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The production and anatomy of a tungsten powder jet
Powder Technology 201 (2010) 296–300
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
The production and anatomy of a tungsten powder jet T.W. Davies b,⁎, O. Caretta a,1, C.J. Densham a,1, R. Woods c a b c
RAL, Chilton, Didcot, OX11 0QX, UK Engineering Department, University of Exeter, UK Gericke Ltd., Cavendish Street, Ashton-under-Lyne, OL6 7DJ, UK
a r t i c l e
i n f o
Article history: Received 5 June 2009 Received in revised form 17 March 2010 Accepted 19 March 2010 Available online 30 March 2010 Keywords: Jet flow Tungsten Powder jet High power target Neutrino factory
a b s t r a c t A tungsten powder jet is a potential candidate technology for a particle production target in a future high power (i.e. Multi-MW) particle accelerator based facility, such as a so-called conventional neutrino Super Beam, a proposed Neutrino Factory, or a future neutron source. To test the viability of producing a suitable powder jet a few simple experiments were performed using standard pneumatic conveying equipment and the encouraging results are presented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction This paper describes some preliminary studies of the production of a horizontal jet of powdered tungsten undertaken to investigate the viability of such a jet for use as a particle production target in a future high power particle accelerator based facility. In such facilities a multi-MW proton beam is required to interact with a dense target material in order to produce sub-atomic particles, e.g. neutrons for a neutron source or pions for a so-called conventional neutrino SuperBeam, neutrino factory or muon collider. The typical requirement is for a target material as dense as possible in order to minimise the production volume of the particles in question. Further the target has to survive high radiation doses and extreme thermal cycling whilst having a layout which allows effective removal of the heat generated by the interaction with the beam (of the order of 1 MW). The current baseline target technology for both a neutrino factory and a muon collider is an open liquid mercury jet, which has been prototyped in the MERIT [1,2] experiment at CERN. Consequently the first powder jet experiments reported here used a geometry consistent with that proposed for the liquid mercury jet. Specific requirements of such a target are, inter alia, that it presents a horizontal interception path of around 20 cm for the incoming horizontal proton beam, and is capable of absorbing the deposition of energy pulses of the order of 10 kJ/kg (see [3,4] for more details on required target properties and applications) whilst allowing off-line cooling of the hot powder. ⁎ Corresponding author. E-mail addresses: [email protected] (T.W. Davies), [email protected] (O. Caretta). 1 Tel.: + 44 1235 446224. 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.03.018
Standard pneumatic conveying equipment was used to produce dense horizontal powder jets (with porosity in the region of 60%) with an initial diameter of 2 cm and an initial horizontal velocity of between 2.4 m s− 1 and 7.6 m s− 1 using both pressurised air or helium as propellants. The jet flows were created by propelling tungsten powder from a storage hopper and along a horizontal 1 m long 2 cm i.d. stainless steel tube into a co-flowing stream of air. The literature on the pneumatic conveying of powders does not contain any information on the handling of material as dense as tungsten (density 19250 kg m− 3), the highest density material previously pneumatically conveyed as a powder being copper (density 8920 kg m− 3) [5,6]. There is little relevant literature on the production and behaviour of powder jets (see for example [7]), and none on the production or behaviour of large scale powder jets. There is however a long tradition of correlating the behaviour two-phase flows (gas/liquid, solid/fluid) on the basis of single phase fluid flows, with physical properties suitably weighted in relation to the volume fractions of the individual phases, and considerable insight into the complex behaviour of two-phase systems is sometimes possible using this approach. Observing the behaviour of the tungsten powder jets produced in our preliminary study indicates that these jets have some features in common with turbulent liquid jets, and it is worth summarising the main factors which define the characteristics of horizontal liquid-in-air jets (free jets). In order to produce a coherent and well behaved liquid jet the flow leading to the nozzle exit must be carefully controlled, and once the jet emerges from the nozzle disruptive interactions with the surrounding fluid must be minimised. The evidence from our preliminary experiments with tungsten jets strongly suggests that the same principles apply.
