外文翻译--工程热力学和制冷循环.doc

英文原文THERMODYNAMICS AND REFRIGERATION CYCLESTHERMODYNAMICS is the study of energy, its transformations, and its relation to states of matter. This chapter covers the application of thermodynamics to refrigeration cycles. The first part reviews the first and second laws of thermodynamics and presents methods for calculating thermodynamic properties. The second and third parts address compression and absorption refrigeration cycles, two common methods of thermal energy transfer.THERMODYNAMICSA thermodynamic system is a region in space or a quantity of matter bounded by a closed surface. The surroundings include everything external to the system, and the system is separated fromthe surroundings by the system boundaries. These boundaries can be movable or fixed, real or imaginary. Entropy and energy are important in any thermodynamic system. Entropy measures the molecular disorder of a system. The more mixed a system, the greater its entropy; an orderly or unmixed configuration is one of low entropy. Energy has the capacity for producing an effect and can be categorized into either stored or transient forms.Stored EnergyThermal (internal) energy is caused by the motion of molecules and/or intermolecular forces.Potential energy (PE) is caused by attractive forces existing between molecules, or the elevation of the system. (1)wherem =massg = local acceleration of gravityz = elevation above horizontal reference planeKinetic energy (KE) is the energy caused by the velocity of molecules and is expressed as(2)where V is the velocity of a fluid stream crossing the system boundary.Chemical energy is caused by the arrangement of atoms composing the molecules.Nuclear (atomic) energy derives from the cohesive forces holding protons and neutrons together as the atoms nucleus.Energy in TransitionHeat Q is the mechanism that transfers energy across the boundaries of systems with differing temperatures, always toward the lower temperature. Heat is positive when energy is added to the system (see Figure 1).Work is the mechanism that transfers energy across the boundaries of systems with differing pressures (or force of any kind),always toward the lower pressure. If the total effect produced in the system can be reduced to the raising of a weight, then nothing but work has crossed the boundary. Work is positive when energy is removed from the system (see Figure 1).Mechanical or shaft work W is the energy delivered or absorbed by a mechanism, such as a turbine, air compressor, or internal combustion engine.Flow work is energy carried into or transmitted across the system boundary because a pumping process occurs somewhere outside the system, causing fluid to enter the system. It can bemore easily understood as the work done by the fluid just outside the system on the adjacent fluid entering the system to force or push it into the system. Flow work also occurs as fluid leaves thesystem.Flow work =pv (3)where p is the pressure and v is the specific volume, or the volume displaced per unit mass evaluated at the inlet or exit.A property of a system is any observable characteristic of the system. The state of a system is defined by specifying the minimum set of independent properties. The most common thermodynamic properties are temperature T, pressure p, and specific volume v or density . Additional thermodynamic properties include entropy, stored forms of energy, and enthalpy.Frequently, thermodynamic properties combine to form other properties. Enthalpy h is an important property that includes internal energy and flow work and is defined as (4)where u is the internal energy per unit mass.Each property in a given state has only one definite value, and any property always has the same value for a given state, regardless of how the