Chapter 6 Fundamentals of Convection Flashcards
What does convection require
fluid motion as well as heat conduction
in a fluid how can heat be transferred
by convection in the presence of bulk fluid motion and by conduction in the absence of it
How can conduction be viewed in relation sto convection
limiting case of convection, corresponding to the case of quiescent fluid
What is the difference between forced and natural convection
forced convection external energy creating motion, natural convection temperature gradient, drives density gradient drives motion
Which has higher heat transfer conduction or convection
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
As velocity of the fluid increases
the rate of heat transfer increases
What does convection heat transfer strongly depend on
fluid properties; dynamic viscosity, thermal conductivity, density, specific heat and fluid velocity
geometery and roughness of the solid surface,
type of fluid flow
Newtons law of cooling =>
q.conv = h(Ts -Tinf) Q.conv = hAs(Ts - Tinf)
Definition of convection heat transfer coefficient
the rate of heat transfer between a solid surface and fluid per unit surface area per unit temperature difference
What is the no slip condition
a fluid in direct contact with a solid sticks to the surface due viscous effects and there is no slip
What is the boundary layer
the flow region adjacent to the wall in which the viscous effects are significant
what fluid property causes no slip condition and boundary layer
the viscosity
what is the implication of the no slip condition
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
expression of no slip heat transfer condition
q.conv = q.cond = -k fluid dT/dy at y=0
How to determines the convection heat transfer coefficient when temperature distribution within the fluid is known
h = ( -k fluid dT/dy at y=0 )/ Ts - Tinf
what happens in general to the heat transfer coefficient along the flow direction
it varies, therefore the mean convection heat transfer coefficient for a surface is determined
how is the mean heat transfer coefficient determined
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
What is happening to the particle touching the wall
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
What is the nusselt number
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
Nu =
hLc/k of fluid
what is the nusselt number a ratio of
convection to conduction
What does a Nu = 1 represent
heat transfer across the layer by pure conduction
What must Nu always be greater than
1
What is q.conv / q.cond equal to
(hdelta T)/(kdeltaT/L) = hLc/k = Nu
Whats the difference between viscous and inviscid flows
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
What is external flow
flow of an unbounded fluid over a surface such as a plate wire or a pipe
What is internal flow
flow in a pipe or duct if the fluid is completely bounded by solid surfaces
What is laminar flow
highly ordered fluid motion characterised by smooth layer of fluid the flow of high viscosity fluids such as oils at low velocities
What is turbulent flow
highly disorded fluid motion that typically occurs at high velocities and is characterised by velocity fluctuations
what is transitional flow
a flow that alternates between laminar and turbulent
What is forced flow
a fluid is forced to flow over a surface or in a pipe by external means such as a pump or fan
what is natural flow
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
What does steady imply
no change at any point with time
What does uniform imply
no change with location over a specified region
what does periodic refer to
kind of unsteady flow in which the flow oscillates about a steady mean
what is a steady flow device
operate for long periods of time under the same conditions ie turbines, compressors boilers
How to decide if a flow is 1D, 2D or 3D
depends on if the flow velocity varies in one, two , or three dimensions
often ignore small variations in a direction
what happens to the velocity boundary layer
it grows as you progress down the stream
What is the velocity boundary layer
region bounded by delta, in which the effects of the viscous shearing forces are felt
what is delta in respect to boundary layers
the boundary layer thickness usually defined as u = 0.99v
What is shear stress
frictional force per unit area
what is shear stress proportional to for most fluids
the velocity gradient
How does the viscosity of gases and liquids vary with with temperature
with liquids, viscosity decreases with temperature,
with gases, viscosity increases with temperature
Equation of shear stress
Shear stress = viscosity * du/dy at y=0
what are Newtonian fluids
fluids where the shear stress is directly proportional to velocity gradient
what is kinematic viscosity
kinematic viscosity = dynamic viscosity / rho
What does kinematic viscosity tell you about a fluid
how fast it responds to shear stress, higher value faster response
What is a thermal boundary layer
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
Thermal boundary layer thickness deltat is defined as
Dist where T-Ts = 0.99(Tinf-Ts)
Draw a thermal boundary layer
not quite like viscosity as at the surface the value of temperature is not zero but the the temperature of the solid
how does the thickness of the thermal boundary layer vary with flow direction
increases as the effects of the heat transfer are felt at greater distances from the surface further down the stream
Whats the relationship between the shape of the temperature profile in the thermal boundary layer and convection
it dictates the convection heat transfer between the solid surface and the fluid flowing over it as the shape represents the temperature gradient
What is the prandtl number
relatives thickness of the velocity and the thermal boundary layers
What does the prandtl number represent
how fast momentum diffuses compares to thermal diffuses
Pr =
molecular diffusivitiy of momentum / molecular diffusivity of heat = kinematic viscosity / alpha = dynamic viscosity * cp / k
What does a prandtl number of 1 represent
momentum and heat dissipate through the fluid at about the same rate
what is the prandtl number a ratio of
deltav ^2 / deltat ^2
What is reynolds number the ratio of
inertial forces to viscous forces
Re =
rho*Vavg * D/dynamic viscosity (mu) = Vavg *D/dynamic viscosity
Transition from laminar to turbulent depends on
geometry, surface roughness, flow velocity, surface temperature and type of fluid
What is happening at a large reynolds number
the inertial forces are large relative to viscous forces, and thus the viscous forces cannot prevent random and rapid fluctuations
Which has a larger velocity gradient laminar or turbulent flow
turbulent (thus creates more shear force)
What is the local nusselt number
non dimensional heat transfer gradient
local nusselt number =
Nu(x) = h(x)*x/k = 0.332 * Pr^1/3 * Re(x)^1/2
