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CHEM 246 > EXAM 4 > Flashcards

EXAM 4 Flashcards

1
Q

where is the resistance to heat transfer located?

A

boundary layer

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2
Q

what does h describe?

A

heat transfer coefficient of convection

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3
Q

what is the nusselt #?

A

hd/k

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4
Q

what is the entrance effect term?

A

1+(d/L)^0.7

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5
Q

what is the Prandtl #?

A

((Cp*viscosity)/k)^1/3

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6
Q

does natural or forced convection have a greater h?

A

forced

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7
Q

how does the heat move in convection with a hot pipe?

A

from hot wall to inside of pipe

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8
Q

how does the heat move in convection with a cold pipe?

A

from inside of pipe to cold wall

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9
Q

what is the delta T of convection?

A

T(hot) - T(cold)

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10
Q

radiation concepts are dependent on their…?

A

temperature

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11
Q

what is absorptivity?

A

energy absorbed by surface/energy incident coming from source

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12
Q

what is emissivity?

A

energy emitted by surface/energy emitted by ideal emitter

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13
Q

what is transmittance?

A

energy transmitted through body/energy incident

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14
Q

what is the fraction of energy reflected?

A

1 - absorptivity - transmittance

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15
Q

if the object and surroundings are at T1, what is true of the heat of the object?

A

does not gain or lose heat

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16
Q

what vary heavily with T and surface type?

A

absorptivity and emissivity

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17
Q

define a greybody

A

object whose absorptivity is the same for all T radiation

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18
Q

what is true of emissivity of a greybody?

A

absorptivity = emissivity = constant at all T

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19
Q

describe the heat transfer from warm room through a wall to the cold outside?

A

convection on inside of wall, conduction through wall, and convection on outside of wall

20
Q

describe the heat loss from the fireplace fire. how do they occur in relation to each other?

A

radiation from flames and convection of moving air. they occur in parallel

21
Q

what term dominates in processes in series?

A

R

22
Q

what term dominated in processes in parallel?

A

C

23
Q

what represents the overall conductances?

A

U

24
Q

how does T move from hot to cold through a wall?

A

it continues to decrease as you are further from hot fluid

25
Q

how does 1/U eqn for a cylindrical pipe differ from through a wall?

A

cylindrical pipe has A in denominator

26
Q

what is the nusselt number in words?

A

convection/conduction

27
Q

when do you use Gz instead of Gr?

A

Gz discusses inside pipe

28
Q

what is the Prandtl number in words?

A

momentum diffusivity/thermal diffusivity

29
Q

what is the Gr number in words?

A

buoyancy/viscous terms

30
Q

why does heat transfer occur from hot to cold?

A

second law of thermodynamics … cold is more stable because of molecule movement (to lower enthalpy)

31
Q

if you replace insulation with insulation of lower thermal conductivity, how would you expect the outside wall temperature to change?

A

it would decrease because it is less effective at conducting the heat

32
Q

is conduction or convection a more efficient heat transfer mechanism? why?

A

convection because more displacement of molecules

33
Q

if you double T of body giving off radiation, how much would you expect radiating heat flux to change?

A

2^4 so 16 times

34
Q

what is characteristic length?

A

volume/surface area

35
Q

what is the dominant mechanism for heat transfer of hot water flowing through a pipe?

A

convection

36
Q

what is the dominant mechanism for heat transfer of searing a piece of meat with a pan?

A

conduction

37
Q

what is the dominant mechanism for heat transfer of sitting in front of a camp fire?

A

radiation

38
Q

what is the dominant mechanism for heat transfer of an ice cube melting as it sits in your hand?

A

conduction

39
Q

what is the dominant mechanism for heat transfer of soaking in a warm bath?

A

convection

40
Q

how is h defined?

A

film boundary/k

41
Q

what is a black body?

A

perfect absorber and emitter … absorptivity = emissivity = 1

42
Q

if unable to calculate h, what should you use for free convection? what about forced?

A

10 for free, 40 for forced

43
Q

what type of heat transfer is the transfer of energy from matter to adjacent matter by direct contact?

A

conduction

44
Q

what type of heat transfer is the energy by mixing of material?

A

convection

45
Q

describe natural convection

A

difference in density of hot and cold fluids causes mixing naturally

46
Q

describe forced convection

A

externally imposted change in P imposed that causes mixing

47
Q

what type of heat transfer is described by emanating from hot body and absorbed by cooler body?

A

radiation

CHEM 246 (7 decks)

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