Talk:Isentropic process

message: I would add to the article the following explanation to make everything more easy for understanding.
Entropy balance for closed system:
dS = dQ/T + dσ, 
where 
  dS is entropy change for the system, 
  dQ/T is entropy generation due to heat transfer to/from the system
  dσ is entropy generation due to internal irreversibilities in the system
=>
S2-S1 = Q/T + σ
By definition isentropic process is S2-S1 = 0. Therefore, the next statements easily follow:
1) Isentropic adiabatic process must be reversible:
   0 =     0     +  σ => σ=0 => reversible process
   (S1=S2) (Q=0)
2) Irreversible isentropic process must be NOT adiabatic:
   0       =  Q/T   +   σ      => σ = -Q/T 
   (S1=S2)    (!=0)     (!=0)     ( heat must be removed from the system, minus sign )
IMHO, it's always better to see formulas instead of verbal explanations..
-
agree IlyaV 18:42, 28 October 2007 (UTC)
thread_title:  Could be more understandable 

message: I removed this notion from the article because I didn't find that it was verifiable:
:for an ideal gas γ is a function of temperature
- Ac44ck (talk) 23:40, 7 November 2008 (UTC)
thread_title:  Removed statement about ideal gas 

message: Is this a mistake, or am I missing something?  If it's not a mistake maybe it could be explained with a note. — Preceding unsigned comment added by 190.21.107.42 (talk) 23:47, 13 August 2014‎
:Not a mistake.  This is because a compressor applies work to the flow to impart a desired pressure ratio (or mass flow or velocity rise).  Due to entropy, you need more work than you would need for an isentropic pressure rise.  This means the isentropic compressor work is less than the actual compressor work.  The isentropic efficiency of any machine that adds energy to the flow will be of the form (isentropic work)/(actual work).
:In the case of the turbine and nozzle, these devices are concerned with extracting energy from the fluid.  Due to entropy, you will extract less energy than you would from an isentropic process.  This is why the compressor seems upside-down to the turbine and nozzle; the compressor performs the opposite process.
:I don't whether or not this merits a note in the article, but a simple way to indicate the difference might be to group the equations as "Devices that impart energy to the flow" and "Devices that extract energy from the flow".  Hopefully someone can think of a less wordy way to say the same thing.  209.87.255.222 (talk) 16:21, 25 November 2016 (UTC)
thread_title:  Why Is Compressor Efficiency "Upside Down"? 

message: The previous lede sentence was heavily qualified and very long, warning the reader that "isentropic" (like most everything in engineering) is often only an approximation or an assumption. I figure people know this anyway, or can obtain this knowledge from another source. 178.38.79.250 (talk) 20:05, 19 January 2015 (UTC)
thread_title: Simplified the long sentence in the lede

message: Parts of the article seem to be lists of equations copied from a handbook without much explanation. And the equations have a lot of redundancy.
I would like to see more conceptual material. In particular, I would like to see concretely some example that is adiabatic but not isentropic (i.e., entropy is generated internally and dS > δQ/T =0). Friction is mentioned; perhaps also viscosity in a fluid? But I want to see how the model is set up, the concrete math of where and how the entropy gets generated, how it is accounted for. It shouldn't just remain a black box (at least not in every situation).
178.38.79.250 (talk) 20:51, 19 January 2015 (UTC)
replies: text: Another example of internal energy generation is sudden expansion of gas into a vacuum. If it takes place in an insulated container, it is adiabatic but not isentropic or reversible. 178.38.77.196 (talk) 20:12, 13 March 2015 (UTC)
thread_title:  This article needs work 

message: In linguistics the idea of minimal pairs exists, which can inter alia help to see a contrast between two letters/sounds without any other confusion.  For example, "tea" and "pea" could illustrate the distinction between "t" and "p" (better than contrasting "table" and "pomelo").  If the distinction being noted between "adiabatic", "reversible adiabatic" and "constant-entropy" processes is significant, then could some directly contrasting examples of such (perhaps with a graph of overload curves) please be presented to illustrate such differences clearly?  Otherwise the distinction may seem too obscure.  
—DIV (120.17.46.63 (talk) 01:48, 9 March 2018 (UTC)
thread_title:  Minimal pairs 

