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CET I > Paper 1 > Flashcards

Paper 1 Flashcards

1
Q

When does Carman-Kozeny apply?

A

Laminar flow through a packed bed for Re_p < 10.

If 10

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2
Q

HMTO column first line

A

Assume dilute sys so L and G remain constant over column. Assume acetone is fully soluble in water, so whatever is lost from gas stream is gained by liquid stream.

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3
Q

When does Nu=2?

A

A sphere losing heat to/gaining heat from an infinite stagnant medium. The heat transfer coeff is k_MEDIUM/RADIUS of sphere.

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4
Q

forced convection in ducts

A

often: turbulent so can use Blasius, film model St Pr=0.5Cf=Nu/Re or Reynolds analogy St=0.5Cf holds

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5
Q

What is Bi?

A

Internal resistance (usually conduction)/External resistance (usually convection) “Irer”. Bi=h_ext/h_int and length scale of h_int given by vol/SA=r/3 for a sphere.

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6
Q

When does Sh=2?

A

diffusion of A from/to a sphere to/from an infinitely large stagnant medium

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7
Q

what density and area for the drag coeff?

A

density of the medium (e.g. air) and cross sectional area of the falling object (e.g. pi r^2)

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8
Q

when is onset of turbulence for film condensation?

A

Re=1000

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9
Q

do pure metals or alloys have larger thermal conductivity?

A

pure metals because long range order helps the flow of electrons

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10
Q

dependence of thermal conductivity of gases on pressure

A

independent over a wide range. low p there are few molecs to carry heat but can travel far, opposite for large p.
very high p k approaches liq values, very low p k approaches insulator values

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11
Q

dependence of thermal conductivity on temp

A

higher temp higher k for gases, lower k for liqs except water and glycerine

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12
Q

What can Fourier’s Law be used for?

A

q=-kdT/dr only for conduction, in contrast to q=h/\T which can be used for both conduction and convection

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13
Q

thermal conductivity of stainless steel

A

20 W/mK

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14
Q

when are contact resistances insignificant?

A

insulating materials and solid-fluid interfaces

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15
Q

what happens when you add lagging to a pipe?

A

Case 1: inner radius small and outer radius not that much bigger. Initially the htc acc increases because the external area is increasing, then as the thickness of lagging increases, the htc will decrease
Case 2: inner radius large: htc will decrease regardless of outer radius or thickness of insulation.

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16
Q

thermal diffusivity

A

alpha=k/(rho*Cp) units of m2/s. Croke park is the alpha stadium.

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17
Q

what is the penetration depth?

A

erfc(delta/sqrt(4at))=erfc(2)=0.005

the depth beyond which there has been no noticeable effect of heat transfer.

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18
Q

what is the heating time?

A

for a finite slab, the time it takes for the centre of the slab to have theta=0.99. It is 2b^2/alpha.
N.B. the penetration time for the finite slab is t=b^2/20alpha vs b^2/16a for the semi-infinite slab for the same depth

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19
Q

what assumptions can be made for low and high Bi?

A

Low Bi=external control where convective transfer is the important one, lumped body “lowbixvlump”
High bi=internal control so assume solid surface at constant temp
Intermediate Bi need to use charts

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20
Q

what diffusion does the course address?

A

binary diffusion for either gases or solutes in excess liquid solvent

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21
Q

what is fick’s law of diffusion an analogue of?

A

Fourier’s law of conduction

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22
Q

mt equation. a) dilute mixt b) ECD c) stagnant B d) diffusion tube

A

yA=0, NA=-NB, NB=0, ECD

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23
Q

effect of Mr on diffusivity. wb large molecs in aqueous solution?

A

low Mr high diffusivity. very low diffusivity

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24
Q

what increases the likelihood of Knudsen diffusion?

A

gases with narrow regions of flow. Kn=Length of free path/diameter of pore. Low pressure. Note that when Kn is high, diffusivity is a maximum and is independent of pressure

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25
Q

mtc and Sh for capillary

A

km=Diffusivity/L and Sh=1

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26
Q

Stefan tube

A

method for measuring diffusivity of the vapour of a volatile liq. assume NB=0, yA at top=0, NA indep of z bc SS
if liquid has low volatility then yA always low. Then Sh=1
It is analagous to heat transfer in a slab.

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27
Q

moth ball

A

mt from an evaporating sphere. Iff dilute case and stagnant gas, Sh=2 so analogous to Nu=2 for heat conduction to infinite stagnant medium.

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28
Q

reaction (kinetics) controlled vs diffusion (mass transfer) controlled

A

if km small then large mt resistance so mt controlled. if kr small then low rate of rxn so kinetics controlled

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29
Q

when can you equate reaction and diffusion rates?

A

when ss holds

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30
Q

sensitivity of diffusivity to ambient temperature

A

kinetic theory says D prop Tamb^1.75

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31
Q

vena contracta

A

section of minimum area, streamlines parallel therefore pressure in the jet is uniform and equal to the surrounding pressure e.g. if vena contracta in air then pressure of fluid at vc is patm

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32
Q

coeff of contraction

A

Avc/Aori

33
Q

coeff of velocity

A

uvc,act=Cv*uvc,ideal

34
Q

what is special about venturi meter?

A

no vena contracta formed, approximately lossless so can apply bern

35
Q

what is the momentum balance that leads to SFEE?

