Shared Flashcard Set

Details

Title

C17

Description

Heat and Mass Transfer

Total Cards

Subject

Science

Level

Undergraduate 3

Created

05/31/2011

Click here to study/print these flashcards.

Create your own flash cards! Sign up here.

Additional Science Flashcards

Cards Return to Set Details

Term

Heat transfer

Definition

energy transfer due to a temperature difference (the driving force is always the temperature difference)

Q=hA(T1-T2)

Q=-kL/A(T1-T2)

Term

Newtonian heating and cooling

Definition

when a Biot number is small (<0.1) temperature gradients in the solid are negligible and as a result the whole plateau decreases/increases with time

Term

Boundary Layer

Definition

the region effected by viscous force and these viscous forces are based on velocity, dynamic viscosity

Term

Viscous Force

Definition

when these forces are dominant, flow is laminar (or stream lined) and fluid flow lines do not touch each other/there is no cross flow but when inertial forces dominate over these viscous forces, regularity is disturbed and turbulent flow is observed

Term

Black body

Definition

a hypothetical ideal surface that is a perfect emitter or absorber. Radiation emitted by a black body varies continuously with wavelength and increases with temperature

ρ=0, τ=0, α=1, ε=1 (black bodies are perfect admitter and absorbers of radiation)

At thermal equilibrium, q=0 and ε=α

Black bodies can be approximated by having an insulated cavity with a small aperture as the absorptivity goes to zero due to repeated absorption and reflection

carbon black has an emissivity very near 1

Term

Radiation

Definition

exists when there is a perfect vacuum between plates. Energy is propagated by electromagnetic phenomena (similar to light and radio wave propagation) and does not require matter to travel [q~0.4]

As radiation has a 4^th power dependence on temperature, at high temperature it can dominate but more often radiation and convection must be considered

Term

Conduction

Definition

a solid material inbetween plates where conduction is due to vibrations through the lattice structure or free electrons. Typically good electrical conductors are also good heat conductors [q~450 copper, q~0.07 mineral wool]

Fourier’s first law: temperature profile does not change with time

The general heat conduction equation: a mathematical expression of the principle of energy conservation in a solid substance

Relevant boundary conditions include – the temperature, heat flux, heat transfer by convection

For a composite system: Q is the same throughout

Term

Convection

Definition

heat transfer due to motion of fluid past surface. We really want to understand h and how it varies with fluid properties. Geometry is also very important and buoyancy effects lead to complex velocity fields

Heat conduction in a solid is influenced by the amount of heat transferred by convection at the surface

Term

Natural Convection

Definition

density difference between air near the hot surface caused also by buoyancy effect [q=0.53]

When heat is applied to a body of fluid volume changes lead to differential velocities and therefore convection

Term

Mass Transfer

Definition

If a fluid is moving, flux will contain two componenets: diffusion from a stationary concentration layer plus a bulk flow

Term

Stoke's Law

Definition

Term

Dimensionless Parameters

-Conduction

Definition

Fourier Number: refers to the extent of ehat penetration by conduction

Biot Number: ratio of convective heat transfer and conductive heat transfer at the surface of a solid (as this number goes to zero there is no temperature gradient in the solid)

Term

Dimensionless Parameters

-Convection

Definition

The relative magnitude of ν/α determines the magnitude of convective heat transfer (Reynolds, Prandtl, and Nusselt) all can be used to estimate the value of heat transfer coefficient. All fluid properties are evaluated at a mean boundary layer temperature (known as Tfilm which is the average of surface and environment temperatures)

Forced convection:

Reynolds number: parameter to determine laminar v. turbulent flow

Natural convection:

Grashof number: a parameter used to evaluate h in free convection

Term

what is the driving force for heat transfer

Definition

temperature difference

Term

boundary conditions for general heat conduction equation

"Fourier's Second Law"

Definition

Convective heat transfer into a fluid at fixed temperature (therefore heat balance at boundary is achieved by q=h(Ts-T∞)=-k(T1-T2)/L at surface edge)

dT/dt=0 (steady state)

surface is maintained at a constant temperature

fixed value of heat flux at a boundAry (ie q is constant)

if q=0: system is adiabatic and also known as a homogeneous boundary condition

Term

How can you find the thermal resistance of a hollow cylinder

Definition

using the 1D heat conduction equation, derive a general equation for T(r) then plug into q=-kdT/dr>Q=Aq>R cylinder

Term

Features of black bodies

Definition

At low T, mostly IR; at high T, mostly UV

with increasing T, max emission is higher and occurs at shorter wavelength

The total rate of emission (emissive power of a black body at T) is the integral if E v. wavelength graphs

E and e are always less than Eb and eb. The proportionality constant is known as emissivity of the material, ε

Term

Grey surface

Definition

one in which the emissivity does not vary with

wavelength (but in real materials it does)

Term

Total irradiation of a material

Definition

G=Gα+Gρ+Gτ

α+ρ+τ=1

α-absorptivity

ρ-reflectivity

τ-transmittivity

α=1, ρ=0, τ=0, for BB

τ=0 for opaque materials

Term

What is the significance of a system being in thermal equilibrium

Definition

q=0

ε=α (consider a real body in an atmosphere of black bodies)

Term

What is the closest approximation to a black body?

Definition

a cavity with a small aperture since radiation entering the cavity is subject to repeated partial absorption and reflection the effective absorptivity is 1

e_hole=εe_b(1/1-ρ)

α+ρ+τ=1, τ=0, therefore α=1-ρ, α=ε (thermal eq)

therefore ehole=eb

Term

significance of moving fluid in mass transfer

Definition

the flux will contain two components: diffusion from a staitionary concentration layer of width S plus a bulk flow

J=-DdC/dx+uC

Term

why might heat transfer coefficient change for different temperature ranges

Definition

at high temperatures just below the melting point, there may be many airpockets preventing heat flow from occuring because of the small surface area contact of fluid with material whereas heat flow is more efficient at lower T with less airpockets and more surface area

Flashcard Machine - create, study and share online flash cards

Shared Flashcard Set

Details

Additional Science Flashcards

Cards Return to Set Details

My Flashcards

Flashcard Library

Browse

About

Help

Mobile