External irreversibilities

A closed system moving slowly through a series of stable states is. said to undergo a reversible process if that process can be completely reversed in all thermodynamic respects, i.e. if the original. state of the system itself can be recovered (internal reversibility) and its surroundings can be restored (external irreversibility). An irreversible process is one that cannot be reversed in this way. 

If the heat transferred from the control volume is not used externally to create work, but is simply lost to the atmosphere in which further entropy is created, then Equj can be said to be equal to /quj, a lost work term, due to external irreversibility. Another form of Eq. (2.23) is thus... 

Although the [CHTJr cycle is internally reversible, external irreversibility is involved in the heat supply from the external reservoir at temperature Tr and the heat rejection to a reservoir at temperature 7. So a consideration of the internal thermal efficiency alone does not provide a full discussion of the thermodynamic performance of the plant. If the reservoirs for heat supply and rejection are of infinite capacity, then it may be shown that the irreversibilities in the heat supply ( r) f d the heat rejection respectively, both positive, are... 

The ideal finite-time Rankine cycle and its T-s diagram are shown in Figs. 7.14 and 7.15, respectively. The cycle is an endoreversible cycle that consists of two isentropic processes and two isobaric heat-transfer processes. The cycle exchanges heats with its surroundings in the two isobaric external irreversible heat-transfer processes. The heat source and heat sink are infinitely large. Therefore, the temperature of the heat source and heat sink are unchanged during the heat-transfer processes. 

To evaluate the energy utilization, energetic and exergetic process analyses are used. Since exergetic/anergetic flowcharts show local internal and external irreversibilities, the locations and quantities of heat losses may be detected, leading to thermodynamic optimization from the consideration of process energy improvements. 

The fluctuation dissipation theorem relates the dissipative part of the response fiinction (x") to the correlation of fluctuations (A, for any system in themial equilibrium. The left-hand side describes the dissipative behaviour of a many-body system all or part of the work done by the external forces is irreversibly distributed mto the infinitely many degrees of freedom of the themial system. The correlation fiinction on the right-hand side describes the maimer m which a fluctuation arising spontaneously in a system in themial equilibrium, even in the absence of external forces, may dissipate in time. In the classical limit, the fluctuation dissipation theorem becomes / /., w) = w). 

In the irreversible limit, the sohidon for combined external resistance and pore diffusion with infinite fluid volume is (Yagi and Knnii) ... 

The objective of the gas turbine designer is to make all the proces.ses in the plant as near to reversible as possible, i.e. to reduce the irreversibilities, both internal and external, and hence to obtain higher thermal efficiency (in a closed cycle gas turbine plant) or higher overall efficiency (in an open gas turbine plant). The concepts of availability and exergy may be used to determine the location and magnitudes of the irreversibilities. 

PV graph shows that workflow into the system Wi is larger than work output The net cycle area for the system 1-2-3-4 measures the work lost by the system—external temperature irreversibilities cause this. All the processes, however, have been considered as internally reversible. 

When a plastic material is subjected to an external force, a part of the work done is elastically stored and the rest is irreversibly (or viscously) dissipated hence a viscoelastic material exists. The relative magnitudes of such elastic and viscous responses depend, among other things, on how fast the body is being deformed. It can be seen via tensile stress-strain curves that the faster the material is deformed, the greater will be the stress developed since less of the work done can be dissipated in the shorter time. 

Boltzmann s H-Theorem. —One of the most striking features of transport theory is seen from the result that, although collisions are completely reversible phenomena (since they are based upon the reversible laws of mechanics), the solutions of the Boltzmann equation depict irreversible phenomena. This effect is most clearly seen from a consideration of Boltzmann s IZ-function, which will be discussed here for a gas in a uniform state (no dependence of the distribution function on position and no external forces) for simplicity. 

As an example of conditional irreversibility may be taken the expansion of a gas. Work is done by the change of bulk in opposition to the external forces, and heat is absorbed from the environment. The conditions which must hold in order that the process actually occurs are ... 

The pressure at every instant during an expansion or contraction of the working substance must be only infinitesimally greater or less respectively, than the external pressure, otherwise turbulent motions occur, the kinetic energy of which is ultimately converted into heat by friction, and this heat production is intrinsically irreversible. 

The accumulation of a number of amino acids from the external medium seems almost irreversible in Saccharomyces cerevisiae. The first detailed study of this phenomenon concerned histidine [13]. Histidine uptake by the specific histidine permease HlPl is an energy dependent process which accumulates free and intact... 

Although uptake and accumulation of most amino acids from the external medium seems to be irreversible, amino acids are excreted into the medium whenever they are overproduced above a given threshold by yeast cells [6], This can occur under a number of specific conditions, namely in mutants with impaired regulation of amino acid biosynthesis, or in the presence of mutations preventing substrate catabolism, or when growth occurs in the presence of metabolic intermediates. It can even occur when growth is arrested under conditions where amino acid synthesis can continue. 

Many elements of the p-block of the periodic table spontaneously adsorb on the surface of a platinum electrode when this is immersed in a solution containing a soluble salt of the element, without an external supply of electricity [Clavilier et al., 1988, 1989a, b, 1990a, b Evans and Attard, 1993 Feliu et al., 1988, 1991, 1993a, b Gomez et al., 1992 Sung et al., 1997, 1998]. The electrode can then be rinsed and transferred to an electrochemical cell that does not contain the corresponding ion of the deposited element, which remains on the surface, irreversibly adsorbed. 

---