姜 峰,李瑞佳,齊國(guó)鵬,李修倫
冷卻水流量對(duì)蒸發(fā)段上移的三相閉式重力熱管傳熱性能的影響
姜 峰1, 2,李瑞佳1,齊國(guó)鵬3,李修倫1
(1. 天津大學(xué)化工學(xué)院,天津 300350;2. 天津市化工安全與裝備技術(shù)重點(diǎn)實(shí)驗(yàn)室,天津 300350;3. 天津職業(yè)大學(xué)生物與環(huán)境工程學(xué)院,天津 300410)
本文將流化床換熱防垢節(jié)能技術(shù)與兩相閉式重力熱管相結(jié)合,設(shè)計(jì)并構(gòu)建了蒸發(fā)段上移的銅/水-碳化硅三相閉式重力熱管.分別采用水和碳化硅顆粒作為液相和固相工質(zhì),考察了冷卻水流量(50~75L/h)、充液率(15%~30%)和加熱功率(100~300W)等操作參數(shù)對(duì)閉式重力熱管傳熱性能的影響.研究結(jié)果表明,碳化硅顆粒的加入可以明顯地降低蒸發(fā)段上移的閉式重力熱管的總熱阻,強(qiáng)化傳熱.實(shí)驗(yàn)范圍內(nèi),總熱阻減少率最大為33.8%,相應(yīng)的充液率為FR=20%,加熱功率=200W,冷卻水流量=100L/h.在較低的冷卻水流量下,兩相和三相閉式重力熱管的總熱阻隨著冷卻水流量的增加明顯減小,然而,隨著冷卻水流量的進(jìn)一步增加,總熱阻減小的幅度降低.加熱功率較低時(shí),三相閉式重力熱管的總熱阻隨充液率的增加先減小,后增大;加熱功率較高時(shí),總熱阻隨充液率的增加呈現(xiàn)出波動(dòng)的趨勢(shì).構(gòu)建了操作參數(shù)對(duì)總熱阻減小率影響的三維圖.研究結(jié)果有助于促進(jìn)三相閉式重力熱管的工業(yè) 應(yīng)用.
三相閉式重力熱管;強(qiáng)化傳熱;蒸發(fā)段上移;冷卻水流量;總熱阻;碳化硅
熱管是依靠自身內(nèi)部工質(zhì)發(fā)生相變來進(jìn)行傳熱的一種元件.閉式重力熱管(TPCT),又稱為熱虹吸管,是結(jié)構(gòu)最為簡(jiǎn)單的一種熱管.其中空的金屬管體,使其具有質(zhì)輕的特點(diǎn),而其快速均溫的特性,則使其具有優(yōu)異的熱超導(dǎo)性能.此外,閉式重力熱管還具有優(yōu)良的熱流密度可變性、環(huán)境適應(yīng)性等優(yōu)點(diǎn)[1-2],目前已被廣泛應(yīng)用于能源[3-5]、空調(diào)系統(tǒng)[6]、電子器件冷卻[7-8]、核反應(yīng)[9]和地?zé)峥刂芠10-12]等領(lǐng)域.
TPCT自下而上可分為蒸發(fā)段、絕熱段和冷凝段.流體在TPCT蒸發(fā)段產(chǎn)生汽相流到TPCT冷凝段,在此段產(chǎn)生液相.冷凝液依靠重力回流至蒸發(fā)段,進(jìn)而完成工質(zhì)的循環(huán),將熱量從高溫區(qū)轉(zhuǎn)移至低溫區(qū)[13].
