空調專業(yè)畢業(yè)設計外文翻譯--工程熱力學和制冷循環(huán)

1、<p><b>　　附錄B 英文翻譯</b></p><p>　　THERMODYNAMICS AND REFRIGERATION CYCLES</p><p>　　THERMODYNAMICS is the study of energy, its transformations, and its relation to states of matter.

2、 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

3、 and third parts address compression and absorption refrigeration cycles, two common methods of thermal energy transfer.</p><p>　　THERMODYNAMICS</p><p>　　A thermodynamic system is a region in sp

4、ace or a quantity of matter bounded by a closed surface. The surroundings include everything external to the system, and the system is separated from</p><p>　　the surroundings by the system boundaries. These

5、 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;

6、 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.</p><p>　　Stored Energy</p><

7、;p>　　Thermal (internal) energy is caused by the motion of molecules and/or intermolecular forces.</p><p>　　Potential energy (PE) is caused by attractive forces existing between molecules, or the elevation

8、 of the system.</p><p><b>　　(1)</b></p><p><b>　　where</b></p><p><b>　　m =mass</b></p><p>　　g = local acceleration of gravity</p&g

9、t;<p>　　z = elevation above horizontal reference plane</p><p>　　Kinetic energy (KE) is the energy caused by the velocity of molecules and is expressed as</p><p><b>　　(2)</b>&l

10、t;/p><p><b>　　where </b></p><p>　　V is the velocity of a fluid stream crossing the system boundary.</p><p>　　Chemical energy is caused by the arrangement of atoms composing

11、 the molecules.</p><p>　　Nuclear (atomic) energy derives from the cohesive forces holding protons and neutrons together as the atom’s nucleus.</p><p>　　Energy in Transition</p><p>　

12、　Heat 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).</p&

13、gt;<p>　　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 ca

14、n 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).</p><p>　　Mechanical or shaft work W is the ene

15、rgy delivered or absorbed by a mechanism, such as a turbine, air compressor, or internal combustion engine.</p><p>　　Flow work is energy carried into or transmitted across the system boundary because a pumpi

16、ng process occurs somewhere outside the system, causing fluid to enter the system. It can be</p><p>　　more easily understood as the work done by the fluid just outside the system on the adjacent fluid enteri

17、ng the system to force or push it into the system. Flow work also occurs as fluid leaves the</p><p><b>　　system.</b></p><p>　　Flow work =pv

18、 (3)</p><p>　　where p is the pressure and v is the specific volume, or the volume displaced per unit mass evaluated at the inlet or exit.</p><p>　　A property of a system is any obser

19、vable 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

20、or density ρ. Additional thermodynamic properties include entropy, stored forms of energy, and enthalpy.</p><p>　　Frequently, thermodynamic properties combine to form other properties. Enthalpy h is an impor

21、tant property that includes internal energy and flow work and is defined as</p><p><b>　　(4)</b></p><p>　　where u is the internal energy per unit mass.</p><p>　　Each prop

22、erty 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.</p><p>　　A process is a change in state

23、 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 sys

24、tem boundaries during the</p><p><b>　　process.</b></p><p>　　A cycle is a process or a series of processes wherein the initial and final states of the system are identical. Therefore,

25、 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 a</p><p><b>　　cycle.</b></p><p>

26、　　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.</p><p>　　If a substance is liquid at the

27、 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 tempe

28、rature 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).</p><p>　　When a substance exis

29、ts 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 pr

30、essure and temperature).Pressure and temperature of saturated substances are not independent properties.</p><p>　　If a substance exists as a vapor at saturation temperature and pressure, it is called a satur

31、ated vapor. (Sometimes the term dry saturated vapor is used to emphasize that the quality is 100%.)</p><p>　　When the vapor is at a temperature greater than the saturation temperature, it is a superheated va

