Is this actually something you can CALCULATE

Or is it something you have to MEASURE. ■

Dear Mr.Satish,

Do you mean "Angle of Repose" ?

Regards, ■

You MEASURE the draw down angle of a bulk material in the same way that you MEASURE the angle of repose.

It is not something you calculate. ■

As Designer notes, its easier to measure than to develop from first principles.

The draw down angle can be calculated, after you have validated its flow properties, and at minimum know the following:

1. Internal friction angle

2. Particle distribution size, shape, and surface texture

3. Moisture effect

4. Elastic modulii of material that may be time dependent

5. Rheology properties (time, force, material dependence, Newtonian or non-Newtonian rheology)

6. Draw down angle also depends on fines filtration (perculation) into reclaim funnel (ie, repeated reclaiming may increase the steepness of the angle.

It is not a simple equation and depends on relaxation time in pile (ie. longer time = stronger resistance to flow). ■

Originally Posted by nordell

It is not a simple equation ....

With all those variables certainly not. I'll stick with measurements ■

I would describe the enquirers phenomenon as the 'Drained Angle of Repose', as the term - 'Drawdown Angle' may be confused with the slope of the internal flow channel developed through a static bed of bulk solids, compared with the current interest in unconfined flow down the surface of a stored material. In a deep pile, the initial flow pattern of funnel flow will consist of an internal 'Core Flow' region up to the free surface, into which an increasing radii of material will drain in an unconfined manner until it teminates with the surface level reducing to expose the outlet. See: - http@//www.erc.ufl.edu/erpt/vol1no1/bate-00.htm for details of local and global flow regimes.

Having said that, the drained angle of repose has two main values that apply in this case because of the slot outlet. Along the length of the slot material will run down the slope in plane flow whereas the ends, being narrow, will be subject to converging flow of decreasing radius. During flow convergence, there is interference between the members that absorbs energy, so the slope has to be steeper to overcome the additional work of 'internal' friction. The ultimate slope depends on the combination of friction between the particulates and the structural stability of their contact points. Even with a cohesionless bulk material it is probably easier to determine this condition be testing, rather than trying to calculate the angle. You just need a pile over an outlet large enough to resist structural arching. See notes on this in the 'ask lyn' section. ■

Originally Posted by Lyn Bates

See: - http@//www.erc.ufl.edu/erpt/vol1no1/bate-00.htm for details of local and global flow regimes.

Was this supposed to be a link to a web page ■

The reference was thought to be a web ref, but an easier one is to search for erpt/flowregimes that I hope is useful for an introduction to this topic. ■

[QUOTE=nordell;689173. ..

5. Rheology properties (time, force, material dependence, Newtonian or non-Newtonian rheology)

[/QUOTE]

Rheology is often defined as the study of non-Newtonian fluids. Innit?■

The draw down angle is akin to the "sluff angle" left behind during the reclaim by a bucket wheel reclaimer. It is a transient unstable condition, typically around 55 degrees for coal whose angle of repose is about 37 degrees. It will drop to the angle of repose if you wait for it. The higher "sluff angle" is held for awhile by cohesion and negative pore pressure.

Joe Dos santos ■

The term sluff angle is used for material stockpiles, in conjunction with stockyard machines. When the stockpile is being formed the material sides will be as per repose angle. However, when stockpile is being reclaimed by say bucket wheel, the cut-face created by it will be at sluff angle which is likely to be around 20 to 30% more than the repose angle (as an average for cut shape). The sluff angle has practical relevance to the positioning of the machine during reclaiming of different layers etc.

It seems the difference arises due to reason that material has downward momentum during stockpile formation tending to reduce sloping side angle as against standstill material being under cut, and material has to collapse on its own, without ‘encouragement’ by momentum.

when the material is draw down at bottom opening, the situation is somewhat analogous but its circular face will tend to enhance the aforesaid angle, and it will also depend upon whether there is rat hole formation etc.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: parimul@pn2.vsnl.net.in ■