1、化学英语证书考试（PEC）-化工热力学常用术语Absolute entropy is entropy calculated relative to the absolute reference point determined by the third law of thermodynamics.Clausius inequality first stated by the German physicist R. J. E. Clausius (1822-1888), is expressed as the cyclic integral of Q/T is always less than
2、or equal to zero. This inequality is valid for all cycles, reversible or irreversible.Entropy (from a classical thermodynamics point of view) is a property designated S and is defined as dS =(dQ/T)int rev.Entropy (from a statistical thermodynamics point of view) can be viewed as a measure of molecul
3、ar 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 pwhere k is the Boltzmann constant.Entropy balance relation for a control v
4、olume is stated as 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
5、 within the boundaries of the control volume as a result of irreversibilities.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 i
6、rreversibilities.Entropy change of a closed system is due to the entropy transfer accompanying heat transfer and the entropy generation within the system boundaries.Entropy generation Sgen is entropy generated or created during an irreversible process, is due entirely to the presence of irreversibil
7、ities, 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.Heat transfer is the area under the process curve on a T-S diagram during an inter
8、nally reversible process. The area has no meaning for irreversible processes.Increase of entropy principle or 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
9、words, the entropy of an isolated system never decreases.Internally reversible process has no irreversibilities occurring within a system undergoing the process.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 valu
10、e in an isentropic manner to the actual work input.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.Isentropic efficiency o
11、f 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.Isentropic process is an internally reversible and adiabatic process. In such a process the entropy rema
12、ins constant.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.Mechanisms of entropy transfer Sin and Soutare heat transfer and mass flow. Entropy transfer is recog
13、nized 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 zer
14、o.Mollier diagram after the German scientist R. Mollier (1863-1935), is the h-s diagram. The Mollier diagram is useful when solving isentropic, steady flow process problems dealing with nozzles, turbines, and compressors.Multistage compression with intercooling is a compression process where a gas i
15、s compressed in stages and cooled between each stage by passing it through a heat exchanger called an intercooler.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
16、 used in isentropic processes of ideal gases where variable specific heats are required.Relative specific volume vr is defined as the quantity T/Pris a function of temperature only and Pr is the relative pressure. Relative specific volume is used in isentropic processes of ideal gases where variable
17、 specific heats are required.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 t
18、he 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.Second law distinction between heat transfer and work states that an energy interaction that is accompanied by e
19、ntropy transfer is heat transfer, and an energy interaction that is not accompanied by entropy transfer is work.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.Third law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero temperature is zero.
