| Absolute entropy | is entropy calculated relative to the absolute reference point determined by the third law of thermodynamics.
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| Bernoulli equation | is the result of the energy analysis for the steady flow of an incompressible liquid through a device that involves no work interactions (such as a nozzle or a pipe section).
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| Boltzmann relation | is the expression of the entropy as a function thermodynamic probability.
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| Boltzmann's constant | k has the value of 1.3806 × 1023 J/K.
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| Clausius inequality | first stated by the German physicist R. J. E. Clausius (1822–1888), is expressed as the cyclic integral of δQ/T is always less than or equal to zero. This inequality is valid for all cycles, reversible or irreversible.
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| Entropy (from a classical thermodynamics point of view) | is a property designated S and is defined as dS = (δQ/T)int rev.
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| Entropy (from a statistical thermodynamics point of view) | can be viewed as a measure of molecular disorder, or molecular randomness. The entropy of a system is related to the total number of possible microscopic states of that system, called thermodynamic probability p, by the Boltzmann relation, expressed as S = k ln p where k is the Boltzmann constant.
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| Entropy balance relation for a control volume | states that the rate of entropy change within the control volume during a process is equal to the sum of the rate of entropy transfer through the control volume boundary by heat transfer, the net rate of entropy transfer into the control volume by mass flow, and the rate of entropy generation within the boundaries of the control volume as a result of irreversibilities.
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| Entropy balance relation in general | is stated as the entropy change of a system during a process is equal to the net entropy transfer through the system boundary and the entropy generated within the system as a result of irreversibilities.
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| Entropy change of a closed system | is due to the entropy transfer accompanying heat transfer and the entropy generation within the system boundaries and is greater than or equal to the integral over the process of δQ/T.
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| Entropy generation Sgen | is entropy generated or created during an irreversible process, is due entirely to the presence of irreversibilities, and is a measure of the magnitudes of the irreversibilities present during that process. Entropy generation is always a positive quantity or zero. Its value depends on the process, and thus it is not a property.
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| Entropy transfer | is the transfer of entropy across a boundary by heat or mass.
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| Heat transfer | is the area under the process curve on a T-S diagram during an internally reversible process. The area has no meaning for irreversible processes.
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| Increase of entropy principle | (see second law of thermodynamics)
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| Internally reversible process | has no irreversibilities occurring within a system undergoing the process.
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| Isentropic efficiency of a compressor | is defined as the ratio of the work input required to raise the pressure of a gas to a specified value in an isentropic manner to the actual work input.
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| Isentropic efficiency of a nozzle | is defined as the ratio of the actual kinetic energy of the fluid at the nozzle exit to the kinetic energy value at the exit of an isentropic nozzle for the same inlet state and exit pressure.
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| Isentropic efficiency of a turbine | is defined as the ratio of the actual work output of the turbine to the work output that would be achieved if the process between the inlet state and the exit pressure were isentropic.
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| Isentropic process | is an internally reversible and adiabatic process. In such a process the entropy remains constant.
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| Isothermal process | is a process in which the temperature maintained constant.
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| Isothermal efficiency of a compressor | is defined as the ratio of the work input to a compressor for the reversible isothermal case and the work input to a compressor for the actual case.
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| Kelvin-Planck statement of the second law | states that no system can produce a net amount of work while operating in a cycle and exchanging heat with a single thermal energy reservoir.
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| Mechanisms of entropy transfer | Sin and Sout are heat transfer and mass flow. Entropy transfer is recognized at the system boundary as it crosses the boundary, and it represents the entropy gained or lost by a system during a process. The only form of entropy interaction associated with a fixed mass or closed system is heat transfer, and thus the entropy transfer for an adiabatic closed system is zero.
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| Mollier diagram | after the German scientist R. Mollier (1863-1935), is the plot of property data on the h-s diagram. The Mollier diagram is useful when solving isentropic, steady flow process problems dealing with nozzles, turbines, and compressors.
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| Multistage compression with intercooling | is a compression process where a gas is compressed in stages and cooled between each stage by passing it through a heat exchanger called an intercooler.
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| Relative pressure Pr | is defined as the quantity exp(s°/R) and is a dimensionless quantity that is a function of temperature only since s° depends on temperature alone. Relative pressure is used to relate the ratio of final to initial pressure in isentropic processes of ideal gases where variable specific heats are required.
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| Relative specific volume vr | is defined as the quantity T/Pr and is a function of temperature only. Pr is the relative pressure. Relative specific volume is used to relate the ratio of final to initial volume in isentropic processes of ideal gases where variable specific heats are required.
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| Reversible steady-flow work | is defined as the negative of the integral of the specific volume-pressure product. The larger the specific volume, the larger the reversible work produced or consumed by the steady-flow device. Therefore, every effort should be made to keep the specific volume of a fluid as small as possible during a compression process to minimize the work input and as large as possible during an expansion process to maximize the work output.
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| Second law distinction between heat transfer and work | states that an energy interaction that is accompanied by entropy transfer is heat transfer, and an energy interaction that is not accompanied by entropy transfer is work.
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| Second law of thermodynamics | is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases.
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| Tds relations | relate the Tds product to other thermodynamic properties. The first Gibbs relation is Tds = du + Pdv. The second Gibbs relation is Tds = dh – vdP.
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| Third law of thermodynamics | states that the entropy of a pure crystalline substance at absolute zero temperature is zero.
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