Isentropic Flow

Constant entropy flow is called isentropic flow

The second law of thermodynamics requires that the adiabatic and frictionless flow of any fluid results in no change in entropy. Constant entropy flow is called isentropic flow.

Based on the equation of entropy in a compressible flow:

ds = cv ln(T2 / T1) + R ln(ρ1 / ρ2)

    = cp ln(T2 / T1) - R ln(p2 / p1)

    = 0             (1)


ds = change in specific entropy  (kJ/kgK)

cv = specific heat capacity at a constant volume process (kJ/kg K)

cp = specific heat capacity at a constant pressure process (kJ/kg K)

T= absolute temperature (K)

R = individual gas constant (kJ/kg K)

ρ = density of gas (kg/m3)

p = absolute pressure (Pa, N/m2)


κ = cp / cv         (2)


κ = specific heat ratio

(1) can be transformed to:

(T2 / T1)(κ/(κ-1)) = 2 / ρ1)κ = (p2 / p1)         (3)

(3) express the useful relationship between temperature, density and pressure for an isentropic flow of an ideal gas.

From (3) the relationship between pressure and temperature can be concluded:

p / ρκ = constant         (4)

Density can be expressed:

ρ = 1 / v         (5)


v = specific volume (m3/kg)

Using (5) in combination with (4) transforms to a common expression:

p vκ = constant         (6)

Related Topics

  • Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.

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