Chapter 9 Natural Convection Flashcards by Jamie Rayson

What is natural convection normally accompanied by

radiation of comparable magnitude except for low emissivity surface

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what is a natural convection current and natural convection heat transfer

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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what is the system of natural convection

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

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Describe basic process to find h

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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What is the upward buoyancy force =>

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

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What is the magnitude of the buoyancy force

= weight of the fluid displaced by the body

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F buoyancy =

rho fluid g V body

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Net vertical force acting on a body Fnet =

(rho body - rho fluid) g * Vbody

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Symbol of the volume expansion coefficient

beta

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What is the volume expansion coefficient

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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volume expansion coefficient =

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

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units of the volume expansion coefficient

1/K

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for gases volume expansion coefficient approx =

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

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ideal gas volume coefficient

1/T

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

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

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What is the flow rate established by in natural convection

dynamic balance of buoyancy and friction

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Whats the difference between velocity for natural and force convection

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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how does boundary layer thickness vary with flow direction

it increases

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Draw the temperature and velocity profile for natural convection

see book

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

they are reversed but same shape

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

see book

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What is the continuity equation

see book

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What is the momentum equation

see book

What is the energy equation

see book

What does the grahof number represent

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

What does the grashof number allow you to

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

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

both natural and forced convection

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

ratio of Gr/Re^2

if Gr/Re^2 << 1

natural convection effects are negligible

if Gr/Re^2 >> 1

free convection dominates and the forced convection effects are negligible

if Gr/Re^2 =1

both effects are significant and must be considered i | mixed convection

What does natural convection depend on

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

Heat transfer relation in natural convection are mostly based on

experimental studies

Nu =

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

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

1/4 for laminar and 1/3 for turbulent

If Gr is big what does that mean

high buoyancy less viscous

what is Gr a ratio of

buoyancy and viscous effects

Draw the boundary layer for a hot cylinder

see book

How does the boundary layer develop over a hot horizontal cylinder

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

What is the local nusselt number highest for a hot clyinder

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

How does a hot cylinder compare to a cold cylinder

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

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

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

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

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

How is a fin approximated

isothermal Ts = const

How is a PCB approximated

qs = const isoflux

What happens in a vertical enclosure and why are enclosures used

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

Draw a diagram of an enclosure

see book

What is the equation for RaL

see book

Fluid in an enclosure thermal conductivity =

kNu as a result of the convection currents

keff =

kNu

Nu = 1

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

What does a nusselt number of 3 for an enclosure indicate

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

Q. = for enclusre

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

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

h = kNu/L

simple power law relations for Nu =

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