目前,對(duì)TPCT傳熱性能強(qiáng)化的研究主要集中在3個(gè)方面:改變熱管的內(nèi)部結(jié)構(gòu)[14-15]、修飾熱管的內(nèi)壁面[16]和選擇適當(dāng)?shù)墓べ|(zhì)[17-20].與其他方法相比,選用適當(dāng)?shù)墓べ|(zhì)可以避免復(fù)雜的結(jié)構(gòu)、降低設(shè)備加工和維護(hù)費(fèi)用.在目前所考察的工質(zhì)中,納米流體被很多研究者證明可以強(qiáng)化TPCT的傳熱[21-23]. Ramezani-zadeh等[21]考察了3種濃度的Ni/甘油-水納米流體對(duì)重力熱管傳熱性能的影響.結(jié)果表明,使用適當(dāng)濃度的納米流體可以降低熱管的總熱阻. Parametthanuwat等[22]分別用水、水基銀納米流體、含油酸等研究了熱管的換熱狀況.他們發(fā)現(xiàn),納米流體的有效性均優(yōu)于水,可以改善熱管的傳熱性能.Huminic等[23]發(fā)現(xiàn)濃度為5.3%的納米流體的傳熱性能優(yōu)于純水.然而,也有一些研究發(fā)現(xiàn)納米流體并不總是能夠改善TPCT的傳熱性能.Khandekar等[24]的研究表明,采用不同的水基納米流體作為工質(zhì),均會(huì)使TPCT的總熱阻增加,傳熱惡化.此外,生產(chǎn)成本高和長(zhǎng)期使用穩(wěn)定性的問題也限制了納米流體在工業(yè)上的應(yīng)用.
流化床換熱技術(shù)中流化的固體顆粒對(duì)換熱壁面進(jìn)行剪切和碰撞,可以破壞、減薄流動(dòng)和傳熱邊界層,增加汽化核心,延長(zhǎng)結(jié)垢的誘導(dǎo)期,降低壁溫,進(jìn)而在線強(qiáng)化傳熱和防、除垢,節(jié)能降耗,延長(zhǎng)換熱設(shè)備的使用壽命.該技術(shù)目前已廣泛應(yīng)用于多個(gè)工業(yè) 領(lǐng)域[25-32].
將流化床換熱防垢節(jié)能技術(shù)和TPCT相結(jié)合,構(gòu)建汽-液-固三相閉式重力熱管(THPCT),是對(duì)TPCT強(qiáng)化傳熱方式的一種有益探索.一些研究者圍繞此方面已開展了初步的研究[33-35].Li等[33]構(gòu)建了碳鋼/碳化硅-水三相閉式重力熱管,在80%的充液率下,考察了固含率、熱通量和顆粒粒徑等參數(shù)對(duì)于其傳熱性能的影響.結(jié)果表明,和TPCT相比,THPCT可以明顯地強(qiáng)化傳熱;顆粒粒徑對(duì)傳熱性能具有顯著的影響.Chen等[34]設(shè)計(jì)并構(gòu)建了碳鋼/水-玻璃珠三相閉式重力熱管,考察了充液率、固含率和加熱功率等操作參數(shù)對(duì)其傳熱性能的影響.結(jié)果表明,玻璃珠的加入增加了蒸發(fā)段的傳熱系數(shù),降低了總熱阻.姜峰 等[35]考察了顆粒類型對(duì)THPCT傳熱性能的影響.其研究結(jié)果表明傳熱性能隨著固含率的增加而波動(dòng),樹脂顆粒具有較好的強(qiáng)化傳熱效果.
閉式重力熱管蒸發(fā)段的傳熱包括底部液池的沸騰傳熱和液池上方的膜狀蒸發(fā).二者傳熱機(jī)理和效果不同.蒸發(fā)段加熱位置的上移將影響兩種傳熱機(jī)理對(duì)蒸發(fā)段傳熱的貢獻(xiàn),進(jìn)而影響顆粒的流化和THPCT的整體傳熱效果[36-37].然而,上述關(guān)于THPCT傳熱性能的研究主要集中在以碳鋼為材質(zhì)的三相閉式重力熱管,其加熱位置固定從蒸發(fā)段底端開始,而對(duì)于蒸發(fā)段加熱位置上移的情況尚鮮有探討.同時(shí),THPCT的傳熱性能是由蒸發(fā)段和冷凝段的傳熱共同控制的,而目前對(duì)于THPCT的研究主要考察的是蒸發(fā)段傳熱條件變化所帶來的影響,如加熱功率和熱通量等,而對(duì)于冷凝段傳熱條件的變化,如冷卻水流量等所產(chǎn)生的影響,則尚鮮見報(bào)道.因此,本文將水和碳化硅顆粒加入TPCT中,將TPCT的蒸發(fā)段上移40mm,研究蒸發(fā)段上移后的THPCT工作狀況.銅作為熱管殼體材料導(dǎo)熱性能好;碳化硅顆粒熱導(dǎo)率高,理化性能好.研究結(jié)果將有助于促進(jìn)對(duì)THPCT的強(qiáng)化傳熱機(jī)理的深入了解,拓展其應(yīng)用范圍,促進(jìn)其工業(yè)化使用.
