Heat transfer occurs when thermal energy moves from one place to another. Atoms and molecules inherently have kinetic and thermal energy, so all matter participates in heat transfer. There are three main types of heat transfer, plus other processes that move energy from high temperature to low temperature.
What Is Heat Transfer?
Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics. The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal). The unit for the rate of heat transfer is the kilowatt (KW).
The Three Types of Heat Transfer With Examples
The three types of heat transfer differ according to the nature of the medium that transmits heat:
- Conduction requires contact.
- Convection requires fluid flow.
- Radiation does not require any medium.
- Conduction is heat transfer directly between neighboring atoms or molecules. Usually, it is heat transfer through a solid. For example, the metal handle of a pan on a stove becomes hot due to convection. Touching the hot pan conducts heat to your hand.
- Convection is heat transfer via the movement of a fluid, such as air or water. Heating water on a stove is a good example. The water at the top of the pot becomes hot because water near the heat source rises. Another example is the movement of air around a campfire. Hot air rises, transferring heat upward. Meanwhile, the partial vacuum left by this movement draws in cool outside air that feeds the fire with fresh oxygen.
- Radiation is the emission of electromagnetic radiation. While it occurs through a medium, it does not require one. For example, it’s warm outside on a sunny day because solar radiation crosses space and heats the atmosphere. The burner element of a stove also emits radiation. However, some heat from a burner comes from conduction between the hot element and a metal pan. Most real-life processes involve multiple forms of heat transfer.
Conduction
Conduction requires that molecules touch each other, making it a slower process than convection or radiation. Atoms and molecules with a lot of energy have more kinetic energy and engage in more collisions with other matter. They are “hot.” When hot matter interacts with cold matter, some energy gets transferred during the collision. This drives conduction. Forms of matter that readily conduct heat are called thermal conductors.
Examples of Conduction
Conduction is a common process in everyday life. For example:
- Holding an ice cube immediately makes your hands feel cold. Meanwhile, the heat transferred from your skin to the ice melts it into liquid water.
- Walking barefoot on a hot road or sunny beach burns your feet because the solid material transmits heat into your foot.
- Iron clothes transfers heat from the iron to the fabric.
- The handle of a coffee cup filled with hot coffee becomes warm or even hot via conduction through the mug material.
Conduction Equation
One equation for conduction calculates heat transfer per unit of time from thermal conductivity, area, thickness of the material, and the temperature difference between two regions:
Q = [K ∙ A ∙ (Thot – Tcold)] / d
- Q is heat transfer per unit time
- K is the coefficient of thermal conductivity of the substance
- A is the area of heat transfer
- Thot is the temperature of the hot region
- Tcold is the temperature of the cold region
- d is the thickness of the body
Convection
Convection is the movement of fluid molecules from higher temperature to lower temperature regions. Changing the temperature of a fluid affects its density, producing convection currents. If the volume of a fluid increases, than its density decreases and it becomes buoyant.
Examples of Convection
Convection is a familiar process on Earth, primarily involving air or water. However, it applies to other fluids, such as refrigeration gases and magma. Examples of convection include:
- Boiling water undergoes convection as less dense hot molecules rise through higher density cooler molecules.
- Hot air rises and cooler air sinks and replaces it.
- Convection drives global circulation in the oceans between the equators and poles.
- A convection oven circulates hot air and cooks more evenly than one that only uses heating elements or a gas flame.
Convection Equation
The equation for the rate of convection relates area and the difference between the fluid temperature and surface temperature:
Q = hc ∙ A ∙ (Ts – Tf)
- Q is the heat transfer per unit time
- hc is the coefficient of convective heat transfer
- A is the area of heat transfer
- Ts is the surface temperature
- Tf is the fluid temperature
Radiation
Radiation is the release of electromagnetic energy. Another name for thermal radiation is radiant heat. Unlike conduction or convection, radiation requires no medium for heat transfer. So, radiation occurs both within a medium (solid, liquid, gas) or through a vacuum.
Examples of Radiation
There are many examples of radiation:
- A microwave oven emits microwave radiation, which increases the thermal energy in food
- The Sun emits light (including ultraviolet radiation) and heat
- Uranium-238 emits alpha radiation as it decays into thorium-234
Radiation Equation
The Stephan-Boltzmann law describes relationship between the power and temperature of thermal radiation:
P = e ∙ σ ∙ A· (Tr – Tc)4
- P is the net power of radiation
- A is the area of radiation
- Tr is the radiator temperature
- Tc is the surrounding temperature
- e is emissivity
- σ is Stefan’s constant (σ = 5.67 × 10-8Wm-2K-4)
More Heat Transfer – Chemical Bonds and Phase Transitions
While conduction, convection, and radiation are the three modes of heat transfer, other processes absorb and release heat. For example, atoms release energy when chemical bonds break and absorb energy in order to form bonds. Releasing energy is an exergonic process, while absorbing energy is an endergonic process. Sometimes the energy is light or sound, but most of the time it’s heat, making these processes exothermic and endothermic.
Phase transitions between the states of matter also involve the absorption or release of energy. A great example of this is evaporative cooling, where the phase transition from a liquid into a vapor absorbs thermal energy from the environment.
References
- Faghri, Amir; Zhang, Yuwen; Howell, John (2010). Advanced Heat and Mass Transfer. Columbia, MO: Global Digital Press. ISBN 978-0-9842760-0-4.
- Geankoplis, Christie John (2003). Transport Processes and Separation Principles (4th ed.). Prentice Hall. ISBN 0-13-101367-X.
- Peng, Z.; Doroodchi, E.; Moghtaderi, B. (2020). “Heat transfer modelling in Discrete Element Method (DEM)-based simulations of thermal processes: Theory and model development”. Progress in Energy and Combustion Science. 79: 100847. doi:10.1016/j.pecs.2020.100847
- Welty, James R.; Wicks, Charles E.; Wilson, Robert Elliott (1976). Fundamentals of Momentum, Heat, and Mass Transfer (2nd ed.). New York: Wiley. ISBN 978-0-471-93354-0.