Difference Between Adiabatic and Isentropic (With Table)

The basic principles of thermodynamics encapsulate the mode of energy transfer between two entities. There are a number of processes through which the said energy transfer takes place, and these various processes are called thermodynamic processes. They are often represented as functions of pressure and volume or temperature and entropy. Adiabatic and Isentropic are two of such processes.

Adiabatic vs Isentropic

The difference between the terms adiabatic and isentropic lies in the energy transfer mechanism involved and the kind of systems they are consequently. The two terms have varied meanings, however with respect to the field of thermodynamics, they are representative of the external conditions imposed on a particular energy system.

The term adiabatic means that there is no heat transfer i.e., heat is neither lost nor gained in the transfer of energy. 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 only variable influencing change in the internal energy of the system is the work done

Isentropic signifies an idealized adiabatic process – one which 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 and n in the aforementioned two processes is 𝛾 and C is a constant. Adiabatic processes occur in a strictly thermally isolated system whereas isentropic processes may not.

Comparison Table Between Adiabatic and Isentropic

Parameters of ComparisonAdiabaticIsentropic
Essential Conditions– Perfectly insulated system
– Swift process to facilitate heat transfer
– Entropy must remain a constant
– Reversible
Ideal Gas relationshipReversible: PV𝛾 = Constant
Irreversible: dU = -P(ext)dV (Function of change in internal energy, pressure and volume)
PV𝛾 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 is equal to the summation of the external heat applied and the work done
Entropy Change (ΔS)Reversible – No change in entropy
Irreversible – Change in entropy represented as a function of net heat transfer and temperature of the system.
Entropy remains unchanged 
Possible Use CasesMeteorological phenomenon of heat burstTurbines

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 does the work 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 are employed in diesel engines

What is Isentropic?

An isentropic process, as the term suggests, is one where there is no net heat exchange and more importantly, entropy of the system 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 the change is entropy does not take place.

The system may be subject to positive entropy and equal and opposite negative entropy. In such a case, the net change in entropy still 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 is what enables the process to be reversible and an idealized adiabatic process.

Main Differences Between Adiabatic and Isentropic

  1. An adiabatic process always occurs in a thermally isolated system whereas an isentropic may not.
  2. 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.
  3. If an adiabatic process is reversible, it is isentropic. However, an isentropic is not always an adiabatic process that is reversible. A process that adheres to the essential conditions of net entropy may also be isentropic.
  4. For an adiabatic process, equilibrium need not be a constant while for an isentropic process, equilibrium is always a constant.
  5. In an adiabatic process, the net internal energy is equal to the work done however this need not necessarily be the case in a isentropic process.
  6. Only if the process is reversible and adiabatic we can deem it isentropic. There exist real life scenarios like in the case of a real compressor where it can be assumed adiabatic but suffers losses due to change in system conditions. Due to these losses the compression becomes irreversible. Thus the compression is not isentropic.

Conclusion

There are a myriad of paths that a thermodynamic process can take. On the basis of the output that the system is required to give, variables such as pressure, work done can be tinkered with. As a result, unique combinations of outcomes emerge. Adiabatic processes and isentropic processes both occur as outcomes of distinct thermodynamic systems where the prerequisites pertain to heat energy and entropy respectively. Although they vary in their systemic conditions, they are not mutually exclusive systems. Both adiabatic processes and isentropic processes have significant use cases in real life.

References

1. https://sci-hub.se/https://aapt.scitation.org/doi/abs/10.1119/1.197364
2. http://www.asimow.com/reprints/PhilTrans_355_255.pdf
3. http://www.mhtl.uwaterloo.ca/courses/me354/lectures/pdffiles/ch2.pdf

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