EXAM 3

Question 1

describe sphericity

Answer 1

most useful single measure for characterizing shape of irregular and non spherical particles

Question 1

eqn for sphericity

Answer 1

(surface of sphere/surface of particle) at same volume

Question 2

what inequality with sphericity will always be true

Answer 2

sphericity ≤ 1

Question 3

how do we measure size of an irregularly shaped particle?

Answer 3

effective diameter (dp)

Question 4

what is the eqn that is always true for dp?

Answer 4

(sphericity)*(dsph)

Question 5

what is dsph?

Answer 5

diameter of sphere with same volume

Question 6

what is the equation for dsph?

Answer 6

(6V/pi)^(1/3)

Question 7

what is bed voidage?

Answer 7

space not taken up in packed bed

Question 8

what is the eqn for bed voidage?

Answer 8

(Vb-Vp)/Vb

Question 9

what is relationship of dense packing to normal packing on bed voidage v. sphericity graph?

Answer 9

shifted down

Question 10

what is relationship of loose packing to normal packing on bed voidage v. sphericity graph?

Answer 10

shifted up

Question 11

Reynolds eqn for packed bed?

Answer 11

Rep = (dpuorho)/(viscosity)

Question 12

what is uo? what does it represent?

Answer 12

superficial velocity … the velocity if there is no packed bed present

Question 13

how many terms are in the ergun eqn for incompressible fluids? what do the terms represent?

Answer 13

2 …. viscous losses & turbulent/inertial losses

Question 14

eqn for uo?

Answer 14

(bed voidage)*u

Question 15

eqn for u with respect to uo?

Answer 15

u = uo/(bed voidage)

Question 16

when can you disregard the turbulent term of the ergun eqn?

Answer 16

Rep < 20

Question 17

when can you disregard the viscous term of the ergun eqn?

Answer 17

Rep > 1000

Question 18

how many terms in the ergun eqn used for compressible fluids?

Answer 18

3 (Go eqn)

Question 19

what happens if you continue to increase the velocity of a fluid flowing upward through solids?

Answer 19

frictional resistance will increase until resistance = weight of solids and solids become suspended (fluidized)

Question 20

what is the uo needed to just fluidize solids called

Answer 20

minimum fluidization velocity (umf)

Question 21

what happens as the flow rate continues to increase beyond umf for liquid?

Answer 21

fluidized bed keeps expanding

Question 22

what happens as the flow rate continues to increase beyond umf for gas?

Answer 22

fluidized bed expands only slightly

Question 23

what does Lm represent?

Answer 23

length of packed bed before fluidization

Question 24

what does εm represent?

Answer 24

bed voidage before fluidization

Question 25

what is the relationship of superficial velocity and umf before fluidization?

Answer 25

uo < umf

Question 26

what does Lmf represent?

Answer 26

length of bed at point of minimum fluidization

Question 27

what does εmf represent?

Answer 27

bed voidage at point of minimum fluidization

Question 28

what is the relationship of superficial velocity and umf at point of minimum fluidization?

Answer 28

uo = umf

Question 29

what does Lf represent?

Answer 29

length of bed while fluidized

Question 30

what does εf represent?

Answer 30

bed voidage while fluidized

Question 31

what is the relationship of superficial velocity and umf during fluidization?

Answer 31

uo > umf

Question 32

what is the difference between liquid and gas fluidization?

Answer 32

liquid bed continues to expand while gas does not, and bubbles seen in gas fluidiz.

Question 33

what does delta Pfr equal? (two things)

Answer 33

rho(g)*frictional loss and the ergun eqn w two terms

Question 34

for low uo, what is true about delta Pfr?

Answer 34

it is proportionate to uo

Question 35

for high uo, what is true about delta Pfr?

Answer 35

it is proportionate to uo^2

Question 36

describe the difference in the uo v delta Pfr graph for packed beds versus fluidization?

Answer 36

fluidization curve follows packed bed curve until reaching mf, where it flattens to horizontal line

Question 37

when does curve of fluidization flatten?

Answer 37

at mf, or when (calculated friction)packed > bed weight

Question 38

when will solids fluidize?

Answer 38

when total drag force by fluid on particles = weight of solids in bed

Question 39

(delta Pfr)*At = ? where At = cross sectional area of tube

Answer 39

At*Lmf*(1-ε)*(|rho(s)-rho(g)|(g)

Question 40

what is the pump requirement eqn?

Answer 40

-Ws = frictional loss of bed = (delta Pfr/rhog)

Question 41

at u>umf, what is true of frictional loss per unit of fluid through bed?

Answer 41

practically unchanged

Question 42

what is true of pump power at u>umf?

Answer 42

will change in proportion to uo

Question 43

the intersection of delta Pfr eqns represents

Answer 43

minimum fluidization condition

Question 44

if bed voidage and sphericity unknown, what eqn do you use?

Answer 44

one including "- 28.7"

Question 45

forces causing particle to accelerate =

Answer 45

net weight of particle - drag force

Question 46

Cd represents

Answer 46

drag coefficient

Question 47

what is true of du/dt at terminal velocity

Answer 47

=0

Question 48

ut represents

Answer 48

terminal velocity

Question 49

how to differentiate ut at low Re

Answer 49

eqn without square root

Question 50

Reynolds eqn at terminal velocity

Answer 50

Resph,t = (dsph*ut*rhog)/viscosity

Question 51

how to differentiate Cd at low Re

Answer 51

eqn containing 24

Question 52

Reynolds eqn for non spherical particle

Answer 52

Re = (dsph*u*rhog)/viscosity

Question 53

when Ret>1, describe how to differentiate eqns used

Answer 53

to -3 or 1/3 power

Question 54

describe relationship between mesh size and diameter?

Answer 54

higher mesh, smaller particle

Question 55

describe relationship between diameter, terminal velocity, and mesh?

Answer 55

larger terminal velocity, larger diameter, smaller mesh

Question 56

what happens to bed voidage as bed fluidizes?

Answer 56

it increases

Question 57

example of packed bed?

Answer 57

cigarette, soil in pot

Question 58

what two terms are equivalent in a force balance at minimum velocity for individual particle?

Answer 58

drag force and gravity

Question 59

steps to calculate sphericity?

Answer 59

1. calculate volume 2. set = vol(sphere) 3. calculate radius of sphere 4. calculate surface area of particle 5. calc surface area of sphere 6. divide surface area of sphere by surface area of particle