the granular material that was dissipated during elutriation experiments, calculated as the product (Ug) of the gas superficial velocity U and the acceleration due to gravity g, was of the order of 10 W/kg. In a real dense pyroclastic gravity current, the mechanical energy dissipation rate, associated with granular frictional stresses, equals the rate of change of potential energy along the incline, i.e. Fig. 9. Elutriation rate as a function of time (sample 1a, Avellino fall deposit).

12 38 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Table 3 Results of elutriation tests performed using the 40-mm ID fluidization column on Avellino eruption samples Sample Size range (Am) Bed mass (kg) U (m/s) T e (s) Peak elutriation rate (s 1 ) Ultimate elutriation rate (s 1 ) 1a b a b T e is the time at which elutriation rate approaches the asymptotic value. U s gsin(h), where U s is the speed of the pyroclastic current and h is the slope of the incline. Assuming typical values of U s (i30 m/s) and of h (i458), the rate of mechanical energy dissipation can be in excess of 200 W/kg, i.e. more than 1 order of magnitude larger than those experienced by the granular material during the experiments. If attrition rates are speculatively scaled as the dissipated mechanical power per unit mass, one might expect attrition rates in real dense pyroclastic gravity currents be on the order of kg of elutriated material/(kg of bed material per second). Accordingly, some 15% of granular material would be subjected to attrition over a time interval of 15 min. Despite the speculative and approximated character of these figures, they help to underline how extensive the generation of fine particulates might be under real pyroclastic flow conditions Attrition tests Attrition tests were carried out as described in Section 2 at a constant gas superficial velocity equal to U 1 W for any given sample. At this velocity, the bed is in a boiling-segregative regime, characterized by moderate boiling (close to incipient bubbling) at the bed bottom and a top region of the bed which is definitely boiling. According to Sparks et al. (1976), this condition could reasonably well represent those of natural pyroclastic flows. During the attrition tests, periodic retrieval of the samples and sieve analysis were directed to quantitative assessment of attritioninduced changes of particle size distribution. Only the Avellino eruption granular material (fall deposit) was tested in this respect. Table 4 and Fig. 10 indicate that significant changes of the particle size distribution occur upon prolonged fluidization. The coarsest grain size fraction becomes less and less populated, and material is transferred to the finer particle size intervals. The distribution of the material among different size classes approaches an asymptote upon prolonged fluidization of the bed material (Fig. 10). On the whole, the modification of the particle size distribution observed is consistent with the extensive occurrence of fragmentation of the granular solids, that is, breakage of mother particles into a relatively small number of fragments. Particle fragmentation takes place at the same time as the generation of elutriable fine particles by surface abrasion. Noteworthy, the Table 4 Results of attrition tests performed using the 40-mm ID fluidization column on Avellino eruption sample Sample Size range (Am) Bed Elutriated Lost mass mass mass (kg) kg) (kg) U Time T a (m/s) (s) (s) Weight change within each grain size interval (kg) Am Am Am Am Am Am Am 1a (b600am) 1b (b500am) 2a (b500am) 2b T a is the time at which steady particle size distribution was achieved. +ve (resp. ve) values indicate increase (resp. decrease) of the weight of bed material in the given size interval. b300 Am

13 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 10. Total weight change of the mass of bed material in each particle size interval along with fluidization. Sample 1b tested in the plant equipped with the 40-mm ID PMMA column. lithological component analysis highlighted the occurrence of an increase in the fraction of loose crystals, probably due to the abrasion of pumice fragments embedding crystal grains SEM Analysis At the end of the various fluidization experiments, the natural granular mixture was investigated by SEM in order to characterize the morphoscopic variation of the juvenile fraction. As to the Avellino eruption juvenile fraction, the pumice fragments changed from acicular and edge-shaped (Fig. 11a) to round-shaped after the fluidization tests (Fig. 11c). Void vesicles of pumice fragments, variously shaped and completely free of dust before the tests (Fig. 11b), became full of fine material produced as a consequence of clast collisions during fluidization (Fig. 11d). SEM analysis of the fine fraction of the Avellino samples 1a and 1b, after the attrition and elutriation tests, clearly shows the occurrence of many glass shards, not observed before the test. Results are consistent with observations of James et al. (2003) who interpreted the presence of the fine fraction to be a consequence of pumice pumice collision The dynamics of particle segregation Fig. 12 reports time-resolved pressure signals measured along the bed after fluidization was suddenly established at U=U 1 W starting from the fixed bed conditions. Data refer to sample 1b and were collected in the 120-mm ID apparatus. At the beginning (t=0) of the reported time interval, the bed is completely fluidized. Correspondingly, the pressure drop measured at each pressure tap equals the weight of bed material above the tap level divided by the bed cross-sectional area. The pressure drop recorded at the bottom is constant and equal to the initial value during the whole test and is not reported in Fig. 12. At any level, pressure decays from the initial value to an asymptotic one corresponding to the segregated state of the bed. Pressure relaxation occurs over a timescale t* which depends on the location of the pressure tap: the shorter the t* is, the larger the distance from the distributor. According to Marzocchella et al. (2000), it is assumed that t* marks the time at which a segregation front passes by the given level. Fig. 13 is a cross-plot of the relaxation time t* as a function of the distance from the gas distributor. The speed V s of the segregation front can be determined as the slope of the plot. This speed turns out to be about 0.4 cm/s for the system considered in Fig. 13. Speeds of the segregation front measured in beds of synthetic dissimilar materials, with grain size between 500 and 100 Am, were of the same order of magnitude (Marzocchella et al., 2000). An order of magnitude assessment of the timescale of segregation in real dense pyroclastic gravity currents can be made by assuming (Sparks et al., 1976): (i) a thickness of the granular bed of the order of meters, 3 8 m; (ii) an average duration of the flow of order of min. Accordingly,