T.W. Davies et al. / Powder Technology 201 (2010) 296–300
Fig. 1. Diagrammatic sketch of the powder jet production system. (1) Feed hopper containing tungsten powder (2) flow valve (3) delivery tube 2 cm i.d., 1 m long (4) plexiglass sheath 14 cm i.d. (5) pressurised propellant gas (6) auxiliary gas injection [not used], (7,8,9) pressure tappings.
Table 1 Size distribution of tungsten powder as determined by a standard sieve test. Screen size (microns)
% retained
212 150 106 75 Below 75
4.5 29.9 19.8 15.2 30.5
Liquid jet flows are inherently unstable due to the interaction between the jet fluid and its surroundings and will break up into slugs and spray at a rate which depends on the conditions of the flow as it leaves the nozzle. Small disturbances on the jet surface are amplified and for a given system a preferential wavelength exists which is exponentially amplified. Liquid jet stability is usually characterised in terms of break up length which is defined as the distance from the nozzle at which the jet is broken for 50% of the time. Theoretically the break up point is where the instability wave amplitude becomes equal to the local jet radius. Break up distance increases linearly with jet nozzle exit speed up to a certain point, but then decreases rapidly thereafter [7,8]. One requirement of the powder jet is that it remains coherent for a considerable distance from the nozzle (at least 20d) and a well engineered liquid jet may not reach break up point until 100d. To preserve coherence it is important to suppress the formation of surface waves on the flow emerging from the nozzle. In water jets these waves may be caused by non-ideal flow immediately upstream of the nozzle exit. Hoyt and Taylor [10] studied wave formation on high velocity water jets using high speed photography and varied the velocity of the surrounding air relative to the jet. For jets emerging into stagnant air axisymmetric instability waves were observed within 1d of the nozzle exit, which then developed into classical Kelvin–Helmholtz helical instability waves as the jet developed and broke up. Coherence of the jet could be dramatically improved by using an axisymmetric co-flowing airstream. Their results demon-
297
strated that the deterioration of liquid jets in air is entirely due to aerodynamic drag acting on the instability waves, and their photographs of water jets show remarkable similarities to our photographs of tungsten powder jets. In order to prolong the coherent section of a liquid jet it has been shown that flow conditioning in the upstream pipework and the nozzle is vital [9,11,12]. In particular it is important to avoid undershoot or overshoot in the pressure gradient at the nozzle exit which leads to rapid disintegration of the jet [13]. In the case of the powder jets observed during our preliminary studies it is evident that high and low supply pressures lead to rapid disaggregation of the jet, whereas intermediate supply pressures result in a desirably coherent jet. Previous work with powder jets has not revealed similar trends. For example Lu and McDonald [14] reported only that a highly parabolic tungsten powder jet could be produced after falling under gravity through an elbow, and Ogata et al. [7] reported the velocity profiles in a freely falling vertical jet of glass beads and compared these with data for a single phase turbulent free jet of air. For an introduction to the principals and practice of bulk handling of powders see [5,6]. An important parameter which is used in the design of dense-phase pneumatic conveying systems is the density of the particulate material to be conveyed. In the case of tungsten powder the density is far beyond the upper limit of previous experience and so no useful design experience was available when considering how to generate the powder jet flow. It was decided to use existing best practice as the starting point for the present study. 2. Experimental equipment Fig. 1 shows a diagrammatic representation of the main elements of the equipment used in our preliminary study. The rig was designed and built by Gericke Ltd [15]. A stainless steel conical hopper of 20 l capacity was used to hold the tungsten powder. The tungsten powder was supplied by Wilbury Metals Ltd and had a resting bulk density of 8600 kg m− 3. The size distribution characterised by sieve analysis is shown in Table 1. The hopper was pressurised by either air or helium in the range 1.5 barg to 3.9 barg which was used to propel the powder from the hopper via a swept elbow and valve into a horizontal stainless steel smooth bore pipe, 1 m long and 2 cm id. This pipe was fitted with three pressure tappings, as indicated; one near the pipe exit, one in the middle of the pipe and one near the pipe entry. Existing best practice indicated the need for a gas injection nozzle in the elbow below the hopper if a dense-phase pneumatic transmission was to be achieved. Initial trials using auxillary gas injection suggested that the ejector action was causing severe duning to occur in the delivery pipe leading to highly disrupted jets, so that subsequent trials were carried out using hopper pressure alone as the driving mechanism, which proved to be sufficient to produce constant and dense powder flow. Initial trials also suggested that the powder jet was being significantly affected by interaction with the surrounding stagnant air, so that a co-flowing air stream was arranged around the emerging jet, which flowed at around 31 m s− 1, in order to minimise aerodynamic interference at the jet surface. The jet nozzle was placed concentrically inside a 14 cm id plexiglass tube which was connected to the inlet of a Roots blower to induce the required flow.
