Heat transfer or heat flow is the energy flow in the form of heat from locations with a higher temperature to locations with a lower temperature. According to the second law of thermodynamics, the opposite is not possible (heat flow from low to high temperature). Therefore, heat will always be distributed as evenly as possible throughout the room. In other words, a thermodynamic system strives to achieve the greatest possible entropy.
Heat transfer plays an important role in many industrial processes and in nuclear energy in particular. The transfer of heat energy is essential to be able to transfer the heat generated in the nuclear reactor to be able to generate steam and drive the steam turbines.
There are three ways to transfer heat energy:
- Heat transfer by conduction.
- Transfer of thermal energy by radiation.
- Heat transfer by convection.
Heat transfer by conduction
Heat transfer by conduction is the transfer of heat within the substance in question, whereby heat flows from particles with higher kinetic energy (temperature) to particles less rich in energy (cooler). The heat flux depends on the temperature difference over the distance (the temperature gradient) and the internal resistance to heat flux of the relevant material, which is called thermal conductivity or thermal conductivity coefficient. This principle is expressed in the Fourier law.
An example of heat transfer by conduction is when we grab a piece of ice by hand. We immediately notice the cold. What is really happening is that we are transferring heat from our body to ice. When performing this thermodynamic transfer the skin lowers the temperature and our nervous system warns us; That's why we noticed the cold.
In any case, heat is always transferred from the body with the highest temperature to the body with the lowest temperature. Otherwise the second principle of the laws of thermodynamics would not be fulfilled.
Transfer of thermal energy by radiation
Transfer of thermal energy by radiation. This is the transfer of heat between two bodies that are not in contact with each other without using an intermediate. One body is hot and, therefore, emits a lot of electromagnetic radiation and, therefore, loses heat, and the other body absorbs part of the incoming radiation and converts it into heat.
The clearest example, in this case, is solar energy. The sun generates thermal energy that is transmitted to the earth by electromagnetic radiation. That is what we call solar radiation. In this case it is obvious that the Earth and the Sun are not in contact nor is there an intermediate: the space is empty.
Convection heat transfer
Convection heat transfer is the transfer of heat through the displacement of a warm liquid or a hot gas, or a cold liquid or a cold gas. When heat drags the flow, its extension can be expressed with the heat transfer coefficient.
Explained with a simple example. If we heat a glass of water in the microwave and then take the glass to the table, we are carrying out a heat transfer. The same goes for wind and hot air currents.
Last review: January 31, 2020