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CHEE 203 > FINAL > Flashcards

FINAL Flashcards

1
Q

What are the two things that the cooling rate depends on?

A

film resistance at surface (h) and rate of heat flow out of interior

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2
Q

What is ks?

A

thermal conductivity

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3
Q

What is Ts?

A

T at any point of object

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4
Q

What is Fourier number?

A

dimensionless measure for conduction

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5
Q

What does Fourier number account for?

A

cooling time and size of object

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6
Q

What is Biot number measure of?

A

relative importance of surface and interior resistance terms

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7
Q

Bi =

A

interior resistance/surface resistance

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8
Q

Describe resistance of small Bi

A

all resistance in film

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9
Q

Describe resistance of intermediate Bi

A

both resistances important

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10
Q

Describe resistance of large Bi

A

all resistance in conduction through solid

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11
Q

What is lumped parameter analysis?

A

analysis of system that has uniform properties throughout

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12
Q

What Bi is considered small?

A

Bi < 0.1

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13
Q

What Bi is considered large?

A

Bi > 40

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14
Q

What happens when negligible surface resistance?

A

surface immediately drops to T of fluid and conduction within object important

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15
Q

The approach to equilibrium (final T) takes a longer time for….

A

larger surface resistance (smaller Bi number)

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16
Q

Short cooling times mean..

A

heat is only lost in outer layer of solid and cooling has not yet reached deep in object

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17
Q

What is true of h (in Bi number) when heat enters/leaves body by both convection and radiation?

A

should be overall coefficient accounting for both mechanisms of heat transfer
h(overall) = h(convection) + h(radiation)

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18
Q

What will happen to the radiation coefficient as the surface temperature of a particle changes?

A

it will change considerably

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19
Q

Heat exchangers are devices for…

A

transferring heat from a hot flowing stream to a cold flowing stream

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20
Q

What are the three broad types of exchangers?

A

recuperator (aka through the wall non-storing), direct-contact, regenerator (aka accumulator or heat-storing)

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21
Q

How does one decide on what type of exchanger to use?

A

depends on nature of transferring phases and mutual solubility of phases involved

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22
Q

Describe the streams and movement of heat of recuperator

A

streams separated by wall and heat has to pass through wall

23
Q

How does recuperators effectiveness compare to direct-contact exchanger?

A

less effective because presence of wall hinders flow of heat

24
Q

When is it smart to use recuperator?

A

when fluids not allowed to contact each other, gas-gas systems, miscible liquids, dissolving liquids, or reactive chemicals

25
Q

Describe the streams and movement of heat of direct-contact non-storing exchangers

A

streams contact each other and the hotter stream gives up heat directly to colder stream

26
Q

When is it smart to use direct-contact non-storing exchanger?

A

when 2 contacting phases mutually insoluble and do not react with each other (cannot be used for gas-gas systems)

27
Q

What are the three types of direct-contact exchangers?

A

gas-solid, fluid-fluid, air-water

28
Q

Describe the fluids of fluid-fluid direct-contact exchangers

A

mutually immiscible

29
Q

What is the outlier of direct-contact exchangers? Why?

A

air-water systems because the water evaporates in the air

30
Q

Describe the streams and movement of heat of direct-contact heat storing exchangers

A

hot stream of gas transfers heat to intermediary (usually solid) that later gives up stored heat to second stream of cold gas

31
Q

What are the two important factors to understand when dealing with recuperators?

A

overall heat transfer coefficient (U), contacting pattern of the two phases

32
Q

What does U account for in recuperator?

A

overall resistance to transfer caused by wall (included individual film resistances and wall resistance)

33
Q

For shell and tube exchangers, M C and T refer to…

A

shell-side

34
Q

For shell and tube exchangers, m c and t refer to…

A

tube-side

35
Q

How is the amount of heat exchanged as the fluids pass through the exchanger measured?

A

enthalpy change of one of the fluids or as fraction of total heat transferred

36
Q

How to differentiate efficiency equation of countercurrent and co-current exchangers?

A

cocurrent as K’

37
Q

What kind of flow is more efficient?

A

countercurrent flow

38
Q

In the absence of shaft work, what is the energy balance of a flowing fluid?

A

DeltaH = Q

39
Q

Why do we use other shell and tube exchangers besides countercurrent?

A

others are more convenient, compact, and less expensive

40
Q

What will the fudge factor be?

A

between 0 and 1

41
Q

Why do we use fudge factor in shell and tube exchangers?

A

to account for lowered contacting efficiency

42
Q

What is true of the change in P for shell side v tube side? What does this tell us about the fluids?

A

tube-side has greater change in pressure, so less viscous fluid should pass through tubes

43
Q

The larger the number of shell and tube passes…

A

the closer flow is to approaching ordinary countercurrent flow with its largest T driving force

44
Q

What are the two types of flow for a phase of crossflow and compact exchangers?

A

well mixed laterally or unmixed

45
Q

How will a fluid with no lateral mixing act in crossflow and compact exchangers?

A

hot fluid will take separate and parallel paths through exchanger and hot fluid at B will be much cooler than hot fluid at A

46
Q

How will a fluid with lateral mixing act in crossflow and compact exchangers?

A

all fluid along AB will be at one and the same T

47
Q

Is unmixed or mixed flow better?

A

unmixed

48
Q

Is single-pass or multi pass contacting better?

A

multi-pass (if done properly)

49
Q

How to find size of exchanger?

A

noting the terminal temperatures of the exchanger

50
Q

What is the difference between Nusselt and Biot numbers?

A

Bi is ratio of internal resistance to surface resistance while Nu is ratio of convective heat transfer to conductive heat transfer

51
Q

Describe graph of using hot oil bath to heat up solid spherical particle (Bi = 100) w T on y axis and L on x axis

A

concave up parabola

52
Q

Describe graph of using hot oil bath to heat up solid spherical particle (Bi = 100) w T on y axis and L on x axis at different times

A

T increase with time and parabolas could get flatter since particle increasing temp

53
Q

In a shell and tube heat exchanger, how is the heat flow in the shell related to the heat flow in the tubes? Why?

A

each Q is equal but opposite in sign and heat flows from one to the other

54
Q

If the overall heat transfer coefficient for a fluid in heat exchanger increases, how would you expect that to change necessary surface area of interaction between hotter and colder fluids? Why?

A

if U increases, A will decrease because of Q eqn

CHEE 203 (3 decks)

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