Fig. 2. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 1.5 barg He.
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Fig. 3. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 2.55 barg He.
Fig. 4. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 3.2 barg He.
3. Test results Figs. 2, 3 and 4 show representative pictures of powder jets produced using helium as the propellant in the equipment described above. The pictures are taken from a high speed video [16] of the jets operating at 6000 frames per second and clearly show the disrupted jets produced with driver pressures of 1.5 barg and 3.2 barg, whilst the jet produced with a 2.55 barg driver pressure is quite coherent over an axial distance of about 30 cm. All jets exhibit a distinct parabolic droop under the action of gravity and the slower the exit jet speed the more pronounced the droop. The corresponding mass flow rates of tungsten powder logged via a load cell were 6.4 kg s− 1 (Fig. 2), 6.9 kg s− 1 (Fig. 3) and 9 kg s− 1 (Fig. 4) and the jet exit velocities were estimated from the video camera to be respectively 2.4 m s− 1 3.2 m s− 1 and 7.6 m s− 1.The conclusion to be drawn from these representative results is that for the jet production system under test an optimum driver pressure exists which produces the most coherent jet. Driver pressures below or above this optimum value result in rapid jet disruption. Figs. 5, 6 and 7 show the time resolved pressures measured at the three sampling points (marked as pressure tappings 7, 8 and 9 in
Fig. 1) for the low, medium and high pressures used to produce the jets shown in Figs. 2, 3 and 4 respectively. It can be seen that for the low and high pressure conditions (Figs. 5 and 7), the delivery pipe pressures exhibit large fluctuations and a gradient in the flow direction, whereas for the 2.55 barg propellant pressure, the pressure along the pipe is low and uniform. All temporal pressure distributions exhibit a peak just before the end of a run corresponding to the remaining contents of the supply hopper being expelled and the hopper pressure being relieved. 4. Discussion and future work Video photography and recordings of delivery pipe pressures suggest that at low propellant pressure (air or helium) the pipe flow occurs as a series of high density slugs of powder suspension (see Fig. 5) and the resulting jet is pulsatile, of low velocity and travels in a pronounced parabolic arc, as exemplified by Fig. 2. Observations of flows generated with high propellant pressures also reveal a slugging or pulsatile pipe flow (see Fig. 6) and a disrupted jet travelling with a more horizontal trajectory (Fig. 4). It was found that at intermediate propellant pressures (in this case 2.55 barg) the pipe flow was well
Fig. 5. Sample of temporal pressure records from the three delivery pipe transducers (7,8,9 in Fig. 1) for a low propellant pressure (1.5 barg). Note the high fluctuating pressures in the upstream pipe section associated with “dune flow” in the pipe.
