Rheological properties of flour

USE

Used for baking and cooking.

TEST EQUIPMENT

Instrument: powder flow tester (PFT)

Tank: 230 cc, 6 inches in diameter (standard volume)

Lid Type: Pallet Lid, 33cc, 6 inch diameter

Wall cover, 2B finish, 6 inches in diameter

Test Type: Flow Function Test, Wall Friction Test

Temperature: Room temperature (70-72 ° F)

Humidity: 24%

METHOD

A Brookfield powder flow tester equipped with Powder Flow Pro software for automated instrument control and data acquisition. The flour was collected in the trough, and the scraping tool was then used to evenly distribute the powder in the trough. After recording the sample weight and entering it into the software, a standard flow function test and then a wall friction test were performed. The time required for each test was 25 minutes and 13 minutes respectively.

MEASURED PARAMETERS

Fluidity: Very cohesive to Cohesive

Wall friction: 12 ° (0.5 kPa) to 9.5 ° (4.75 kPa)

Apparent density: 530 kg / m 3 (filling density) to 880 kg / m 3

TO ANALYSE

Hopper shape: Conical

Critical arch dimension: Single hopper: 71.7 mm

Conical hopper: 143.3 mm

Rat hole diameter: Depends on tank diameter

RESULTS

Figure 1 shows the flowability of flour at different levels of consolidation stress. These results show that the flour is generally cohesive except at very low levels of consolidation stress where it begins to fall into the very cohesive range (less than 2 kPa).

Note: Flow function data is indicated by the red line. When interpreting a flow function graph, the data is read from right to left. The rightmost data point indicates the powder flow rate when the hopper is full; the leftmost data point indicates the powder flow when the hopper is nearly empty. Purple lines are standard flow indices that distinguish different types of flow behavior across levels of consolidation, ranging from “free fluid” (lower segment) to “non-fluid” (upper upper segment).

Figure 1 :

Breaking strength of the graph of the function of flour flow versus consolidation stress

Figure 2 shows the friction angle of the walls at different levels of normal stress. The wall friction angle represents the friction between the sliding powder and the wall of the hopper or chute at the start of flow. In this test, a 304 stainless steel cover was used, illustrating what the friction would look like if the flour was in a stainless steel hopper. Wall friction angles> 30 ° indicate that the material will have difficulty sliding against this surface. At a low normal stress of about 0.5 kPa, the effective wall friction angle is about 12 ° and drops to about 9.5 ° at higher levels of normal stress (4.75 kPa).

Note: Wall friction test and flow function test are independent tests. The flow function test measures the flow of powder through the orifice while the wall friction test measures the flow of powder against a specific building material.

Figure 2: Flour wall friction graph

Effective angle of the wall to the normal stress

Figure 3 shows the bulk density of the material at different levels of consolidation stress. This graph tells us that the flour has a filling density of about 530 kg / m³ and amounts to about 880 kg / m³ at about 4.5 kPa of consolidation stress. In general, a free flowing powder will exhibit very small changes (less than 30%) in bulk density, while a cohesive or poor flow powder will generally show a large increase (greater than 30%) of bulk density. In this case, the change in bulk density is 66%, indicating a more cohesive material and more difficult to flow.

Figure 3: Density of Flour Bulk Density vs. Consolidation Stress Plot

CONCLUSION

Flour is a very cohesive powder at low levels of consolidation stress and cohesive at high levels of consolidation stress. This means that the flour may have problems with flow when the hopper empties. Potential issues include camber (when the powder forms a cohesive bridge over the outlet) and rat hole. The critical dimension of the rat hole depends on the diameter of the tank. Powder Flow Pro can automatically calculate the diameter of the rathole once the diameter of the bin is entered. The critical vault dimension was determined to be 4.192 inches (104.8 mm) and provides a conservative estimate to prevent vaulting, provided the minimum hopper outlet dimension exceeds this value. The large arch dimension in this case indicates that this material will be difficult to pour.