蔡驥馳,王瑞祥,徐榮吉,張一灝,丁思源
(北京建筑大學(xué)北京市建筑能源綜合高效綜合利用工程技術(shù)研究中心,北京 100044)
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SDBS對(duì)銅-水脈動(dòng)熱管啟動(dòng)及傳熱性能影響
蔡驥馳,王瑞祥,徐榮吉,張一灝,丁思源
(北京建筑大學(xué)北京市建筑能源綜合高效綜合利用工程技術(shù)研究中心,北京 100044)
脈動(dòng)熱管工質(zhì)氣泡的產(chǎn)生、脈動(dòng)、分裂、合并是脈動(dòng)熱管運(yùn)行特性的重要影響因素。而表面張力是氣泡產(chǎn)生、脈動(dòng)、分裂、合并運(yùn)行規(guī)律的重要影響參數(shù)。以表面活性劑十二烷基苯磺酸鈉(sodium dodecyl benzene sulfonate,SDBS)為溶質(zhì),以去離子水為基液,配制了質(zhì)量分?jǐn)?shù)為0.001%、0.005%和0.009%的3種溶液。測(cè)試發(fā)現(xiàn)表面活性劑的添加對(duì)水的表面張力改變較大,濃度為0.005%時(shí)表面張力降低了42.2%,而對(duì)水的比熱容、密度、黏度影響較小。在自行設(shè)計(jì)搭建的脈動(dòng)熱管傳熱性能實(shí)驗(yàn)臺(tái)上,對(duì)比實(shí)驗(yàn)測(cè)試了在不同濃度、不同加熱功率下表面張力對(duì)脈動(dòng)熱管啟動(dòng)及傳熱性能的影響規(guī)律。研究結(jié)果發(fā)現(xiàn):表面活性劑可以減小脈動(dòng)熱管啟動(dòng)時(shí)間,在低加熱功率條件下(≤90 W),0.001% SDBS水溶液的啟動(dòng)時(shí)間最快,最快比水快了11.2%;而在大功率加熱條件下,SDBS的添加對(duì)啟動(dòng)時(shí)間影響較小。而且表面活性劑可以改變脈動(dòng)熱管傳熱性能,使其熱阻出現(xiàn)不同程度的減小。
傳熱傳質(zhì);脈動(dòng)熱管;表面張力;啟動(dòng)時(shí)間;傳熱性能
DOI:10.11949/j.issn.0438-1157.20151160
脈動(dòng)熱管因具有體積小、散熱效果佳、成本低、結(jié)構(gòu)簡(jiǎn)單等特點(diǎn)[1-4],自Akachi[5]在20世紀(jì)90年代提出后,便受到學(xué)者們的廣泛關(guān)注。工質(zhì)是脈動(dòng)熱管傳熱性能的重要影響因素[6-8],一直是熱點(diǎn)研究問題。有學(xué)者認(rèn)為[9-10]可以通過改善脈動(dòng)熱管中工質(zhì)的熱導(dǎo)率和輸運(yùn)能力來提高脈動(dòng)熱管的性能。Ma等[11]發(fā)現(xiàn)用寶石納米流體作為脈動(dòng)熱管的工質(zhì),加熱功率為336 W時(shí),熱阻只有0.03℃·W-1,而且此時(shí)熱導(dǎo)率為1.0032 W·m-1·℃-1。商福民等[12]研究了Cu-H2O納米流體對(duì)脈動(dòng)熱管傳熱性能的影響,進(jìn)一步證明了納米流體能起到強(qiáng)化換熱的效果。除熱導(dǎo)率外,工質(zhì)的表面張力也是脈動(dòng)熱管傳熱性能的重要影響因素[13-14]。郝婷婷等[15]研究了疏水表面對(duì)脈動(dòng)熱管性能的影響,發(fā)現(xiàn)疏水處理后的脈動(dòng)熱管的熱阻高于普通紫銅脈動(dòng)熱管。Wang等[16]研究了表面活性劑硬質(zhì)酸鈉對(duì)脈動(dòng)熱管性能的影響,發(fā)現(xiàn)向脈動(dòng)熱管工質(zhì)中添加表面活性劑后,脈動(dòng)熱管的熱阻有一定的減小,且濃度為0.004%,充液率為58%,加熱功率為160 W時(shí),與去離子水相比,熱阻減小了0.13 K·W-1,文中并沒有提到表面活性劑對(duì)水熱物性的改變。