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Fig. 6. Sample of temporal pressures measured at the three pressure transducers for an intermediate propellant pressure (2.55 barg). Note the uniformly low and relatively steady pipe pressures which produced the coherent jet shown in Fig. 3.
behaved and the resulting powder jet acceptably coherent over a distance which would satisfy the assumed requirements of about 20 cm (Fig. 3). Factors which are likely to have an important bearing on the overall performance of the jet production and the jet anatomy are the particle shapes and size distribution. Fig. 8 show a micrograph of a sample of the powder used in the present tests. A powder with a significantly different size distribution and particle shape will very
probably produce a different behaviour pattern in the delivery pipe and the resulting jet. These initial results are sufficiently encouraging to justify a more detailed study of the processes involved in producing dense jets of heavy powder. Careful study of the video footage reveals that surface particles from the upper surface of the jet travel in a falling curve towards the bottom of the jet during transit from the nozzle and over
Fig. 7. Sample from the temporal pressures in the delivery pipe for a high propellant pressure (3.2 barg). Note the high pressure at the pipe inlet leading to jet disruption.
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Fig. 8. Micrograph of the tungsten powder used in the tests.
the field of view (approximately 30 cm). It appears that the upper part of the jet is less dense than the lower part, particularly so during duning (e.g. Figs. 2 and 4) when the crests of the dunes can be seen to be travelling at a much higher speed than the lower part of the jet. An estimate of the mean void fraction of the jet can be calculated from the mass flow rate, the mean velocity and the jet cross sectional area, giving a value of around 60%. This can be compared with the resting void fraction of the powder in the supply hopper of (9650/19250) or 50%, which can be considered as the minimum achievable value. More studies are now in progress with the aim of elucidating the mechanisms responsible for the behaviour described above and to establish the effects of particle size, particle shape, particle size distribution, driver pressure, delivery pipe geometry and the effect of a converging nozzle on the structure of the powder jet. Particular attention will be given to establishing those conditions which maximise the density and uniformity of the jet. Plunging the jet into water and capturing the gas bubbles should allow accurate estimates of the void fraction, as previously shown in water jet studies [17,18] and continued use of high speed video photography will provide the evidence for desirable jet anatomy. 5. Conclusions The tests reported here have demonstrated that it is possible to create a dense and coherent jet of tungsten powder which has the potential to form the basis of a new target design for a high power particle accelerator based facility. The jet production process is simple, involving only the release of pressurised powder from a hopper through a horizontal pipe. Further experiments have established that the powder can be collected, cooled and pneumatically recycled so providing continuous replenishment of the supply hopper. Acknowledgements Many thanks to the Engineering Instrument Pool [16] for loaning the high speed video equipment used for this work.
References [1] H.G. Kirk, H.J. Park, T. Tsang, et al., A high-power target experiment at the CERN PS, H.G, IEEE particle accelerator conference, vol. 1-11, 2007, pp. 1216–1218. [2] I. Efthymiopoulos, A. Fabich, et al., The MERIT(nTOF-11) high intensity liquid mercury target experiment at the CERN PS, MOPC087 Proceedings of EPAC08, Genoa, Italy, 2008. [3] C.J. Densham, O. Caretta, P. Loveridge, T.W. Davies, R.M. Woods, The potential of fluidised powder target technology in high power accelerator facilities, Proceedings of PAC09, Vancouver, Canada, 2009. [4] O. Caretta, C.J. Densham, T.W. Davies, R. Woods, Preliminary experiments on a fluidised powder target, WEPP161 Proceedings of EPAC08, Genoa, Italy, 2008. [5] W. Gericke, K.-E. Wirth, Pneumatic Conveyors for Bulk Material, PowtetekGericke Ltd, Ashton-under-Lyne, 1991. [6] D. Mills, M.G. Jones, V.K. Agarwal, Handbook of Pneumatic Conveying Engineering, Marcel Dekker0-8247-4790-9, 2004. [7] K. Ogata, K. Funatsu, Y. Tomita, Experimental investigation of a free falling powder jet and the air entrainment, Powder Technol. 