Abstract
The often used Nusselt number is critically questioned with respect to its physical meaning. Based on a rigorous dimensional analysis, alternative assessment numbers are found that in a systematic way separately account for the quantitative and qualitative aspect of a heat transfer process. The qualitative aspect is related to the entropy generated in the temperature field of a real, irreversible heat transfer. The irreversibility can be quantified by referring it to the so-called entropic potential of the energy involved in the transfer process.
Highlights
One of the first quantities introduced in almost every lecture about heat transfer is the convective heat transfer coefficient: qw h” (1)∆T or its nondimensional version, the Nusselt number: q L L Nu ” w “ h k∆T k (2)They combine the heat flux qw at a heat transfer surface and a certain temperature difference ∆T due to which the heat transfer occurs
It shows that only 0.37% of the entropic potential is used for the heat transfer in the steam power cycle (SPC)-case, but almost 5% in the organic Rankine cycle (ORC)-case “even though” both heat transfer situations have the same Nusselt number
Convective heat transfer situations areThis oftenkind characterized by theisnondimensional groupwhen of the entropic potential is used
Summary
One of the first quantities introduced in almost every lecture about heat transfer is the convective heat transfer coefficient: qw h”. In order to give a more substantial answer to the question about the information associated with a certain Nusselt number a thorough dimensional analysis of convective heat transfer problems is followed by a critical look at the nondimensional groups obtained. This analysis is performed from a thermodynamics point of view, keeping in mind that heat transfer is a fundamental process in thermodynamics; see [4] for more details about this often ignored relationship. This study is conceptual in nature and relevant for practical applications alike
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