Thermal irreversibility demystified
International Journal of Numerical Methods for Heat & Fluid Flow
ISSN: 0961-5539
Article publication date: 14 October 2022
Issue publication date: 5 January 2023
Abstract
Purpose
This study aims to understand the difference between irreversibility in heat and work transfer processes. It also aims to explain that Helmholtz or Gibbs energy does not represent “free” energy but is a measure of loss of Carnot (reversible) work opportunity.
Design/methodology/approach
The entropy of mass is described as the net temperature-standardised heat transfer to mass under ideal conditions measured from a datum value. An expression for the “irreversibility” is derived in terms of work loss (Wloss) in a work transfer process, unaccounted heat dissipation (Qloss) in a heat transfer process and loss of net Carnot work (CWnet) opportunity resulting from spontaneous heat transfer across a finite temperature difference during the process. The thermal irreversibility is attributed to not exploiting the capability for extracting work by interposing a combination of Carnot engine(s) and/or Carnot heat pump(s) that exchanges heat with the surrounding and operates across the finite temperature difference.
Findings
It is shown, with an example, how the contribution of thermal irreversibility, in estimating reversible input work, amounts to a loss of an opportunity to generate the net work output. The opportunity is created by exchanging heat with surroundings whilst transferring the same amount of heat across finite temperature difference. An entropy change is determined with a numerical simulation, including calculation of local entropy generation values, and results are compared with estimates based on an analytical expression.
Originality/value
A new interpretation of entropy combined with an enhanced mental image of a combination of Carnot engine(s) and/or Carnot heat pump(s) is used to quantify thermal irreversibility.
Keywords
Acknowledgements
The author confirms that the views presented in the manuscript are his own personal and impartial views. These are not influenced by any form of financial incentives and nationalism. He does not have bias against, or towards, any thermodynamic concept, including entransy. He has attempted to address all open and unanswered questions in the literature relating to entropy and has tried to contribute to the fundamental understanding irreversibility during heat and work transfer processes.
The author would like to thank Mr William P.C. Procter, a final year student at the Department of Mechanical Engineering, Swansea University, for completing the ANSYS simulation and producing results presented in Section 6.
The author wishes to thank all referees for a detailed scrutiny of this paper, and in particular, to one anonymous referee for suggesting the ANSYS simulation. This has certainly enhanced the usefulness of this paper.
Citation
Ransing, R.S. (2023), "Thermal irreversibility demystified", International Journal of Numerical Methods for Heat & Fluid Flow, Vol. 33 No. 2, pp. 682-711. https://doi.org/10.1108/HFF-02-2022-0079
Publisher
:Emerald Publishing Limited
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