32、por. 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 superhe

33、ated vapors.</p><p>　　FIRST LAW OF THERMODYNAMICS</p><p>　　The first law of thermodynamics is often called the law of conservation of energy. The following form of the first-law equation is vali

34、d only in the absence of a nuclear or chemical reaction.</p><p>　　Based on the first law or the law of conservation of energy for any system, open or closed, there is an energy balance as</p><p>

35、;　　Net amount of energy Net increase of stored</p><p><b>　　=</b></p><p>　　added to system energy in system</p><p><b>　　or</b>&

36、lt;/p><p>　　[Energy in] – [Energy out] = [Increase of stored energy in system]</p><p>　　Figure 1 illustrates energy flows into and out of a thermodynamic system. For the general case of multiple ma

37、ss flows with uniform properties in and out of the system, the energy balance can be written</p><p><b>　　(5)</b></p><p>　　where subscripts i and f refer to the initial and final stat

38、es,respectively.</p><p>　　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.

39、 Consequently,</p><p><b>　　(6)</b></p><p>　　where h = u + pv as described in Equation (4).</p><p>　　A second common application is the closed stationary system for which

40、 the first law equation reduces to</p><p><b>　　(7)</b></p><p>　　SECOND LAW OF THERMODYNAMICS</p><p>　　The second law of thermodynamics differentiates and quantifies proc

41、esses 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 irrever

42、sibility 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</p&

43、gt;<p>　　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,

44、 a cycle attains its maximum ideal efficiency. In an open system, the second law of thermodynamics can be described in terms of entropy as</p><p><b>　　(8)</b></p><p><b>　　where

45、</b></p><p>　　dS = total change within system in time dt during process system</p><p>　　δm s = entropy increase caused by mass entering (incoming)</p><p>　　δm s = entropy decr

46、ease caused by mass leaving (exiting)</p><p>　　δQ/T = entropy change caused by reversible heat transfer between system and surroundings at temperature T</p><p>　　dI = entropy caused by irreversi

47、bilities (always positive)</p><p>　　Equation (8) accounts for all entropy changes in the system. Rearranged, this equation becomes</p><p><b>　　(9)</b></p><p>　　In integr

48、ated 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</p><p><b>　　(10)</b></p><

49、p>　　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 product

50、ion caused by irreversibilities in the process, can be determined by rearranging Equation (10):</p><p><b>　　(11)</b></p><p>　　Equation (6) can be used to replace the heat transfer qu

51、antity.Note that the absolute temperature of the surroundings with which the system is exchanging heat is used in the last term. If the temper-</p><p>　　ature of the surroundings is equal to the system tempe

52、rature, heat istransferred reversibly and the last term in Equation (11) equals zero. </p><p>　　Equation (11) is commonly applied to a system with one mass flow in, the same mass flow out, no work, and negli

53、gible kinetic or potential energy flows. Combining Equations (6) and (11) yields</p><p><b>　　(12)</b></p><p>　　In a cycle, the reduction of work produced by a power cycle (or the inc

54、rease 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 an

55、y refrigeration cycle, theoretical or real, operating under the same conditions, becomes</p><p><b>　　(13)</b></p><p>　　THERMODYNAMIC ANALYSIS OF</p><p>　　REFRIGERATION C

56、YCLES</p><p>　　Refrigeration 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,

57、at temperature T0, the temperature of the surroundings.</p><p>　　The first and second laws of thermodynamics can be applied to individual components to determine mass and energy balances and the irreversibil

58、ity of the components. This procedure is illustrated in later sections in this chapter.</p><p>　　Performance of a refrigeration cycle is usually described by a coefficient of performance (COP), defined as th

59、e benefit of the cycle (amount of heat removed) divided by the required energy input to operate the cycle:</p><p>　　Useful refrigerating effect</p><p>　　COP≡Useful refrigeration effect/Net energ

60、y supplied from external sources (14)</p><p>　　Net energy supplied from external sources For a mechanical vapor compression system, the net energy supplied is usually in the form of work, mechanical

