What is heat transfer?
Heat is a form of energy that is transferred when there is a temperature difference. Heat transfer deals with the rates of heat energy being transferred and the variation of temperature. Energy transferred from heat is always from the higher to lower temperatures and stops when there is no temperature difference.
Why should you care?
Before and during the early nineteenth century, heath was seen in a different way from what we know it is today. They used to think that heat was an invisible fluid (a caloric) that was tasteless, colorless, massless, and odorless. It was believed that this fluid like substance could be transferred from one body into another.
This belief was thankfully dismissed during the mid-nineteenth century by James P. Joule. His experiments demonstrated that heat was not a substance and open the path to true understanding of the physical properties of heat.
Due to this understanding, we have developed heating and air conditioning systems, refrigeration systems, power plants, and all the electronics we have. Without the knowledge of heat transfer our houses and water will be too cold or too hot, our electronics will be too hot to use or explode due to combustion, and we wouldn’t know how to generate efficient power and fuel energy.
How is heat transferred?
Conduction, convection, and radiation are the three different ways in wish heat can be transferred
Conduction
Conduction happens when energetic particles interact and therefore pass their energy to less energetic particles. For example, in liquid and gasses the collisions and diffusion of their molecules during motion causes conduction. In solids, conduction happens when molecules vibrate and when free electrons cause energy transport.
The equation above represents the rate of heat conduction when going through a plane. It means that heat conduction is proportional to the temperature
difference happening in the area across the layer. It is also inversely proportional to the layer thickness.
Fourier’s law of heat conduction:
In the equation the temperature gradient (dt/dx) is the slope of the temperature curve on a T-x diagram The Thermal conductivity (k) is the materials ability to conduct heat.
Remember that heat will be conducted in direction of decreasing temperature, making the temperature gradient more negative as temperature decreases and x increases. Therefore, there is a negative in the equation, it makes sure that the heat being transferred is positive when moving in the x-direction.
High thermal conductivity indicates good heat conduction & a low one will be a good insulator Note that metals and pure crystals show higher thermal conductivities, while gases and other insulating materials show low thermal conductivity. However, a material thermal conductivity will vary with temperature.
The thermal diffusivity property of a material will represent how fast heat will move through a material:
In this equation C_p is the specific heat (J/kg · °C) or the heat capacity per unit mass. This will make ρC_p Heat capacity, (J/m3·°C) or heat capacity per unit volume. α represents the thermal diffusivity, and as stated before represents how fast heat will diffuse through a material.
Convection
Convection explains how energy is transfer from a solid surface to a liquid or gas in motion. How convection involves a solid, it will involve the effects of conduction with the fluid in motion. As the fluid motion increases so will the heat transfer through convection. However, if the fluid is not in motion then the heat transfer between them will be only due to conduction.
Newton’s law of cooling:
· 
is the temperature of the fluid away from the surface.
· 
is the temperature at the surface.
· 
will be the surface area where the convection heat transfer happens
· 
represents the convection heat transfer coefficient in W/m2 · °C
Note that the convection heat transfer coefficient (h) is not a property of the fluid, but a parameter dependent on all variables that influence convection. Meaning that the coefficient is usually experimentally determined due to geometry, fluid motion, properties of fluids, etc.
Radiation
Radiation is energy emitted by matter in what we know to be electromagnetic waves (photos). This is caused by changes in the configurations of the molecules or atoms.
Of the three forms of heat transfer, radiation is the fastest (speed of light fast) and doesn’t suffer attenuation in a vacuum. A good example of this will be the energy emitted by the sun.
Thermal radiation is radiation emitted by a body’s temperature. Therefore, radiation is known to be a volumetric phenomenon because all solids, liquids, and gases absorb, transmit, and emit radiation. Even though radiation is mostly considered to be a surface phenomenon in solids.
Stefan-Boltzmann law:
Radiation emitted by real surfaces:
· Emissivity (
is a measurement representing how close a surface approximates a blackbody when 
of the surface. 0 ≤ 
≤ 1. A blackbody radiation is the maximum amount of radiation emitted from a surface at a specified temperature
Kirchhoff’s Law:
Here 
represents absorptivity, which is incidental radiation energy absorbed by a surface
Net radiation heat transfer is the difference between the rates of radiation emitted and that wish is absorbed by the surface. Determining this between two surfaces can be complex due to its dependence on diverse properties and conditions.
Use the following equation only when the following conditions apply:
· a surface is completely enclosed by a larger surface at a temperature 
,
· these surfaces are separated by a gas that doesn’t intervene with radiation
If radiation and convection are occurring concurrently between a surface and a gas:
Simultaneous modes of heat transfer
In opaque solids, heat transfer happens only by conduction. In semitransparent solids, it can be by conduction and radiation. When exposed to a fluid or other surface a solid may involve convection and/or radiation.
Heat transfer in a vacuum happens by radiation. In a still fluid heat transfer is by conduction and maybe radiation. However, in a flowing fluid by convection and radiation. If there is no radiation, then it can be by conduction or convection.
