The basic principles of thermodynamics encapsulate the mode of energy transfer between two entities. There are several processes through which the said energy transfer takes place, and these various processes are called thermodynamic processes.
They are represented as functions of pressure and volume or temperature and entropy. Adiabatic and Isentropic are two such processes.
Key Takeaways
- The adiabatic process refers to a thermodynamic process where no heat enters or leaves the system, while the Isentropic process refers to a thermodynamic process with no entropy change.
- The adiabatic process can be either reversible or irreversible, while the Isentropic process is always reversible.
- In the Adiabatic process, the temperature can change while the internal energy remains constant, while in the Isentropic process, both temperature and internal energy remain constant.
Adiabatic vs Isentropic
Adiabatic processes refer to changes in temperature and pressure that occur without exchanging heat or matter. Isentropic processes refer to changes in temperature and pressure that occur with no change in entropy. Adiabatic processes can be isentropic, but not all adiabatic processes are isentropic.
Adiabatic means no heat transfer, i.e., heat is neither lost nor gained in energy transfer. Therefore, it constitutes a thermally insulated system. It represents an ideal energy transfer process.
It may be reversible (where the total internal energy remains unchanged) or irreversible (the total internal energy is altered). In an adiabatic process, the total heat exchanged between the system and its surrounding is zero.
As a result, the work done is the only variable influencing change in the system’s internal energy.
Isentropic signifies an idealized adiabatic process that is reversible and suffers no change in entropy. Both isentropic processes and adiabatic reversible processes are types of polytropic processes.
Polytropic processes are those which obey the PVn = C.
In this case, P represents pressure, V represents volume, n is the polytropic index, and C is a constant. Adiabatic processes occur in a strictly thermally isolated system, whereas isentropic processes may not.
Comparison Table
| Parameters of Comparison | Adiabatic | Isentropic |
|---|---|---|
| Essential Conditions | – Perfectly insulated system – Swift process to facilitate heat transfer | – Entropy must remain a constant – Reversible |
| Ideal Gas relationship | Reversible: PVn = Constant Irreversible: dU = -P(ext)dV (Function of change in internal energy, pressure, and volume) | PVn is always a constant |
| Total Internal Energy (U = Q + W) | Internal energy is equal to the work done since the system is thermally isolated (Q = 0) | Internal energy equals the summation of the external heat applied and the work done. |
| Entropy Change (ΔS) | Reversible – No change in entropy Irreversible – Change in entropy is represented as a function of the system’s net heat transfer and temperature. | Entropy remains unchanged. |
| Possible Use Cases | The meteorological phenomenon of heat burst. | Turbines |
What is Adiabatic?
Adiabatic processes can be of two types – adiabatic expansion and adiabatic compression. In the adiabatic expansion of an ideal gas, the ideal gas within the system works, and therefore the system’s temperature drops.
Owing to the drop in the temperature, this constitutes adiabatic cooling. On the contrary, in the adiabatic compression of an ideal gas, work is done on the system comprising the gas in a thermally isolated environment.
As a result, the temperature of the gas rises. This gives rise to what is called adiabatic heating.
Consequently, these properties are used in specific real-life applications. For instance, expansion properties are employed in cooling towers and compression properties in diesel engines.
What is Isentropic?
As the term suggests, an isentropic process is one where there is no net heat exchange, and more importantly, the system’s entropy is a constant. In reversible adiabatic processes, the entropy change is zero.
Therefore, all reversible adiabatic processes also constitute isentropic processes. However, the vice versa isn’t always implied in this case.
There exist isentropic processes that are not adiabatic. The pivotal point to note in the case of isentropic processes is that the change in entropy does not occur.
The system may be subject to positive entropy and equal and opposite negative entropy. In such a case, the net change in entropy remains zero since the two entropy values balance each other out.
Such a system is not adiabatic (since it is not a thermally isolated system) but is isentropic. Most isentropic systems are also majorly characterized by the lack of friction.
This lack of friction makes the process reversible and an idealized adiabatic process.
Main Differences Between Adiabatic and Isentropic
- An adiabatic process always occurs in a thermally isolated system, whereas an isentropic may not.
- The net change in entropy may be encountered in an adiabatic process wherein it would be irreversible. An isentropic process cannot accommodate a change in entropy.
- If an adiabatic process is reversible, it is isentropic. However, an isentropic is not always a reversible adiabatic process. A process that adheres to the essential conditions of net entropy may also be isentropic.
- For an adiabatic process, equilibrium need not be a constant, while equilibrium is always a constant for an isentropic process.
- In an adiabatic process, the net internal energy equals the work done. However, this need not necessarily be the case in an isentropic process.
- Only if the process is reversible and adiabatic can we deem it isentropic. There exist real-life scenarios, like in the case of an actual compressor, where it can be assumed adiabatic but suffers losses due to changes in system conditions. Due to these losses, the compression becomes irreversible. Thus the compression is not isentropic.
- https://sci-hub.se/https://aapt.scitation.org/doi/abs/10.1119/1.197364
- http://www.asimow.com/reprints/PhilTrans_355_255.pdf
- http://www.mhtl.uwaterloo.ca/courses/me354/lectures/pdffiles/ch2.pdf
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