表面張力對(duì)脈動(dòng)熱管傳熱性能的研究還處于起步階段,表面活性劑的添加對(duì)工質(zhì)熱物性的影響、對(duì)脈動(dòng)熱管啟動(dòng)性能的影響鮮有報(bào)道。本文首先嘗試分析了表面張力對(duì)熱管傳熱性能影響的機(jī)理;然后測(cè)試了表面活性劑對(duì)水熱物性的影響。以此為基礎(chǔ),在自行設(shè)計(jì)的脈動(dòng)熱管傳熱性能實(shí)驗(yàn)臺(tái)上對(duì)比測(cè)試了不同濃度溶液脈動(dòng)熱管啟動(dòng)及傳熱性能。為表面張力對(duì)脈動(dòng)熱管運(yùn)行特性的影響規(guī)律研究提供基礎(chǔ)。
與傳統(tǒng)熱管靠氣液相變傳熱不同,脈動(dòng)熱管的工作原理是:將毛細(xì)管抽真空后充注部分工質(zhì),由于管徑足夠小,管內(nèi)將形成氣塞和液塞的隨機(jī)分布。在蒸發(fā)段,工質(zhì)吸熱產(chǎn)生氣泡、膨脹升壓推動(dòng)液塞流向冷凝段。在冷凝段,氣泡冷卻收縮并破裂,壓力下降。由于兩端的壓力差和相鄰管子之間的壓力不平衡,使得工質(zhì)在蒸發(fā)段和冷凝段之間振蕩流動(dòng),從而實(shí)現(xiàn)熱量的傳遞。工質(zhì)氣泡的產(chǎn)生、脈動(dòng)、分裂、合并是脈動(dòng)熱管運(yùn)行特性的重要影響因素。而表面張力是氣泡產(chǎn)生、脈動(dòng)、分裂、合并運(yùn)行規(guī)律的重要影響參數(shù)。在工質(zhì)中添加少量的表面活性劑將會(huì)大大降低工質(zhì)的表面張力[17-18],而表面張力的改變將影響加熱壁面上成核所需要的過熱度,從而影響工質(zhì)氣泡的生成。壁面上產(chǎn)生沸騰所需的最大過熱度為[19]
從式(1)中分析發(fā)現(xiàn),當(dāng)表面張力減小時(shí),加熱壁面上成核所需要的過熱度也會(huì)隨之變小,從而導(dǎo)致成核率變高。所以表面張力越小,越容易成核。表面張力的變化除了能影響液體的沸騰,還將影響液塞在脈動(dòng)熱管中運(yùn)動(dòng)時(shí)的毛細(xì)阻力(圖1)[20-21]。根據(jù)Young-Laplace方程
可以看出,對(duì)于純水單個(gè)液塞的毛細(xì)阻力為3.24× 10-4N,而脈動(dòng)熱管是由眾多間隔的氣塞和液塞組成,由于表面張力所引起的毛細(xì)阻力不能忽略,且對(duì)給定脈動(dòng)熱管表面張力越小,毛細(xì)阻力越小。而且毛細(xì)阻力會(huì)影響氣塞的脈動(dòng)、合并和分裂,從而影響脈動(dòng)熱管的傳熱性能。
圖1 脈動(dòng)熱管毛細(xì)阻力分析Fig.1 Capillary resistance analysis of PHP
添加少量表面活性劑后不僅對(duì)表面張力產(chǎn)生影響,也會(huì)對(duì)黏度產(chǎn)生影響。根據(jù)牛頓內(nèi)摩擦力公式
可以看出黏度與內(nèi)摩擦力呈正比關(guān)系。當(dāng)工質(zhì)中添加少量表面活性劑后,會(huì)對(duì)黏度產(chǎn)生影響,從而影響內(nèi)摩擦力,最終導(dǎo)致脈動(dòng)熱管中流體流動(dòng)阻力產(chǎn)生變化。
綜上,通過在去離子水里添加一定濃度表面活性劑來改變其表面張力,使管內(nèi)工質(zhì)更容易成核,同時(shí)減小毛細(xì)阻力,最終達(dá)到增加脈動(dòng)熱管的傳熱性能的目的。
