彭冬根,羅丹婷,李順意
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內(nèi)熱型除濕溶液再生器溶液再生方式及裝置性能分析
彭冬根,羅丹婷,李順意
(南昌大學(xué)建筑工程學(xué)院,南昌 330031)
內(nèi)熱型再生器作為一種高效溶液再生裝置其性能主要由溶液加熱形式、熱水流向、傳熱單元數(shù)及溶液空氣相對(duì)流向等決定。該文基于裝置內(nèi)溶液、空氣、熱水三者間的能量和質(zhì)量守恒,分別建立預(yù)熱、內(nèi)熱再生器在2種熱水流向下的順流、逆流、叉流的數(shù)學(xué)模型,并進(jìn)行理論性能模擬和比較。數(shù)值模擬發(fā)現(xiàn)大部分工況下內(nèi)熱型再生器再生性能為預(yù)熱型的2~4倍,且受溶液–空氣流量比和熱水–空氣流量比影響較大。熱水與溶液流向相反時(shí)的再生性能要優(yōu)于相同時(shí),最大可高于5%。再生性能隨溶液–空氣傳熱單元數(shù)和溶液–熱水傳熱單元數(shù)的增大而提高,且存在性能增長(zhǎng)最快的組合曲線。另外再生過(guò)程中大部分情況下溶液和空氣呈順流時(shí)的出入口濃度差最大,叉流為其0.97倍左右,逆流最低時(shí)僅達(dá)到其0.87倍左右。該文研究結(jié)果為內(nèi)熱型溶液再生器設(shè)計(jì)優(yōu)化提供理論依據(jù)。
模型;再生器;濃度;內(nèi)熱;性能;傳熱單元數(shù)
作為傳統(tǒng)電制冷空調(diào)替代技術(shù)之一的溶液除濕空調(diào)因其環(huán)境友好、驅(qū)動(dòng)能源品位低、蓄能密度高、具有改善室內(nèi)空氣品質(zhì)等優(yōu)勢(shì),受到越來(lái)越多國(guó)內(nèi)外研究人員的青睞[1-3]。溶液再生技術(shù)作為溶液除濕空調(diào)系統(tǒng)中的關(guān)鍵技術(shù),是其大規(guī)模推廣應(yīng)用的前提[4-6]?,F(xiàn)有溶液再生方法有電滲析法[7-8]、膜再生[9-10]及填料再生等。電滲析法和膜再生由于系統(tǒng)裝置復(fù)雜,成本高而限制其大規(guī)模應(yīng)用。填料裝置結(jié)構(gòu)簡(jiǎn)單,既可用于溶液再生,也可用于溶液除濕。在溶液除濕方面,內(nèi)冷型的填料除濕器具有較高性能。Zhang等[11]對(duì)于使用廢熱的內(nèi)冷型除濕器進(jìn)行試驗(yàn)研究,發(fā)現(xiàn)性能系數(shù)(coefficient of performance, COP)能達(dá)到4.2至6.5。常曉敏等[12]建立理論模型對(duì)內(nèi)冷型溶液除濕器的熱質(zhì)交換和流型進(jìn)行研究分析。填料再生裝置由于可利用低品位熱能而越來(lái)越受到國(guó)內(nèi)外學(xué)者關(guān)注,其最早是一種絕熱再生方式[13-15]。但是由于絕熱再生伴隨溶液溫度上升導(dǎo)致再生效果不高,因此內(nèi)熱型溶液再生技術(shù)近年來(lái)逐漸受到學(xué)者重視。Mun等[16]采用試驗(yàn)方法研究影響內(nèi)熱型再生性能的主要因素。Liang等[17]對(duì)基于熱源塔的內(nèi)熱型溶液再生進(jìn)行了研究。Yin等[18-19]設(shè)計(jì)一種內(nèi)熱型平板降膜溶液再生器,對(duì)其進(jìn)行試驗(yàn)研究和理論建模,并和絕熱型再生裝置進(jìn)行對(duì)比,發(fā)現(xiàn)內(nèi)熱型再生方式優(yōu)于絕熱再生。