外文翻譯 工程熱力學和制冷循環(huán)

1、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

2、methods for calculating thermody namic prop erties. The sec ond and third parts address comp ressi on and absor pti on refrigerati on cycles, two com mon methods of thermal en ergy tran sfer.THERMODYNAMICSA thermody namic system is a regi on in space or a qua ntity of matter boun ded by a closed sur

3、face. The surro undings in clude everyth ing exter nal to the system, and the system is sep arated 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

4、disorder of a system. The more mixed a system, the greater its entropy; an orderly or unm ixed configuration is one of low entropy. Energy has the capacity for producing an effect and can be categorized into either stored or tran sie nt forms.Stored En ergyThermal (i ntern al) en ergy is caused by t

5、he moti on of molecules an d/or in termolecular forces.Pote ntial en ergy (PE) is caused by attractive forces existi ng betwee n molecules, or the elevatio n of the system.P E =mgzwherem =massg = local accelerati on of gravityz = elevati on above horiz on tal reference planeKin etic en ergy (KE) is

6、the en ergy caused by the velocity of molecules and is exp ressed as卡whereV is the velocity of a fluid stream cross ing the system boun dary.Chemical en ergy is caused by the arran geme nt of atoms composing the molecules.Nuclear (atomic) en ergy derives from the cohesive forces holdi ng protons and

7、 n eutr ons together as the atom s nucleus.En ergy in Tran siti onHeat Q is the mechanism that transfers energy across the boundaries of systems with differing temp eratures, always toward the lower temp erature. Heat is po sitive whe n en ergy is added to the system (see Figure 1).Work is the mecha

8、nism 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 rais ing of a weight, the n nothing but work has crossed the boun dary. Work is po sitive whe

9、 n en ergy is removed from the system (see Figure 1).Mecha ni cal or shaft work W is the en ergy delivered or absorbed by a mecha ni sm, such as a turbi ne, air comp ressor, or internal combusti on engine.Flow work is energy carried into or transmitted across the system boundary because a pumping pr

10、o cess occurs somewhere outside the system, caus ing fluid to en ter the system. It can be more easily un derstood as the work done by the fluid just outside the system on the adjace nt fluid en teri ng the system to force or push it into the system. Flow work also occurs as fluid leaves the system.

11、Flow work = pv(3)where p is the p ressure and v is the sp ecific volume, or the volume dis placed per un it 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 defi ned by sp ecify ing the minimum set of independent prop

12、 erties. The most com mon thermody namic properties are temperature T, pressure p, and specific volume v or densityp . Additional thermodynamicprop erties in elude entropy, stored forms of en ergy, and en tha Ipy.Frequently, thermodynamic properties combine to form other properties. EnthaIpy h is an

13、 important property that in eludes internal en ergy and flow work and is defi ned ash 三 u + pvwhere u is the internal en ergy per un it mass.Each property in a give n state has only one defi nite value, and any property always has the same value for a give n state, regardless of how the substa nee a

14、rrived at that state.A pro cess is a cha nge in state that can be defi ned as any cha nge in the prop erties of a system. A pro cess is described by sp ecify ing the in itial and final equilibrium states, the p ath (if iden tifiable), and the in teract ions that take p lace across system boun daries

15、 duri ng the pro cess.A cycle is a p rocess or a series of pro cesses where in the in itial and final states of the system are iden tical. Therefore, at the con clusi on of a cycle, all the prop erties have the same value they had at the begi nning. Refrigera nt circulati ng in a closed system un de

16、rgoes a cycle.A pure substa nee has a homoge neous and in variable chemical comp ositi on. It can exist in more than one p hase, but the chemical compo siti on is the same in all p hases.If a substa nee is liquid at the saturati on temp erature and p ressure,it is called a saturated liquid. If the t

17、emperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid (the temp erature is lower tha n the saturati on temp erature for the give n pressure) or a compressed liquid (the pressure is greater than the saturation pressure for

18、the given temp erature).When a substa nee exists as part liquid and part vapor at the saturati on temp erature, its quality is defi ned as the ratio of the mass of vapor to the total mass. Quality has meaning only whe n the substa nee is saturated (i.e., at saturation pressure and temperature).Press

19、ure and temperature of saturatedsubsta nces are not independent prop erties.If a substa nee exists as a vapor at saturatio n temp erature and p ressure, it is called a saturated vapor. (Sometimes the term dry saturated vapor is used to emp hasize that the quality is 100%.)When the vapor is at a temp

20、erature greater than the saturation temperature, it is a superheated vapor. Pressure and temperature of a superheated vapor are independent properties, because the temp erature can in crease while p ressure rema ins con sta nt. Gases such as air at room temp erature and p ressure are highly sup erhe

