A concentric cylinder viscometer may be formed by rotating the inner member of a pair of closely fitting cylinders (see Fig). The annular gap is small so that a linear velocity profile will exist in the liquid sample. Consider a viscometer with an inner cylinder of 4 in. diameter and 8 in. height, and a clearance gap width of 0.001 in., filled with castor oil at 90°F. Determine the torque required to turn the inner cylinder at 400 rpm.

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Answer:

T = 52.76 ft . lbf

Explanation:

Given data:

inner diameter of cylinder 4 in

height of cylinder 8 in

gap width = 0.001 in

we know shear force is given as

[tex]F =\tau A = \tau (2\pi RH) [/tex]

Where, [tex]\tau[/tex] is torque acting on cylinder

[tex]\tau = \mu \frac{V}{d} = \mu \frac{ R\omega}{d}[/tex]

[tex]\mu[/tex] is kinematic viscosity, for castor oil at 90 degree F  = 0.259 N s/m^2

substituing all value in   shear force formula we get

[tex]F = \frac{2\pi \mu R^2\omega h}{d}[/tex]

we know that

[tex]Torque, T  = force \times R[/tex]

[tex]T = \frac{2\pi \mu R^3\omega h}{d}[/tex]

[tex]T = \frac{2\pi (0.259\times 2.09\times 10^{-2}\times 2^3\times 400\times 8 }{10^{-3}} \times 2\pi \times \frac{1 min}{60} \times \frac{1 ft^3 }{1728 in^3}[/tex]

T = 52.76 ft . lbf

The torque required to turn the inner cylinder at 400 rpm is; 77.4 lb.ft

What is the Torque?

We are given;

Diameter; d = 4 in

radius; R = 2 in = 2/12 ft

Clearance Gap = 0.001 in

angular speed; ω = 400 rpm = 41.8879 rad/s

height; h = 8 in

The formula to find the torque from different derivations is;

T = 2πμR³ωh/d

where;

μ is viscosity

R is radius

ω is angular speed

h is height

d is clearance gap

At 90°F castor oil, μ = 0.007942 lbf.s/ft²

Thus;

T = (2π * 0.007942 * (2/12)³ * 41.8879 * 8)/0.001

T = 77.4 lb.ft

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