1 - Introduction And Basic Concepts Flashcards
Define heat
The form of energy that can be transferred from one system to another as a result of temperature difference.
Define thermodynamics
Concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.
Define heat transfer
Deals with the determination of the rates of such energy transfers as well as variation of temperature.
Points about heat transfer
Always from high-temperature medium to low-temperature medium.
Stops when the two mediums reach the same temperature.
Can be transferred in three different modes: conducton; convection and radiation.
What do rating problems deal with?
The determination of the heat transfer rate for an existing system at a specified temperature difference.
What do sizing problems deal with?
The determination of the size of a system in order to transfer heat at a specified rate for a specified temperature difference.
Advantages of the experimental approach
We deal with the actual physical system.
The desired quantity is determined by measurement within the limits of experimental error.
Disadvantages of the experimental approach
Expensive
Time-consuming
Often impractical
Advantages of the analytical approach (including numerical approach)
Fast
Inexpensive
Disadvantages of the analytical approach
Results obtained are subject to the accuracy of the assumptions, approximations and idealisations made in the analysis.
Define specific heat
The energy required to raise the temperature of a unit mass of a substance by one degree
What does specific heat depend on at low pressures?
Temperature only, as at low pressures all real gases approach ideal gas behaviour.
Define incompressible substance
A substance whose specific volume/density does not change with temperature or pressure.
What are the specific heats of an incompressible substance?
Constant-volume and constant-pressure specific heats are identical.
Specific heat depends on temperature only.
Mechanisms for energy transfer
Heat transfer
Work
Define heat transfer rate
The amount of heat transferred per unit time.
Define heat flux
The rate of heat transfer per unit area normal to the direction of heat transfer.
What is the first law of thermodynamics?
Energy can neither be created nor destroyed during a process; it can only change forms.
(Conservation of energy principle)
Define mass flow rate
The amount of mass flowing through a cross-section of a flow device per unit time.
Define volume flow rate
The volume of a fluid flowing through a pipe or duct per unit time.
What is surface energy balance?
Energy remains constant during a process as a surface contains no volume or mass, and thus no energy (E in = E out).
This relation is valid for both steady and transient conditions, and the surface energy balance does not involve heat generation since a surface does not have a volume.
Define conduction
The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles.
What is conduction caused by in gases and liquids?
Collisions and diffusion of the molecules during their random motion.
What is conduction caused by in solids?
The combination of vibrations of the molecules in a lattice and the energy transport by free electrons.
Define thermal conductivity (k)
A measure of the ability of a material to conduct heat.
Define temperature gradient (dT/dx)
The slope of the temperature curve on a T-x diagram.
Define specific heat
Heat capacity per unit mass
Define thermal diffusivity (alpha)
Represents how fast heat diffuses through a material
Effect of larger thermal diffusivity
High thermal conductivity
Low heat capacity
Faster propagation of heat into medium
Effect of smaller value of thermal diffusivity
Heat is mostly absorbed by the material
A small amount of heat is conducted further
Define convection
The mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.
Define forced convection
If the fluid is forced to flow over the surface by external means such as a fan, pump or the wind.
Define natural/free convection
The fluid motion is caused by the buoyancy forces that are induced by density differences due to the variation of temperature in the fluid.
Which heat transfer process is used to describe one that involves a change of phase?
Convection because of the fluid motion induced during the process, such as the rise of the vapour bubbles during boiling or the fall of the liquid droplets during condensation.
What does the convection heat transfer coefficient (h) depend on?
Surface geometry
Nature of fluid motion
Fluid properties
Bulk fluid velocity
Define radiation
The energy emitted by matter in the form of EM waves or photons as a result of the changes in the electronic configurations of the atoms or molecules.
Points about radiation
Does not require an intervening medium.
Heat transfer by radiation is fastest (speed of light) and it suffers no attenuation in a vacuum.
All bodies at a temperature above absolute zero emit thermal radiation.
Radiation is a volumetric phenomenon.
All solids, liquids and gases emit, absorb, or transmit radiation to varying degrees.
However, radiation is usually considered to be a surface phenomenon for solids.
Define blackbody
The idealised surface that emits radiation at the maximum rate.
Define emissivity (epsilon)
A measure of how closely a surface approximates a blackbody for which epsilon = 1 for the surface.
What is blackbody radiation?
Represents the maximum amount of radiation that can be emitted from a surface at a specified temperature.
Define absorptivity (alpha)
The fraction of the radiation energy incident on a surface that is absorbed by the surface (0-1).
A blackbody absorbs the entire radiation incident on it (alpha = 1).
What is Kirchhoff’s law?
The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.
Define net radiation heat transfer
The difference between the rates of radiation emitted by the surface and the radiation absorbed.
What does the determination of the net rate of heat transfer by radiation depend on?
Surface properties
The surfaces’ orientation relative to each other
The interaction of the medium between the surfaces with radiation
When is radiation significant?
It’s significant relative to conduction or natural convection, but negligible relative to forced convection.
What type of heat transfer in opaque solids?
Conduction only
What type of heat transfer in semitransparent solids?
Conduction and radiation
When can a solid involve convection and/or radiation?
On surfaces exposed to a fluid or other surfaces.
Heat transfer process in a still fluid
Conduction and possibly radiation.
Heat transfer process in a flowing fluid
Convection and radiation.
In the absence of radiation, what is the heat transfer process through a fluid?
Either conduction or convection, depending on the presence of any bulk fluid motion.
Convection = conduction + fluid motion
Heat transfer process through a vaccuum
Radiation
Which phase is a strong absorber of radiation?
Liquid
How many mechanisms of heat transfer may a medium involve simultaneously?
Only two
