Adiabatic vs Isentropic

The term adiabatic is of Greek origin whereby if translated would generally mean impassable. Thus, it is one of the most important processes in thermodynamics that’s characterized by the absence of heat transfer either from or to the fluid being worked on.

Also known as isocaloric process, the adiabatic process is also considered as something that falls directly under the isocaloric process itself. The term adiabatic can also describe a boundary wherein if the boundary is insulated then the boundary will become impassable to the transfer of heat. Another description can also be for flame temperatures wherein the flame temperature is said to be adiabatic if there’s no significant heat loss throughout the environment.

Adiabatic can also describe a kind of heating or cooling. Changing the overall pressure of the gas attributes to adiabatic temperature changes, that is, granting that there’s no addition or subtraction of any form of heat whatsoever.

In adiabatic heating, this phenomenon occurs during an increase in gas pressure because of some variables happening within its immediate surroundings as in the case of a piston. In diesel engines, the principle of adiabatic heating is used so as to ignite the gasoline. Conversely, there’s also such a thing as adiabatic cooling wherein the substance’s pressure is reduced. Nevertheless, this process does not require fluids.

Adiabatic processes can either be reversible and irreversible. Given the variables δQ as the energy that the system will gain through heating, T is for the system temperature and dS stands for entropy; if it is the latter (irreversible adiabatic process) then δQ is said to be equal to 0 but is not equal to TdS. In writing the expression, it will appear as δQ=0≠TdS because TdS is greater than 0 (TdS>0). An example of which is when gas is allowed to expand inside a vacuum. Contrarily for the reversible process, all variables are equal. Because of such the variables are expressed as δQ=0=TdS. For this instance, this process will also be known as an isentropic one (sometimes called isoentropic process).

In summary:

1. Adiabatic process is the process wherein there’s absolutely no heat loss and gain in the fluid being worked on whereas isentropic process is still an adiabatic process (there’s no heat energy transfer) and is the reversible type (no entropy change).

2. Adiabatic process can either be reversible and irreversible but isentropic process is specifically a reversible adiabatic process itself.

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