圖1所示為本文設(shè)計(jì)并構(gòu)建的一套蒸發(fā)段加熱位置上移的三相閉式重力熱管(THPCT),其主要由閉式重力熱管、真空系統(tǒng)和數(shù)據(jù)采集系統(tǒng)三部分構(gòu)成.閉式重力熱管的管殼材料為銅,長(zhǎng)度為600mm,直徑為22mm×1.5mm.熱管自下而上可分為蒸發(fā)段、絕熱段和冷凝段,其中,冷凝段長(zhǎng)度為220mm.蒸發(fā)段分為4段,自下而上長(zhǎng)度分別為40mm、120mm、40mm和40mm,每段均可以由電加熱帶(材料為鎳鉻電阻絲)獨(dú)立加熱,總額定功率為600W.本文主要考察蒸發(fā)段加熱位置上移后三相閉式重力熱管的傳熱性能,蒸發(fā)段距離熱管底部40mm,如圖2所示.
1—電腦;2—數(shù)字顯示儀表;3—冷卻水進(jìn)口;4—轉(zhuǎn)子流量計(jì);5—冷卻水夾套;6—測(cè)溫?zé)犭娕迹?—壓力傳感器;8—三通;9—冷卻水出口;10—真空泵;11—金屬架;12—電阻絲;13—管壁;14—金屬螺母
在該設(shè)備壁面上對(duì)稱地安裝有11對(duì)熱電偶以測(cè)量其外壁溫,其中蒸發(fā)段5對(duì)、絕熱段2對(duì)、冷凝段4對(duì),如圖3所示.冷凝段安裝冷凝夾套,冷卻水的進(jìn)、出口溫度由熱電偶測(cè)量,流量由轉(zhuǎn)子流量計(jì)測(cè)量.整個(gè)熱管外面包有保溫棉,以減少熱損失.
圖2 蒸發(fā)段設(shè)置
圖3 熱電偶的分布
熱管頂部裝有壓力傳感器,通過三通與2XZ-4旋片式真空泵相連,可測(cè)量熱管內(nèi)的操作壓力.該設(shè)備抽真空后初始操作壓力為95kPa(真空度).熱電偶和壓力傳感器在使用前已要求制造商對(duì)其進(jìn)行了校準(zhǔn),對(duì)壓力傳感器進(jìn)行了充分補(bǔ)償,對(duì)熱電偶進(jìn)行 了冷端補(bǔ)償和非線性補(bǔ)償.相關(guān)儀表的規(guī)格如表1 所示.
表1 相關(guān)儀表的規(guī)格
實(shí)驗(yàn)過程中每組實(shí)驗(yàn)條件重復(fù)3次,以檢查數(shù)據(jù)的重復(fù)性.
實(shí)驗(yàn)中,選擇與銅相容性良好的自來水作為熱管的液相工質(zhì),選擇碳化硅顆粒作為惰性固體顆粒.碳化硅顆粒理化性質(zhì)穩(wěn)定,導(dǎo)熱性能好,適于工業(yè)化推廣應(yīng)用.顆粒的相關(guān)物性如表2所示.
充液率為加入的液相工質(zhì)體積與熱管總?cè)莘e之比,實(shí)驗(yàn)中,充液率分別設(shè)置為15%、20%、25%和30%.冷卻水流量分別為50L/h、75L/h、100L/h和125L/h.加熱功率分別為100W、200W和300W.實(shí)驗(yàn)中的固含率固定為15%,這里的固含率是指添加的固體顆粒堆體積與液相工質(zhì)體積之比.
表2 顆粒的相關(guān)物性
注:沉降速度在20℃和常壓下的水中測(cè)得.
