Stagnation enthalpy
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thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
and
fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...
, the stagnation enthalpy of a fluid is the static enthalpy of the fluid at a
stagnation point In fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero.Clancy, L.J. (1975), ''Aerodynamics'', Pitman Publishing Limited, London. A plentiful, albeit surprising, example of such points seem ...
. The stagnation enthalpy is also called total enthalpy. At a point where the flow does not stagnate, it corresponds to the static enthalpy of the fluid at that point assuming it was brought to rest from velocity V isentropically.Van Wylen, G.J. and Sonntag, R.E. (1976) ''Fundamentals of Classical Thermodynamics'', section 14.1 (SI Version 2e), John Wiley & Sons, Inc., New York That means all the
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
was converted to internal energy without losses and is added to the local static enthalpy. When the potential energy of the fluid is negligible, the mass-specific stagnation enthalpy represents the total energy of a flowing fluid stream per unit mass. Stagnation enthalpy, or total enthalpy, is the sum of the static enthalpy (associated with the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
static pressure In fluid mechanics the term static pressure has several uses: * In the design and operation of aircraft, ''static pressure'' is the air pressure in the aircraft's static pressure system. * In fluid dynamics, many authors use the term ''static pres ...
at that point) plus the enthalpy associated with the
dynamic pressure In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5 :q = \frac\rho\, u^2 where (in SI units): * is the dynamic pressure in pascals ( ...
, or velocity. This can be expressed in a formula in various ways. Often it is expressed in specific quantities, where specific means mass-specific, to get an
intensive In grammar, an intensive word form is one which denotes stronger, more forceful, or more concentrated action relative to the root on which the intensive is built. Intensives are usually lexical formations, but there may be a regular process for for ...
quantity: : h_0 = h + \frac where: :h_0 = mass-specific total enthalpy, in /kg:h = mass-specific static enthalpy, in /kg:V = fluid velocity at the point of interest, in /s:\frac = mass-specific
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
, in /kg The volume-specific version of this equation (in units of energy per volume, /m^3is obtained by multiplying the equation with the fluid density \rho: : h_0^* = h^* + \rho\frac where: :h_0^* = volume-specific total enthalpy, in /m^3:h^* = volume-specific static enthalpy, in /m^3:V = fluid velocity at the point of interest, in /s:\rho = fluid density at the point of interest, in g/m^3:\rho\frac = volume-specific
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
, in /kg The non-specific version of this equation, that means extensive quantities are used, is: : H_0 = H + m\frac where: :H_0 = total enthalpy, in :H = static enthalpy, in :m = fluid mass, in g:V = fluid velocity at the point of interest, in /s:m\frac =
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
, in The suffix ‘0’ usually denotes the stagnation condition and is used as such here. Enthalpy is the energy associated with the temperature plus the energy associated with the pressure. The stagnation enthalpy adds a term associated with the kinetic energy of the fluid mass. The total enthalpy for a real or ideal gas does not change across a shock. The total enthalpy can not be measured directly. Instead, the static enthalpy and the fluid velocity can be measured. Static enthalpy is often used in the energy equation for a fluid.


See also

*
Stagnation pressure In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.Clancy, L.J., ''Aerodynamics'', Section 3.5 At a stagnation point the fluid velocity is zero. In an incompressible flow, stagnation pressure is equ ...
*
Stagnation temperature In thermodynamics and fluid mechanics, stagnation temperature is the temperature at a stagnation point in a fluid flow. At a stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy and is a ...
* Rothalpy


References

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External links

http://ocw.mit.edu/ans7870/16/16.unified/thermoF03/chapter_6.htm Fluid dynamics Enthalpy