1) To measure the angle of repose of the sand.
2) To study the factor that can influence the angle of repose of the sand.
2) To study the effect of glidant on the angle of repose.
Introduction:
When bulk granular material is poured on a horizontal surface of conical pile, it will form the internal angle between the surface of the pile and the horizontal surface is known as angle of repose. Angle of repose or the critical angle of repose is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90°.
In this experiment, we were measuring the angle of repose of the sand with 355 micron, 500 micron, 850 micron and various sizes without and with the addition of glidants. The experiment is done with a view to assessing the angle of repose of a substance and the factors that can influence the angle of repose.
Apparatus and Materials :
100g of 355, 500, 850 micron and various size of sand
Glidants
ruler
Procedure :
- The 355 micron sand (without addition of the glidant yet) was poured in a level surface allowing it to build from the top.
- The height of the pile from the peak to the ground eas measured by using the ruler
- The horizontal distance from the middle of the pile to the edge was measured by using the ruler.
- The equation tan-1 (height/width) had been used to find the angle of repose.
- The procedure 1-5 was repeated with the addition of glidant.
- The procedure 1-6 was repeated by using 500 micron, 850 micron and various size of sand
Result :
1. Sand
without Glidant
Size of particles of sand (micron)
|
Height (cm)
|
Width (cm)
|
Angle of repose
|
355
|
2.0
|
2.4
|
39.810
|
500
|
2.1
|
2.4
|
41.190
|
850
|
1.8
|
2.4
|
36.870
|
Various Size
|
2.0
|
2.4
|
39.810
|
2. Sand
is mixed with Glidant
Size of particles of sand (micron)
|
Height (cm)
|
Width (cm)
|
Angle of repose
|
355
|
2.0
|
2.4
|
39.810
|
500
|
1.9
|
2.4
|
38.370
|
850
|
1.7
|
2.4
|
35.310
|
Various Size
|
1.9
|
2.4
|
38.370
|
Calculation :
Tan θ = Height
Width
Discussion:
What
is the angle of repose for each materials?
Size of particles of sand (micron)
|
Angle of repose
(Without Glidant)
|
Angle of repose
(With Glidant)
|
355
|
39.810
|
39.810
|
500
|
41.190
|
38.370
|
850
|
36.870
|
35.310
|
Various Size
|
39.810
|
38.370
|
What
other factor that will influence angle of repose for the materials?
Internal
factor
- Particle size,coarser particles have high angles of repose than fine particles
- Particle shape
- Cohesiveness, fine particles may reveal cohesiveness owing to spherical particles having a greater tendency to roll
External
Factors
- Methods of measurement,Ledge and erater method give higher angle of repose than from the heap formation methods
- Presence of other components example glidants
- Moisture, Angle of repose of loose dry powder increases by compacting as well as by introducing by moisture.Moist sand has a much higher angle of repose than dry sand.
- The individual material will affect the angle of repose, a reflection of the different coefficients of friction between different substances.
What
other method can be used to calculate the angle of repose for the materials?
Tilting box method
This
method is appropriate for fine-grained, non-cohesive materials, with individual
particle size less than 10 mm. The material is placed within a box with a
transparent side to observe the granular test material. It should initially be
level and parallel to the base of the box. The box is slowly tilted at a rate
of approximately 0.3 degrees/second. Tilting is stopped when the material
begins to slide in bulk, and the angle of the tilt is measured.
Fixed funnel method
The
material is poured through a funnel to form a cone. The tip of the funnel
should be held close to the growing cone and slowly raised as the pile grows,
to minimize the impact of falling particles. Stop pouring the material when the
pile reaches a predetermined height or the base a predetermined width. Rather
than attempt to measure the angle of the resulting cone directly, divide the
height by half the width of the base of the cone. The inverse tangent of this
ratio is the angle of repose.
Revolving /Rotating cylinder method
Where
a sealed hollow cylinder half full of powder surface is rotated until the
powder surface shows its maximum angle with the horizontal.The material is
placed within a cylinder with at least one transparent face. The cylinder is
rotated at a fixed speed and the observer watches the material moving within
the rotating cylinder. The effect is similar to watching clothes tumble over
one another in a slowly rotating clothes dryer. The granular material will
assume a certain angle as it flows within the rotating cylinder. This method is
recommended for obtaining the dynamic angle of repose, and may vary from the
static angle of repose measured by other methods. When describing the angle of
repose for a substance, always specify the method used.
Crater Method /Discharge Method
Where
circular tube is placed vertically on a plate with an orifice in the center.The
height of the remaining powder against the wall of the tube is measured at
eight equidistant points around the circumference to determine the angle of
repose.
Dynamic Angle of Repose
Determine
in the apparatus consisting of a drum with a roughened internal surface that is
half filled with powder and slowly rotated around its horizontal axis.Within a
certain range of rotation speeds (usually from 2.5 to 6 rpm) the surface of the
powder in the drum comes to a sufficient steady condition.The maximum angle of
bed inclination just before slump occurs is designated as the dynamic angle of
repose.
Ledge Method
Where
the powder is initially charged into a rectangular box.A slot at the base of
one vertical wall is closed by a board.The closure board is then removed to
allow the material to flow slowly through the narrow slot.The angle with the
horizontal plane of the surface of the powder equilibrium when the flow stops
is calculated as the angle of repose.
Conclusion:
A
Glidant's effect is due to a counter-action to factors resulting in poor
flowability of powders. For instance, correcting surface irregularity, reducing
interparticular friction & decreasing surface charge. The result is a
decrease in the angle of repose which is an indication of an enhanced powder's
flowability.
References:
http://books.google.com.my/books?id=6aP3te2hGuQC&pg=PA40&lpg=PA40&dq=other+factors+influence+the+angle+of+repose&source=bl&ots=wP2LxPO4Uc&sig=TeOQ70O_DQE54n5bY2hHN9iMimg&hl=en&sa=X&ei=2g6OUvZtjcmtB6i1gcAF&ved=0CD8Q6A
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