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

More documents

Recommendations

Info

88 a, b – product-dependent coefficients, V 0 – superficial air velocity, f E – friction factor, %– porosity, !– density of air, D p – specific surface equivalent particle diameter, k 1 , k 2 – product-dependent coefficients, (Re) dp – Reynolds number based on volume equivalent diameter, d p – volume equivalent particle diameter. Airflow resistance by Ergun’s equation was used to predict pressure drop across a column of corn, soft white winter wheat, soft red winter wheat and soybeans at three moisture content levels and two bulk densities [112]. The data collected indicated that Ergun’s equation could be successfully applied to grain aeration design and analysis. Previous work indicated that Ergun’s equation would not be applicable to grain aeration due to packing effects within the bin. However, previous research indicated also that variations in bulk density and porosity could be estimated using granular mechanics models. The overall error using Ergun’s equation was less than 23 Pa m -1 , when the pressure drop was less than 500 Pa m -1 . When all data were included up to a pressure drop of 1800 Pa m -1 , the standard error averaged 76 Pa m -1 . The effect of grain orientation that would be typical in storage bins was negligible, less than 10%, increase in airflow resistance. However, the fill method and resulting bulk density increased the airflow resistance by an order of magnitude. Ergun’s equation, with an appropriate model of porosity variation during storage, could be utilized for the design and analysis of grain aeration systems. 12. FLOW RATE THROUGH ORIFICES Numerous cases of design of storage and processing equipment require estimation of flow rate of granular material through orifices. ASAE standard D274.1 [6] gives recommended procedure to estimate the flow rate of specific grains and oilseeds through horizontal and vertical orifices. Recommended graphs and equations can be applied to mass flow from bins and hoppers during emptying. The standard distinguishes between small orifice – that is one whose hydraulic diameter is less than 15 times the minor diameter of the particle, and large orifices with larger hydraulic diameters. The rate of flow of grain or oilseeds through a horizontal or vertical orifice can be predicted by the following equation: where: Q – volume flow rate, A 0 – area of the orifice, Q = C A , (12.1) $ 1 0D
89 D – hydraulic diameter of the orifice, C 1 – coefficient from the table, different for horizontal and vertical orifices $– exponent from the table with a value between 0.5 and 1.0. The equation has been validated for square and circular orifices in both horizontal and vertical directions. The authors propose also a simplified version of the equation with exponent $ set equal to 0.7. With such simplification, the equation is usually accurate with ±6% for large orifices and ±12% for small orifices. Flow rate was found independent of grain depth in the experimental studies cited by the standard. The authors stated that for depths below 1 m flow can be affected by depth. Increased moisture content was found to reduce mass flow rate for corn and wheat, but increased mass flow rate for sorghum. An orifice in the floor of the silo adjacent to the smooth wall discharged approximately 15% more grain than an orifice in the center of the silo floor. 13. INDICES OF STRENGTH AND FLOWABILITY USED BY PROCESS TECHNOLOGY Some specialists (see [16]) consider the Jenike method of testing flowability to be much more complex and time consuming than less sophisticated measures. These authors state that the Jenike process takes at least 4 to 6 hours per sample, and the specific ramifications of the resulting flow function are understandable for only very few engineers practicing in the USA. Testing a bulk material with a Jenike cell takes more time and skill than the average producer or consumer of powders is willing to invest, unless there is a major financial stake such as the design of a new silo. Current state of theory and technology of granular materials does not allow for wider standardization of material parameters and methods of their determination. However, industrial practice needs material parameters for design of processes, as well as measures of quality of products. Increasing number of consulting firms appear on the market and offer help in solutions of technological difficulties or determination of material parameters. High credibility is gained by firms led by specialists of respect established earlier in academia. Probably a good illustration of the demand from practice and proposed solutions are the offers of consulting groups founded by A. Jenike or by J.R. Johanson. These two specialists were earlier employees of academia. The establishment of Jenike addresses its offer to the following industries: chemical, environment protection, food, glass, metallurgy, mineral solids, paper, pharmaceuticals, mining, plastics and metal powders. Jenike and Johanson [75] propose standard tests for determination of cohesion, coefficient of wall friction, angle of internal friction, compressibility, permeability and angle of chute inclination to allow for stable flow. Determination of material parameters may
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 40 and 41: 40 In the case of agricultural mate
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 90 and 91: 90 also be performed as a function
Page 92 and 93: 92 reliable processing and stable v
Page 94 and 95: 94 unconfined yield strength to pow
Page 96 and 97: 96 The Chute Index (CI) is recommen
Page 98 and 99: 98 obtained the maximum deviation o
Page 100 and 101: 100 efficient flow control using op
Page 102 and 103: 102 In the field of experimental in
Page 104 and 105: 104 22. Britton M.G., Moysey E.B.:
Page 106 and 107: 106 69. Horabik J., Rusinek R.: Pre
Page 108 and 109: 108 114. Morland L.W., Sawicki A.,
Page 110 and 111: 110 158. 6WSQLHZVNL$ Influence of m
Page 112 and 113: 112 16. APPENDIX - Physical Propert
Page 114 and 115: 114 16.2. Density and porosity Tabl
Page 116 and 117: 116 Table 16.7. Mean values (±St.
Page 126 and 127: 126 Table 16.16. Cont. 1 2 3 4 Icin
Page 136 and 137: 136 Table 16.25. Cont. 1 2 3 4 Icin
Page 138 and 139:
138 Table 16.27. Mean values (±St.
Page 140 and 141:
140 Table 16.29. Cont. Table sugar
Page 142 and 143:
142 Table 16.31. Mean values (±St.
Page 144 and 145:
144 Table 16.35. Mean values (± St
show all

Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...

Create successful ePaper yourself

Delete template?

Save as template?

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