115 (1) (2001) 90–95. [8] R.E. Phinney, Breakup of a turbulent liquid jet in a low pressure atmosphere, AIChem. E. J. 21 (5) (1975) 996–999. [9] M.J. McCarthy, N.A. Molloy, Review of stability of liquid jets and the influence of nozzle design, Chem. Eng. J. 7 (1974) 1–20. [10] J.W. Hoyt, J.J. Taylor, Waves on water jets, J Fluid Mech 83 (1) (1977) 119–127. [11] J.W. Hoyt, J.J. Taylor, Effect of nozzle shape and polymer additives on water jet appearance, Trans ASME, J Fluids Eng. 101 (1979) 304–308. [12] A.P. Hatton, M.J. Osborne, The trajectories of large fire fighting jets, Int. J. Heat Fluid Flow 1 (1) (1979) 37–41. [13] R. Narasimha, K.R. Sreenivasan, Relaminarisation in highly accelerated turbulent boundary layers, J. Fluid Mech. 61 (3) (1973) 417–447. [14] C. Lu, K.T. McDonald, Flowing Tungsten Powder for Possible Use as the Primary Target at a Muon Collider Source, 1998 Princeton/μμ/98-10. [15] R. Woods, Private Communication, Gericke Ltd, Ashton-under-Lyne, October 2007. [16] EPSRC Engineering Instrument Pool, www.eip.rl.ac.uk. [17] E. van de Sande, J.M. Smith, Surface entrainment of air by high velocity water jets, Chem. Eng. Sci. 28 (1973) 1161–1168. [18] E.J. McKeogh, D.A. Ervine, Air entrainment rate and diffusion patterns of plunging liquid jets, Chem. Eng. Sci. 36 (1981) 1161–1172.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
The production and anatomy of a tungsten powder jet T.W. Davies b,⁎, O. Caretta a,1, C.J. Densham a,1, R. Woods c a b c
RAL, Chilton, Didcot, OX11 0QX, UK Engineering Department, University of Exeter, UK Gericke Ltd., Cavendish Street, Ashton-under-Lyne, OL6 7DJ, UK
a r t i c l e
i n f o
Article history: Received 5 June 2009 Received in revised form 17 March 2010 Accepted 19 March 2010 Available online 30 March 2010 Keywords: Jet flow Tungsten Powder jet High power target Neutrino factory
a b s t r a c t A tungsten powder jet is a potential candidate technology for a particle production target in a future high power (i.e. Multi-MW) particle accelerator based facility, such as a so-called conventional neutrino Super Beam, a proposed Neutrino Factory, or a future neutron source. To test the viability of producing a suitable powder jet a few simple experiments were performed using standard pneumatic conveying equipment and the encouraging results are presented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction This paper describes some preliminary studies of the production of a horizontal jet of powdered tungsten undertaken to investigate the viability of such a jet for use as a particle production target in a future high power particle accelerator based facility. In such facilities a multi-MW proton beam is required to interact with a dense target material in order to produce sub-atomic particles, e.g. neutrons for a neutron source or pions for a so-called conventional neutrino SuperBeam, neutrino factory or muon collider. The typical requirement is for a target material as dense as possible in order to minimise the production volume of the particles in question. Further the target has to survive high radiation doses and extreme thermal cycling whilst having a layout which allows effective removal of the heat generated by the interaction with the beam (of the order of 1 MW). The current baseline target technology for both a neutrino factory and a muon collider is an open liquid mercury jet, which has been prototyped in the MERIT [1,2] experiment at CERN. Consequently the first powder jet experiments reported here used a geometry consistent with that proposed for the liquid mercury jet. Specific requirements of such a target are, inter alia, that it presents a horizontal interception path of around 20 cm for the incoming horizontal proton beam, and is capable of absorbing the deposition of energy pulses of the order of 10 kJ/kg (see [3,4] for more details on required target properties and applications) whilst allowing off-line cooling of the hot powder. ⁎ Corresponding author. E-mail addresses: [email protected] (T.W. Davies), [email protected] (O. Caretta). 1 Tel.: + 44 1235 446224. 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.03.018
