Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...

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40 In the case of agricultural materials and food powders, apart form the classifications mentioned above attention should be paid also to a number of additional features, such as: friable, easily degradable, freezing, hygroscopic, toxic properties, flammable, explosive, very dusty, decomposes, deteriorates in storage. The tendency of certain materials to freeze may constitute a serious problem during winter and demand for heating equipment. Explosive powders require the application of suitable construction materials, protective devices, and following fire prevention rules. In some cases wetting of transported material is applied. 5. DENSITY OF GRANULAR MATERIALS Density, that is mass divided by volume, is one of the fundamental parameters of granular materials. Knowledge of the exact value of the density of a deposit of granular material is very important for numerous practical applications. The density of a material has a significant effect on its mechanical characteristics. It is one of the three basic parameters, along with the friction coefficient and the pressure ratio, that are used in the determination of granular material pressure against the structure of the bin or silo. It is also necessary for accurate estimation of container capacity. With relation to their bulk density, granular materials are classified as: light (peat, sawdust, bran, cereal meal, dried plant material) ρ < 600 kg m -3 , medium (cereal grain, fertilizers, soil) 600 < ρ ≤ 1100 kg m -3 , heavy (mineral raw materials, sand, gravel) 1100 < ρ ≤ 2000 kg m -3 , very heavy (minerals, stone) ρ > 2000 kg m -3 . 5.1. Bulk density A popular method for the determination of the bulk density is based on measurement of the mass of a granular material poured freely into a cylindrical container of constant volume, typically 0.25 or 1 dm 3 [22, 137 138]. The values of bulk density of typical agricultural materials presented in Table 5.1 differ significantly from the density of those materials in silos [22, 31]. That density is a function of moisture, pressure, degree of contamination, manner and rate of filling, and falling height of the grain [135] Cereal grain density usually varies within a relatively broad range, depending on the species and cultivar, manner of bin or silo filling, height of deposit, degree of contamination of the grain, and other factors.
41 Tapped density provides information on the susceptibility of a granular material to compaction through vibrations. The relevant standard provides for a measurement consisting in bringing a known mass of a granular material to the lowest volume possible through the application of vibrations of constant amplitude and frequency [138. The tapped density of cereal grain is higher than the bulk density by several percent, and in some cases even by over twenty percent [159. Chang et al. [31] showed that distributed filling of silo increases the density of granular material by from 5.1 to 9.2% as compared to filling from centrally located spout of conveyor. Stephens and Foster [155] observed increases in density of the order of 3 to 5% above the bulk density values in condensed filling from spout of a conveyor, and 7% in the case of distributed filling. Versavel and Britton [166] showed that density depends on the falling height, the degree of contamination, and on the filling rate. The researchers noted a considerable increase in density, of the order of 8-10%, and in the case of high filling rates a decrease in density. Similar relations were found by Schott and Britton [150] in laboratory studies. With increasing grain falling height the kinetic energy of the grain increases, which increases the packing density of the material [116]. That effect disappears above a certain height, due to increasing aerodynamic drag during the free fall of the grain. Table 5.1. Bulk density, porosity and specific gravity of grain [3] Grain Barley Rape Maize Linseed Oat Rice Rye Soy Wheat Bulk density (kg m -3 ) 618 669 721 721 412 579 721 772 772 Moisture (%) 9.7-10.7 6.5-6.7 9-15 5.8 9.4-10.3 11.9-12.4 9.7 6.9-7.0 9.8 Porosity (%) 39.5-57.6 38.4-38.9 40.0-44.0 34.6 47.6-55.5 46.5-50.4 41.2 33.8-36.1 39.6-42.6 Specific gravity (kN m -3 ) 12.1-13.3 11.0-11.5 11.9-13.0 11.0 9.5-10.6 11.1-11.2 12.3 11.3-11.8 12.9-13.2 In view of the wide range of variation and considerable number of factors affecting density, attempts are made at developing methods for the determination of „apparent density” that would correspond to the density of a material in a silo. Therefore, it is necessary to search for general rules applicable to the determination of the actual density of granular media in containers that would be common for as extensive a class of materials as possible. Basing on experimental results obtained so far it is recommended to estimate the density of a granular material in a silo by assuming an average density increase of 6% with relation to the density value determined from the mass of 1 hectolitre [22]. It appears, however, that application of more accurate methods for the prediction of density of granular material deposit is a necessity.
