EXAM 4 Flashcards
where is the resistance to heat transfer located?
boundary layer
what does h describe?
heat transfer coefficient of convection
what is the nusselt #?
hd/k
what is the entrance effect term?
1+(d/L)^0.7
what is the Prandtl #?
((Cp*viscosity)/k)^1/3
does natural or forced convection have a greater h?
forced
how does the heat move in convection with a hot pipe?
from hot wall to inside of pipe
how does the heat move in convection with a cold pipe?
from inside of pipe to cold wall
what is the delta T of convection?
T(hot) - T(cold)
radiation concepts are dependent on their…?
temperature
what is absorptivity?
energy absorbed by surface/energy incident coming from source
what is emissivity?
energy emitted by surface/energy emitted by ideal emitter
what is transmittance?
energy transmitted through body/energy incident
what is the fraction of energy reflected?
1 - absorptivity - transmittance
if the object and surroundings are at T1, what is true of the heat of the object?
does not gain or lose heat
what vary heavily with T and surface type?
absorptivity and emissivity
define a greybody
object whose absorptivity is the same for all T radiation
what is true of emissivity of a greybody?
absorptivity = emissivity = constant at all T
describe the heat transfer from warm room through a wall to the cold outside?
convection on inside of wall, conduction through wall, and convection on outside of wall
describe the heat loss from the fireplace fire. how do they occur in relation to each other?
radiation from flames and convection of moving air. they occur in parallel
what term dominates in processes in series?
R
what term dominated in processes in parallel?
C
what represents the overall conductances?
U
how does T move from hot to cold through a wall?
it continues to decrease as you are further from hot fluid
how does 1/U eqn for a cylindrical pipe differ from through a wall?
cylindrical pipe has A in denominator
what is the nusselt number in words?
convection/conduction
when do you use Gz instead of Gr?
Gz discusses inside pipe
what is the Prandtl number in words?
momentum diffusivity/thermal diffusivity
what is the Gr number in words?
buoyancy/viscous terms
why does heat transfer occur from hot to cold?
second law of thermodynamics … cold is more stable because of molecule movement (to lower enthalpy)
if you replace insulation with insulation of lower thermal conductivity, how would you expect the outside wall temperature to change?
it would decrease because it is less effective at conducting the heat
is conduction or convection a more efficient heat transfer mechanism? why?
convection because more displacement of molecules
if you double T of body giving off radiation, how much would you expect radiating heat flux to change?
2^4 so 16 times
what is characteristic length?
volume/surface area
what is the dominant mechanism for heat transfer of hot water flowing through a pipe?
convection
what is the dominant mechanism for heat transfer of searing a piece of meat with a pan?
conduction
what is the dominant mechanism for heat transfer of sitting in front of a camp fire?
radiation
what is the dominant mechanism for heat transfer of an ice cube melting as it sits in your hand?
conduction
what is the dominant mechanism for heat transfer of soaking in a warm bath?
convection
how is h defined?
film boundary/k
what is a black body?
perfect absorber and emitter … absorptivity = emissivity = 1
if unable to calculate h, what should you use for free convection? what about forced?
10 for free, 40 for forced
what type of heat transfer is the transfer of energy from matter to adjacent matter by direct contact?
conduction
what type of heat transfer is the energy by mixing of material?
convection
describe natural convection
difference in density of hot and cold fluids causes mixing naturally
describe forced convection
externally imposted change in P imposed that causes mixing
what type of heat transfer is described by emanating from hot body and absorbed by cooler body?
radiation