內(nèi)冷型溶液除濕和內(nèi)熱型溶液再生都是基于相同的傳熱傳質(zhì)原理[20],因此很多學(xué)者都是對(duì)兩者同時(shí)進(jìn)行研究。Liu等[21]設(shè)計(jì)一種由導(dǎo)熱塑料構(gòu)成的內(nèi)熱/內(nèi)冷型的溶液再生/除濕器,顯示具有較好性能。Qi等[22]在香港搭建了內(nèi)冷/內(nèi)熱型溶液除濕空調(diào)系統(tǒng),并針對(duì)各項(xiàng)性能進(jìn)行研究分析。同時(shí)Qi等[23]建立數(shù)學(xué)模型對(duì)內(nèi)熱/內(nèi)冷型溶液除濕系統(tǒng)的焓效率、溫度效率及濕度效率進(jìn)行研究。但是目前針對(duì)內(nèi)熱型溶液再生裝置的研究依然相對(duì)較少,并且均針對(duì)特定裝置結(jié)構(gòu)和流型的再生器性能進(jìn)行研究。為使得研究結(jié)果更具普適性,本文采用基于傳熱單元數(shù)的數(shù)學(xué)模型,對(duì)影響內(nèi)熱型溶液再生裝置性能的所有再生流型及傳熱單元數(shù)進(jìn)行研究,以期為內(nèi)熱型溶液再生器設(shè)計(jì)優(yōu)化提供理論依據(jù)。
文章構(gòu)建溶液和空氣在流向上呈順流、逆流、叉流的3種再生器模型,并根據(jù)熱水流向分為與溶液流向一致(流向Ⅰ)及與溶液流向相反(流向Ⅱ)的2種情況,如圖1所示。假設(shè)再生器裝置高度為,長(zhǎng)度為,寬度為。
溶液再生過(guò)程是一個(gè)從非穩(wěn)態(tài)到穩(wěn)態(tài)的過(guò)程,文章只研究裝置在穩(wěn)態(tài)過(guò)程的性能;在實(shí)際裝置中,熱源主要分布在裝置內(nèi)部并加熱其他流體,而裝置內(nèi)靠近壁面的流體與裝置外環(huán)境溫度相差不大,所以假設(shè)整個(gè)裝置絕熱,與外界環(huán)境之間不存在熱濕交換;文中裝置采用填料使得裝置中溶液分布均勻,可以忽略垂直于紙面方向的溶液熱濕傳遞,只考慮在溶液和空氣流動(dòng)方向上的熱濕傳遞,因此順、逆流再生過(guò)程可以簡(jiǎn)化成一維傳熱傳質(zhì)問(wèn)題,叉流除濕過(guò)程簡(jiǎn)化為二維傳熱傳質(zhì)問(wèn)題;文中假設(shè)熱水管路均勻布置;對(duì)流傳熱和對(duì)流傳質(zhì)作為該裝置的主要傳熱方式和傳質(zhì)方式使得文章在假設(shè)過(guò)程中僅考慮溶液、空氣和熱水在其流動(dòng)方向的對(duì)流傳熱、傳質(zhì),忽略流體自身的導(dǎo)熱和質(zhì)量擴(kuò)散。
注:H為再生器高度,m;L為再生器長(zhǎng)度,m;dx為微元再生器高度,m;dy為微元再生器長(zhǎng)度,m。
根據(jù)上述模型假設(shè),可得到順、逆流溶液再生過(guò)程傳熱傳質(zhì)數(shù)學(xué)模型[2]:
式中a,s為空氣和溶液焓值,kJ/kg;e為與溶液平衡空氣焓值,kJ/kg;為水汽化潛熱,kJ/kg;1為溶液與空氣的流向系數(shù),其中順流為-1,逆流為1;2為熱水的流向系數(shù),熱水流向與溶液相同(流向Ⅰ)時(shí)為-1,相反(流向Ⅱ)時(shí)為1;為劉易斯數(shù);a為空氣含濕量,kg/kg;e為與溶液平衡空氣含濕量,kg/kg;w,s為熱水和溶液溫度,℃;a,w,salt為空氣、熱水和溶液中鹽分質(zhì)量流量,kg/s;pa,pw為空氣和熱水比熱,kJ/(kg·K);為水鹽比,kg/kg;為空氣和溶液間傳熱系數(shù),kW/(m2·K);w為熱水和溶液間傳熱系數(shù),kW/(m2·K);m為空氣和溶液間傳質(zhì)系數(shù)kg/(m2·s);1為溶液和空氣接觸面積,m2;2為溶液和熱水接觸面積,m2;NTU1為溶液和空氣間的傳熱單元數(shù);NTU2為溶液和熱水間的傳熱單元數(shù)。
