Читать книгу Fundamentals of Heat Engines - Jamil Ghojel - Страница 53

Stagnation properties are those thermodynamic properties a flowing compressible fluid would possess if it were brought to rest adiabatically and reversibly, i.e. isentropically, and without heat and work transfer. The stagnation state is a convenient hypothetical state that simplifies many of the equations involving flow by taking account of the kinetic energy terms in the steady flow energy equation implicitly.

The stagnation enthalpy h_t is the enthalpy that a gas stream of enthalpy h and velocity C would possess when brought to rest adiabatically and without work transfer. The energy equation thus becomes

(1.63)

For a perfect gas, h = c_pT and the corresponding stagnation temperature T_t is

(1.64)

Applying the concept of stagnation properties to an adiabatic compression, the energy Eq. (1.60) becomes

Rearranging, we get

(1.65)

Temperature‐measuring devices such as thermometers and thermocouples in reality measure the stagnation temperature of the flow and not the static temperature. Thus, introduction of stagnation temperatures simplifies solving the energy equation by eliminating the kinetic energy term and the need to measure flow velocity.

The stagnation pressure p_t is defined as the pressure the gas stream would possess if the gas were brought to rest adiabatically and reversibly. Using Eqs. (1.48) and (1.64), p_t can be written as

(1.66)