Isentropic process

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In thermodynamics, an isentropic process or isoentropic process (ισον = "equal" (Greek); εντροπία entropy = "disorder"(Greek)) is one in which, for purposes of simplified engineering analysis and calculations, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the system remains constant throughout.[1][2] It can be proven that any reversible adiabatic process is an isentropic process. A simple more common definition of isentropic would be one that produces "No change in entropy".

Background[edit]

The second law of thermodynamics states that,

TdS \ge \delta Q

where \delta Q is the amount of energy the system gains by heating, T is the temperature of the system, and dS is the change in entropy. The equal sign will hold for a reversible process. For a reversible isentropic process, there is no transfer of heat energy and therefore the process is also adiabatic. For an irreversible process, the entropy will increase. Hence removal of heat from the system (cooling) is necessary to maintain a constant entropy for an irreversible process in order to make it isentropic. Thus an irreversible isentropic process is not adiabatic.

For reversible processes, an isentropic transformation is carried out by thermally "insulating" the system from its surroundings. Temperature is the thermodynamic conjugate variable to entropy, thus the conjugate process would be an isothermal process in which the system is thermally "connected" to a constant-temperature heat bath.

Isentropic Processes in Thermodynamic Systems[edit]

T-s (Entropy vs. Temperature) diagram of an isentropic process, which is a vertical line segment.

The entropy of a fixed mass does not change during a process that is internally reversible and adiabatic. A process during which the entropy remains constant is called an isentropic process.[3]

Isentropic Process:  \bigtriangleup s=0 or  s_1 = s_2

Many thermodynamic devices are essentially isentropic when the irreversibilities, for example friction, are idealized as 0 or minimized to almost 0.
Some isentropic thermodynamic devices include:
Pump
Gas compressor
Turbine
Nozzle
Diffuser

Isentropic Efficiencies of Steady-Flow Devices in Thermodynamic Systems[edit]

Most steady-flow devices operate under adiabatic conditions, and the ideal process for these devices is the isentropic process.The parameter that describes how efficiently a device approximates a corresponding isentropic device is called isentropic or adiabatic efficiency.[4]


Isentropic efficiency of Turbines:

 \eta _T = \frac{Actual\;Turbine \; Work}{Isentropic \;Turbine \;Work}=\frac{W_a}{W_s} \cong \frac{h_1-h_{2a}}{h_1-h_{2s}}


Isentropic efficiency of Compressors

 \eta _C = \frac{Isentropic \;Compressor\;Work}{Actual\;Compressor\; Work}=\frac{W_s}{W_a} \cong \frac{h_{2s}-h_1}{h_{2a}-h_1}


Isentropic efficiency of Nozzles

 \eta _N = \frac{Actual \;KE \; at \;Nozzle \; Exit}{Isentropic \;KE \; at \;Nozzle \; Exit}=\frac{V_{2a}^2}{V_{2s}^2} \cong \frac{h_1-h_{2a}}{h_1-h_{2s}}




For all above equations:

 h_1 is the enthalpy at the entrance state
 h_{2a} is the enthalpy at the exit state for the actual process
 h_{2s} is the enthalpy at the exit state for the isentropic process

Isentropic Devices in Thermodynamic Cycles[edit]

Ideal Rankine Cycle 1->2 Isentropic compression in a pump
Ideal Rankine Cycle 3->4 Isentropic expansion in a turbine
Ideal Carnot Cycle 2->3 Isentropic expansion
Ideal Carnot Cycle 4->1 Isentropic compression
Ideal Otto Cycle 1->2 Isentropic compression
Ideal Otto Cycle 3->4 Isentropic expansion
Ideal Diesel Cycle 1->2 Isentropic compression
Ideal Diesel Cycle 3->4 Isentropic expansion
Ideal Brayton Cycle 1->2 Isentropic compression in a compressor
Ideal Brayton Cycle 3->4 Isentropic expansion in a turbine
Ideal Vapor-compression refrigeration Cycle 1->2 Isentropic compression in a compressor
NOTE: The isentropic assumptions are only applicable with ideal cycles. Real world cycles have inherent losses due to inefficient compressors and turbines. The real world system are not truly isentropic but are rather idealized as isentropic for calculation purposes.

Isentropic flow[edit]

An isentropic flow is a flow that is both adiabatic and reversible. That is, no heat is added to the flow, and no energy transformations occur due to friction or dissipative effects. For an isentropic flow of a perfect gas, several relations can be derived to define the pressure, density and temperature along a streamline.

Note that energy can be exchanged with the flow in an isentropic transformation, as long as it doesn't happen as heat exchange. An example of such an exchange would be an isentropic expansion or compression that entails work done on or by the flow.

Derivation of the isentropic relations[edit]

For a closed system, the total change in energy of a system is the sum of the work done and the heat added,

dU = \delta W + \delta Q\,\!

The reversible work done on a system by changing the volume is,

dW = -pdV\,\!

where p is the pressure and V is the volume. The change in enthalpy (H = U + pV\,\!) is given by,

dH = dU + pdV + Vdp\,\!

