By measuring the yield locus, several physical properties can describe the behavior of the bulk material. For the classification of powders, and to express the flowability in a single parameter, the flowability factor (FL = ffc = σ1 / σc) was developed. The flowability factor provides a quantitative characterization of the flowability of a powder. This factor is dimensionless and enables the classification of bulk materials into classes:
|25 >||FL||> 15||slightly cohesive|
|15 >||FL||> 5||cohesive|
|5 >||FL||> 2||very cohesive|
|2 >||FL||> 1||plastic|
For materials which very different densities, an extended calculation basis for the flow factor was developed:
Relative flow factor: FLR = ( σ1 - σ2 ) / σc
Absolute flow factor: FLA = FLR * ρb0
Each measured yield locus is based on the individual consolidation stress or the major principal stress (σ1). It is only possible to compare materials using the same reference stress.
The flowability factor is a easy way to quantitatively select substances with regard to their flowability. When evaluating the numerical values, however, the exponential increase in the flow factor must be taken into account.
Bulk solids with the same flowability factors should have the same flow behavior. Higher values indicate better flow properties. In the case of lower values, poorer flowability must be expected and the suitability for use in the silo or the possibility of processing must be checked.
This method is therefore a good way of ensuring that technical processes are used during product changes or for different batches of raw or product materials.
By measuring several yield loci at different consolidation stresses the flow function is obtained, which describes the flow factor over an operational range.
Often the flow function is represented as a linear equation, which can be used more easily in the calculation of the necessary outlet diameter to avoid bridging .In the area of lower stresses, however, there is often a curved gradient. It is also theoretically understandable beause a flowability factor smaller than one is not possible and the compressive strength cannot be higher than the greatest principal stress.
For technical calculations or if the stresses that occur are only insufficiently known, the flow function is used to have the possible parameters available.
To determine the flow function, flow locations are measured at at least three different consolidation stresses. These should be chosen so that they cover the area of application. The function that is used for further calculations is obtained through mathematical approximation methods. This also allows differences between individual substances to be shown over a larger range of stresses.
Flow functions with associated flowability factors for three different materials. Automatically generated analysis of the flow function(s) according to the measurements.