選用應(yīng)用較為廣泛的十二烷基苯磺酸鈉(sodium dodecyl benzene sulfonate,SDBS)作為表面活性劑,分析純,由國藥集團(tuán)化學(xué)試劑有限公司提供,基液為去離子水。分別配制了質(zhì)量分?jǐn)?shù)為0.001%、0.005%、0.009% 3種不同濃度的SDBS水溶液。并將配制好的溶液放入超聲波振蕩器中振蕩2 h,然后取出放在溫度為25℃的室內(nèi),靜置1周后觀察發(fā)現(xiàn)沒有沉淀產(chǎn)生。
采用西安夏溪電子科技有限公司提供的熱物性測(cè)試儀器,分別對(duì)純水及這3種濃度的SDBS水溶液進(jìn)行表面張力、黏度、密度、比熱容、熱導(dǎo)率的測(cè)量。表1所示為在室溫25℃時(shí),各溶液的物性參數(shù)測(cè)量值。首先用去離子水對(duì)物性測(cè)試儀器的精準(zhǔn)度進(jìn)行校驗(yàn),如表1所示測(cè)試值與標(biāo)準(zhǔn)值符合良好,其誤差在精度范圍內(nèi)。從表可以看出,3種SDBS水溶液的表面張力都出現(xiàn)了明顯的降低,且隨濃度的升高而降低。對(duì)于濃度為0.001%、0.005%和0.009%的SDBS水溶液表面張力比純水分別降低了31.4%、42.2%和54.9%。SDBS水溶液的黏度隨表面活性劑濃度的升高而增大。對(duì)于0.001%的SDBS水溶液其黏度與純水相比變化很小,僅增加了1.15%。0.005%和0.009%的SDBS水溶液黏度分別增加了5.6%和9.8%。但表面活性劑SDBS的添加對(duì)水的密度、比熱容、熱導(dǎo)率影響很小,可以忽略。
3.1實(shí)驗(yàn)系統(tǒng)
自行設(shè)計(jì)的實(shí)驗(yàn)系統(tǒng)主要由脈動(dòng)熱管元件、加熱裝置、冷卻裝置、數(shù)據(jù)采集裝置4部分組成,如圖2所示。其中,脈動(dòng)熱管由外徑4 mm、內(nèi)徑2.5 mm的銅管彎曲而成,共有5個(gè)彎頭,蒸發(fā)段、絕熱段、冷凝段長度均為10 cm。蒸發(fā)段用自帶絕緣層電阻絲纏繞并加熱,由兆信RXN-605A型號(hào)直流電源供電,外部加保溫層。脈動(dòng)熱管的加熱方式為垂直底部加熱模式,實(shí)驗(yàn)過程中通過調(diào)節(jié)直流電源輸入電壓來改變輸入功率,從而研究不同加熱功率下,脈動(dòng)熱管傳熱性能。冷凝段采用西安交通大學(xué)研制的RC3030恒溫水箱(精度為0.1℃)降溫,始終保持冷凝段冷卻水進(jìn)口溫度為21℃。
圖2 脈動(dòng)熱管實(shí)驗(yàn)系統(tǒng)Fig.2 Schematic diagram of experimental system of PHP
溫度測(cè)試系統(tǒng)主要由計(jì)算機(jī)、安捷倫34970A數(shù)據(jù)采集儀以及熱電偶組成。熱電偶為歐米伽公司提供的K型熱電偶,其半徑為0.254 mm,精度為0.1℃,熱電偶測(cè)點(diǎn)布置如圖2中1~15處。溫度信號(hào)由熱電偶傳入數(shù)據(jù)進(jìn)行AD轉(zhuǎn)化采集,最后傳入計(jì)算機(jī)由自帶軟件記錄并顯示。
3.2測(cè)試工況及誤差分析
實(shí)驗(yàn)主要對(duì)比測(cè)試了純水和質(zhì)量分?jǐn)?shù)為0.001%、0.005%、0.009%SDBS水溶液4種工質(zhì),在50、70、90、110、130 W 5種加熱功率下的啟動(dòng)時(shí)間和傳熱性能。測(cè)試過程中脈動(dòng)熱管的傾角為90°,充液率為60%。