式(9)-式(13)為叉流熱濕交換數(shù)學(xué)模型。
式中為溶液流動(dòng)方向;為空氣流動(dòng)方向;為再生裝置高度,m;為再生裝置長(zhǎng)度,m。
再生器為預(yù)熱型時(shí),相當(dāng)于將內(nèi)熱型中的熱水和溶液傳熱單元數(shù)(NTU2)移到裝置外構(gòu)成預(yù)加熱器,因此在模型建立中,只需要將上述內(nèi)熱型模型中的NTU2設(shè)置為0,而再生器的入口溶液受預(yù)熱器加熱作用溫度升高。假設(shè)預(yù)熱再生器入口溶液與熱水在預(yù)熱器中進(jìn)行逆流換熱,換熱器效能由公式(14)得出,則再生器的溶液入口溫度可經(jīng)公式(15)計(jì)算得到。
式中為換熱效率;(p)max,(p)min分別為流體質(zhì)量流量和比熱乘積的最大和最小值,kW/℃;in,out分別為流體進(jìn)出口溫度,℃;w,in,s,in,分別為熱水和溶液進(jìn)口溫度,℃。
為驗(yàn)證預(yù)熱和內(nèi)熱溶液再生模型的精確性,將文獻(xiàn)[24-27]中通過(guò)試驗(yàn)及理論得到的57個(gè)傳濕率(蒸發(fā)率或除濕率)結(jié)果作為對(duì)比參數(shù),其中規(guī)整填料和散裝填料的形狀參數(shù)計(jì)算方法在文獻(xiàn)[28-29]中給出,溶液物性參數(shù)根據(jù)文獻(xiàn)[30]中給出的計(jì)算方式進(jìn)行計(jì)算。文獻(xiàn)中的溶液、空氣和水的初始參數(shù)如表1所示。圖2對(duì)比了文獻(xiàn)和本文數(shù)值模擬的傳濕率,其中圖2a為模擬的蒸發(fā)率與文獻(xiàn)[24,27]給出的預(yù)熱型逆流及叉流再生試驗(yàn)數(shù)據(jù)的結(jié)果對(duì)比;圖2b為模擬的蒸發(fā)率及除濕率與文獻(xiàn)[25-26]分別給出的內(nèi)熱/冷型逆流理論解及叉流內(nèi)熱再生試驗(yàn)數(shù)據(jù)的結(jié)果對(duì)比。其中文獻(xiàn)[25]中還對(duì)比了內(nèi)冷除濕率的結(jié)果,雖不是再生結(jié)果,但采用相同的理論模型也能驗(yàn)證模型的精確性。由圖2可以看出,模擬數(shù)據(jù)和對(duì)比數(shù)據(jù)之間誤差除去1個(gè)數(shù)據(jù)點(diǎn)外,均在20%的誤差范圍內(nèi),平均誤差為8.6%??紤]到試驗(yàn)儀器和測(cè)量上的誤差,一般工程上20%的誤差在可接受范圍內(nèi)[31],驗(yàn)證了模擬程序的精確性。
表1 對(duì)比文獻(xiàn)的流體入口參數(shù)
a. 預(yù)熱模型
a. Pre-heated model
b. 內(nèi)熱模型
b. Internally heated model
注:圖2a的蒸發(fā)率的數(shù)據(jù)來(lái)自文獻(xiàn)[24]和[27];圖2b的蒸發(fā)率及除濕率的數(shù)據(jù)來(lái)自文獻(xiàn)[25]和[26];圖中橫縱坐標(biāo)均為對(duì)數(shù)坐標(biāo)。
Note: Evaporativity data of Fig.2a is from Refs. [24] and [27]; Evaporativity and dehumidificativity data of Fig.2b is from Refs. [25] and [26]. The-axis and-axis are logarithmic coordinate.
