# What is Isentropic: Definition and 57 Discussions

In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. 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. This is idealized as 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 requires the process to be 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could also occur in a system where the work done on the system includes friction internal to the system, and heat is withdrawn from the system in just the right amount to compensate for the internal friction, so as to leave the entropy unchanged. However, in relation to the universe, the entropy of the universe would increase as a result.

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1. ### Enthelpy and Isentropic compression/expansion

In a certain thermodynamics textbook, specific work done by an isentropic compressor/pump in an ideal rankine cycles, is given by the following; Wpump = h2 - h1 Wpump = v(P2 - P1), where v = v1 When I carry out these two calculations between any two states, I get vastly different answers...
2. ### I How can a process be isentropic but not reversible or adiobatic?

In the book for our thermodynamics, it states that a process that is internally reversible and adiabatic, has to be isentropic, but an isentropic process doesn't have to be reversible and adiabatic. I don't really understand this. I always thought isentropic and reversible mean the same thing...
3. ### Engineering Isentropic Efficiency of a Compressor

We know that if potential and kinetic energy our negligible, the isentropic efficiency of a compressor is isentropic enthalpy change divided by actual enthalpy change: n=(h2s-h1)/(h2a-h1) So I already answered this question and here is the answer: n1 is the isentropic efficiency of the...
4. ### Enthelpy change in isentropic condition (air-con)

h and s can be obtained from "Saturated refrigerant-134a—Pressure table" however, how to get h2? it is not on the curve, and neither p or dV is given in the question. Thank you
5. ### Finding Isentropic Enthelpy, knowing Isentropic Entropy

A short background: My question focuses solely on the part of the refrigeration cycle to do with the compressor, where the cycle begins. The first state is before the refrigerant enters the compressor, and the second state is after the refrigerant leaves the compressor. My goal is to obtain...
6. ### I Isentropic Process, General Results for dU and dH

Hello forumites, I've been working with the following expression for the change in internal energy in an isentropic scenario. $$dU = n*c_v *dT = -pdV$$ However, I'm a bit stumped here, the left hand side of the expression (or middle rather), states the change in internal energy is the...
7. ### Relation Between Ma and Re for Isentropic Expansion

Homework Statement Consider the isentropic expansion of air from a fixed given reservoir (i.e. total pressure and temperature). Investigate the behaviour of the value of the Reynolds number of the flow, as a function of the Mach Number M of the expanded flow. For small values of M, the...
8. ### Isentropic Expansion Within a Steam Turbine System

Homework Statement Steam is expanded from 90 bar, 412°C to 4.0 bar saturation in a high pressure turbine, after-which a certain percentage of the steam is bled off to a feed heater. The remaining steam is then expanded through a low pressure turbine to 1.0 bar and dryness fraction of 0.93. All...
9. ### Isentropic process (temperature and pressure relationship)

Homework Statement for isentropic process, which of the following is true about the relationship of temperature and pressure? (1): P2/P1=(T2/T1)^[γ/(γ-1)] (2): P2/P1=(T2/T1)^[(γ-1)/γ] where γ = 1.4 for air Homework Equations stated above The Attempt at a Solution I have found equation (1)...
10. ### Isentropic Efficiency and Entropy Production Rate of Turbine

Homework Statement Water vapor at 6 MPa, 600C enters a turbine operating at steady state and expands to 10 kPa. The mass flow rate is 2 kg/s, and the power developed is 2626 kW. Stray heat transfer and kinetic and potential energy effects are negligible. Determine (a) the isentropic turbine...
11. ### Thermodynamics - isentropic, polytropic and compressibility

Several questions: What does compression actually means? In the case of isentropic calculations a changed in density (or specific volume) is included in the calculations (isentropic.jpg) so isentropic means that compressibility is included in those calculations or what?In a previous thread, a...
12. ### Conditions for optimum isentropic efficiency

Homework Statement Ive looked through several published journals online trying to find an experiment that I can use as background research for my report, something I can compare my findings to but I am just wasting lots of time that I could be doing the rest of my report so what I am trying to...
13. ### Calculating isentropic efficiency of a compressor