Standard pneumatic conveying equipment was used to produce dense horizontal powder jets (with porosity in the region of 60%) with an initial diameter of 2 cm and an initial horizontal velocity of between 2.4 m s− 1 and 7.6 m s− 1 using both pressurised air or helium as propellants. The jet flows were created by propelling tungsten powder from a storage hopper and along a horizontal 1 m long 2 cm i.d. stainless steel tube into a co-flowing stream of air. The literature on the pneumatic conveying of powders does not contain any information on the handling of material as dense as tungsten (density 19250 kg m− 3), the highest density material previously pneumatically conveyed as a powder being copper (density 8920 kg m− 3) [5,6]. There is little relevant literature on the production and behaviour of powder jets (see for example [7]), and none on the production or behaviour of large scale powder jets. There is however a long tradition of correlating the behaviour two-phase flows (gas/liquid, solid/fluid) on the basis of single phase fluid flows, with physical properties suitably weighted in relation to the volume fractions of the individual phases, and considerable insight into the complex behaviour of two-phase systems is sometimes possible using this approach. Observing the behaviour of the tungsten powder jets produced in our preliminary study indicates that these jets have some features in common with turbulent liquid jets, and it is worth summarising the main factors which define the characteristics of horizontal liquid-in-air jets (free jets). In order to produce a coherent and well behaved liquid jet the flow leading to the nozzle exit must be carefully controlled, and once the jet emerges from the nozzle disruptive interactions with the surrounding fluid must be minimised. The evidence from our preliminary experiments with tungsten jets strongly suggests that the same principles apply.
T.W. Davies et al. / Powder Technology 201 (2010) 296–300
Fig. 1. Diagrammatic sketch of the powder jet production system. (1) Feed hopper containing tungsten powder (2) flow valve (3) delivery tube 2 cm i.d., 1 m long (4) plexiglass sheath 14 cm i.d. (5) pressurised propellant gas (6) auxiliary gas injection [not used], (7,8,9) pressure tappings.
Table 1 Size distribution of tungsten powder as determined by a standard sieve test. Screen size (microns)
% retained
212 150 106 75 Below 75
4.5 29.9 19.8 15.2 30.5
Liquid jet flows are inherently unstable due to the interaction between the jet fluid and its surroundings and will break up into slugs and spray at a rate which depends on the conditions of the flow as it leaves the nozzle. Small disturbances on the jet surface are amplified and for a given system a preferential wavelength exists which is exponentially amplified. Liquid jet stability is usually characterised in terms of break up length which is defined as the distance from the nozzle at which the jet is broken for 50% of the time. Theoretically the break up point is where the instability wave amplitude becomes equal to the local jet radius. Break up distance increases linearly with jet nozzle exit speed up to a certain point, but then decreases rapidly thereafter [7,8]. One requirement of the powder jet is that it remains coherent for a considerable distance from the nozzle (at least 20d) and a well engineered liquid jet may not reach break up point until 100d. To preserve coherence it is important to suppress the formation of surface waves on the flow emerging from the nozzle. In water jets these waves may be caused by non-ideal flow immediately upstream of the nozzle exit. Hoyt and Taylor [10] studied wave formation on high velocity water jets using high speed photography and varied the velocity of the surrounding air relative to the jet. For jets emerging into stagnant air axisymmetric instability waves were observed within 1d of the nozzle exit, which then developed into classical Kelvin–Helmholtz helical instability waves as the jet developed and broke up. Coherence of the jet could be dramatically improved by using an axisymmetric co-flowing airstream. Their results demon-
297
strated that the deterioration of liquid jets in air is entirely due to aerodynamic drag acting on the instability waves, and their photographs of water jets show remarkable similarities to our photographs of tungsten powder jets. In order to prolong the coherent section of a liquid jet it has been shown that flow conditioning in the upstream pipework and the nozzle is vital [9,11,12]. In particular it is important to avoid undershoot or overshoot in the pressure gradient at the nozzle exit which leads to rapid disintegration of the jet [13]. In the case of the powder jets observed during our preliminary studies it is evident that high and low supply pressures lead to rapid disaggregation of the jet, whereas intermediate supply pressures result in a desirably coherent jet. Previous work with powder jets has not revealed similar trends. For example Lu and McDonald [14] reported only that