A

rate of acc of momentum in ctrl vol = rate of mom entering CV - rate of mom leaving CV + sum of forces acting on sys = 0 if steady flow

36
Q

what does bernoulli assume

A

no viscous or frictional dissipation, incomrpressible flow, steady flow, applies along a streamline only

37
Q

for reaction on a vertical pipe bend, should you use gauge or atmospheric pressure?

A

gauge

also assume radial velocity variation negligible

38
Q

where streamlines are parallel,

A

dp/dx is const with y and du/dy is const with x

39
Q

when is the film model Nu=0.5CfPr i.e. StRePr=0.5CfPr ok?

A

turb flow with Pr ~ 1 i.e. a gas

ht btw wall of pipe and the fluid flowing through it

40
Q

nusselt’s analysis for condensation on a vertical surface assumptions

A

viscous laminar flow in condensate film (Re<1000), saturated vapour with no superheat/sensible effects, low vapour velocity so that we can ignore shear stress at interface, steady state, negligible interfacial shear stress, isobaric, ignore curvature, fluid moves only vertically, pressure does not change with x, no acceleration, no slip at wall, thickness at top = 0, no ht resistance in metal, no mt resistance in gas, assume liquid Newtonian st tau=mu dv/dx

41
Q

going from condensation on vertical plate to horizontal cylinder

A

replace g with gsintheta and dy with rdtheta

also say that gamma=0 when theta=0

42
Q

when would you use vertical tubes for condensation rather than horizontal?

A

if subcooling required or if L<3D.

43
Q

what is bleeding a condenser?

A

regularly passing gas through it to purge inerts

44
Q

can you use Q=UA(/\T)lm for co-current? When does the formula have issues

A

yes

when M.Cp=m.cp now Q=UA(t1-T1)

45
Q

fouling

A

dynamic formation of unwanted surface deposits

chemical rxn, crystallisation, corrosion, particulates, biofouling

46
Q

why are plant heat exchangers at ground level

A

so they can isolated, opened up and cleaned because of fouling

47
Q

what is special about continuous contact between phases compared with discrete contact?

A

equilibrium btw the bulk phases does not occur

48
Q

what does the HMTO mass transfer course assume?

A

plug flow, no dispersion, isothermal, dilute i.e. constant molar overflow

49
Q

how do you prevent fluidisation in a packed column?

A

hold-down plate

50
Q

advantages of packed over plate columns

A

smaller liq hold-up so can minimise inventory of flammable/toxic liqs, cheaper for small diameter columns (<0.6 m) since plates difficult to install, dirty packing can be replaced rather than cleaned in situ, cheaper to construct for handling of corrosive liqs/vaps, pressure drops lower, so suitable for vacuum columns, good at handling foaming systems

51
Q

advantages of plate over packed

A

wider range of flow rates (lower flowrates), number of plates more certain than packing height, withdrawal of side streams easier, shorter, cooling and heating easier, easier to clean

52
Q

For N2 or NH3 is it that Kg approx kg?

A

NH3, which is very soluble in the liquid

53
Q

is the thickness of the film for Whitman two film model a function of the component (e.g. N2 vs NH3)?

A

No

54
Q

height of an equivalent theoretical plate

A

z=n*HETP

55
Q

what is liq hold-up

A

volume frac of liq`

56
Q

onset of flooding

A

when /\P tends to rhogz

57
Q

relationship btw mean shear rate produced by an agitator in a mixing tank and the agitator rotational speed

A

proportional

i.e. gamma.mean proportional to N

58
Q

two requirements of liq mixing devices

A

no stagnant zones

inhomogeneities broken down

59
Q

Reynolds regime within agitated/stirred vessel

A

Re<10 lam

Re>10,000 turb

60
Q

Re for a mixing impeller

A

Re=D^2 N rho/mu

61
Q

reasons for baffles

A

prevent vortexing and rotation of liquid mass as a whole

62
Q

for pumps, when can you say that CH=f(CQ)

A

If Re=rhoND^2/mu > 10^5 then you don’t need dynamic similarity and CH=f(CQ)

63
Q

pump efficiency as a fraction

A

eta = rhogHQ/P = CHCQ/CP

64
Q

geometric similarity, kinematic similarity, dynamic similarity

A

lengths/velocity/forces scale by constant factor

65
Q

what does a low specific speed Ns of a pump correspond to?

A

low Q and high H

NB specific speed is measured at the point of max efficiency

66
Q

when is Froude unimportant for fluid mixing?

A

when baffles are used or Re < 300

67
Q

if two tanks have different sizes but are geometrically similar, are their power curves the same?

A

Yes

68
Q

packed bed superficial velocity

A

volumetric flowrate/total cross sectional area of bed

69
Q

packed bed Reynolds regime

A

Re’ < 10 laminar

Re’ > 2000 turbulent

70
Q

when can you use Carman Kozeny

A

Laminar flow through a packed bed

Also for filtration

71
Q

when can you use Ergun

A

laminar and turbulent flow through packed bed

72
Q

when applying Bernoulli across a packed bed, what simplification can you often make?

A

to ignore velocity heads if laminar flow

73
Q

what do you do if you have non-spherical particles for packed beds?

A

use the volume-surface diameter

d=6V/A all wrt particles

74
Q

what is elutriation

A

particles being swept out of bed because velocity so high

75
Q

When is Richardson Zaki applicable

A

liquid-solid systems particulate fluidisation

76
Q

distinguishing feature of a fluid

A

inability to resist shearing forces while remaining in static equilibrium

77
Q

shear thickening

A

viscosity increases with increasing shear rate

78
Q

power law fluid equation

A

tau = K * gamma. ^n

CET I (5 decks)

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