Gas-fluidized bed rheology for granular media

Transcription

1 FlowingMatter 2017 Porto / Portugal Gas-fluidized bed rheology for granular media Denis Schütz, Jörg Läuger Anton Paar Germany

2 Powder Rheology Background Powder as raw material, intermediate and final product Industries Chemical industry (washing powder, make-up powder, superabsorber ) Construction industry (cement, maraging steel ) Food industry (flour, coffee powder, dried spices ) Applications and processes: Pneumatic transport Flow through nozzles Filling and dosing Storage Influencing parameters: Load Temperature Moisture Particle size

3 Fluidization General Concept Air stream through a porous material at the base of the powder column into the powder Gas stream overcomes gravity and inter-particle adhesive forces Powder behavior changes from a static solid-like state to a dynamic fluid-like state, depending on the air velocity through the powder Fluidization clears all memory, i.e. residual stresses, agglomerates, etc. reproducible starting conditions for measurements Spouting powder bed

4 Setup for fluidized bed rheology powder cell for fluidized bed rheology in a rheometer make use of large ranges in rotational speed and torque of the rheometer All kinds of different rheological tests

5 Setup for fluidized bed rheology Flow meter Mass flow controller Measuring geometry Dust protection hood Dust Protection Hood Measuring cell Poreous fritted glass disk Pressure sensor (Below fritted glass disk) Air flow inlet Pressure sensor Measuring cell holder

6 Setup for fluidized bed rheology Stirrer Warren Springs Smooth cylinder Sandblasted cylinder Profiled cylinder Inner diameter of cup: 50mm Glass beads with particle size: µm

7 Pressure Drop Measurement Pre-test for fluidization behavior of unknown powder Pressure difference between empty and powder loaded measuring cell Increase in volumetric flow rate Determination of minimum volumetric flow rate for full fluidization fluidization glass beads 40-80µm de-aeration full fluidization Small Q: linear build up of pressure Maximum: start of fluidization, i.e. breakdown of initial cohesion Larger Q: pressure constant, i.e. equilibrium between air pressure and weight of powder

8 Cohesive Strength of fluidized Metal Powders Powder Based Selective Laser Sintering (SLS) (3D-Printing) Powder BEFORE and AFTER several sintering steps Two-plate stirrer with 8rpm, Geometry factor by calibration Transient startup of flow Steady state flow Repeatability Degradation leads to higher cohesion and can cause problems and defects in the final sintered part

9 Cohesive Strength of fluidized coffee creamer Two different commercial coffee creamer Two-plate stirrer with 8rpm, 3 repeats for each sample Good reproducibility Clear distinguishing between the two creamers

10 Pressure Drop Measurement and Flow Curves Profiled cylinder R cup = 25mm, R bob = 12.08mm Speed ramps after applying different increasing gas flow rates Q max (2l/min) for 5min, actual Q for 5min then measurement at actual Q glass beads 40-80µm increasing volumetric flow rate

11 Shear Stress vs. Shear Rate at various fluidization levels 0 l/min 1.5 l/min Increasing volumetric flow rate No fluidization: high stress plateau, i.e. dynamic yield point For increasing fluidization stress plateau is lower Indermediate fluidization: minimum in stress, i.e. non-monotonic flow curve High fluidization: no stress plateau Strong increase at larger rates most likely due to onset of inertial flows

12 Shear Stress vs. Shear Rate at various fluidization levels 0 l/min Gas fluidization Glass beads: µm 1.5 l/min Increasing volumetric flow rate Dijksman et al. Phys. Rev. Lett. (2011) 107, Fluidization by weak vibration Glass beads: mm Similar behavior for gas fluidization and fluidization by weak vibration

13 Viscosity vs. Shear Rate at various fluidization levels Glass beads with profiled cylinder R cup = 25mm, R bob = 12.08mm Increasing volumetric flow rate 0 l/min 1.5 l/min

14 Viscosity vs. Shear Rate for different geometries 0 l/min Smooth Sandblasted Profiled 1.5 l/min No fluidization: no slip at small shear rates for profiled cylinder Full fluidization: adhesion of powder particles on smooth cylinder at small rates; slip on smooth and sandblasted cylinder at large rates

15 Comparsion to other materials 0.6 l/min a, b: MRFs 1.5 l/min c, d: ERFs e, f: Suspensions Generality of shear thickening in dense suspensions. Brown E et.al. Nat Mater Mar;9(3): Similar behavior to MRF, ERF and to dense suspensions in general

16 Conclusions Easy to use set-up to measure and characterize powders flow Direct measurement of viscosity in a fluidized bed New possibilities for powder measurements and research Better understanding of phenomena governing powder flow Simple quality control tool for many powder processes, like pneumatic transport or dosage Future work: More complex rheological testing: stress control, oscillatory testing, transient testing. Adding more parameters: temperature and humidity Investigation powders with different sizes, size distributions, interactions, etc. Bubble rheology: defined gas flow in liquids

17 Thank you for your attention