Homework Statement We did an experiment which included having an electric motor provide power to drive the compressor and we had instrumentation available to measure the compressor shaft torque and shaft rotational speed and 6 water filled manometer tubes were used as measurements of the...
14. ### Interpreting graphs from isentropic efficiency of a compressor lab

Homework Statement We did a lab experiment for isentropic efficiency of a compressor and was asked to plot the following graphs, change in stagnation pressure across the compressor (∆p0) vs air mass flow rate, and isentropic efficiency vs air mass flow rate. I have plotted the graphs but not...
15. ### Why is Bernoulli's Equation Isentropic

I have trouble understanding why we classify an inviscid adiabatic incompressible flow along a streamline as isentropic I understand this from a Thermodynamic definition/explanation $$dS = dQ/T$$ Adiabatic Invsicid $$dQ =0= dS$$ So no heat added or lost no change in entropy I'm fine with that...
16. ### Low Air Compressor CFM Still Works?

Halfway through my thesis on a mach 2 ramjet, and I unfortunately realize that I need 50-72 CFM to test it. The only ACs I have are 2 CFM portable pancakes. Is it possible to run say a 25 CFM AC at 50 CFM for a short period of 10-60 seconds? Is there a quantitative way to find how long the AC...
17. ### Isentropic efficiency of a pump

Homework Statement Find the isentropic efficiency of the pump: effpump=(h4s-h3)/(h4-h3) state 3: p3=1.5 bar, h3=467.11, x3=0, v3=1.0528/1000 m3/kg ,s3=1.4336 kJ/kg*K, T3=111.4 deg C state 4: p4=60 bar, h4=474.14, compressed liquid state 4s: p4=60 bar, s3=s4s The values in red are the ones I...
18. ### Calculate Isentropic Efficiency: Tips & Tricks

Hi can anyone help me about the following ? if I have the experimental data of inlet and outlet of expander (temperature and pressure) working in ORC. Now if I want to find the Isentropic efficiencie : the Isentropic expansion can be found by (entropy in = entropy out) but how to find the...
19. ### Adiabatic or Isothermal? Compressed air energy storage

Hi, I'm doing a high school physics project and am trying to figure out if a certain setup that I'm using is adiabatic or isothermal, in order to determine what equation I can use to calculate the work that my setup does-- the threads I've come across so far only explain the difference, but not...
20. ### I Why do we see the claim that an isentropic process is adiabatic and reversible?

Why do we always see the claim that an isentropic process for a system is adiabatic and reversible? The change in entropy for a process is the sum of the entropy transfer accompanying heat and the entropy production. The entropy production term is always at least zero, and the transfer term...
21. ### PV^lamba=constant in an isentropic Flow

if a gas expands isentropically (consider the process reversible adiabatic), from point 1 in a nozzle to point 2 downstream of the nozzle. If you know the pressure and temperature at point 1, can you use the relationship pv=1/M RT to work out the specific volume at point 1? And similarly if you...
22. ### Isentropic Compressor Efficiency

Homework Statement Nitrogen enters an insulated compressor operating at steady state at 1 bar, 310 K with a mass flow rate of 1000 kg/h and exits at 10 bar. Change in KE and PE is zero. Heat transfer is zero. Assume ideal gas with k=1.391. Find minimum theoretical power input required and the...
23. ### Speed of sound in an isentropic ideal gas

Homework Statement \rho_0, c_0 is the mean density, the mean speed of sound in the ideal gas. Is the following correct? c(\rho)=c_0\left(\frac{\rho}{\rho_0}\right)^{\frac{\kappa-1}{2}} Homework Equations p = const * \rho^\kappa, c=\sqrt{\frac{\partial p}{\partial \rho}} The Attempt at...
24. ### Are "isentropic" and "adiabatic" same?