a highly parabolic tungsten powder jet could be produced after falling under gravity through an elbow, and Ogata et al. [7] reported the velocity profiles in a freely falling vertical jet of glass beads and compared these with data for a single phase turbulent free jet of air. For an introduction to the principals and practice of bulk handling of powders see [5,6]. An important parameter which is used in the design of dense-phase pneumatic conveying systems is the density of the particulate material to be conveyed. In the case of tungsten powder the density is far beyond the upper limit of previous experience and so no useful design experience was available when considering how to generate the powder jet flow. It was decided to use existing best practice as the starting point for the present study. 2. Experimental equipment Fig. 1 shows a diagrammatic representation of the main elements of the equipment used in our preliminary study. The rig was designed and built by Gericke Ltd [15]. A stainless steel conical hopper of 20 l capacity was used to hold the tungsten powder. The tungsten powder was supplied by Wilbury Metals Ltd and had a resting bulk density of 8600 kg m− 3. The size distribution characterised by sieve analysis is shown in Table 1. The hopper was pressurised by either air or helium in the range 1.5 barg to 3.9 barg which was used to propel the powder from the hopper via a swept elbow and valve into a horizontal stainless steel smooth bore pipe, 1 m long and 2 cm id. This pipe was fitted with three pressure tappings, as indicated; one near the pipe exit, one in the middle of the pipe and one near the pipe entry. Existing best practice indicated the need for a gas injection nozzle in the elbow below the hopper if a dense-phase pneumatic transmission was to be achieved. Initial trials using auxillary gas injection suggested that the ejector action was causing severe duning to occur in the delivery pipe leading to highly disrupted jets, so that subsequent trials were carried out using hopper pressure alone as the driving mechanism, which proved to be sufficient to produce constant and dense powder flow. Initial trials also suggested that the powder jet was being significantly affected by interaction with the surrounding stagnant air, so that a co-flowing air stream was arranged around the emerging jet, which flowed at around 31 m s− 1, in order to minimise aerodynamic interference at the jet surface. The jet nozzle was placed concentrically inside a 14 cm id plexiglass tube which was connected to the inlet of a Roots blower to induce the required flow.
Fig. 2. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 1.5 barg He.
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Fig. 3. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 2.55 barg He.
Fig. 4. Tungsten powder jet leaving a 20 mm ID cylindrical nozzle propelled by 3.2 barg He.
3. Test results Figs. 2, 3 and 4 show representative pictures of powder jets produced using helium as the propellant in the equipment described above. The pictures are taken from a high speed video [16] of the jets operating at 6000 frames per second and clearly show the disrupted jets produced with driver pressures of 1.5 barg and 3.2 barg, whilst the jet produced with a 2.55 barg driver pressure is quite coherent over an axial distance of about 30 cm. All jets exhibit a distinct parabolic droop under the action of gravity and the slower the exit jet speed the more pronounced the droop. The corresponding mass flow rates of tungsten powder logged via a load cell were 6.4 kg s− 1 (Fig. 2), 6.9 kg s− 1 (Fig. 3) and 9 kg s− 1 (Fig. 4) and the jet exit velocities were estimated from the video camera to be respectively 2.4 m s− 1 3.2 m s− 1 and 7.6 m s− 1.The conclusion to be drawn from these representative results is that for the jet production system under test an optimum driver pressure exists which produces the most coherent jet. Driver pressures below or above this optimum value result in rapid jet disruption. Figs. 5, 6 and 7 show the time resolved pressures measured at the three sampling points (marked as pressure tappings 7, 8 and 9 in
