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CHEE 203 > EXAM 3 > Flashcards

EXAM 3 Flashcards

1
Q

describe sphericity

A

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

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1
Q

eqn for sphericity

A

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

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2
Q

what inequality with sphericity will always be true

A

sphericity ≤ 1

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3
Q

how do we measure size of an irregularly shaped particle?

A

effective diameter (dp)

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4
Q

what is the eqn that is always true for dp?

A

(sphericity)*(dsph)

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5
Q

what is dsph?

A

diameter of sphere with same volume

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6
Q

what is the equation for dsph?

A

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

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7
Q

what is bed voidage?

A

space not taken up in packed bed

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8
Q

what is the eqn for bed voidage?

A

(Vb-Vp)/Vb

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9
Q

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

A

shifted down

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10
Q

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

A

shifted up

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11
Q

Reynolds eqn for packed bed?

A

Rep = (dpuorho)/(viscosity)

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12
Q

what is uo? what does it represent?

A

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

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13
Q

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

A

2 …. viscous losses & turbulent/inertial losses

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14
Q

eqn for uo?

A

(bed voidage)*u

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15
Q

eqn for u with respect to uo?

A

u = uo/(bed voidage)

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16
Q

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

A

Rep < 20

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17
Q

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

A

Rep > 1000

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18
Q

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

A

3 (Go eqn)

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19
Q

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

A

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

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20
Q

what is the uo needed to just fluidize solids called

A

minimum fluidization velocity (umf)

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21
Q

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

A

fluidized bed keeps expanding

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22
Q

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

A

fluidized bed expands only slightly

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23
Q

what does Lm represent?

A

length of packed bed before fluidization

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24
Q

what does εm represent?

A

bed voidage before fluidization

25
Q

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

A

uo < umf

26
Q

what does Lmf represent?

A

length of bed at point of minimum fluidization

27
Q

what does εmf represent?

A

bed voidage at point of minimum fluidization

28
Q

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

A

uo = umf

29
Q

what does Lf represent?

A

length of bed while fluidized

30
Q

what does εf represent?

A

bed voidage while fluidized

31
Q

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

A

uo > umf

32
Q

what is the difference between liquid and gas fluidization?

A

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

33
Q

what does delta Pfr equal? (two things)

A

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

34
Q

for low uo, what is true about delta Pfr?

A

it is proportionate to uo

35
Q

for high uo, what is true about delta Pfr?

A

it is proportionate to uo^2

36
Q

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

A

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

37
Q

when does curve of fluidization flatten?

A

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

38
Q

when will solids fluidize?

A

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

39
Q

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

A

AtLmf(1-ε)*(|rho(s)-rho(g)|(g)

40
Q

what is the pump requirement eqn?

A

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

41
Q

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

A

practically unchanged

42
Q

what is true of pump power at u>umf?

A

will change in proportion to uo

43
Q

the intersection of delta Pfr eqns represents

A

minimum fluidization condition

44
Q

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

A

one including “- 28.7”

45
Q

forces causing particle to accelerate =

A

net weight of particle - drag force

46
Q

Cd represents

A

drag coefficient

47
Q

what is true of du/dt at terminal velocity

A

=0

48
Q

ut represents

A

terminal velocity

49
Q

how to differentiate ut at low Re

A

eqn without square root

50
Q

Reynolds eqn at terminal velocity

A

Resph,t = (dsphutrhog)/viscosity

51
Q

how to differentiate Cd at low Re

A

eqn containing 24

52
Q

Reynolds eqn for non spherical particle

A

Re = (dsphurhog)/viscosity

53
Q

when Ret>1, describe how to differentiate eqns used

A

to -3 or 1/3 power

54
Q

describe relationship between mesh size and diameter?

A

higher mesh, smaller particle

55
Q

describe relationship between diameter, terminal velocity, and mesh?

A

larger terminal velocity, larger diameter, smaller mesh

56
Q

what happens to bed voidage as bed fluidizes?

A

it increases

57
Q

example of packed bed?

A

cigarette, soil in pot

58
Q

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

A

drag force and gravity

59
Q

steps to calculate sphericity?

A
  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

CHEE 203 (3 decks)

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