21、ated vapors.FIRST LAW OF THERMODYNAMICSThe first law of thermod yn amics is often called the law of con servati on of en ergy. The follow ing form of the first-law equati on is valid only in the abse nee of a nu clear or chemical react ion.Net in crease of storedBased on the first law or the law of

22、con servati on of en ergy for any system, open or closed, there is an en ergy bala nee asNet amount of en ergyadded to systemoren ergy in systemEnergy in -Energy out = Increase of stored energy in systemFigure 1 illustrates en ergy flows into and out of a thermod yn amic system. For the gen eral cas

23、e of mult iple mass flows with uniform prop erties in and out of the system, the en ergy bala nee can be writte n-v2V22- min(u + pv + gz)in -Z mout(u + pv + gz)out +Q-W =2 2rV2V2mf(u+ pv+寧z)f-mi(u + pv+ 亍zh 爲where subscri pts i and f refer to the in itial and final states,res pectively.Nearly all im

24、portant engineering processes are commonly modeled as steady-flow processes.Steady flow sig ni fies that all qua ntities associated with the system do not vary with time. Con seque ntly.2 2Z m(h +J+ gz)-2 m(h +乞+ gz)+ Q-W = 0all2all2streamstreamen teri ngleavi ngwhere h = u + pv as described in Equa

25、ti on (4).A sec ond com mon app licati on is the closed stati onary system for which the first law equati on reduces toQ-W = m(uf -uZsystemSECOND LAW OF THERMODYNAMICSThe sec ond law of thermody namics differe ntiates and qua ntifies pro cesses that only pro ceed in acerta in direct ion (irreversibl

26、e) from those that are reversible. The sec ond 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 pro cess. The concept of irreversibility p rovides added in sight into the op erati on of cycles. For exa mpl

27、e, the larger the irreversibility in a refrigerati on cycle op erat ing with a give n refrigeration load between two fixed temperature levels, the larger the amount of work required to op erate the cycle. Irreversibilities in clude p ressure drops in lines andheat exchangers, heat transfer between f

28、luids of different temperature, and mechanical friction. Reduci ng total irreversibility in a cycle imp roves cycle p erforma nee. In the limit of no irreversibilities, a cycle atta ins its maximum ideal efficie ncy. In an open system, the sec ond law of thermody namics can be described in terms of

29、entropy as(8)dSsystern =單 +6mSi6meSe +dlwheredS = total cha nge with in system in time dt duri ng pro cess system5 m s = entropy in crease caused by mass en teri ng (incoming)5 m s = entropy decrease caused by mass leaving (exiting)5 Q/T = entropy change caused by reversible heat transfer between sy

30、stem and surroundings at temp erature TdI = entropy caused by irreversibilities (always po sitive)(9)Equatio n (8) acco unts for all entropy cha nges in the system. Rearra nged, this equati on becomes =T (eSe -亦is)+dSsys-dlIn in tegrated form, if inlet and outlet prop erties, mass flow, and in terac

31、t ions with the surro undings do not vary with time, the gen eral equati on for the sec ond law is(10)(Sf -Si)system =網/T 吃(ms), -Z (ms)out +1revIn many app licati ons, the p rocess can be con sidered to op erate steadily with no cha nge in time. The cha nge in entropy of the system is therefore zer

32、o. The irreversibility rate, which is the rate of entropy p roducti on caused by irreversibilities in the pro cess, can be determ ined by rearra nging Equatio n (10):(11)1=2 (ms)out -送(msn S 丁T surrEquati on (6) can be used to rep lace the heat tran sfer qua ntity.Note that the absolute temp erature

33、 of the surro undings with which the system is excha nging heat is used in the last term. If the temperature of the surroundings is equal to the system temp erature, heat istra nsferred reversibly and the last term in Equati on (11) equals zero.Equatio n (11) is com monly app lied to a system with o

34、ne mass flow in, the same mass flow out, no work, and n egligible kin etic or poten tial en ergy flows. Comb ining Equati ons (6) and (11) yieldsI =m(sout -sQ -hout hinTsurr(12)In a cycle, the reduct ion of work p roduced by a po wer cycle (or the in crease in work required by a refrigeratio n cycle

35、) equals the absolute ambie nt temp erature multi plied by the sum of irreversibilities in all pro cesses in the cycle. Thus, the differe nee in reversible and actual work for any refrigerati on cycle, theoretical or real, op erati ng un der the same con diti ons, becomes(13)actual Wreversible +% IT

36、HERMODYNAMIC ANAL YSIS OFto one of higherREFRIGERATION CYCLESRefrigeratio n cycles tran sfer thermal en ergy from a regi on of low temp erature Ttemperature. Usually the higher-T r temperature heat sink is the ambient air or cooling water, at temp erature To, the temp erature of the surroundin gs.Th

