TY - JOUR
N2 - Thermodynamics deals with irreversible transformations of substances. Every thermodynamic property of a substance, as a function of two parameters describing its state, can be illustrated as a simply connected manifold. The term manifold stands for the Methods of Geometrical Representation of Thermodynamic Properties of Substances by Means of Surfaces. Generally, every transformation of a substance changes its energy (or enthalpy) by heat transfer and work done on it. All such changes (transformations) are considered to be irreversible and can be described using appropriate manifolds. Studies show that every transformation is associated with the degradation of energy. Such relations (between heat, work and other forms of energy or enthalpy) can be described by the Pfaff formulas and their integrations. This article discusses the issue of irreversible energy degradation in heat transfer between two fluids. Irreversible heat transfer between separated fluids most often occurs through surface heat exchangers. All such processes are determined by convective heat transfer in thermal boundary layers and conduction through the wall. Consequently, entropy changes of fluids in heat and mass transfer can be observed in these layers. While the entropy rate of the heating fluid is negative and that of the heated medium is positive, the sum of entropy changes of all substances involved in the heat transfer process is always positive. These sums, known as entropy increase (entropy generation), can be interpreted as the measure of irreversible degradation of energy in heat transfer processes. The consequence of this degradation is that an arbitrary engine powered by the degraded (lower-temperature) heat flux will operate at a lower efficiency. The significance of this discussion relates especially to cases in power plants and cooling systems where surface heat exchangers are used. In the discussion proposed is the entropy increase as a criterion of irreversible energy degradation in heat transfer. Such introduced measure of effectiveness leads to an analysis of local overall heat transfer coefficient optimization on the cone-shaped manifold.
L1 - http://journals.pan.pl/Content/118780/PDF/art07.pdf
L2 - http://journals.pan.pl/Content/118780
PY - 2020
IS - No 4
EP - 190
DO - 10.24425/ather.2020.135858
KW - Heat transfer
KW - Analysis
KW - Manifolds
A1 - Drożyński, Zbigniew
PB - The Committee of Thermodynamics and Combustion of the Polish Academy of Sciences and The Institute of Fluid-Flow Machinery Polish Academy of Sciences
VL - vol. 41
DA - 2020.06.30
T1 - Irreversible energy degradation in heat transfer between two fluids – analysis on manifolds
SP - 169
UR - http://journals.pan.pl/dlibra/publication/edition/118780
T2 - Archives of Thermodynamics
ER -