Transformation of energy

Adiabatic process: without heat exchange

Adiabatic process: without heat exchange

An adiabatic process is a process in which the system does not exchange heat with its surroundings. A reversible adiabatic process is called isentropic.

The adiabatic process provides a rigorous conceptual basis for the theory used to expose the first law of thermodynamics.

The term adiabatic refers to elements that prevent heat transfer with the environment. An isolated wall is quite close to an adiabatic limit. Hence the name adiabatic wall appears.

An adiabatic process does not involve the transfer of heat or matter inside or outside a system. When it happens, the prosses is adiabatically isolated. It is frequent to simplify a process assuming that it is adiabatic.

The adiabatic isolation assumption of a system is useful. It is often combined with others to make it possible to calculate the behavior of the system. Such beliefs are idealizations, they are approximate, but they are not real.

The behavior of real machines deviates from these idealizations. The assumption of such perfect behavior provides a useful first approximation of how the real world works.

Examples of adiabatic processes

To better understand this concept, let's look at some examples:

Is a refrigerator an adiabatic system?

A working refrigerator is not an adiabatic process. However, when the engine stops, it acts as an adiabatic system.

When the engine of the refrigerator is running, it transfers heat from the inside to the outside. But without the motor, the walls prevent heat transfer because it is thermally insulated.

Cycles of heat engines

Part of the thermal engine cycle is adiabatic processes. 

In this example, we assume that the piston-cylinder is our thermodynamic system.

One assumes that the compression of a gas inside a cylinder of a heat engine occurs so quickly. Because of that, it is considered as quasi-static. It means that only a small part of the system’s energy can be transferred as heat to the surroundings.

Although heat engine cylinders are not insulated and relatively conductive, that process is idealized to be adiabatic. During compression, the temperature increases.

The expansion process of such a system due to the work done by the gas happens the same thing during the expansion of an ideal gas, the temperature of the gas decrease. In this process, energy is released in the form of work.

This process is idealized to be adiabatic.

Adiabatic heating and cooling

Adiabatic compression of gas causes an increase in the gas temperature. Adiabatic expansion against pressure, or a spring, causes a drop in temperature. In contrast, free expansion is an isothermal process for an ideal gas.

Such temperature changes can be quantified using the ideal gas law or the hydrostatic equation for atmospheric processes.

According to the law of ideal gasses, when the heat is added at constant volume, the heat energy goes entirely into increasing the internal kinetic energy. If the heat is added at constant pressure, the gas can expand while the heat is added. The gas does work on expanding the system.

In practice, no process is truly an adiabatic process. Many processes depend on a massive difference in time scales of the process of interest. They also rely on the rate of heat energy dissipation across a system boundary.

Therefore, they are approximated by using an adiabatic assumption. There is always some heat loss, as there are no perfect insulators.

Adiabatic heating

Heating in a called adiabatic condition occurs when the gas’s pressure increases due to work added. An example of adiabatic heating is a heat engine piston that compresses a gas contained within a cylinder. Gas compression leads to a temperature rise. In many practical situations, the conduction of heat is assumed to be null.

This feature has practical use in diesel heat engines.

Adiabatic cooling

Adiabatic cooling occurs when the pressure on an isolated system decreases, allowing it to expand. This expansion makes it work in your environment. When the pressure applied to an air pack is reduced, the pack’s air is allowed to grow. As the volume increases, the temperature decreases as its internal energy decreases.

Adiabatic cooling does not have to involve a fluid. One technique used to reach shallow temperatures is through adiabatic demagnetization. In adiabatic demagnetization, the change in the magnetic field’s magnetic field is used to provide adiabatic cooling. Furthermore, the content of an expanding universe can be described (first-order) as an adiabatic cooling fluid.

Publication Date: March 19, 2018
Last Revision: June 28, 2020