37、e first and sec ond laws of thermody namics can be app lied to in dividual components to determine mass and energy balances and the irreversibility of the components. This procedure is illustrated in later secti ons in this cha pter.Performa nee of a refrigerati on cycle is usually described by a co

38、efficie nt of p erforma nee (COP), defi ned as the ben efit of the cycle (am ount of heat removed) divided by the required en ergy input to op erate the cycle:Useful refrigerati ng effectCOP 三 Useful refrigerati on effect/Net en ergy supp lied from exter nal sources(14)Net energy supplied from exter

39、nal sources For a mechanical vapor compression system, the net en ergy supp lied is usually in the form of work, mecha ni cal or electrical, and may in clude work to the comp ressor and fans or pumps. Thus,(15)In an absor pti on refrigerati on cycle, the net en ergy supp lied is usually in the form

40、of heat i nto the gen erator and work into the pumps and fans, orQevapCOP =Qgen +Wnet(16)13In many cases, work supp lied to an absor pti on system is very small comp ared to the amount of heat supp lied to the gen erator, so the work term is ofte n n eglected.Applying the sec ond law to an en tire r

41、efrigeratio n cycle shows that a comp letely reversible cycle op erati ng un der the same con diti ons has the maximum p ossible COP. Dep arture of the actual cycle from an ideal reversible cycle is give n by the refrigerat ing efficie ncy:n n COPR(17)(CO P)tevThe Carnot cycle usually serves as the

42、ideal reversible refrigerati on cycle. For multistage cycles, each stage is described by a reversible cycle.工程熱力學和制冷循環(huán)工程熱力學是研究能量及其轉換和能量與物質狀態(tài)之間的關系。 這個章節(jié)講述了 工程熱力學在制冷循環(huán)中的應用。第一部分回顧了熱力學第一定律、第二定律以及計 算熱力學參數(shù)的方法。第二部分和第三部分講述了壓縮和吸收式兩種制冷循環(huán)， 兩種 最尋常的能量轉換形式。工程熱力學熱力學系統(tǒng)是被一個封閉曲面包圍的一個空間區(qū)域或者一定量的物質。對于這個系統(tǒng)而言，周圍的環(huán)境都是外界物質。

43、也就是說，這個系統(tǒng)的界面把系統(tǒng)與環(huán)境分開。 邊界是可移動的也可以是固定的， 可以是真實的也可以是假定的。熵是系統(tǒng)分子無序 性的量度。系統(tǒng)越復雜，熵就越大；一個有序簡單系統(tǒng)的熵就會很小。能量可以產生 作用，并且可以分為儲存形式和短暫形式兩種。1、儲存能熱能（內能）是分子的運動或者分子間的相互作用產生的。勢能是由分子間的吸引或者是系統(tǒng)位置被提升而產生的。P E = m g z式中：m質量；g重力加速度；z距水平基準面的高度 動能的產生是由于分子具有速度。其表達式如下：mV2KE =2式中：V流體流過邊界面的速度化學能是由組成分子的原子的排列產生的。原子能是起源于把質子與中子聚在一起組成原子的那種聚

44、合力2、不穩(wěn)定能熱量Q的工作原理是用不同的溫度把能量傳出系統(tǒng)的邊界，通常是高溫傳到低 溫。當熱量被加入到系統(tǒng)中時，熱量的符號為正 （可看圖1）。機械功或者軸功是由機 械裝置傳出或者傳入的能量。例如：這些裝置有汽輪機、空氣壓縮機、內燃機。流動功是由在系統(tǒng)外部產生的流動流經過系統(tǒng)界面而帶入的能量， 從而把流體帶 入這個系統(tǒng)。也可以這樣理解，系統(tǒng)的外部空間有兩股相鄰的流體， 后面的一股推動 前面的一股流進系統(tǒng)，這種作用的來源就是流動功。當流體流出系統(tǒng)時，流動功同樣 產生。流動功（每單位）=pv式中：P代表壓力，V代表比容，即：物質流在流進或流出的每單位質量的體積。 一個系統(tǒng)的參數(shù)是該系統(tǒng)非常明顯的特

45、征，系統(tǒng)的狀態(tài)由指定的獨立的參數(shù)來定 義。最常用的熱力學參數(shù)是溫度 T、壓力P、比容V和密度P其他的熱力學參數(shù)包 括熵、內能和焓。一般情況下，最基本的熱力學參數(shù)組合到一起組成其它的參數(shù)。 焓h是一個重 要的參數(shù)，它包括內能和流動功。其定義如下：h 三 U + pv其中：u是單位質量的內能。每一個給定狀態(tài)的參數(shù)有唯一的確定的值，并且不論物質處于什么樣的狀態(tài)，任何一個參數(shù)只要處于給定的狀態(tài)下，就會有同樣的值。系統(tǒng)中任何一個參數(shù)變化了，就可以確定整個系統(tǒng)發(fā)生了變化。 一個過程可以由 系統(tǒng)的初狀態(tài)和處于平衡態(tài)的末狀態(tài)來描述。這個過程中路徑和相互作用超出了系統(tǒng) 的邊界。一個閉式的制冷過程就是一個循環(huán)是經

