1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    h=h0.

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Total enthalpy concept question
Loading...