Isentropic means a process where entropy remains constant. Now formula for entropy is ΔS = ΔQ/T now in an isentropic process, ΔS=0...so that means ΔQ = 0 ...right? but if ΔQ = 0, that is an adiabatic process. so are isentropic and adiabatic processes are...
25. ### Spherical bubble rises to surface, Ideal Gas, Thermal Energy

Homework Statement A spherical air bubble in a lake expands as it rises slowly to the surface. At the point it starts to rise, the pressure is 2.00 atm, the temperature of the water is 10.0 ∘C, and the radius of the bubble is 5.00 × 10^−3 m. At the surface, the pressure is 1.00 atm and the...
26. ### Isentropic Compression of Saturated Water: Workdone Calculation

Workdone= integral Pdv a kg of saturated water was compressed isentropically from 1 bar to 10 bar. I solved it in the following logic: Since water is incompressible dv=0 , work =0 But my answers was wrong. The solution was integral Vdp where V is Vf at 1 bar My doubt is , is workdone...
27. ### Supercharger - Isentropic charts - Pressure Ratio and temperature

I am trying to understand my superchargers isentropic chart. For an example case where a supercharger outputs 10m3/hr but the engine only takes 6.57m3/hr in capacity, I get a volume ratio (VR) of 1.52 - easy... I can then work out a pressure ratio by VR^k = 2.0^1.4 = 1.8 The theoretical...
28. ### Isentropic efficiency vs polytropic efficiency of a gas turbine

My classmates and I are working on a senior design project where we optimize a baseline turbofan for supersonic cruise. We are a little confused about when to use polytropic vs isentropic efficiencies. I have read to begin with isentropic, however is there anyone who could explain a little more...
29. ### Determination of the equation of isentropic processes

I have been given a fundamental equation of a system as u = \frac{s^4}{v^2} After writing down the 3 equations of state, namely: T = 4\frac{S^3}{VN} P = \frac{1}{2}\frac{S^4}{V^{3}N} \mu = -\frac{S^4}{VN^{2}} I need to determine the equation of isentropic (dS = 0) processes on the P-V...
30. ### Thermodynamics - Isentropic Compression

Hello all, Its review time again for another Thermodynamics midterm. As such, I have a practice exam to try for optional extra review work. I've come across a problem that I'm somewhat stumped on. I've tried the problem, but I feel as though I've made too many assumptions in trying to solve...
31. ### Isentropic Nozzle Behavior Explanation

Hello. I was working on a sample problem on my thermodynamics book about isentropic nozzles. Pressures, velocities, and temperatures of steam entering to a non isentropic nozzle and exiting it are known. The question asks me to find the velocity of the steam exiting, if the nozzle were...
32. ### Thermodynamics ( steam tables ) isentropic

Homework Statement The power output of an adiabatic steam turbine is 5 MW. If the device can be assumed to operate as a steady flow device with isentropic expansion, determine the following: i) The dryness fraction, x, at the exit from the turbine; ii) The work output per unit mass of steam...
33. ### Isentropic Relations for Real Gas in Converging-Diverging nozzle

Hello, I am looking at a problem concerning flow through a converging-diverging nozzle. The governing equations are relatively straight-forward for gasses that closely follow the ideal gas law. However I am looking at an unusual gas which is certainly not represented by the ideal gas...
34. ### Thermodynamic Properties; Isentropic Expansion

Homework Statement An amount of saturated steam at a pressure of 10 Bar, undergoes an isentropic (constant entropy) expansion. The final temperature is 30C. Find the final specific enthalpy and specific volume. Homework Equations Saturated Steam tables, Linear Interpolation methods? No...
35. ### Why isentropic mass is taken for calculation of area of nozzels

i was reading an example of area calculation of a nozzle,in which mass flow rate was and coeff. of discharge (ratio of actual mass flow rate to isentropic mass flowrate) was given. i though the mass flow rate given in example is isentropic and we will first calculate actual mass flowrate from...
36. ### Calculating quality of steam after isentropic expansion

Homework Statement Saturated steam with a pressure of 30 bars is isentropically expanded to a pressure of 1 bar. What is the quality (x) of the steam after expansion? Homework Equations No clue. I do have a table containing all necessary data such as temperature, specific volume...
37. ### Need help with a tricky thermodynamics question involving isentropic efficiency

Homework Statement An engine operates on an air cycle consisting of the following non-flow processes in succession: 1-2 isentropic compression from an initial state of 80 kPa, 27°C and the volume of 0.071 m 3 through the volume compression ratio of 17.75; 2-3 reversible heat...
38. ### Temperature change in an isentropic flow of an ideal gas

