【正文】 e 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 coefficients B’ , C’ , D’ , etc., and B, C, D, etc., are the virial coefficients. B’ and B are second virial coefficients。176。這個(gè)章節(jié)講述了工程熱力學(xué)在制冷循環(huán)中的應(yīng)用。對(duì)于這個(gè)系統(tǒng)而言，周圍的環(huán)境都是外界物質(zhì)。系統(tǒng)越復(fù)雜，熵就越大；一個(gè)有序簡(jiǎn)單系統(tǒng)的熵就會(huì)很小。 mgzPE? (1) 式中： m—— 質(zhì)量； g—— 重力加速度； z—— 距水平基準(zhǔn)面的高度 動(dòng)能的產(chǎn)生是由于分子具有速度。機(jī)械功或者軸功是由機(jī)械裝置傳出或者傳入的能量。當(dāng)流體流出系統(tǒng)時(shí)，流動(dòng)功同樣產(chǎn)生。其他的熱力學(xué)參數(shù)包括熵、內(nèi)能和焓。 每一個(gè)給定狀態(tài)的參數(shù)有唯一的確定的值，并且不論物質(zhì)處于什么樣的狀態(tài)，任何一個(gè)參數(shù)只要處于給定的狀態(tài)下，就會(huì)有同樣的值。 一個(gè)循環(huán)是經(jīng)過一個(gè)過程或幾個(gè)過程，系統(tǒng)的初狀態(tài)與末狀態(tài)是相同的。這種物質(zhì)可以處在多個(gè)相態(tài)，但是在所有的相態(tài)中它的化學(xué)成分不變。干度只有在飽和狀態(tài) (飽和溫度與飽和壓力 )下才有意義。過飽和蒸氣的壓力和溫度是相互獨(dú)立的參數(shù)，因?yàn)楫?dāng)壓力保持穩(wěn)定時(shí)，溫度可以上升。 進(jìn)入系統(tǒng)的凈能量 =系統(tǒng)儲(chǔ)存能的凈增量 或者 進(jìn)入的能量 — 流出的能量 =系統(tǒng)儲(chǔ)存能的增量 圖 1 表明一個(gè)熱力學(xué)系統(tǒng)能量的流進(jìn)與流出。因此： 0)2()2( 22 ???????? ?? WQgzVhmgzVhml e a v i n gs t r e a ma l le n t e r i n gs t r e a ma l l (6) 式中： h = u + pv 的含義與公式 (4)代表的含義相同。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。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。 eesm? 由質(zhì)量的流出引起的熵減。熱力學(xué)第二定律的一般公式可以寫為： ImsmsTQSS outinr e vs y s t e mif ????? ??? )()(/)( ? (10) 在很多應(yīng)用中，這個(gè)過程被認(rèn)為是一個(gè)穩(wěn)態(tài)過程。 ??? ??? s ur rinout T QmsmsI )()( (11) 公式 (6)可以用來代替熱交換量。把公式 (6)和公式 (11)聯(lián)立可以得到 ? ?s ur r inoutinout T hhssmI ???? )( (12) 在一個(gè)循環(huán)中，一個(gè)能量循環(huán)產(chǎn)生功的降低， (或者一個(gè)制冷循環(huán)所需要的功的增加 )等于周圍環(huán)境的絕對(duì)溫度乘以在循環(huán)中各 個(gè)不可逆的總量。這個(gè)過程在以下的幾個(gè)章節(jié)中會(huì)講到。所以： genev apWQ QC O P ?? (16) 在很多情況下，給吸收式系統(tǒng)提供的功相對(duì)于給發(fā)生器提供的熱量是非常小的。對(duì)于多級(jí)循環(huán)，每一個(gè)階段都可以由一個(gè)可 逆循環(huán)來描述。維里方程表示為相互的擴(kuò)張壓力 p 或每單位質(zhì)量體積 v 的值 (19) (20) 系數(shù) B’ ,C’,D 39。維里系數(shù)是溫度的函數(shù) ,的值在方程 (19)和相應(yīng)的系數(shù) (20)是相關(guān)的。當(dāng)前最好的 R 值是 也就是說 ,Z = pv / RT 或 維里形成的一個(gè)優(yōu)勢(shì)是統(tǒng)計(jì)力學(xué)被用來預(yù)測(cè)低階系數(shù) 并提供物理維里系數(shù)的意義。然而 ,第二個(gè)和第三個(gè)系數(shù)的實(shí)驗(yàn)值優(yōu)先。數(shù)字計(jì)算機(jī)允許使用非常復(fù)雜的狀態(tài)方程在計(jì)算 pvT 價(jià)值觀 ,甚至高的密度。的 MartinHou 方程如下