14 40 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 11. SEM micrographs of Avellino eruption sample before and after fluidization and attrition tests. (a) Pumice fragment falling in the size range Am, before the fluidization test (100); (b) close up on a pumice fragment falling in the Am size range before the fluidization test (750); (c) pumice fragment, falling in the size range Am, at the end of the attrition test (50); (d) close up of a pumice fragment falling in the size range Am after the fluidization test (1500). Fig. 12. Time-resolved pressure profiles at different levels along the bed. The large-frequency oscillations are related to bed fluctuations associated with the passage of bubbles. Material: sample 1b. U=0.46 m/s. Symbols are described in Fig. 3A.

15 T. Gravina et al. / Journal of Volcanology and Geothermal Research 138 (2004) Fig. 13. Cross-plot of pressure tap level versus pressure relaxation time t* obtained from time-resolved pressure profiles reported in Fig. 12. and assuming V s of about 0.4 cm/s, segregation time would be of the order of min, that is, comparable with the pyroclastic flow lifetime. 4. Conclusions The reported experimental campaign addressed some features of the fluidization behaviour of natural granular solids. In particular, incipient and complete fluidization velocities, onset of particle segregation due to size and density polydispersity of the samples, elutriation of attrition-induced fine particulates and fragmentation-induced changes of particle size distributions have been characterized in purposely designed experiments. When comparing the fluidization behaviour of the Avellino and of the Neapolitan Yellow Tuff samples, it is concluded that only the former exhibits significant segregation. This different behaviour is interpreted in the light of the features of the native material. The Neapolitan Yellow Tuff, in fact, being a pyroclastic flow deposit, already underwent selective transport and associated segregation. The mixture extracted from the Avellino fall deposit presents a much more polydisperse nature as regards lithological composition, and its grain size distribution is only dependent on selective air transport driven by terminal velocity of single clasts. The ground layer sometimes present in the basal part of a pyroclastic flow deposit can be compared with the bottom layer of the fluidized bed, enriched in lithic fragments and crystals, produced after the fluidization experiment of the Avellino fall starting material. The absence of this layer at the bottom of the Neapolitan Yellow Tuff is attributed to the narrow density range in which the particles of the starting mixture fall. Results of elutriation and attrition experiments highlight the transient nature of particle attrition phenomena: generation of fines by attrition of relatively coarse granular solids is a maximum in the early stage of fluidization, decaying significantly thereafter. This feature, which is commonly experienced also in studies of attrition of fluidized solids used in the process industry, is related to the removal of surface asperities from angular byoungq solids, followed by rounding off of the particles which reduces their propensity to attrition. Generation of fine particles by attrition, which is expected to be very extensive in the head of a pyroclastic flow, is bound to affect in a significant manner the mobility of the granular flow by reducing the complete fluidization velocity of the mixture and enhancing gas solids momentum exchange. In addition, elutriation tests indicate that the fine fraction generated by attrition is only partly elutriated and is partly retained in the bed. Fragmentation of particles into relatively coarse fragments takes place along with fluidization in parallel with generation of fine particulates by surface abrasion. Acknowledgments The authors wish to gratefully thank O. Roche and S. Lane for thoughtful and constructive reviews of the paper. This work is part of the project of the Regional Center of Competence banalysis and Monitoring of the Environmental RiskQ supported by the European Community on Provision 3.16.

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