Fig. 1) for the low, medium and high pressures used to produce the jets shown in Figs. 2, 3 and 4 respectively. It can be seen that for the low and high pressure conditions (Figs. 5 and 7), the delivery pipe pressures exhibit large fluctuations and a gradient in the flow direction, whereas for the 2.55 barg propellant pressure, the pressure along the pipe is low and uniform. All temporal pressure distributions exhibit a peak just before the end of a run corresponding to the remaining contents of the supply hopper being expelled and the hopper pressure being relieved. 4. Discussion and future work Video photography and recordings of delivery pipe pressures suggest that at low propellant pressure (air or helium) the pipe flow occurs as a series of high density slugs of powder suspension (see Fig. 5) and the resulting jet is pulsatile, of low velocity and travels in a pronounced parabolic arc, as exemplified by Fig. 2. Observations of flows generated with high propellant pressures also reveal a slugging or pulsatile pipe flow (see Fig. 6) and a disrupted jet travelling with a more horizontal trajectory (Fig. 4). It was found that at intermediate propellant pressures (in this case 2.55 barg) the pipe flow was well
Fig. 5. Sample of temporal pressure records from the three delivery pipe transducers (7,8,9 in Fig. 1) for a low propellant pressure (1.5 barg). Note the high fluctuating pressures in the upstream pipe section associated with “dune flow” in the pipe.
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Fig. 6. Sample of temporal pressures measured at the three pressure transducers for an intermediate propellant pressure (2.55 barg). Note the uniformly low and relatively steady pipe pressures which produced the coherent jet shown in Fig. 3.
behaved and the resulting powder jet acceptably coherent over a distance which would satisfy the assumed requirements of about 20 cm (Fig. 3). Factors which are likely to have an important bearing on the overall performance of the jet production and the jet anatomy are the particle shapes and size distribution. Fig. 8 show a micrograph of a sample of the powder used in the present tests. A powder with a significantly different size distribution and particle shape will very
probably produce a different behaviour pattern in the delivery pipe and the resulting jet. These initial results are sufficiently encouraging to justify a more detailed study of the processes involved in producing dense jets of heavy powder. Careful study of the video footage reveals that surface particles from the upper surface of the jet travel in a falling curve towards the bottom of the jet during transit from the nozzle and over
Fig. 7. Sample from the temporal pressures in the delivery pipe for a high propellant pressure (3.2 barg). Note the high pressure at the pipe inlet leading to jet disruption.
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Fig. 8. Micrograph of the tungsten powder used in the tests.
the field of view (approximately 30 cm). It appears that the upper part of the jet is less dense than the lower part, particularly so during duning (e.g. Figs. 2 and 4) when the crests of the dunes can be seen to be travelling at a much higher speed than the lower part of the jet. An estimate of the mean void fraction of the jet can be calculated from the mass flow rate, the mean velocity and the jet cross sectional area, giving a value of around 60%. This can be compared with the resting void fraction of the powder in the supply hopper of (9650/19250) or 50%, which can be considered as the minimum achievable value. More studies are now in progress with the aim of elucidating the mechanisms responsible for the behaviour described above and to establish the effects of particle size, particle shape, particle size distribution, driver pressure, delivery pipe geometry and the effect of a converging nozzle on the structure of the powder jet. Particular attention will be given to establishing those conditions which maximise the density and uniformity of the jet. Plunging the jet into water and capturing the gas bubbles should allow accurate estimates of the void fraction, as previously shown in water jet studies [17,18] and continued use of high speed video photography will provide the evidence for desirable jet anatomy. 5. Conclusions The tests reported here have demonstrated that it is possible to create a dense and coherent jet of tungsten powder which has the potential to form the basis of a new target design for a high power particle accelerator based facility. The jet production process is simple, involving only the release of pressurised powder from a hopper through a horizontal pipe. Further experiments have established that the powder can be collected, cooled and pneumatically recycled so providing continuous replenishment of the supply hopper. Acknowledgements Many thanks to the Engineering Instrument Pool [16] for loaning the high speed video equipment used for this work.
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