Page 2 and 3: EC Centre of Excellence AGROPHYSICS
Page 4 and 5: 4 9.3. Factors influencing the coef
Page 6 and 7: 6 BASIC NOTATION c - cohesion [kPa]
Page 8 and 9: 8 Numerous granular materials when
Page 10 and 11: 10 the tensor of plastic strain inc
Page 12 and 13: 12 Fig. 2.1. Yield curves with comp
Page 14 and 15: 14 Stable or unstable behaviour of
Page 16 and 17: 16 In turn, the model of Lade, also
Page 18 and 19: 18 same porosity need not have iden
Page 20 and 21: 20 where: A = ∫ E( β) sin βdβ
Page 22 and 23: 22 (β = 0 o ), positioning the out
Page 24 and 25: 24 grain was poured into the mould
Page 26 and 27: 26 axes of the ellipse characterizi
Page 28 and 29: 28 [176] for the determination of p
Page 30 and 31: 30 A different approach to the desc
Page 32 and 33: 32 where: I - moment of inertia, k
Page 34 and 35: 34 a) b) f s f s m M f n f n Contac
Page 36 and 37: 36 The micropolar model yields resu
Page 38 and 39: 38 Fig. 3.17. Model of shear band f
Page 42 and 43: 42 5.2. Density of consolidated mat
Page 44 and 45: 44 (cereal grain, sand, soil), the
Page 46 and 47: 46 Multiple wetting and drying of g
Page 48 and 49: 48 vertical pressure within the ran
Page 50 and 51: 50 of the curve defined by A, where
Page 52 and 53: 52 6. Vertical consolidation refere
Page 54 and 55: 54 poured into the box without vibr
Page 56 and 57: 56 7.2.2. Bulk density Stress-strai
Page 58 and 59: 58 near the box wall and lifted alo
Page 60 and 61: 60 and the upper boundary was 18.5
Page 62 and 63: 62 A -1 5 -1 0 -5 1 0 5 -5 5 1 0 1
Page 64 and 65: 64 Fig. 8.2. Yield locus and effect
Page 66 and 67: 66 in such a way that friction woul
Page 68 and 69: 68 a) b) µ = tg ϕw ϕ w c) d) e)
Page 70 and 71: 70 9.3. Factors influencing the coe
Page 72 and 73: 72 galvanized steel received from d
Page 74 and 75: 74 9.3.4. Velocity of sliding Becau
Page 76 and 77: 76 1+ sinϕ k = . 1 − sinϕ (10.3
Page 78 and 79: 78 N ϕ VLQϕ 3UHVVXUHUDWLRN
Page 80 and 81: 80 10.4.1. Friction force mobilizat
Page 82 and 83: 82 10.4.3. Moisture content With in
Page 84 and 85: 84 Table 10.1. Measured (k s ) and
Page 86 and 87: 86 conventional engineering analyse
Page 88 and 89: 88 a, b - product-dependent coeffic
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90 also be performed as a function
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92 reliable processing and stable v
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94 unconfined yield strength to pow
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96 The Chute Index (CI) is recommen
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98 obtained the maximum deviation o
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100 efficient flow control using op
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102 In the field of experimental in
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104 22. Britton M.G., Moysey E.B.:
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106 69. Horabik J., Rusinek R.: Pre
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108 114. Morland L.W., Sawicki A.,
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110 158. 6WSQLHZVNL$ Influence of m
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112 16. APPENDIX - Physical Propert
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114 16.2. Density and porosity Tabl
Page 116 and 117:
116 Table 16.7. Mean values (±St.
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126 Table 16.16. Cont. 1 2 3 4 Icin
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136 Table 16.25. Cont. 1 2 3 4 Icin
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140 Table 16.29. Cont. Table sugar
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144 Table 16.35. Mean values (± St
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