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MEC203 > Chapter 6 Fundamentals of Convection > Flashcards

Chapter 6 Fundamentals of Convection Flashcards

1
Q

What does convection require

A

fluid motion as well as heat conduction

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2
Q

in a fluid how can heat be transferred

A

by convection in the presence of bulk fluid motion and by conduction in the absence of it

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3
Q

How can conduction be viewed in relation sto convection

A

limiting case of convection, corresponding to the case of quiescent fluid

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4
Q

What is the difference between forced and natural convection

A

forced convection external energy creating motion, natural convection temperature gradient, drives density gradient drives motion

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5
Q

Which has higher heat transfer conduction or convection

A

convection as it brings warmer and cooler chunks of fluid into contact initiating higher rates of conduction at a greater number of sites in a fluid

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6
Q

As velocity of the fluid increases

A

the rate of heat transfer increases

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7
Q

What does convection heat transfer strongly depend on

A

fluid properties; dynamic viscosity, thermal conductivity, density, specific heat and fluid velocity
geometery and roughness of the solid surface,
type of fluid flow

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8
Q

Newtons law of cooling =>

A 
q.conv = h(Ts -Tinf)
Q.conv = hAs(Ts - Tinf)
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9
Q

Definition of convection heat transfer coefficient

A

the rate of heat transfer between a solid surface and fluid per unit surface area per unit temperature difference

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10
Q

What is the no slip condition

A

a fluid in direct contact with a solid sticks to the surface due viscous effects and there is no slip

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11
Q

What is the boundary layer

A

the flow region adjacent to the wall in which the viscous effects are significant

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12
Q

what fluid property causes no slip condition and boundary layer

A

the viscosity

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13
Q

what is the implication of the no slip condition

A

the heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction since the fluid layer is motionless

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14
Q

expression of no slip heat transfer condition

A

q.conv = q.cond = -k fluid dT/dy at y=0

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15
Q

How to determines the convection heat transfer coefficient when temperature distribution within the fluid is known

A

h = ( -k fluid dT/dy at y=0 )/ Ts - Tinf

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16
Q

what happens in general to the heat transfer coefficient along the flow direction

A

it varies, therefore the mean convection heat transfer coefficient for a surface is determined

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17
Q

how is the mean heat transfer coefficient determined

A

properly averging the local convection heat transfer coefficients over the entire surface area As or length L as
h = 1/L integral(0 to L) of h(x) dx
and h = 1/As integral(As) hlocal dAs

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18
Q

What is happening to the particle touching the wall

A

it is not moving but spinning able to do heat transfer balance on it as can not store energy but also means the temperature of the wall and temperature of the fluid are the same

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19
Q

What is the nusselt number

A

dimensionless convection heat transfer coefficient
represents the enhancement of heat transfer through a fluid layer as a result of convection relatives to conduction across the same layer

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20
Q

Nu =

A

hLc/k of fluid

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21
Q

what is the nusselt number a ratio of

A

convection to conduction

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22
Q

What does a Nu = 1 represent

A

heat transfer across the layer by pure conduction

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23
Q

What must Nu always be greater than

A

1

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24
Q

What is q.conv / q.cond equal to

A

(hdelta T)/(kdeltaT/L) = hLc/k = Nu

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25
Q

Whats the difference between viscous and inviscid flows

A

viscous flows in which the frictional effects are significant -> boundary layer
inviscid flows -> typically found in regions not close to solid surfaces where viscous forces are negligible

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26
Q

What is external flow

A

flow of an unbounded fluid over a surface such as a plate wire or a pipe

27
Q

What is internal flow

A

flow in a pipe or duct if the fluid is completely bounded by solid surfaces

28
Q

What is laminar flow

A

highly ordered fluid motion characterised by smooth layer of fluid the flow of high viscosity fluids such as oils at low velocities

29
Q

What is turbulent flow

A

highly disorded fluid motion that typically occurs at high velocities and is characterised by velocity fluctuations