substance arrived at that state.A process is a change in state that can be defined as any change in the properties of a system. A process is described by specifying the initial and final equilibrium states, the path (if identifiable), and the interactions that take place across system boundaries during theprocess.A cycle is a process or a series of processes wherein the initial and final states of the system are identical. Therefore, at the conclusion of a cycle, all the properties have the same value they had at the beginning. Refrigerant circulating in a closed system undergoes acycle.A pure substance has a homogeneous and invariable chemical composition. It can exist in more than one phase, but the chemical composition is the same in all phases.If a substance is liquid at the saturation temperature and pressure,it is called a saturated liquid. If the temperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid (the temperature is lower than the saturation temperature for the given pressure) or a compressed liquid (the pressure is greater than the saturation pressure for the given temperature).When a substance exists as part liquid and part vapor at the saturation temperature, its quality is defined as the ratio of the mass of vapor to the total mass. Quality has meaning only when the substance is saturated (i.e., at saturation pressure and temperature).Pressure and temperature of saturated substances are not independent properties.If a substance exists as a vapor at saturation temperature and pressure, it is called a saturated vapor. (Sometimes the term dry saturated vapor is used to emphasize that the quality is 100%.)When the vapor is at a temperature greater than the saturation temperature, it is a superheated vapor. Pressure and temperature of a superheated vapor are independent properties, because the temperature can increase while pressure remains constant. Gases such as air at room temperature and pressure are highly superheated vapors.FIRST LAW OF THERMODYNAMICSThe first law of thermodynamics is often called the law of conservation of energy. The following form of the first-law equation is valid only in the absence of a nuclear or chemical reaction.Based on the first law or the law of conservation of energy for any system, open or closed, there is an energy balance asNet amount of energy Net increase of stored=added to system energy in systemorEnergy in Energy out = Increase of stored energy in systemFigure 1 illustrates energy flows into and out of a thermodynamic system. For the general case of multiple mass flows with uniform properties in and out of the system, the energy balance can be written (5)where subscripts i and f refer to the initial and final states,respectively.Nearly all important engineering processes are commonly modeled as steady-flow processes. Steady flow signifies that all quantities associated with the system do not vary with time. Consequently, (6)where h = u + pv as described in Equation (4).A second common application is the closed stationary system for which the first law equation reduces to (7)SECOND LAW OF THERMODYNAMICSThe second law of thermodynamics differentiates and quantifies processes that only proceed in a certain direction (irreversible) from those that are reversible. The second law may be described in several ways. One method uses the concept of entropy flow in an open system and the irreversibility associated with the process. The concept of irreversibility provides added insight into the operation of cycles. For example, the larger the irreversibility in a refrigeration cycle operating with a given refrigeration load between two fixed temperature levels, the larger the amount of work required to operate the cycle. Irreversibilities include pressure drops in lines andheat 