Isentropic process

Background

The second law of thermodynamics states that,

T d S ≥ δ Q

where δ Q is the amount of energy the system gains by heating, T is the temperature of the system, and d S is the change in entropy. The equal sign refers to a reversible process, which is an imagined idealized theoretical limit, never actually occurring in physical reality. For an isentropic process, which by definition is reversible, there is no transfer of energy as heat because the process is adiabatic. In an irreversible process of transfer of energy as work, entropy is produced within the system; consequently, in order to maintain constant entropy within the system, energy must be removed from the system as heat during the process.

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

The entropy of a given 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, written △ s = 0 or s 1 = s 2 . Some isentropic thermodynamic devices include: pumps, gas compressors, turbines, nozzles, and diffusers.

Isentropic efficiencies of steady-flow devices in thermodynamic systems

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.

Isentropic efficiency of Turbines:

η T = Actual Turbine Work Isentropic Turbine Work = W a W s ≅ h 1 − h 2 a h 1 − h 2 s

Isentropic efficiency of Compressors

η C = Isentropic Compressor Work Actual Compressor Work = W s W a ≅ h 2 s − h 1 h 2 a − h 1

Isentropic efficiency of Nozzles

η N = Actual KE at Nozzle Exit Isentropic KE at Nozzle Exit = V 2 a 2 V 2 s 2 ≅ h 1 − h 2 a h 1 − h 2 s

For all the above equations:

h 1 is the specific enthalpy at the entrance state h 2 a is the specific enthalpy at the exit state for the actual process h 2 s is the specific enthalpy at the exit state for the isentropic process

Isentropic devices in thermodynamic cycles

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

In fluid dynamics, an isentropic flow is a fluid 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.

For an isentropic flow, entropy density can vary between different streamlines. If the entropy density is the same everywhere, then the flow is said to be homentropic.

Derivation of the isentropic relations

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

d U = δ W + δ Q

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

d W = − p d V

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

d H = d U + p d V + V d p

Then for a process that is both reversible and adiabatic (i.e. no heat transfer occurs), δ Q r e v = 0 , and so d S = δ Q r e v / T = 0 . All reversible adiabatic processes are isentropic. This leads to two important observations,

d U = δ W + δ Q = − p d V + 0 , and d H = δ W + δ Q + p d V + V d p = − p d V + 0 + p d V + V d p = V d p

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

d U = n C v d T , and d H = n C p d T .

Using the general results derived above for d U and d H , then

d U = n C v d T = − p d V , and d H = n C p d T = V d p .

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

γ = C p C V = − d p / p d V / V

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

p V γ = constant i.e. p 2 p 1 = ( V 1 V 2 ) γ

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

T V γ − 1 = constant proof: P V γ = c o n s t a n t ⇒ P V V γ − 1 = c o n s t a n t ⇒ n R T V γ − 1 = c o n s t a n t as nR= constant itself so we can divide both sides by "nR" and still have constant term in the R.H.S.Ultimate result, T V γ − 1 = c o n s t a n t p γ − 1 T γ = constant

also, for constant C p = C v + R (per mole),

V T = n R p and p = n R T V S 2 − S 1 = n C p ln ⁡ ( T 2 T 1 ) − n R ln ⁡ ( p 2 p 1 ) S 2 − S 1 n = C p ln ⁡ ( T 2 T 1 ) − R ln ⁡ ( T 2 V 1 T 1 V 2 ) = C v ln ⁡ ( T 2 T 1 ) + R ln ⁡ ( V 2 V 1 )

Thus for isentropic processes with an ideal gas,

T 2 = T 1 ( V 1 V 2 ) ( R / C v ) or V 2 = V 1 ( T 1 T 2 ) ( C v / R )

Derived from:

p V γ = constant p V = m R s T p = ρ R s T

p = Pressure

V = Volume

γ = 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

ρ = Density

C p = Specific heat at constant pressure

C v = Specific heat at constant volume