實(shí)驗(yàn)中,溫度和冷卻水流量的最大測(cè)量誤差分別為0.1℃和1L/h.傳熱速率、總熱阻和對(duì)流傳熱系數(shù)的不確定度可分別計(jì)算如下:
實(shí)驗(yàn)范圍內(nèi),傳熱速率、熱管總熱阻、蒸發(fā)段和冷凝段對(duì)流傳熱系數(shù)的最大不確定度分別為2.15%、2.55%、2.21%和2.18%,滿足工程計(jì)算的要求.
圖4所示為加熱位置上移的三相閉式重力熱管的總熱阻減少率隨操作參數(shù)的變化規(guī)律.由圖可知,在加熱位置上移后,碳化硅顆粒的加入仍然可以明顯地降低TPCT的總熱阻,強(qiáng)化傳熱.實(shí)驗(yàn)范圍內(nèi),總熱阻減少率最大為33.8%,相應(yīng)的充液率為FR=20%,加熱功率=200W,冷卻水流量=100L/h.
在THPCT中,顆粒存在于蒸發(fā)段.蒸發(fā)段液池的沸騰,導(dǎo)致顆粒流化.流化的顆粒會(huì)碰撞和剪切加
熱壁面,減薄和破壞流動(dòng)與傳熱邊界層,減小熱阻,強(qiáng)化對(duì)流傳熱;同時(shí),顆粒與壁面的作用,還會(huì)增加壁面上的汽化核心,加快汽泡的生成和脫離頻率,促進(jìn)沸騰傳熱.此外,顆粒的流化還有利于將液池中的大汽泡破碎成小汽泡,減小汽泡離開液池時(shí)的霧沫夾帶,有利于減小冷凝段的液膜厚度,降低冷凝傳熱熱阻,強(qiáng)化傳熱.因此,在上述因素的綜合作用下,碳化硅顆粒的加入,明顯地降低了蒸發(fā)段和冷凝段的壁 面溫差,減小了熱管的總熱阻,強(qiáng)化了傳熱,如圖5所示.
圖6和圖7分別為冷卻水流量對(duì)兩相和三相閉式重力熱管總熱阻的影響.由圖可知,隨著冷卻水流量的增加,兩相和三相閉式重力熱管的總熱阻呈現(xiàn)出相似的變化趨勢(shì).冷卻水流量較小時(shí),總熱阻隨著冷卻水流量的增加而迅速降低.然而,隨著冷卻水流量的進(jìn)一步增加,總熱阻下降的幅度逐漸減小,甚至出現(xiàn)增大的趨勢(shì).
圖4 操作參數(shù)對(duì)閉式重力熱管總熱阻減少率的影響
一方面,冷卻水流量的增加,增大了冷卻水側(cè)的對(duì)流傳熱系數(shù),導(dǎo)致冷凝段總傳熱系數(shù)也隨之增大,降低了冷凝段的壁溫,增加了蒸汽冷凝速率.熱管冷凝段和蒸發(fā)段液池上方的操作壓力下降,蒸發(fā)段液相工質(zhì)的沸點(diǎn)降低.在一定的加熱功率下,蒸發(fā)段的壁溫也隨之下降,如圖8所示,因而有利于減小熱損失,增加蒸發(fā)段的實(shí)際輸入功率,促進(jìn)沸騰傳熱,加劇對(duì)液池的擾動(dòng),促進(jìn)顆粒的流化.顆粒流化程度的提高則有利于促進(jìn)顆粒和加熱壁面的相互作用,將大汽泡破碎成小汽泡,增加汽泡的脫離頻率,促進(jìn)傳熱,減小汽泡脫離液池上升到冷凝段時(shí)的霧沫夾帶,進(jìn)而減小冷凝段液膜的平均厚度,減小冷凝熱阻.
圖5 操作參數(shù)對(duì)蒸發(fā)段與冷凝段內(nèi)壁溫差的影響
圖6 冷卻水流量對(duì)兩相閉式重力熱管總熱阻的影響
圖7 冷卻水流量對(duì)三相閉式重力熱管總熱阻的影響
圖8 冷卻水流量對(duì)三相閉式重力熱管平均內(nèi)壁溫的影響
另一方面,冷卻水流量增加所引起的冷凝速率增大,也會(huì)導(dǎo)致冷凝段平均液膜厚度增加,增大蒸汽冷凝傳熱熱阻.同時(shí),操作壓力的降低,也增加了液體工質(zhì)的黏度和表面張力,不利于沸騰傳熱,進(jìn)而削弱了顆粒的流化程度,不利于碳化硅對(duì)THPCT的換熱,最終呈現(xiàn)出如圖9和圖10所示的變化趨勢(shì).
