Stagnation enthalpy

The total enthalpy, also called total enthalpy, Stagnationsenthalpie or Ruheenthalpie is a size in compressible flowing media and is required in fluid mechanics, in particular, in calculations for heat engines, such as steam turbines and rocket engines. The total enthalpy is defined as the sum of the enthalpy and the kinetic energy of a flowing particle,

Wherein the enthalpy of the mass and the velocity of the particle.

Since in the mass flow is maintained, the specific total enthalpy, which is the total enthalpy per unit mass is often used:

The significance of the total enthalpy is that the enthalpy can be converted into kinetic energy of the flowing medium (in this case, results in expansion and most of temperature decrease ), and vice versa, the kinetic energy in enthalpy ( back pressure, so compression, consequently, generally increasing the temperature ). The total enthalpy is a measure of how much " working capacity " a medium, for example steam at a certain point of a steam turbine, or inherent in, regardless of whether the energy is provided as thermal energy, pressure and kinetic energy.

If the flowing medium is decelerated to the speed ( for example by an obstacle ), then its increased enthalpy of the total enthalpy and the temperature of the so-called stagnation temperature or temperature Total

Wherein the specific heat at constant pressure ( It has been assumed here that between and is independent of temperature ). The stagnation temperature is used in aircraft and rocket construction for the thermal load on surfaces in supersonic flows of importance.

The total enthalpy (and thus also for ideal gases ) remains constant in flows, as long as neither mechanical work is done or heat between the moving fluid and the ambient flow; this also applies to shock waves of supersonic flow.

More energy terms can be added ( for example, for chemical reactions, phase changes, motion fields). In the gravitational field, the potential energy can be considered:

For example, the first law of thermodynamics can then write as ( for an open system ):

Here w is the specific work (so, the mass element ), q is the specific heat, h is the specific enthalpy, u is the velocity, g is the gravitational acceleration and z is the height.

• Aerodynamics
• Turbomachinery
• Thermodynamic state variable