圖2 數(shù)值模型的精確性驗(yàn)證
Fig.2 Validation of numerical model
為研究?jī)?nèi)熱和預(yù)熱2種不同的溶液再生方式對(duì)再生器再生性能的影響,文章針對(duì)溶液和空氣呈順流、逆流、叉流3種流型及熱水和溶液分別處于同向(流向Ⅰ)和反向(流向Ⅱ)的6種不同組合情況進(jìn)行模擬,并與相應(yīng)情況下的預(yù)熱結(jié)果進(jìn)行比較。各模擬工況所選擇的參數(shù)范圍見表2,表中給出19組初始參數(shù),結(jié)合內(nèi)熱型再生的6種流型及預(yù)熱型再生的3種流型組合共計(jì)得到171個(gè)模擬結(jié)果,如圖3所示。模擬中、、a分別為0.4、0.5 m和1 kg/s。模擬結(jié)果顯示大部分工況下內(nèi)熱型再生器的溶液出入口濃度差是預(yù)熱型的2~4倍,即內(nèi)熱型再生器表現(xiàn)出明顯的性能優(yōu)勢(shì)。這是由于預(yù)熱型再生器中溶液的溫度會(huì)隨著熱濕交換的發(fā)生而下降,而溶液溫度的下降影響著裝置的再生性能。內(nèi)熱型再生器由于熱水與溶液的熱交換及溶液與空氣的熱濕交換同時(shí)進(jìn)行,能一定程度上抑制溫度下降,使溶液與熱水的溫度勢(shì)差更均勻。僅當(dāng)s/a以及w/a在一定范圍內(nèi)變化時(shí),部分工況的預(yù)熱型再生器優(yōu)于內(nèi)熱型再生器。
為進(jìn)一步了解內(nèi)熱和預(yù)熱再生性能受溶液與空氣流量比以及熱水與空氣流量比的影響,文章分析溶液-空氣質(zhì)量流量比(s/a)從0.1上升至0.65,熱水-空氣流量比(w/a)從0.4上升至0.95的范圍中,溶液和空氣順流,且熱水自下而上流動(dòng)(流向Ⅱ)時(shí),內(nèi)熱型再生器與預(yù)熱型的出入口濃度差比值,見圖4所示。模擬中參數(shù)取如下基準(zhǔn)值:熱水空氣流量比w/a=0.7,溶液空氣流量比s/a=0.2;空氣入口溫度a, in=30 ℃、溶液入口溫度s, in=27 ℃,熱水入口溫度w, in=80 ℃;空氣入口含濕量a, in=0.012kg/kg,溶液入口水鹽比in=1.85 kg/kg;溶液空氣間傳熱單元數(shù)NTU1=0.5,溶液熱水間傳熱單元數(shù)NTU2=0.9。
表2 不同工況所選擇的初始參數(shù)
注:圖中數(shù)據(jù)包含內(nèi)熱型及預(yù)熱型再生中的溶液與空氣在順、逆、叉3種不同流向下的比較,其中內(nèi)熱型再生包含2種熱水流向(流向Ⅰ和流向Ⅱ);長(zhǎng)度L=0.4 m;高度H=0.5 m;空氣質(zhì)量流量ma=1( kg·s-1),下同。
預(yù)熱溶液的出口溫度逐漸升高并趨于一個(gè)定值,但溶液總體的所得熱能大幅下降,而整體的流量下降也導(dǎo)致在熱濕交換中溫度下降更快,再生能力變差。而在內(nèi)熱裝置中,由于溶液流量的減小,其入口段溫升越快,再生段的長(zhǎng)度增加,再生能力增強(qiáng)。即使熱水流量持續(xù)增加,預(yù)熱型再生器中再生溶液的溫度增加到一個(gè)定值后便不再遞增,隨后再生溶液進(jìn)入再生器中進(jìn)行再生過(guò)程時(shí),由于熱濕傳遞使得溶液溫度下降較大,再生能力也大幅下降。而內(nèi)熱型再生器中的再生溶液依靠大流量的熱水保持溫度基本穩(wěn)定,且隨著熱水流量增加,溶液溫度基本與熱水溫度相等,這就使得溶液一直保持著較好的再生性能。
注:溶液和空氣順流;熱水自下而上流動(dòng)(流向Ⅱ);模擬中參數(shù)基準(zhǔn)值:ta, in=30 ℃、ts, in=27 ℃,tw, in=80 ℃;ωa, in=0.012 (kg·kg-1),Xin=1.85 (kg·kg-1);NTU1=0.5,NTU2=0.9,下同。A點(diǎn)為內(nèi)熱最大點(diǎn) (A:mw/ma=0.95, ms/ma=0.1); B點(diǎn)為內(nèi)熱最大點(diǎn) (B:mw/ma=0.65, ms/ma=0.4)
圖中顯示隨著s/a減小以及w/a增加,內(nèi)熱型再生器具有更好的性能。即單位空氣流量下,溶液流量越小,或是熱水流量越大,內(nèi)熱型再生器相對(duì)于預(yù)熱型再生器的再生性能越好。這是由于隨著溶液流量的減小,由圖4可知,在給定的模擬范圍內(nèi),內(nèi)熱型再生器的溶液出入口濃度差最高可達(dá)到預(yù)熱的10.85倍,最低僅有預(yù)熱的0.15倍。將圖中的交界線擬合成曲線,得到內(nèi)熱效果較好的工況,流量比之間滿足關(guān)系式(s/a)<(0.628 6w/a?0.055 9)。另外,通過(guò)計(jì)算蒸發(fā)率,得到該模擬范圍內(nèi)的內(nèi)熱最大值點(diǎn)(:s/a=0.1,w/a=0.95)時(shí),溶液中的水蒸發(fā)率為0.0105kg/s,得到該模擬范圍內(nèi)的預(yù)熱最大值點(diǎn)(:s/a=0.4,w/a=0.65)時(shí),溶液的水蒸發(fā)率僅為5.29×10-4kg/s,即內(nèi)熱最佳點(diǎn)的蒸發(fā)量是預(yù)熱的19.84倍,內(nèi)熱型再生器明顯優(yōu)于預(yù)熱型。因此,選用內(nèi)熱型再生方式進(jìn)行研究。A、B兩點(diǎn)分別為基于NTU1和NTU2的基準(zhǔn)值模擬得到,其隨NTU1與NTU2的變化規(guī)律將在下文研究中闡述。