圖9 冷卻水流量對(duì)三相閉式重力熱管蒸發(fā)段對(duì)流傳熱系數(shù)的影響
圖10 冷卻水流量對(duì)三相閉式重力熱管冷凝段對(duì)流傳熱系數(shù)的影響
在蒸發(fā)段和冷凝段傳熱效果的共同作用下,冷卻水流量較小時(shí),總熱阻隨著冷卻水流量的增加而迅速降低.然而,隨著冷卻水流量的進(jìn)一步增加,總熱阻下降的幅度逐漸減小,甚至出現(xiàn)增大的趨勢(shì),如圖7所示.
圖11所示為充液率對(duì)三相閉式重力熱管總熱阻的影響.由圖可知,在不同的加熱功率下,總熱阻隨充液率的增加呈現(xiàn)出不同的變化趨勢(shì).加熱功率較低時(shí),總熱阻隨著充液率的增加先減小,后增大;而加熱功率較高時(shí),總熱阻則隨著充液率的增加呈現(xiàn)出波動(dòng)的趨勢(shì).
圖11 充液率對(duì)三相閉式重力熱管總熱阻的影響
如前所述,閉式重力熱管蒸發(fā)段的傳熱包括底部液池的沸騰傳熱和液池上方的膜狀蒸發(fā).充液率的增大,增加了蒸發(fā)段液池的高度,一方面提高了液池中的沸騰傳熱對(duì)蒸發(fā)段傳熱的貢獻(xiàn),有利于增加沸騰核心和汽泡的生成頻率,加劇對(duì)液池的擾動(dòng),促進(jìn)顆粒的流化.顆粒流化程度的提高則有利于促進(jìn)顆粒和加熱壁面的相互作用,將大汽泡破碎成小汽泡,增加汽泡的脫離頻率,減小汽泡脫離液池上升到冷凝段時(shí)的霧沫夾帶,進(jìn)而減小冷凝段液膜的平均厚度,減小冷凝熱阻.另一方面,液池高度的進(jìn)一步增加,也增大了汽泡從液池中逸出的難度,因此抑制了沸騰傳熱,削弱了汽泡對(duì)液池的擾動(dòng)程度,不利于顆粒的流化,增大了汽泡脫離液池時(shí)的尺寸,加劇了霧沫夾帶.因此,在上述因素的共同作用下,加熱功率較低時(shí),蒸發(fā)段和冷凝段的對(duì)流傳熱系數(shù)基本上隨著充液率的增加先增大,后減小,如圖12(a)和圖13(a)所示,進(jìn)而導(dǎo)致總熱阻隨著充液率的增加先減小,后增大.
圖12 充液率對(duì)三相閉式重力熱管蒸發(fā)段對(duì)流傳熱系數(shù)的影響
圖13 充液率對(duì)三相閉式重力熱管冷凝段對(duì)流傳熱系數(shù)的影響
加熱功率的增大,增加了蒸發(fā)段壁面的過熱度和沸騰核心,強(qiáng)化了沸騰傳熱,增加了蒸發(fā)段的上升蒸汽量,加劇了對(duì)冷凝段液膜的擾動(dòng),有利于傳熱;但同時(shí),蒸汽量的增加也加劇了霧沫夾帶,增加了冷凝液膜沿壁面下流的阻力.此時(shí),充液率的增加對(duì)蒸發(fā)段沸騰核心和霧沫夾帶影響的程度減小,如圖12(b)和圖13(b)所示,但蒸發(fā)段和冷凝段的對(duì)流傳熱系數(shù)均隨著充液率的增加而波動(dòng),因此也導(dǎo)致總熱阻隨著充液率的增加呈現(xiàn)出波動(dòng)的趨勢(shì).
