FLOW PASSED IMMERSED OBJECTS. the flow of fluids over bodies that are immersed in a fluid, called external flow , with emphasis on the resulting lift and drag forces. . Many applications in food and chemical engineering -flow passed spheres ( settling )
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the flow of fluids over bodies that are immersed ina fluid, called external flow, with emphasis on the resulting lift and dragforces.
-flow passed spheres (settling)
-packed beds( drying, fluidising, filtration)
-flow passed tubes( heat exchangers)
These processesare equivalent to each other; what matters is the relative motion between thefluid and the body.
Force in the direction of flow exerted by the fluid on the solid is called drag.
Due to turbulence, the pressure on the downstream side of the sphere will never fully recovered to that on the upstream side, and there will be a form drag( pressure drag) to the right of the sphere. (For purely laminar flow, the pressure recovery is complete, and the form drag is zero.)
In addition, because of the velocity gradients that exist near the sphere, there will also be a net viscous drag (also called as skin drag, wall drag, friction drag) to the right (In potential flow there is no wall drag). The sum of these two effects is known as the (total) drag force.
The component of the resultantpressure and shear forces that acts in the flow direction is called the drag force, and the component that acts normal to the flow direction iscalled the lift.
This is accomplished by stream lining.
The frictiondrag is also proportional to the surface area.(airplanesreduce their total surface area and thus their drag by retractingtheir wing extensions when they reach cruising altitudes to save fuel.)
The pressure drag is proportional to the frontal area and to the differencebetween the pressures acting on the front and back of the immersed body.
Therefore, the pressure drag is usually dominant for blunt bodies, negligiblefor streamlined bodies
FD=CDЛR2 (1/2)ρv02 FB=4/3ЛR3ρg
For NRE<1 FD=6Лμv0 R
So terminal velocity v0 =2/9* R2 (ρp-ρ)ρ/μ
Settling time t=h/v0
If large number of particles exist, suraounding particles interfere with the individual motion of particles.
Due to Upward movement of surrounding fluid so relative velocity decreases.
Settling will be slower than one calculated from Stokes law. (true drag force is greater)
μm=μ/ψP where ψP =1/101.82(1-ε)
where ε=volume fr. of liq in slurry
ρm= ερ+(1-ε)ρP ρm: slurry density
v0 =2/9* (R2 (ρp-ρ)ρm/μ)* (ε2 ψP )
For non-spherical particles: an equivalent diameter is defined.
The equiv. D of a non-spherical particle is defined as a sphere having the same volume as the particle.
Sphericity is the ratio of surface area of this sphere to the actual surface area of particle. (table3.1-1)
The formula for sphericity is reduced to Фs = 6vp/ (DpSp)
Where vp is the volume of particle, Dp is the characteristic dimension of particle, and Sp is the surface area of particle.
For non-spherical particle Ergun equation is given by,
requirements of a tower packing are:
It must be chemically inert .
It must be strong without excessive weight.
It must contain adequate passageswithout excessive pressure drop.
It must provide good contact
It should be reasonable in cost.
Then there is no space to expend further so particles will be entrained on top.
At this point velocity is called as entrainment velocity v’t
L1A(1- ε1 )= L2A(1- ε2 )
Фs εmf 3≈1/14 and 1- εmf / Фs2εmf 3 ≈11
Then above eq becomes
NRe,mf =[(33.7)2 +0.0408Dp3ρ(ρp –ρ)g/μ2]-33.7
Valid for 0.001 Deviation +/- 25%
Deviation +/- 25%
-the solid is vigorously agitated by the fluid passing through the bed
-no temperature gradients in the bed even with quite exothermic or endothermic reactions.
-Erosion of vessel internals
-Attrition of solids. the size of the solid particles is getting reduced and possibility for entrapment of solid particles with the fluid are more.