Page 151 - Modelling in Transport Phenomena A Conceptual Approach

PROBLEMS 131

c) In fluidization, the drag force on each particle should support its effective weight,
i.e., weight minus buoyancy. Show that the drag force is given by

FD=AL(1-€)(Pp--)gE (6)
where L is the length of the bed, p and pp are the densities of the fluid and solid
particle, respectively. Note that in the calculation of the buoyancy force, the volume
occupied by solid particles should be multiplied by the density of suspension, Le.,
EP + (1 - c)pp, instead of p.
Combine Eqs. (5) and (6) to get

which is a well-known equation in fluidization.

4.19 A 15 x 90 m lawn is covered by a layer of ice of 0.15 mm thick at - 4 "C. The
wind at a temperature of 0°C with 15% relative humidity blows in the direction
of the short side of the lawn. If the wind velocity is 10m/s, estimate the time
required for the ice layer to disappear by sublimation under steady conditions.
The vapor pressure and the density of ice at - 4 "C are 3.28 mmHg and 917 kg/ m3,
respectively.
(Answer: 33 min)