EXAM 4

Question 1

where is the resistance to heat transfer located?

Answer 1

boundary layer

Question 2

what does h describe?

Answer 2

heat transfer coefficient of convection

Question 3

what is the nusselt #?

Answer 3

hd/k

Question 4

what is the entrance effect term?

Answer 4

1+(d/L)^0.7

Question 5

what is the Prandtl #?

Answer 5

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

Question 6

does natural or forced convection have a greater h?

Answer 6

forced

Question 7

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

Answer 7

from hot wall to inside of pipe

Question 8

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

Answer 8

from inside of pipe to cold wall

Question 9

what is the delta T of convection?

Answer 9

T(hot) - T(cold)

Question 10

radiation concepts are dependent on their…?

Answer 10

temperature

Question 11

what is absorptivity?

Answer 11

energy absorbed by surface/energy incident coming from source

Question 12

what is emissivity?

Answer 12

energy emitted by surface/energy emitted by ideal emitter

Question 13

what is transmittance?

Answer 13

energy transmitted through body/energy incident

Question 14

what is the fraction of energy reflected?

Answer 14

1 - absorptivity - transmittance

Question 15

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

Answer 15

does not gain or lose heat

Question 16

what vary heavily with T and surface type?

Answer 16

absorptivity and emissivity

Question 17

define a greybody

Answer 17

object whose absorptivity is the same for all T radiation

Question 18

what is true of emissivity of a greybody?

Answer 18

absorptivity = emissivity = constant at all T

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

what term dominates in processes in series?

Answer 21

R

Question 22

what term dominated in processes in parallel?

Answer 22

C

Question 23

what represents the overall conductances?

Answer 23

U

Question 24

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

Answer 24

it continues to decrease as you are further from hot fluid

Question 25

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

Answer 25

cylindrical pipe has A in denominator

Question 26

what is the nusselt number in words?

Answer 26

convection/conduction

Question 27

when do you use Gz instead of Gr?

Answer 27

Gz discusses inside pipe

Question 28

what is the Prandtl number in words?

Answer 28

momentum diffusivity/thermal diffusivity

Question 29

what is the Gr number in words?

Answer 29

buoyancy/viscous terms

Question 30

why does heat transfer occur from hot to cold?

Answer 30

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

Question 31

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

Answer 31

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

Question 32

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

Answer 32

convection because more displacement of molecules

Question 33

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

Answer 33

2^4 so 16 times

Question 34

what is characteristic length?

Answer 34

volume/surface area

Question 35

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

Answer 35

convection

Question 36

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

Answer 36

conduction

Question 37

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

Answer 37

radiation

Question 38

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

Answer 38

conduction

Question 39

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

Answer 39

convection

Question 40

how is h defined?

Answer 40

film boundary/k

Question 41

what is a black body?

Answer 41

perfect absorber and emitter ... absorptivity = emissivity = 1

Question 42

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

Answer 42

10 for free, 40 for forced

Question 43

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

Answer 43

conduction

Question 44

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

Answer 44

convection

Question 45

describe natural convection

Answer 45

difference in density of hot and cold fluids causes mixing naturally

Question 46

describe forced convection

Answer 46

externally imposted change in P imposed that causes mixing

Question 47

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

Answer 47

radiation