1. Hardy Ankit Assistant Professor Food Engineering Department National Institute of Technology

2. Friction of solids and flow of granular solids Angle of repose Coefficient of friction Aero and hydrodynamic characteristics Application of frictional properties in grain Handling Processing Conveying 3. Designing of storage bins, hoppers, chutes, pneumatic conveyors, screw conveyors, threshers, harvesters. Rolling friction: Rolling of round shape agricultural materials, conveying of fruits and vegetables by gravity flow. Conveying and mechanical systems: material moves or slides in direct contact with your troughs, casings and other components of the machine. So power requirement to drive the machine as frictional losses is one of the factors to be overcome by providing additional power to the machine. 4. Friction: Friction is the force that resists motion of objects that are in contact with each other. 5. Frictional force (F) is proportional to the load (N) F N , F = N, Where, F = Force = Coefficient of friction (static or dynamic) N = Load 6. i) Static coefficient of friction (s): predicts the force at the point in time when motion is initiated. ii) Dynamic coefficient of friction (d): predicts the force required to maintain the motion once it is initiated. d s (why???) Reason: For a given applied normal force, the force required to maintain motion is less than or equal to the force required to initiate motion. 7. In grain conveying systems : Two types of friction 1. External friction/wall friction: Friction between grain mass of kernels and wall of bin or a surface 2. Internal friction: Friction of the kernels or grains against each other. Higher angle of internal friction indicate the material is cohesive. Coefficient of friction () between granular materials is equal to tangent of the angle of internal friction for that material. 8. Grains Angle of internal friction, degree Minimum Maximum Wheat 24 26 Maize 26 29 Barley 25 29 Rice 24 26 9. 1. Static : It is an angle of friction taken up by granular material to just slide upon itself . 2. Dynamic : When bulk of the grain is in motion like discharge from bins and hoppers. It is an important property which helps to estimate lateral pressures in storage silos. It is also used in designing : Storage bins (both shallow and deep) and hopper for gravity discharge. 3. Effect of moisture content: increases with the increase in moisture content. Reason: Surface layer of moisture surrounding the particle holds the aggregate of grain together by the surface tension. 10. It is the angle between the base and slope of the cone formed on a free vertical fall of the angular material to a horizontal plane. For understanding: The angle between the horizontal and inclination of heap is called angle of repose. Angle of repose=Tan-1 (2H/B) 11. Indicator of flow ability of product. Lower angle of repose: easy flow ability Cohesive material has larger angle of repose Factors affecting angle of repose: Size, shape, moisture content and orientation of the grains. Each product has its own angle of repose. Examples Barley (13% mc): 31 (20% mc): 34 12. Flow characteristics with respect to angle of repose Flow characteristics angle of repose, degree Very, free flowing 25-30 Free flowing 30-38 Cohesive 45-55 Very cohesive Beyond 55 13. 1.Emptying angle of repose: Angle of repose in situations where the material is being emptied from a bin. 2. Piling angle of repose : Angle of repose formed when material is allowed to flow from a spout or elevator outlet so as to form a pile. Use: For calculating the quantity of granular material that can be placed in piles or on storage structures with flat surfaces. 14. Through the orifice of given size and shape 1. Type of flow 2. Rate of flow two important factors in designing and operation of processing facilities Type of flow: Two types of flow 1. Mass flow 2. Funnel flow 15. Mass flowFunnel flow 16. 1) Piping: When bulk solid above the outlet is flowing out remaining bulk solids stays in place. called dead regions 2) Arching: It is a stable obstruction that forms over the point of narrowest cross section of the storage vessel (usually the discharge outlet). 17. Based on concept of plane of rupture Plane of rupture: is that surface down which is wedge of material bounded by one wall face, the free face surface and the plane of rupture would start sliding if bounding wall were to move. a) Shallow bin: A bin in which plane of rupture meets the grain surface before it strikes the opposite side. b) Deep bin: A bin in which plane of rupture meets the opposite side before it emerges from the grain 18. According to Rankins theory Shallow bin: hL tan(90+)/2 h= depth of storage bin L=breadth of storage bin = Angle of repose 19. For shallow bins 1. Rankin equation P=KWh P= Lateral pressure on the wall bin by grains W= Wt density of grains h=depth of grains from the top of the bin surface K= Rankin coefficient= (1-sin)/(1+sin) =tan2 (45- /2) where = angle of internal friction of grains 20. 2. Airy equation Where, W=grain bulk density, h= height/depth of grain = Coefficient of internal friction= tan = Coefficient of external friction= tan For deep bins Jannsen equation K= Pressure ratio= (1-sin)/(1+sin) R= hydraulic radius , =Coefficient of external friction 21. Rolling friction W and r(directly proportional) 1/r (inversely proportional) Fr = rN/r Where r = Coefficient of rolling friction N= Normal force r= radius Example : Separation of potatoes and stones.