圖14為操作參數(shù)對(duì)三相閉式重力熱管總熱阻影響的三維圖.該圖可以綜合反映操作參數(shù)對(duì)THPCT傳熱性能的影響,有助于確定閉式重力熱管的最優(yōu)操作條件,并為其工業(yè)應(yīng)用提供參考.
圖14 操作參數(shù)對(duì)THPCT總熱阻影響的三維圖
(1) 碳化硅顆粒的加入可以明顯地強(qiáng)化蒸發(fā)段上移的閉式重力熱管的傳熱性能.實(shí)驗(yàn)范圍內(nèi),總熱阻減少率最大為33.8%,相應(yīng)的充液率為FR=20%,加熱功率=200W,冷卻水流量=100L/h.
(2) 冷卻水流量較小時(shí),總熱阻隨著冷卻水流量的增加而迅速降低.隨著冷卻水流量的進(jìn)一步增加,總熱阻下降的幅度逐漸減小,甚至出現(xiàn)增大的趨勢(shì).
(3) 總熱阻在加熱功率較低時(shí),隨著充液率的增加先減小,后增大;而在加熱功率較高時(shí),隨著充液率的增加呈現(xiàn)出波動(dòng)的趨勢(shì).
(4) 構(gòu)建了操作參數(shù)對(duì)總熱阻影響的三維圖,以反映THPCT傳熱性能的變化規(guī)律.
(5) 后續(xù)研究將進(jìn)一步考察固含率和顆粒類型等操作參數(shù)對(duì)蒸發(fā)段上移的三相閉式重力熱管傳熱性能的影響,同時(shí)開展相應(yīng)的數(shù)值模擬.
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Effect of Cooling Water Flow Rate on the Thermal Performance of a Three-Phase Closed Thermosyphon with an Upward-Moving Evaporation Section
Jiang Feng1, 2,Li Ruijia1,Qi Guopeng3,Li Xiulun1
(1. School of Chemical Engineering and Technology,Tianjin University,Tianjin 300350,China;2. Tianjin Key Laboratory of Chemical Process Safety and Equipment Technology,Tianjin 300350,China;3.School of Biological and Environmental Engineering,Tianjin Vocational Institute,Tianjin 300410,China)
In this study,a copper(Cu)/water-silicon carbide(SiC)three-phase closed thermosyphon with an upward-moving evaporation section is designed and built by combining fluidized bed heat transfer and fouling prevention technology with a two-phase closed thermosyphon. Water and SiC solid particles are used as the liquid and solid working media,respectively. The effect of the operating parameters,namely the cooling water flow rate(50—75L/h),filling ratio(15%—30%),and heating power(100—300W),on the heat transfer performance of the three-phase closed thermosyphon is studied. The results show that adding SiC particles can obviously reduce the overall thermal resistance and enhance the heat transfer of the closed thermosyphon with an upward-moving evaporation section. The maximum reduction rate of the overall thermal resistance was up to 33.8% at FR=20%,=200W,and=100L/h within the experimental range. The overall thermal resistance of the two- and three-phase closed thermosyphons decreased with the increase in the cooling water flow rate when the cooling water flow rate was low. However,the extent of the decline decreased with a further increase in the cooling water flow rate. The overall thermal resistance of the three-phase closed thermosyphon initially decreased,and then increased with the increase in the filling ratio at a low heating power but fluctuated with the increase in the liquid filling ratio at a high heating power. Three-dimensional diagrams of the effect of the operating parameters on the reduction rate of the overall thermal resistance are established. The obtained findings are beneficial to the industry application of the three-phase closed thermosyphon.
three-phase closed thermosyphon;heat transfer enhancement;upward-moving evaporation section;cooling water flow rate;overall thermal resistance;silicon carbide
TQ051.5
A
0493-2137(2022)09-0909-10
10.11784/tdxbz202104035
2021-04-19;
2021-06-08.
姜 峰(1975— ),男,博士,副教授.
姜 峰,jiangfeng@tju.edu.cn.
化學(xué)工程國(guó)家重點(diǎn)實(shí)驗(yàn)室開放基金資助項(xiàng)目(SKL-ChE-18B03).
the Open Foundation of State Key Laboratory of Chemical Engineering(No. SKL-ChE-18B03).
(責(zé)任編輯:田 軍)