影響內(nèi)熱型溶液再生器性能的因素包括各種流體流型和傳熱單元數(shù)。流型主要有熱水流動(dòng)方向、溶液和空氣相對(duì)流動(dòng)方向。傳熱單元數(shù)是基于溶液、空氣和熱水三者間傳熱系數(shù)和傳熱面積的綜合變量。下文針對(duì)這3個(gè)因素對(duì)內(nèi)熱型再生的性能(溶液進(jìn)出口濃度差)進(jìn)行影響分析。
圖5中展示了內(nèi)熱型再生器在不同熱水流向下溶液出入口濃度差的變化情況。
由圖5可以看出,熱水的流向影響比較小,但是總體上流向Ⅱ要優(yōu)于流向Ⅰ,溶液與空氣呈順流、逆流、叉流時(shí)均滿足此規(guī)律,且在順流工況下流向Ⅱ相對(duì)流向Ⅰ表現(xiàn)更優(yōu),其再生性能最大可提高5%。這是由于在熱水與溶液逆向流動(dòng)的過(guò)程中,其傳熱溫差比較穩(wěn)定,所以傳熱效果更理想。
圖5 熱水流向Ⅰ和流向Ⅱ?qū)υ偕阅艿挠绊?/p>
傳熱單元數(shù)受裝置傳熱面積和裝置內(nèi)流體間傳熱系數(shù)變化的影響,是一種影響性能的重要因素。在內(nèi)熱型再生器中,溶液與空氣間的傳熱單元數(shù)(NTU1)以及熱水與溶液間的傳熱單元數(shù)(NTU2)是反映溶液和空氣間及溶液和熱水間的傳熱面積和傳熱系數(shù)的綜合影響因素,是影響裝置熱濕交換性能的2個(gè)關(guān)鍵性參數(shù)。
圖6為NTU1和NTU2對(duì)溶液出入口濃度差的影響,其中NTU1和NTU2均在0.2到1.65之間變化。
圖6 不同NTU對(duì)順流內(nèi)熱再生器出入口濃度差的影響
為比較溶液與空氣在順流、逆流、叉流3種再生方式下的再生效果,圖6b繪制了逆流與順流濃度差比以及叉流與順流濃度差比變化的2種情況。由圖可以看出,整體上,順流最優(yōu),逆流其次,叉流最差。但在NTU1偏大且NTU2極小時(shí)(NTU1大于0.7,NTU2小于0.4),逆流優(yōu)于叉流;特別是當(dāng)NTU2小于0.35,NTU1大于1.45時(shí),逆流與順流比高于1,逆流優(yōu)于順流,但最高僅達(dá)到1.03優(yōu)勢(shì)不明顯。叉流與順流之比大部分處于0.97,當(dāng)NTU2小于0.5時(shí)叉流與順流的濃度差之比下降到0.88;而逆流與順流的濃度差之比的曲面呈現(xiàn)馬鞍狀,當(dāng)NTU1與NTU2均小于0.7或均大于1.5時(shí),逆流與順流的濃度差之比均下降至0.87。隨著基準(zhǔn)點(diǎn)的變化,對(duì)具體濃度差會(huì)有一定的影響,但總體趨勢(shì)不變。
為分析在模擬范圍內(nèi)大部分工況下順流最優(yōu)的原因,本節(jié)對(duì)于模擬范圍內(nèi)順流最優(yōu)工況(工況:NTU1=1.2,NTU2=0.8)和逆流優(yōu)勢(shì)工況(工況:NTU1=1.8,NTU2=0.2)進(jìn)行溶液與空氣含濕量變化趨勢(shì)研究,分別見圖7a和7b。圖中同一流向中溶液與空氣含濕量的交點(diǎn)表示由除濕過(guò)程轉(zhuǎn)變至再生過(guò)程的NTU1位置,稱為臨界點(diǎn),內(nèi)熱再生過(guò)程中到達(dá)臨界點(diǎn)的先后決定了該過(guò)程的再生性能優(yōu)劣。由圖可看出工況中順流的臨界點(diǎn)比逆流提前,即提前進(jìn)入再生狀態(tài),而工況中逆流臨界點(diǎn)靠前,即逆流更快到達(dá)再生狀態(tài)。分析2種工況下溶液與空氣含濕量可得到流體流動(dòng)過(guò)程中的平均含濕量,以及得到溶液和空氣的含濕量差值。工況中,逆流含濕量差為5.85 g/kg,小于6.19 g/kg順流含濕量差;而工況中,逆流含濕量差為0.21 g/kg,大于0.207 g/kg順流含濕量差。含濕量差越大意味著傳質(zhì)驅(qū)動(dòng)力越大,在相同傳熱單元數(shù)下,再生性能越好。上述結(jié)果解釋了圖6b所呈現(xiàn)的結(jié)果。工況中的含濕量差遠(yuǎn)大于工況中的含濕量差,這說(shuō)明在順流工況下再生性能優(yōu)于逆流工況下再生性能。
注:圖中為順流最優(yōu)工況(工況Ⅰ:NTU1=1.2,NTU2=0.8)。
Note: Parallel type is in the best condition in the figure (Condition Ⅰ: NTU1=1.2, NTU2=0.8).
a. 順流為優(yōu)勢(shì)流型的情況
a. Parallel type in best condition
注:圖中為逆流最優(yōu)工況(工況Ⅱ:NTU1=1.8,NTU2=0.2)。
Note: Counter type is in the best condition in the figure (Condition Ⅱ: NTU1=1.8, NTU2=0.2).
b. 逆流為優(yōu)勢(shì)流型的情況
b. Counter type is best condition
圖7 2種優(yōu)勢(shì)流型下溶液與空氣含濕量變化趨勢(shì)
Fig.7 Change of moisture content of solution and air under two kinds of flow types
文章基于溶液、空氣以及熱水的三者間能量和質(zhì)量守恒,建立基于傳熱單元數(shù)的預(yù)熱、內(nèi)熱再生器在2種熱水流向下的順流、逆流、叉流的數(shù)學(xué)模型,分析不同裝置結(jié)構(gòu)對(duì)再生性能的影響,得到如下結(jié)論:
1)文章在分析預(yù)熱及內(nèi)熱結(jié)構(gòu)對(duì)再生性能的影響時(shí),發(fā)現(xiàn)大部分工況下內(nèi)熱型再生器的溶液出入口濃度差是預(yù)熱型的2~4倍,即內(nèi)熱型再生器表現(xiàn)出明顯的性能優(yōu)勢(shì)。且整體受流量比影響,隨著溶液-空氣流量比減小以及熱水-空氣流量比增加,內(nèi)熱型再生器相對(duì)于預(yù)熱型再生器的再生性能越好。
2)在研究熱水的流向?qū)υ偕阅艿挠绊憰r(shí),發(fā)現(xiàn)熱水流向?qū)ρb置性能影響比較小,但是總體上熱水與溶液流向相反時(shí)要優(yōu)于相同時(shí),兩者濃度差最大可達(dá)到5%。
3)在分析傳熱單元數(shù)對(duì)再生性能的影響時(shí),發(fā)現(xiàn)當(dāng)溶液-熱水傳熱單元較小時(shí)(0.2~0.35)再生效果隨著溶液-空氣流量比的增加,呈現(xiàn)先增加后減少的趨勢(shì),當(dāng)溶液-熱水傳熱單元較大時(shí)(大于0.35)濃度差隨著溶液-空氣流量比的增加先增長(zhǎng)然后逐漸趨于穩(wěn)定??傮w上當(dāng)空氣與溶液處于順流狀態(tài)時(shí),再生效果最優(yōu),僅當(dāng)溶液-空氣流量比大于0.7,溶液-熱水傳熱單元小于0.4時(shí),空氣與溶液處于逆流的再生效果優(yōu)于叉流;當(dāng)溶液-熱水傳熱單元小于0.35,溶液-空氣流量比大于1.45時(shí)逆流優(yōu)于順流。
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Analysis of performance and regeneration method of internally-heated liquid desiccant regenerator
Peng Donggen, Luo Danting, Li Shunyi
(,,330031,)
Liquid desiccant cooling system, as a good alternative to traditional electric refrigeration air conditioner, is environmentally friendly and can be driven by low-grade energy while it can improve indoor air quality and has high energy storage capacity. The regeneration technique,a key technique in the liquid desiccant cooling system,must be developed before it is widely applied in variety of buildings. The present methods of solution regeneration have electrodialysis, membrane energy exchanger, ultrasonic atomization regeneration, packing tower, and so on. The first three have complicated structures and high costs for their application on a large scale. The packing tower regenerator because of its simple structure and being driven possibly by low-grade energy has attracted many attentions across the world. In packing tower regenerators, the internally-heated liquid desiccant regenerator is a kind of high-efficient solution regeneration device. To improve the reliability and economy of internally-heated regeneration technique, the mathematical models of pre-heated and internally-heated regeneration are established based on the energy and mass conservation between solution and air as well as the energy conservation between heated water and solution in this paper, which include parallel flow, counter flow and cross flow with 2 kinds of different flow directions of heated water respectively, and their theoretical performances are numerically simulated and compared with each other. As for the regeneration performances