工程熱力學(xué)和制冷循環(huán)畢業(yè)論文外文翻譯(編輯修改稿)

　

【文章內(nèi)容簡介】 ed temperature levels, the larger the amount of work required to operate the cycle. Irreversibilities include pressure drops in lines and heat exchangers, heat transfer between fluids of different temperature, and mechanical friction. Reducing total irreversibility in a cycle improves cycle performance. In the limit of no irreversibilities, a cycle attains its maximum ideal efficiency. In an open system, the second law of thermodynamics can be described in terms of entropy as dIsmsmdS eeiiTQs y s t e m ???? ??? (8) where dS = total change within system in time dt during process system δ m s = entropy increase caused by mass entering (ining) δ m s = entropy decrease caused by mass leaving (exiting) δ Q/T = entropy change caused by reversible heat transfer between system and surroundings at temperature T dI = entropy caused by irreversibilities (always positive) Equation (8) accounts for all entropy changes in the system. Rearranged, this equation bees ? ?IddSsmsmTQ s y siiee ???? )( ??? (9) In integrated form, if inlet and outlet properties, mass flow, and interactions with the surroundings do not vary with time, the general equation for the second law is ImsmsTQSS outinr e vs y s t e mif ????? ??? )()(/)( ? (10) In many applications, the process can be considered to operate steadily with no change in time. The change in entropy of the system is therefore zero. The irreversibility rate, which is the rate of entropy production caused by irreversibilities in the process, can be determined by rearranging Equation (10): ??? ??? s ur rinout T QmsmsI )()( (11) Equation (6) can be used to replace the heat transfer that the absolute temperature of the surroundings with which the system is exchanging heat is used in the last term. If the temper ature of the surroundings is equal to the system temperature, heat istransferred reversibly and the last term in Equation (11) equals zero. Equation (11) is monly applied to a system with one mass flow in, the same mass flow out, no work, and negligible kiic or potential energy flows. Combining Equations (6) and (11) yields ? ?s ur r inoutinout T hhssmI ???? )( (12) In a cycle, the reduction of work produced by a power cycle (or the increase in work required by a refrigeration cycle) equals the absolute ambient temperature multiplied by the sum of irreversibilities in all processes in the cycle. Thus, the difference in reversible and actual work for any refrigeration cycle, theoretical or real, operating under the same conditions, bees ??? ITWW r e v e r s i b l ea c t u a l 0 (13) THERMODYNAMIC ANALYSIS OF REFRIGERATION CYCLES Refrigeration cycles transfer thermal energy from a region of low temperature T to one of higher temperature. Usually the higherTR temperature heat sink is the ambient air or cooling water, at temperature T0, the temperature of the surroundings. The first and second laws of thermodynamics can be applied to individual ponents to determine mass and energy balances and the irreversibility of the ponents. This procedure is illustrated in later sections in this chapter. Performance of a refrigeration cycle is usually described by a coefficient of performance (COP), defined as the benefit of the cycle (amount of heat removed) divided by the required energy input to operate the cycle: Useful refrigerating effect COP≡ Useful refrigeration effect/Net energy supplied from external sources (14) Net energy supplied from external sources For a mechanical vapor pression system, the energy supplied is usually in the form of work, mechanical or electrical, and may include work to the pressor and fans or pumps. Thus, evapWQCOP? (15) In an absorption refrigeration cycle, the energy supplied is usually in the form of heat into the generator and work into the pumps and fans, or ne tge nev apWQ QC O P ?? (16) In many cases, work supplied to an absorption system is very small pared to the amount of heat supplied to the generator, so the work term is often neglected. Applying the second law to an entire refrigeration cycle shows that a pletely reversible cycle operating under the same conditions has the maximum possible COP. Departure of the actual cycle from an ideal reversible cycle is given by the refrigerating efficiency: tevR COPCOP)(?? (17) The Carnot cycle usually serves as the ideal reversible refrigeration cycle. For multistage cycles, each stage is described by a reversible cycle. EQUATIONS OF STATE The equation of state of a pure substance is a mathematical relation between pressure, specific volume, and temperature. When the system is in thermodynamic equilibrium, (18) The principles of statistical mechanics are used to (1) explore the fundamental properties of matter, (2) predict an equation of state based on the statistical nature of a particular system, or (3) propose a functional form for an equation of state with unknown parameters that are determined by measuring thermodynamic properties of a substance. A fundamental equation with this basis is the virial equation. The virial equation is expressed as an expansion in pressure p or in reciprocal values of volume per unit mass v as (19) (20) where coe