30
Q

what is transitional flow

A

a flow that alternates between laminar and turbulent

31
Q

What is forced flow

A

a fluid is forced to flow over a surface or in a pipe by external means such as a pump or fan

32
Q

what is natural flow

A

fluid motion is due to natural means such as the buoyancy effect (rise of warmer and thus lighter luid and the fall of cooler and thus denser fluid

33
Q

What does steady imply

A

no change at any point with time

34
Q

What does uniform imply

A

no change with location over a specified region

35
Q

what does periodic refer to

A

kind of unsteady flow in which the flow oscillates about a steady mean

36
Q

what is a steady flow device

A

operate for long periods of time under the same conditions ie turbines, compressors boilers

37
Q

How to decide if a flow is 1D, 2D or 3D

A

depends on if the flow velocity varies in one, two , or three dimensions
often ignore small variations in a direction

38
Q

what happens to the velocity boundary layer

A

it grows as you progress down the stream

39
Q

What is the velocity boundary layer

A

region bounded by delta, in which the effects of the viscous shearing forces are felt

40
Q

what is delta in respect to boundary layers

A

the boundary layer thickness usually defined as u = 0.99v

41
Q

What is shear stress

A

frictional force per unit area

42
Q

what is shear stress proportional to for most fluids

A

the velocity gradient

43
Q

How does the viscosity of gases and liquids vary with with temperature

A

with liquids, viscosity decreases with temperature,

with gases, viscosity increases with temperature

44
Q

Equation of shear stress

A

Shear stress = viscosity * du/dy at y=0

45
Q

what are Newtonian fluids

A

fluids where the shear stress is directly proportional to velocity gradient

46
Q

what is kinematic viscosity

A

kinematic viscosity = dynamic viscosity / rho

47
Q

What does kinematic viscosity tell you about a fluid

A

how fast it responds to shear stress, higher value faster response

48
Q

What is a thermal boundary layer

A

develops when a fluid at a specified temperature flows over a surface that is a difference temperature,
the region over the surface in which the temperature variation in the direction normal to the surface is significant

49
Q

Thermal boundary layer thickness deltat is defined as

A

Dist where T-Ts = 0.99(Tinf-Ts)

50
Q

Draw a thermal boundary layer

A

not quite like viscosity as at the surface the value of temperature is not zero but the the temperature of the solid

51
Q

how does the thickness of the thermal boundary layer vary with flow direction

A

increases as the effects of the heat transfer are felt at greater distances from the surface further down the stream

52
Q

Whats the relationship between the shape of the temperature profile in the thermal boundary layer and convection

A

it dictates the convection heat transfer between the solid surface and the fluid flowing over it as the shape represents the temperature gradient

53
Q

What is the prandtl number

A

relatives thickness of the velocity and the thermal boundary layers

54
Q

What does the prandtl number represent

A

how fast momentum diffuses compares to thermal diffuses

55
Q

Pr =

A

molecular diffusivitiy of momentum / molecular diffusivity of heat = kinematic viscosity / alpha = dynamic viscosity * cp / k

56
Q

What does a prandtl number of 1 represent

A

momentum and heat dissipate through the fluid at about the same rate

57
Q

what is the prandtl number a ratio of

A

deltav ^2 / deltat ^2

58
Q

What is reynolds number the ratio of

A

inertial forces to viscous forces

59
Q

Re =

A

rho*Vavg * D/dynamic viscosity (mu) = Vavg *D/dynamic viscosity

60
Q

Transition from laminar to turbulent depends on

A

geometry, surface roughness, flow velocity, surface temperature and type of fluid

61
Q

What is happening at a large reynolds number

A

the inertial forces are large relative to viscous forces, and thus the viscous forces cannot prevent random and rapid fluctuations

62
Q

Which has a larger velocity gradient laminar or turbulent flow

A

turbulent (thus creates more shear force)

63
Q

What is the local nusselt number

A

non dimensional heat transfer gradient

64
Q

local nusselt number =

A

Nu(x) = h(x)*x/k = 0.332 * Pr^1/3 * Re(x)^1/2

MEC203 (12 decks)

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