Generated entropy

Thus, an analysis of the lost work, made by calculation of the fraction that each individual lost-work term represents of the total lost work, is the same as an analysis of the rate of entropy generation, made by expressing each individual entropy-generation term as a fraclion of the sum of all entropy-generation terms. 

An alternative to the lost-work or entropy-generation analysis is a work analysis. This is based on Eq. (4-366), written... 

In both cases the mass flow, mechanical work, and density are constant. The flow is denoted by subscripts 1 before the fan and 2 after the fan. If there is no entropy generation, subscript 2 is replaced by subscript Is. 

Real fan total pressure difference is smaller for the same volume flow than that of an isentropic, theoretical fan. This is a result of the fan losses. These arise from the entropy generation in adiabatic systems. We investigate the losses separately, i.e., entropy generation in the impeller and casing. 

The term Ap = plj T ds is the entropy generation given as pressure loss. 

In a straight duct where Zj = Z2 and the cross-sectional area is uniform, then Cl = C2- This gives Ap = pi -p2 > 0 < P is the duct pressure loss, which is a result of entropy generation. Entropy generation is due to the flow friction, which is the reason for pressure loss. 

The internal friction due to vortices occurs in rapid expansion, diverging, and regulation valves. The entropy generation due to those vortices is taken into consideration in the local resistance. The entropy generation is directly proportional to c- thus. 

If the outlet sp>ecific entropy. s-j, is determined in this way the gross entropy generation in the whole process is also obtained. 

The reader is referred to the original papers for detailed analysis, where the various components of entropy generation and irreversibility are defined. The advantage of this work is not only that it involves less approximation but also that it is revealing in terms of the basic thermodynamics. It should also be used by designers who should be able to see how design changes relate to increased or decreased local loss. 

Relations (5) and (6) are combined in (4). Inequality (6) shows that if a system undergoes any irreversible change, there will have been a positive amount of entropy generated in the system itself during that change, since the entropy absorbed from outside is always less than the amount by which the entropy of... 

But if the given process is conducted irreversibly, we have proved that there is always more entropy generated in the system than is lost by bodies outside the system, and the excess is called the non-compensatcd entropy. It may happen, and frequently does, that the entropy of the system itself decreases in a particular change, but we have proved that there is an increase outside the system which is greater than the decrease in the system, or at best equal to it in the case of reversible changes. 

A human body generates heat at the rate of about 100. W (1 W = l J-s ). (a) At what rate does your body heat generate entropy in your surroundings, taken to be at 20.°C (b) How much entropy do you generate each day (c) Would the entropy generated he greater or less if you were in a room kept at 30.°C Explain your answer. 

The same property of entropy generation holds for other processes. In electrical conductance, charged particles move from higher to lower electrical potentials. In diffusion phenomena, all chemical entities are transferred from higher to lower chemical potentials. 

Figure 2.29 Dimensionless entropy generation rate as a function of Reynolds number for a circular channel of 610 pm diameter and different ratios of the fluid inlet temperature and the wall temperature, taken from [104. ...

We note that the resulting entropic force is particular to macromolecular systems, where the conformational entropy generates a force stabilizing the most favorable conformation. 

J.S. Shiner (ed.) Entropy and Entropy Generation. Fundamentals and Applications. 1996 ISBN 0-7923-4128-7... 

A second application is being conducted with the aim of developing new geometries. With this goal the entropy generation minimization technique (Bejan, 1996) has been applied to a tubular cell whose shape is made to change in order to reduce as much as possible the sources of irreversibility. 

Figures 7.15 and 7.16 show the velocity profiles and the voltage drop in a tubular cell characterized by trapezoidal shape channels, which is the geometry considered as the base case for entropy generation minimization (Sciacovelli, 2006).

Bejan A. (1996) Entropy Generation Minimization. The Method of Thermodynamic Optimization of Finite-Size Systems and Finite-Time Processes, CRC Press, Boca Raton, FL. 

Notice that the direction of the process and time have been determined This has been called the arrow of time [2], Time proceeds in the direction of entropy generation, that is, toward a state of greater probability for the total of the system and its environment, which, in the widest sense, makes up the universe. Finally, we wish to point out that an interesting implication of Equation 2.10 is that for substances in the perfect crystalline state at T = 0 K, the thermodynamic probability Q = 1 and thus S = 0. 

If a mixing process or chemical transformation is brought about, spontaneously or by applying work on the system, the process will take place with entropy generation ... 

We conclude this example with the observation that the second law for real processes expresses that entropy generation is positive and that the implication is that the real amount of work required is larger than that calculated for... 

In this chapter, we show that it is not so much energy that is consumed but its quality, that is, the extent to which it is available for work. The quality of heat is the well-known thermal efficiency, the Carnot factor. If quality is lost, work has been consumed and lost. Lost work can be expressed in the products of flow rates and driving forces of a process. Its relation to entropy generation is established, which will allow us later to arrive at a universal relation between lost work and the driving forces in a process. 

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