46、過一個過程或幾個過程，系統(tǒng)的初狀態(tài)與末狀態(tài)是相同的。因此， 由循環(huán)可以得到一個結論，所有的參數(shù)值與初狀態(tài)相同。個循環(huán)。這種物質可以處在多個相態(tài),一種純凈的物質含有均一的、不變的化學組成成分。 但是在所有的相態(tài)中它的化學成分不變。這時液體被稱為飽和液體。如如果一種物質在其飽和壓力和飽和溫度下是液態(tài)，果液體的溫度在給定的壓力下低于其飽和溫度，被稱為過冷液體，如果液體的壓力在給定的溫度下高于其飽和壓力，被稱為壓縮液體。當一種物質在其飽和溫度下，一部分是液體一部分是氣體，規(guī)定飽和干度為氣體 的質量與總質量之比。干度只有在飽和狀態(tài)（飽和溫度與飽和壓力）下才有意義。飽和 物質的壓力和溫度不是相互獨立的參數(shù)

47、。如果物質在飽和溫度與壓力下是處于液態(tài)，那么它被稱為飽和蒸氣（有時候干飽 和蒸氣的說法是為了強調干度是 100%）。當蒸氣的溫度高于它的飽和溫度時，此時的蒸氣被稱為過飽和蒸氣。過飽和蒸氣 的壓力和溫度是相互獨立的參數(shù)， 因為當壓力保持穩(wěn)定時，溫度可以上升。在室內的 溫度和壓力下，氣體一般都是過飽和蒸氣。熱力學第一定律熱力學第一定律常常又被稱為能量守恒定律。熱力學守恒定律的以下公式僅在沒 有原子變化和化學反應時成立。進入系統(tǒng)的凈能量=系統(tǒng)儲存能的凈增量或者進入的能量一流出的能量=系統(tǒng)儲存能的增量圖1表明一個熱力學系統(tǒng)能量的流進與流出。 在一般的情況下，對于多種物質以 不同的參數(shù)流進與流出，能量的

48、平衡公式可以寫為：PVPV2I：min(u+ p2 +血in 一送皿如 Pv+2 +血0ut+Q 一吩mf(u + pv 斗 gz)fmi(u + pv 耳 gz)i 爲2 2 y式中：下腳標i和f分別指的是系統(tǒng)的處狀態(tài)和末狀態(tài)。幾乎所有的熱力學過程都是以穩(wěn)流為模型的。穩(wěn)流指的是與系統(tǒng)有關的流體量 不隨著時間而變化。因此：2 2V2V2Z m(h+gz)-2 m(h + gz) +Q-W=Oall2all2streamstreamen teri ngleavi ng式中：h = u + pv的含義與公式(4)代表的含義相同。熱力學第一定律的另一種應用是用于閉式的固定系統(tǒng)。熱力學第一定律的表達式

49、 可以寫為：Q -W = m(Uf -uj、f I / system熱力學第二定律熱力學第二定律做出了與可逆過程的區(qū)別和量化了只在不可逆中發(fā)生的過程。熱力學第二定律可以有多種敘述方法。一種方法可以用在開式系統(tǒng)里熵流的概念和過程 的不可逆性來描述。不可逆性的概念為系統(tǒng)循環(huán)的運作提供了更深入的研究。例如， 在給定的兩個溫度之間，有給定的制冷負荷，這個制冷循環(huán)的不可逆性越大，它的運 行就需要更大功。不可逆產生的原因包括壓力的線性下降，在熱交換過程中熱交換器 的熱量損失，以及各種不可避免的機械摩擦。循環(huán)系統(tǒng)中減低總的不可逆性可以提高 系統(tǒng)的循環(huán)特性。在沒有不可逆性時，這個系統(tǒng)達到最大理想效率。在一個開式系統(tǒng) 里，熱力學第二定律用熵表達為：(8)dSsystem 異+和i S -SmeSfe + dl式中：dssysten在這個系統(tǒng)的熱力學過程中dt時間內總的交換量。5m Si由質量的流進引起的熵增。 和eSe由質量的流出引起的熵減。g/T在一定的溫度下由系統(tǒng)與環(huán)境的熱交換產生的可逆引起的熵的變化。dI由于不可逆引起的熵(總是正的)公式(8)說明了在系統(tǒng)中所有的熵