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ODE with a function of variable crazy equ please help

  1. Feb 17, 2012 #1
    How to solve a equation of this kind

    T' = a*(T^4 - r^4) + b*(T^4 - s^4) + P*(1 - η(T))

    The above equation is driving me nuts... the 'η' is a function of T(Temperature) the efficiency and initial value of T is known.
    Say at t = 0 T is 298
    Need help! Please!

    I need to find the temperature of the system after a given time say 60s...
     
  2. jcsd
  3. Feb 17, 2012 #2
    Ok, i'm assuming that T' means dT/dt, and that a, b, P, r and s are not functions of t.

    If it were the case, this is a separable DE, and can be solved by the standard way.

    The only problem is you will get t as a function of T, instead of T as a function of t as wished, but i'm afraid this is the closest you can get of an analytical solution.

    However, if you need only numbers, i strongly suggest integrating it numerically, as it is a much easier way to get the answer you need.
     
  4. Feb 17, 2012 #3

    epenguin

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    My amateur answer :uhh: is I would have thought:
    you can solve

    T' = a*(T^4 - r^4)

    It is a 'straightforward' integration - I think I have done it buried in one of my early posts here somewhere. Standardish anyway. Edit: now I remember one way was to express (T4 - r 4) as (T2 - r2)(T2 - r2) then you have a simple partial fractions problem and then standard integrals.

    You can integrate each term. Then any linear combination of those integrals plus and arbitrary constant determined by the physics should be a solution of the overall equation.

    Then you have two problems.

    One is whether this is the most general solution. On a quick look it seemed to me it is but anyway get a solution first.

    The other is that last term. As we don't know the form of η we can only write for the integral of the last term P*(x - ∫ η(T) dT ) . Then if η is given as data integrate that graphically or numerically. But I would guess that this is a function varying much more lowly than the fourth powers you have in your other terms so is a relatively small correction, and that it is adequately modelled by a simple expression perhaps empirical, e.g. linear or quadratic.
     
    Last edited: Feb 17, 2012
  5. Feb 17, 2012 #4
    Hi all,

    Thanks for taking time and answering my question. I know to solve for T^4 terms. Thats not the problem.
    And I don't require analytical solution. I am interested only in numbers. I think you would have realized that the equation is an energy balance equation. 'η' is the efficiency of the system which is a function of 'T' the temperature of the system. If η were a constant, I would simply integrate it. But, η(T) will vary as T increases. When power 'P' i.e. heat is incident, T will rise... will the rise be determined only by material constants? No...

    Rise in T will also be affected by the behavior of η(T). η is a complex program.
    My intention is this...
    I have written a program for η... I want to call it in the energy balance equation and pass 'T' as a parameter into η. As η decreases with T... Rise in T will be more dramatic at higher temperatures...

    Please help...
     
  6. Feb 17, 2012 #5
    Is this form of energy balance equation correct?

    ρ*A*C*T'n = σ(T^4 - con1^4) + σ(T^4 - con2^4) + Pin(1 - η(Tn-1))

    That is passing 'n-1' value of T as a parameter into η function...

    By T' I mean dT/dt

    at t = 0; T = 298
     
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