Chap3b

Chap3b - Drag flows Majority of commercial rheometers...

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1 1 Drag flows Drag flows • Majority of commercial rheometers Æ rotating devices • Three possible geometries: – Coaxial cylinders (Couette) – Cone and plate – Concentric disks • Enable characterization of viscosity and other important rheological functions • Commercial instruments now available with computer control and data analysis software packages 2 Drag flows Drag flows (Morrison, 2001)
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2 3 Parallel plate (sliding) rheometers Parallel plate (sliding) rheometers fluid Sample x 1 H (Morrison, 2001) 4 • The total strain imposed is limited by the length of the rheometer • The gap h should be made as small as possible Parallel plate (sliding) rheometers Parallel plate (sliding) rheometers h x y x = = γ h V dt x d h dt d = = γ = γ 1 &
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3 5 v 1 0 = f(x 2 ) v 2 = v 3 = 0 v 1 = Vx 2 /H σ x1x2 = F/A For a Newtonian fluid: σ x1x2 = η V/H A = W x L Total force on the plate: F = WL η V/H Parallel plate (sliding) rheometers Parallel plate (sliding) rheometers 6 Parallel plate (sliding) rheometers Parallel plate (sliding) rheometers • Sources of errors: – End and edge effects Æ use of a pressure-transducer to measure the stress locally – Pumping of the fluid due to N 2 (secondary flow) Reimers and Dealy (1996)
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4 7 3.2 Coaxial 3.2 Coaxial -cylinder rheometers cylinder rheometers Couette flow 8 Coaxial Coaxial -cylinder rheometers cylinder rheometers • Probably the first rotating devices to measure viscosity • Named after Maurice Couette (1890), who invented it • Assumed that the resistance to flow occurs in the small gap • Used to determine the viscosity of low-viscosity fluids Couette geometry for Bohlin VOR rheometer with evaporation control
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5 9 Calculation of viscosity Calculation of viscosity • Two coaxial cylinders of length L and radii R and KR • One cylinder rotates at a known
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Chap3b - Drag flows Majority of commercial rheometers...

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