affected by the ways of heating solution and flow directions of heated water, the simulation results show that regeneration performances of internally-heated type are about 2-4 times that of pre-heated type in most conditions, which means the internally-heated regenerator has a better performance. And the regeneration performances are greatly influenced by the flow-rate ratios of solution to air and heated water to air. With the decrease in flow-rate ratio of solution to air and the increase in flow-rate ratio of heated water to air, the regeneration performances of the internally-heated regenerator are increasingly better than that of the pre-heated regenerator. At the maximum point of the internally-heated (flow-rate ratio of solution to air is 0.1 and flow-rate ratio of heated water to air is 0.95, flow-rate ratio of air is1 kg/s), the rate of evaporation is calculated to be 20 times that of the pre-heated at its maximum point (flow-rate ratio of solution to air is0.4, flow-rate ratio of heated water to air is0.65). The flow direction of heated water in internally-heated regenerator is divided into 2 conditions: Heated-water is parallel to solution (DirectionⅠ) or counter to solution (DirectionⅡ). It is also found the regeneration performances, when the heated water flows counter to solution, are superior to heated water paralleling to solution and are increased by 5% at most. As for the effects of the numbers of heat transfer units (NTU1andNTU2), the regeneration performances in general increase with the increase in NTU1and NTU2, and a fitted curve combining NTU1with NTU2occurs that presents the rapidest increase in regeneration performance with the increasing of NTU1and NTU2. Besides, it is also exposed that parallel type shows the largest concentration difference of solution and the cross type is about 97% of that, while the counter type only reaches about 87% as much as parallel type in the worst condition. The results in this paper can offer theoretical supports for the optimal design of internally-heated regenerator.
models; regenerators; concentration; internally-heated; performance; numbers of heat transfer units
10.11975/j.issn.1002-6819.2017.18.022
TU831.6
A
1002-6819(2017)-18-0165-08
2017-05-24
2017-08-07
國(guó)家自然科學(xué)基金項(xiàng)目(51766010);江西省研究生創(chuàng)新專項(xiàng)資金項(xiàng)目(YC2017-S012)
彭冬根,男,博士,副教授,主要從事太陽(yáng)能制冷空調(diào)研究。南昌 南昌大學(xué)建筑工程學(xué)院,330031。Email:ncu_hvac2013@163.com