message: This edit by Editor Bubaff is covered by the following edit summary: "Conservative opinions based in a classical, limited, and conservative understanding of physical systems only garnish condescension. The notion of an ideal machine was not a theorist reaching for a phantom limb. To use words like fictial, or idyllic (or ...)".
The words used were "fictive idealized". According to the second law, reversible processes do not occur in nature. The term 'isentropic' is not widely used in good quality reliable sources, and when it is used in them, it is usually defined rather narrowly. It originated in the writing of Gibbs.Chjoaygame (talk) 13:44, 3 September 2015 (UTC)
replies: text: If any of the cited sources use the phrase "fictive idealized", it would be a good idea to make that clear in the Notes. --50.53.59.40 (talk) 21:15, 6 September 2015 (UTC)
thread_title:  edit with slighting summary 

message: In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible.[1][2][3][4][5][6] The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process is useful in engineering as a model of and basis of comparison for real processes.[7] This process is idealized because reversible processes do not occur in reality; thinking of a process as both adiabatic and reversible would show that the initial and final entropies are the same, thus, the reason it is called isentropic (entropy does not change). Thermodynamic processes are named based on the effect they would have on the system (ex. isovolumetric: constant volume, isenthalpic: constant enthalpy). Even though in reality it is not necessarily possible to carry out an isentropic process, some may be approximated as such.  The word "isentropic" can be interpreted in another way, since its meaning is deducible from its etymology. It means a process in which the entropy of the system remains unchanged; as mentioned, this could occur if the process is both adiabatic and reversible. However, this could also occur in a system where the work done on the system includes friction internal to the system, and heat is withdrawn from the system in just the right amount to compensate for the internal friction, so as to leave the entropy unchanged.[8] However, in relation to the universe, the entropy of the universe would increase as a result, in agreement with the Second Law of Thermodynamics.
Male voice 2400:1A00:B111:16A9:48CC:4BA8:9E6D:95B3 (talk) 14:52, 13 September 2022 (UTC)
thread_title:  Isentropic process 

message: "In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible."
This is only true in the converse: "In thermodynamics, an isentropic process is an idealized thermodynamic process where entropy remains constant. An adiabatic and reversible process is by definition isentropic (see Second Law of Thermodynamics)."
A square is a rectangle, but not all rectangles are squares. 208.91.55.70 (talk) 14:54, 5 October 2023 (UTC)
replies: text: Thanks for taking an interest in our article. The topic you have raised has been raised before, although I suspect it didn’t ever reach a conclusion that satisfied everyone. See above at #Definition seems convoluted. Dolphin (t) 00:18, 6 October 2023 (UTC)
replies: text: Thank you for your quick response. I strongly disagree with the conclusion from that discussion to keep this wording of the definition. As a thermodynamics student, speaking on behalf of others, Wikipedia returning this statement (which I will argue is false) is detrimental to our understanding of the material and our success in the class. This by itself is not grounds to change the wording, but please understand my background and context for this comment. Regardless of whether this is a fictive process, the point still stands that an isentropic process need not be adiabatic or reversible. For instance, logic says that if one has a Carnot engine, that is reversible, yet by no means adiabatic. It is reversible because it returns to its original state, but it is not adiabatic because we have two heat flows across the boundary - one from the hot source and one to the cold source. True, the individual processes may not be isentropic, but the overall behavior is such. Let us assume a different scenario where a temperature gradient is present across a gaseous medium between a hot source and a cold source. Temperature gradients beget entropy, yet within the boundaries of the gaseous medium at equilibrium, despite entropy being generated constantly, there is a constant entropy across the whole system (or, rather it can be modeled as such). In the case of an infinitesimally small distance, this is by all means true. The entropy being generated is transferred to the cold source alongside the entropy moved from the hot source. In this case, we have an isentropic process yet again, which is neither adiabatic nor reversible. Truthfully, neither process exists in the real world as conceptually phrased, but for the purposes of modeling it is important to approach scenarios with the correct assumptions and methods (all models are wrong, but some are useful). This is why I find issue with the current wording - it imposes a false assumption and a false model, which makes it useless as a modeling tool. I admit that I have no sources for this line of thought, and I don't care enough to collect some beyond seeing that all other top websites and the way that thermodynamics is taught at the university level disagree with Wikipedia. At some point, I think we have to get with the times and concede that the word may have evolved past its original usage and now unmistakably refers to something broader. 208.91.53.187 (talk) 03:14, 12 October 2023 (UTC)
thread_title:  Definition of isentropic fundamentally wrong