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11.3.2: Speed of Sound in Ideal and Perfect Gases

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  • The speed of sound can be obtained easily for the equation of state for an ideal gas (also perfect gas as a sub set) because of a simple mathematical expression. The pressure for an ideal gas can be expressed as a simple function of density, \(\rho\), and a function "molecular structure'' or ratio of specific heats, \(k\) namely

    \[ P= constant\times \rho^{k}

    and hence

    \[ \label{gd:sd:eq:idealGas1}
    c = \sqrt{\dfrac{\partial P}{\partial \rho}}
    = k \times constant \times \rho^{k-1} =k \times
    \dfrac{\overbrace{ constant \times \rho^k}^{P} }{ \rho}
    = k \times \dfrac{P }{\rho }
    \] Remember that \(P / \rho \) is defined for an ideal gas as \(RT\), and equation (12) can be written as

    Ideal Gas Speed Sound

    c = \sqrt{ k\, R\, T}

    Example 11.2

    Calculate the speed of sound in water vapor at \(20 [bar]\) and \(350^{\circ}C\), (a) utilizes the steam table, and (b) assuming ideal gas.

    Solution 11.2

    The solution can be estimated by using the data from steam table
    c \sim \sqrt{ \dfrac{\Delta P}{ \Delta \rho} }_{s=constant}
    At \(20[bar]\) and \(350^{\circ}C\): s = 6.9563 \(\left[ \dfrac{kJ }{ K\, kg}\right]\) \(\rho \) = 6.61376 \(\left[ \dfrac{kg }{ m^3} \right]\)
    At \(18[bar]\) and \(350^{\circ}C\): s = 7.0100 \(\left[\dfrac{ kJ }{ K\, kg}\right]\) \(\rho \) = 6.46956 \(\left[ \dfrac{ kg }{ m^3}\right]\)
    At \(18[bar]\) and \(300^{\circ}C \): s = 6.8226 \(\left[\dfrac{ kJ }{ K\, kg}\right]\) \(\rho \) = 7.13216 \(\left[ \dfrac{kg }{ m^3} \right] \)
    After interpretation of the temperature:
    At \(18[bar]\) and \(335.7^{\circ}C \): s \(\sim\) 6.9563 \(\left[\dfrac{ kJ }{ K\, kg} \right]\) \(\rho \sim\) 6.94199 \(\left[ \dfrac{kg }{ m^3} \right]\)
    and substituting into the equation yields
    c = \sqrt{ 200000 \over 0.32823} = 780.5 \left[ m \over
    sec \right]
    for ideal gas assumption (data taken from Van Wylen and Sontag, Classical Thermodynamics, table A 8.)
    c = \sqrt{k\,R\,T} \sim \sqrt{ 1.327 \times 461 \times
    (350 + 273)} \sim
    771.5 \left[ \dfrac{m }{ sec} \right]
    Note that a better approximation can be done with a steam table, and it \(\cdots\),

    Contributors and Attributions

    • Dr. Genick Bar-Meir. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or later or Potto license.