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 (8)wheredS = total change within system in time dt during process systemm s = entropy increase caused by mass entering (incoming)m s = entropy decrease caused by mass leaving (exiting)Q/T = entropy change caused by reversible heat transfer between system and surroundings at temperature TdI = entropy caused by irreversibilities (always positive)Equation (8) accounts for all entropy changes in the system. Rearranged, this equation becomes (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 (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): (11)Equation (6) can be used to replace the heat transfer quantity.Note 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 commonly applied to a system with one mass flow in, the same mass flow out, no work, and negligible kinetic or potential energy flows. Combining Equations (6) and (11) yields (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, becomes (13)THERMODYNAMIC ANALYSIS OFREFRIGERATION CYCLESRefrigeration cycles transfer thermal energy from a region of low temperature T to one of higher temperature. Usually the higher-TR 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 components to determine mass and energy balances and the irreversibility of the components. 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 effectCOPUseful refrigeration effect/Net energy supplied from external sources (14)Net energy supplied from external sources For a mechanical vapor compression system, the net energy supplied is usually in the form of work, mechanical or electrical, and may include work to the compressor and fans or pumps. Thus, (15)In an absorption refrigeration cycle, the net energy supplied is usually in the form of heat into the generator and work into the pumps and fans, or (16)In many cases, work supplied to an absorption system is very small compared 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 completely 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: (17)The Carnot cycle usually serves as the ideal reversible refrigeration cycle. For multistage cycles, each stage is described by a reversible cycle.工程热力学和制冷循环工程热力学是研究能量及其转换和能量与物质状态之间的关系。这个章节讲述了工程热力学在制冷循环中的应用。第一部分回顾了热力学第一定律、第二定律以及计算热力学参数的方法。第二部分和第三部分讲述了压缩和吸收式两种制冷循环，两种最寻常的能量转换形式。工程热力学热力学系统是被一个封闭曲面包围的一个空间区域或者一定量的物质。对于这个系统而言，周围的环境都是外界物质。也就是说，这个系统的界面把系统与环境分开。边界是可移动的也可以是固定的，可以是真实的也可以是假定的。熵是系统分子无序性的量度。系统越复杂，熵就越大；一个有序简单系统的熵就会很小。能量可以产生作用，并且可以分为储存形式和短暂形式两种。 1、 储存能热能(内能)是分子的运动或者分子间的相互作用产生的。势能是由分子间的吸引或者是系统位置被提升而产生的。 (1)式中：m质量；g重力加速度；z距水平基准面的高度动能的产生是由于分子具有速度。其表达式如下： (2)式中：V流体流过边界面的速度化学能是由组成分子的原子的排列产生的。原子能是起源于把质子与中子聚在一起组成原子的那种聚合力 2、不稳定能 热量Q的工作原理是用不同的温度把能量传出系统的边界，通常是高温传到低温。当热量被加入到系统中时，热量的符号为正(可看图1)。机械功或者轴功是由机械装置传出或者传入的能量。例如：这些装置有汽轮机、空气压缩机、内燃机。流动功是由在系统外部产生的流动流经过系统界面而带入的能量，从而把流体带入这个系统。也可以这样理解，系统的外部空间有两股相邻的流体，后面的一股推动前面的一股流进系统，这种作用的来源就是流动功。当流体流出系统时，流动功同样产生。流动功(每单位)=pv (3)式中：p代表压力，v代表比容，即：物质流在流进或流出的每单位质量的体积。一个系统的参数是该系统非常明显的特征，系统的状态由指定的独立的参数来定义。最常用的热力学参数是温度T、压力P、比容v和密度。其他的热力学参数包括熵、内能和焓。 一般情况下，最基本的热力学参数组合到一起组成其它的参数。焓h是一个重要的参数，它包括内能和流动功。其定义如下： (4)其中：u是单位质量的内能。每一个给定状态的参数有唯一的确定的值，并且不论物质处于什么样的状态，任何一个参数只要处于给定的状态下，就会有同样的值。系统中任何一个参数变化了，就可以确定整个系统发生了变化。一个过程可以由系统的初状态和处于平衡态的末状态来描述。这个过程中路径和相互作用超出了系统的边界。一个循环是经过一个过程或几个过程，系统的初状态与末状态是相同的。因此，由循环可以得到一个结论，所有的参数值与初状态相同。一个闭式的制冷过程就是一个循环。一种纯净的物质含有均一的、不变的化学组成成分。这种物质可以处在多个相态，但是在所有的相态中它的化学成分不变。如果一种物质在其饱和压力和饱和温度下是液态，这时液体被称为饱和液体。如果液体的温度在给定的压力下低于其饱和温度，被称为过冷液体，如果液体的压力在给定的温度下高于其饱和压力，被称为压缩液体。当一种物质在其饱和温度下，一部分是液体一部分是气体，规定饱和干度为气体的质量与总质量之比。干度只有在饱和状态(饱和温度与饱和压力)下才有意义。饱和物质的压力和温度不是相互独立的参数。如果物质在饱和温度与压力下是处于液态，那么它被称为饱和蒸气(有时候干饱和蒸气的说法是为了强调干度是100%)。当蒸气的温度高于它的饱和温度时，此时的蒸气被称为过饱和蒸气。过饱和蒸气的压力和温度是相互独立的参数，因为当压力保持稳定时，温度可以上升。在室内的温度和压力下，气体一般都是过饱和蒸气。热力学第一定律热力学第一定律常常又被称为能量守恒定律。热力学守恒定律的以下公式仅在没有原子变化和化学反应时成立。进入系统的净能量=系统储存能的净增量或者进入的能量流出的能量=系统储存能的增量图1表明一个热力学系统能量的流进与流出。在一般的情况下，对于多种物质以不同的参数流进与