61、or electrical, and may include work to the compressor and fans or pumps. Thus,</p><p><b>　　(15)</b></p><p>　　In an absorption refrigeration cycle, the net energy supplied is usually

62、 in the form of heat into the generator and work into the pumps and fans, or</p><p><b>　　(16)</b></p><p>　　In many cases, work supplied to an absorption system is very small compared

63、 to the amount of heat supplied to the generator, so the work term is often neglected.</p><p>　　Applying the second law to an entire refrigeration cycle shows that a completely reversible cycle operating und

64、er the same conditions has the maximum possible COP. Departure of the actual cycle from an ideal reversible cycle is given by the refrigerating efficiency:</p><p><b>　　(17)</b></p><p&g

65、t;　　The Carnot cycle usually serves as the ideal reversible refrigeration cycle. For multistage cycles, each stage is described by a reversible cycle.</p><p>　　工程熱力學和制冷循環(huán)</p><p>　　工程熱力學是研究能量及其轉換

66、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。</p><p><b>　　工程熱力學</b></p><p>　　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)

67、境都是外界物質。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(chǎn)生作用，并且可以分為儲存形式和短暫形式兩種。</p><p><b>　　1、 儲存能</b></p><p>　　熱能(內能)是分子的運動或者分子間的相互作用產(chǎn)生的

68、。</p><p>　　勢能是由分子間的吸引或者是系統(tǒng)位置被提升而產(chǎn)生的。</p><p><b>　　(1)</b></p><p>　　式中：m——質量；g——重力加速度；z——距水平基準面的高度</p><p>　　動能的產(chǎn)生是由于分子具有速度。其表達式如下：</p><p><b&g

69、t;　　(2)</b></p><p>　　式中：V——流體流過邊界面的速度</p><p>　　化學能是由組成分子的原子的排列產(chǎn)生的。</p><p>　　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合力 </p><p><b>　　2、不穩(wěn)定能</b></p><p>　　

70、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。</p><p>　　流動功是由在系統(tǒng)外部產(chǎn)生的流動流經(jīng)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

71、是流動功。當流體流出系統(tǒng)時，流動功同樣產(chǎn)生。</p><p>　　流動功(每單位)=pv (3)</p><p>　　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。</p><p>　　一個系統(tǒng)的參數(shù)是該系統(tǒng)非常明顯的特征，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、

72、壓力P、比容v和密度ρ。其他的熱力學參數(shù)包括熵、內能和焓。</p><p>　　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：</p><p><b>　　(4)</b></p><p>　　其中：u是單位質量的內能。</p><p>　　每一個給定狀態(tài)的參

73、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。</p><p>　　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。</p><p>　　一個循環(huán)是經(jīng)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

74、有的參數(shù)值與初狀態(tài)相同。一個閉式的制冷過程就是一個循環(huán)。</p><p>　　一種純凈的物質含有均一的、不變的化學組成成分。這種物質可以處在多個相態(tài)，但是在所有的相態(tài)中它的化學成分不變。</p><p>　　如果一種物質在其飽和壓力和飽和溫度下是液態(tài)，這時液體被稱為飽和液體。如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

75、體。</p><p>　　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)。</p><p>　　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%)。</p><p>　　當蒸氣

76、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。</p><p><b>　　熱力學第一定律</b></p><p>　　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。</p>

77、<p>　　進入系統(tǒng)的凈能量=系統(tǒng)儲存能的凈增量</p><p><b>　　或者</b></p><p>　　進入的能量—流出的能量=系統(tǒng)儲存能的增量</p><p>　　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，能量的平衡公式可以寫為：</p><p>&

78、lt;b>　　(5)</b></p><p>　　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)。</p><p>　　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：</p><p><b>　　(6)</b></p><p>　　式中：h = u +