I am a bit confused by the definition of an isentropic process in the flow of an ideal gas. isentropic implies reversible & adiabatic. for a process to be reversible, there are no losses to friction (viscosity in this case), for a process to be adiabatic, there is no heat transfer with...
39. ### Isentropic Effeciencies of steam turbines

Hi, I'm have big problems calculating the Isentropic Effeciencies of steam turbines. There is a HP, IP and LP turbine in the system. I know the calculation is (h1 - h2)/(h1 - h2'), i have the temperatures and pressures in and out for the first two turbines so i can easily get h1 and h2 from the...
40. ### Thermodynamics: isentropic efficiency and ideal gas model

Air at 1175 K, 8 bar enters a turbine operating at steady state and expands adiabatically to 1 bar. The isentropic turbine efficiency is 92%. Employing the ideal gas model with k=1.4 for the air, determine (a) the work developed by the turbine, in kJ per kg of air flowing, and (b) the...
41. ### Isentropic Specific Enthelpy HELP

Hi guys, I'm struggling with some equations regarding the enthalpy change over a compressor in a vcc system. I'm currently working on a model for the whole system and I think I've got all equations except the one describing the enthalpy difference betweeen the inlet and outlet of the...
42. ### Can Realistic Assumptions Help Design an Efficient Air Acceleration System?

I came up with my own design problem, after completing a refrigeration design problem on the completion of my thermo I class. My design problem is this: I want to accelerate air to 56m/s via a compressor and then a nozzle. I'm trying to do a cost analysis of how much power I would save if the...
43. ### Calculating Power Output of Isentropic Steam Turbine | Helpful Guide

I know the answer is 6328kW, I just not sure on how the book came up with that answer. Can anyone help? An isentropic steam turbine processes 5 kg/s of steam at 4MPa,which is exhausted at 50 kPa and 100oC. 5% of this flow is diverted for feed water heating at 700kPa. Determine the power...
44. ### Calculate Work, Isentropic Head & Efficiency of a Centrifugal Gas Compressor

Hi everyone! I have a question and I would be so thankful if you could help me! For a Centrifugal Gas Compressor, if I have the flow rate, the input and output temperatures and pressures in specified rpm, then could I calculate its needed work, isentropic head and efficiency? how? Thanks! Sara
45. ### Paradox in isentropic expansion? (statistical mechanics)

If I have a container containing a liquid mixed with some other substance that has much a higher boiling point (i.e. water and salt). This liquid will be in equilibrium with its vapor (the salt vapor pressure is negligible). Now I quasi-statically adiabatically expand this vapor. Isentropic...
46. ### Real gas tables: isentropic work

Hello. I am writing a Matlab program for calculating properties and processes of dry air using real gas tables. The only tables I could find were from Perry's Chemical Engineer's Handbook (7th edition, 1999). Mostly I make use of cubic interpolation and in some critical cases numerical...
47. ### Isentropic Process and Reversible Process

Hi all, Will like to check if my below thoughts are right: 1) For a totally reversible process, it has to be both internally and externally reversible. And the definition of externally reversible means heat transfer between a system and reservoir where the temperature are the same. 2)...
48. ### Isentropic efficiency vs polytropic efficiency of a gas turbine

definition wise i got the difference between these two efficiencies. but in practice how does these two differ? i was referring to this book: Jet propulsion: a simple guide to the aerodynamics and thermodynamic design ... By N. A. Cumpsty he has mentioned this: "using polytropic...
49. ### What is the final temperature of an isentropic expansion with a given function?

Homework Statement I have a system u =Av-2exp(s/R) N moles of this substance initially at T0 and P0 are expanded isentropically until the pressure is halved. What is the final temperature? Homework Equations du = \partialu\partials ds + \partialu\partialv dv du = Tds - Pdv...
50. ### Determine the isentropic efficiency of a turbine

Homework Statement Neon gas at the rate of 100kg/min enters an adiabatic turbine at a temperature and pressure of 1073K and 1.5 MPa. The gas leaves the turbine at a pressure of 200kPa. If the real(actual) power output of the turbine is 370 kW, determine the isentropic efficiency of the...