Then for a process that is both reversible and adiabatic (i.e. no heat transfer occurs),  \delta Q_{rev} = 0\,\!, and so dS=\delta Q_{rev}/T = 0\,\!. All reversible adiabatic processes are isentropic. This leads to two important observations,

dU = \delta W + \delta Q = -pdV + 0\,\! , and
dH = \delta W + \delta Q + pdV + Vdp = -pdV + 0 + pdV + Vdp = Vdp\,\!

Next, a great deal can be computed for isentropic processes of an ideal gas. For any transformation of an ideal gas, it is always true that

dU = nC_vdT\,\!, and dH = nC_pdT\,\!.

Using the general results derived above for dU and dH, then

dU = nC_vdT = -pdV\,\!, and
dH = nC_pdT = Vdp\,\!.

So for an ideal gas, the heat capacity ratio can be written as,

\gamma = \frac{C_p}{C_V} = -\frac{dp/p}{dV/V}\,\!

For an ideal gas \gamma\,\! is constant. Hence on integrating the above equation, assuming a perfect gas, we get

 pV^{\gamma} = \mbox{constant} \, i.e.
\frac{p_2}{p_1} = \left(\frac{V_1}{V_2}\right)^{\gamma}

Using the equation of state for an ideal gas, p V = n R T\,\!,

 TV^{\gamma-1} = \mbox{constant} \,
 \frac{p^{\gamma -1}}{T^{\gamma}} = \mbox{constant}

also, for constant C_p = C_v + R (per mole),

 \frac{V}{T} = \frac{nR}{p} and p = \frac{nRT}{V}
 S_2-S_1 = nC_p \ln\left(\frac{T_2}{T_1}\right) - nR\ln\left(\frac{p_2}{p_1}\right)
 \frac{S_2-S_1}{n} = C_p \ln\left(\frac{T_2}{T_1}\right) - R\ln\left(\frac{T_2 V_1}{T_1 V_2}\right ) = C_v\ln\left(\frac{T_2}{T_1}\right)+ R \ln\left(\frac{V_2}{V_1}\right)

Thus for isentropic processes with an ideal gas,

 T_2 = T_1\left(\frac{V_1}{V_2}\right)^{(R/C_v)} or  V_2 = V_1\left(\frac{T_1}{T_2}\right)^{(C_v/R)}

Table of isentropic relations for an ideal gas[edit]

\Bigg\lbrack \frac{T_2}{T_1} \Bigg\rbrack =\,\!  \left (\frac{p_2}{p_1} \right )^\frac {\gamma-1}{\gamma} =\,\! \left (\frac{V_1}{V_2} \right )^{(\gamma-1)} =\,\!  \left (\frac{\rho_2}{\rho_1} \right )^{(\gamma - 1)}
 \left (\frac{T_2}{T_1} \right )^\frac {\gamma}{\gamma-1} =\,\! \Bigg\lbrack \frac {p_2} {p_1} \Bigg\rbrack =\,\! \left (\frac{V_1}{V_2} \right )^{\gamma} =\,\!  \left (\frac{\rho_2}{\rho_1} \right )^{\gamma}
 \left (\frac{T_1}{T_2} \right )^\frac {1}{\gamma-1} =\,\!  \left (\frac{p_1}{p_2} \right )^\frac {1}{\gamma} =\,\! \Bigg\lbrack \frac{V_2}{V_1} \Bigg\rbrack =\,\! \frac{\rho_1}{\rho_2}
 \left (\frac{T_2}{T_1} \right )^\frac {1}{\gamma-1} =\,\!  \left (\frac{p_2}{p_1} \right )^\frac {1}{\gamma} =\,\! \frac{V_1}{V_2} =\,\! \Bigg\lbrack \frac{\rho_2}{\rho_1} \Bigg\rbrack

Derived from:


pV^{\gamma} = \text{constant}
\,\!

pV = m R_s T
\,\!

p = \rho R_s T\,\!
\,\!
Where:
  • p\,\! = Pressure
  • V\,\! = Volume
  • \gamma\,\! = Ratio of specific heats = C_p/C_v\,\!
  • T\,\! = Temperature
  • m\,\! = Mass
  • R_s\,\! = Gas constant for the specific gas = R/M\,\!
  • R\,\! = Universal gas constant
  • M\,\! = Molecular weight of the specific gas
  • \rho\,\! = Density
  • C_p\,\! = Specific heat at constant pressure
  • C_v\,\! = Specific heat at constant volume

References[edit]

  • Van Wylen, G.J. and Sonntag, R.E. (1965), Fundamentals of Classical Thermodynamics, John Wiley & Sons, Inc., New York. Library of Congress Catalog Card Number: 65-19470

Notes[edit]

  1. ^ Van Wylen, G.J. and Sonntag, R.E., Fundamentals of Classical Thermodynamics, Section 7.4
  2. ^ Massey, B.S. (1970), Mechanics of Fluids, Section 12.2 (2nd edition) Van Nostrand Reinhold Company, London. Library of Congress Catalog Card Number: 67-25005
  3. ^ Cengel, Yunus A., and Michaeul A. Boles. Thermodynamics: An Engineering Approach. 7th Edition ed. New York: Mcgraw-Hill, 2012. Print.
  4. ^ Cengel, Yunus A., and Michaeul A. Boles. Thermodynamics: An Engineering Approach. 7th Edition ed. New York: Mcgraw-Hill, 2012. Print.

See also[edit]