79、pv的含義與公式(4)代表的含義相同。</p><p>　　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可以寫為：</p><p><b>　　(7)</b></p><p><b>　　熱力學第二定律</b></p><p>　　熱力學第二定律做出了與可逆過程的區(qū)別和量化

80、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(chǎn)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)

81、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：</p><p><b>　　(8)</b></p><p>　　式中：在這個系統(tǒng)的熱力學過程中dt時間內總的交換量。</p><p>　　由質量的流進引起的熵增。</p><p>　　由質量的流出引起的熵減。</p><p>　　在一

82、定的溫度下由系統(tǒng)與環(huán)境的熱交換產(chǎn)生的可逆引起的熵的變化。</p><p>　　由于不可逆引起的熵(總是正的)</p><p>　　公式(8)說明了在系統(tǒng)中所有的熵變。重新整理以下，這個公式可以寫為：</p><p><b>　　(9)</b></p><p>　　整體上，如果系統(tǒng)中流進與流出的參數(shù)，質量流量和同環(huán)境的交換

83、量不隨時間而變化。熱力學第二定律的一般公式可以寫為：</p><p><b>　　(10)</b></p><p>　　在很多應用中，這個過程被認為是一個穩(wěn)態(tài)過程。所以，系統(tǒng)的熵增為零。不可逆的比率指的是由不可逆性產(chǎn)生的熵占總熵的比率。這個比率可以由公式(10)計算出來。</p><p><b>　　(11)</b><

84、;/p><p>　　公式(6)可以用來代替熱交換量。注意的一點是：環(huán)境與系統(tǒng)的絕對溫度是熱交換要用到的最后一個條件。如果環(huán)境的溫度與系統(tǒng)的溫度是相同的，那么熱傳遞是可逆的，并且在公式(11)的最后一個條件為零。</p><p>　　公式(11)通常用于同質量流進同質量流出的系統(tǒng)中，沒有功，可以忽略動能、勢能。把公式(6)和公式(11)聯(lián)立可以得到</p><p><

85、;b>　　(12)</b></p><p>　　在一個循環(huán)中，一個能量循環(huán)產(chǎn)生功的降低，(或者一個制冷循環(huán)所需要的功的增加)等于周圍環(huán)境的絕對溫度乘以在循環(huán)中各個不可逆的總量。因此，在同等的條件下，任何制冷循環(huán)中不管是理論還是實際中，可逆過程的功與實際的功會變?yōu)椋?lt;/p><p><b>　　(13)</b></p><p>

86、　　制冷循環(huán)的熱力學分析</p><p>　　制冷循環(huán)是把熱能從一個低溫的區(qū)域傳遞到另一個高溫的區(qū)域。通常較高溫度TR是周圍環(huán)境中的空氣或者冷卻水的溫度，T0是環(huán)境的溫度。</p><p>　　熱力學第一和第二定律可以應用到單個成分中去決定質量和能量的平衡，同時也可以來分析其的不可逆能力。這個過程在以下的幾個章節(jié)中會講到。</p><p>　　制冷循環(huán)的性能通常用性

87、能參數(shù)來衡量，性能參數(shù)定義為循環(huán)可以移走的熱量除以系統(tǒng)</p><p>　　輸入的所必需的能量。</p><p>　　COP≡ (14)</p><p>　　對于蒸氣壓縮系統(tǒng)來說，提供的凈能量通常是以功、機械能或電能的形式出現(xiàn)，并且還包括壓縮機、風機、水泵所需要的能量。因此：</p><p><b&

88、gt;　　(15)</b></p><p>　　在吸收式制冷循環(huán)中，提供的凈能量是以熱量的形式傳送到發(fā)生器和功傳送到泵與風機中去。所以：</p><p><b>　　(16)</b></p><p>　　在很多情況下，給吸收式系統(tǒng)提供的功相對于給發(fā)生器提供的熱量是非常小的。所以功常?？梢员缓雎浴?lt;/p><p&g