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MEC203 > Chapter 9 Natural Convection > Flashcards

Chapter 9 Natural Convection Flashcards

1
Q

What is natural convection normally accompanied by

A

radiation of comparable magnitude except for low emissivity surface

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2
Q

what is a natural convection current and natural convection heat transfer

A

motion that results from the continual replacement of heated air by cooler air nearby and the heat transfer that is enhanced as a result of this current is called natural convection heat transfer

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3
Q

what is the system of natural convection

A

nabla temperature drives nabla density, drives change in mass, drives force by newtons second law, drives velocity drives energy change

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4
Q

Describe basic process to find h

A

Nufree = function of the rayleigh number
Nu = hlc / kfluid = cRa^alpha where alpha, and c depend on geom
Ra = grashof number * Pr
properties evaluated at film temp Tf = Ts + Tinf /2

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5
Q

What is the upward buoyancy force =>

A

the upward force exterted by a fluid on a body completely or partially immersed in a gravitation field

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6
Q

What is the magnitude of the buoyancy force

A

= weight of the fluid displaced by the body

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7
Q

F buoyancy =

A

rho fluid g V body

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8
Q

Net vertical force acting on a body Fnet =

A

(rho body - rho fluid) g * Vbody

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9
Q

Symbol of the volume expansion coefficient

A

beta

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10
Q

What is the volume expansion coefficient

A

variation of the density of a fluid with temperature at constant pressure
measure of the change in volume of a substance with temperature at constant pressure

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11
Q

volume expansion coefficient =

A

1/specific volume * (pard specific volume/ pardT) = -1/rho*(pard rho / pard T)

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12
Q

units of the volume expansion coefficient

A

1/K

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13
Q

for gases volume expansion coefficient approx =

A

-1/rho * DELTArho/DELTA T = -1/rho * ((rho inf - rho)/ (Tinf -T) at constant P

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14
Q

ideal gas volume coefficient

A

1/T

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15
Q

what happens as temperature difference between the fluid adjacent to a hot surface and the fluid away from it increases for natural convection

A

bigger the buoyancy force, stronger the natural convection, the higher the heat transfer

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16
Q

What is the flow rate established by in natural convection

A

dynamic balance of buoyancy and friction

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17
Q

Whats the difference between velocity for natural and force convection

A

the fluid velocity is zero at the outer edge of the velocity boundary layer as well as at the surface of the plate

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18
Q

how does boundary layer thickness vary with flow direction

A

it increases

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19
Q

Draw the temperature and velocity profile for natural convection

A

see book

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20
Q

How do the the shape of temperature and velocity profiles vary with cold surfaces

A

they are reversed but same shape

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21
Q

Derive the equation of motion that governs natural convection flow in laminar boundary layer

A

see book

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22
Q

What is the continuity equation

A

see book

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23
Q

What is the momentum equation

A

see book

24
Q

What is the energy equation

A

see book

25
Q

What does the grahof number represent

A

the natural convection effects in momentum equation

is a measure of the relative magnitudes of the buoyancy force and opposing viscous force acting on the fluid

26
Q

What does the grashof number allow you to

A

determine whether the fluid flow is laminar or turbulent in natural convection,
critical grashof number is 10^9

27
Q

If a body is subject to external flow what does the problem involve

A

both natural and forced convection

28
Q

When natural and forced convection are involved how do you determine which is dominant

A

ratio of Gr/Re^2

29
Q

if Gr/Re^2 &laquo_space;1

A

natural convection effects are negligible

30
Q

if Gr/Re^2&raquo_space; 1

A

free convection dominates and the forced convection effects are negligible

31
Q

if Gr/Re^2 =1

A

both effects are significant and must be considered i

mixed convection

32
Q

What does natural convection depend on

A

the geom of the surface
the orientation
variation of temp on the surface
thermophyscial properties of the fluid

33
Q

Heat transfer relation in natural convection are mostly based on

A

experimental studies

34
Q

Nu =

A

hLc/k = C (Gr Pr)^n = C Ra^n where c and n depend on the geom and the flow regime

35
Q

what is the value of n usually for Nu = C Ra^n

A

1/4 for laminar and 1/3 for turbulent

36
Q

If Gr is big what does that mean

A

high buoyancy less viscous

37
Q

what is Gr a ratio of

A

buoyancy and viscous effects

38
Q

Draw the boundary layer for a hot cylinder

A

see book

39
Q

How does the boundary layer develop over a hot horizontal cylinder

A

develop at the bottom increasing in thickness along the circumference and forming a rising plume at the top

40
Q

What is the local nusselt number highest for a hot clyinder

A

at the bottom and lowest at the top of the cylinder when the boundary layer flow remains laminar

41
Q

How does a hot cylinder compare to a cold cylinder

A

Everything is reversed, boundary layer develops at the top of the cylinder and ending with a descending plume at the botoom

42
Q

For two parallel plates (fins or PCBs) where does the boundary layer start to develop

A

at the lower ends of opposing surfaces and eventually merges at the mid plane if the plates are vertical and sufficiently long will have fully developed channel flow

43
Q

For two parallel plates what happens if the boundary layers never meet

A

natural convection flow on a surface is not affected by the presence of the opposing surface, problem anaylsed as two independent plates

44
Q

How is a fin approximated

A

isothermal Ts = const

45
Q

How is a PCB approximated

A

qs = const isoflux

46
Q

What happens in a vertical enclosure and why are enclosures used

A

fluid adjacent to the hotter surface rises and the fluid adjacent to the cooler one falls, setting of rotationary motion within the enclosure that enhances heat transfer through the enclosure

47
Q

Draw a diagram of an enclosure

A

see book

48
Q

What is the equation for RaL

A

see book

49
Q

Fluid in an enclosure thermal conductivity =

A

kNu as a result of the convection currents

50
Q

keff =

A

kNu

51
Q

Nu = 1

A

the effective thermal conductivity of the enclosure is equal to the conductivity of the fluid, ie pure conduction

52
Q

What does a nusselt number of 3 for an enclosure indicate

A

heat transfer through the enclosure but natural convection is three time that by pure conduction

53
Q

Q. = for enclusre

A

hAs (T1 - T2) = kNu As (T1 - T2)/ Lc

54
Q

what does the the convection heat transfer coefficient equal for an enclosure

A

h = kNu/L

55
Q

simple power law relations for Nu =

A

CRa^n where C and n are constants, usually applicable to a narrow range of prandltl and rayleigh numbers and aspect ratios

MEC203 (12 decks)

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