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Total enthalpy concept question

  1. Sep 21, 2014 #1
    1. The problem statement, all variables and given/known data
    Total enthalpy is defined such that:

    h+(V^2)/2 = constant ...... (1)

    h0 is the stagnation enthalpy. Stagnation means V=0. Plug this V into the above, get


    Hence h+(V^2)/2 = h0 ......... (2)

    My question is we already defined V=0, so how could there still be a V^2/2 term in (2)? The total enthalpy is defined as the sum of the static enthalpy plus the kinetic energy. But we've already defined V=0!!! So shouldn't there be no kinetic energy? I'm really confused on this.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 8, 2016 #2
    Firstly you need to understand the definition of stagnation enthalpy. Stagnation enthalpy of a fluid is the enthalpy it attains when it is isentropically decelerated to zero velocity. We are not defining the velocity as zero. Here h is the enthalpy at initial state and h0 is the enthalpy at the final stage when isentropic deceleration is over. If subscript 0 denotes the stagnation state of the fluid simple energy balance will give us the equation:
    h+(V^2)/2 = h0+(V0^2/2)...(assuming all other variables of steady flow energy equation to be 0)
    But V0=0 since velocity is 0 at stagnation state
    Substitute this and you get your equation.
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