啟動(dòng)時(shí)間的規(guī)定:溫度波動(dòng)是脈動(dòng)熱管管內(nèi)工質(zhì)流型變化的重要標(biāo)志。脈動(dòng)熱管啟動(dòng)時(shí),蒸發(fā)段由氣泡產(chǎn)生,溫度會(huì)由過熱突然降低。因此,規(guī)定從加熱開始到溫度曲線出現(xiàn)第1次明顯波動(dòng)時(shí),這一時(shí)間段為脈動(dòng)熱管啟動(dòng)時(shí)間。當(dāng)脈動(dòng)熱管的蒸發(fā)段溫度在某個(gè)溫度區(qū)間內(nèi)持續(xù)60 min脈動(dòng)變化時(shí),認(rèn)為其達(dá)到了穩(wěn)定運(yùn)行狀態(tài),采集30 min脈動(dòng)熱管蒸發(fā)段溫度及冷凝段溫度,并通過式(4)計(jì)算得到脈動(dòng)熱管傳熱熱阻。
表1 R不同濃度溶液的物性參數(shù)Table 1 Physical properties of different concentration solution
式中,Q為脈動(dòng)熱管的加熱量,忽略漏熱損失。分別為加熱段和冷凝段5個(gè)測(cè)點(diǎn)的平均溫度。
加熱功率的相對(duì)誤差:電壓和電流均由直流穩(wěn)壓穩(wěn)流電源讀取,其顯示精度均為1%。電壓量程0~60 V,電流量程0~5 A。本實(shí)驗(yàn)最小電壓和最小電流為28 V和1.8 A,則功率的最大相對(duì)誤差為[22]
熱阻的相對(duì)誤差:標(biāo)定后熱電偶的測(cè)溫誤差為±0.1℃,根據(jù)熱阻計(jì)算公式,且在加熱功率為130 W時(shí),0.009%工質(zhì)的Te-Tc為最小值,為53.56℃,熱阻的最大相對(duì)誤差為
4.1表面活性劑SDBS對(duì)脈動(dòng)熱管啟動(dòng)時(shí)間的影響
應(yīng)用4種工質(zhì)的脈動(dòng)熱管啟動(dòng)時(shí)間列于表2。由表可以看出,在加熱功率較低時(shí), 熱管啟動(dòng)時(shí)間很長, 不同工質(zhì)的啟動(dòng)時(shí)間差別較大,濃度為0.001%的SDBS水溶液的啟動(dòng)時(shí)間最短,且隨著工質(zhì)濃度的增加,啟動(dòng)時(shí)間逐漸變長。隨著加熱功率的增加,啟動(dòng)時(shí)間縮短。當(dāng)加熱功率≥100 W時(shí),4種工質(zhì)的啟動(dòng)時(shí)間基本相同。當(dāng)加熱功率從50 W增加到130 W時(shí),水、0.001%、0.005%和0.009% SDBS水溶液的啟動(dòng)時(shí)間分別提高了51.9%、46.9%、57.9%和61.3%。這是由于隨著加熱功率的增加,在蒸發(fā)段更容易形成局部過熱,工質(zhì)越易沸騰,啟動(dòng)時(shí)間越短。
表2 R不同工質(zhì)脈動(dòng)熱管啟動(dòng)時(shí)間Table 2 Start-up time of PHPs with different working fluid PHP/s
圖3 90 W時(shí)不同工質(zhì)脈動(dòng)熱管啟動(dòng)時(shí)間Fig.3 Start-up time of PHPs with different working fluidsunder heat input power of 90 W
在低加熱功率條件下,3種濃度的SDBS水溶液的啟動(dòng)時(shí)間隨著濃度的增加逐漸增大,其中0.001%的SDBS溶液?jiǎn)?dòng)時(shí)間要小于純水的啟動(dòng)時(shí)間,而當(dāng)濃度到達(dá)0.005%之后,SDBS溶液的啟動(dòng)時(shí)間要大于純水脈動(dòng)熱管的啟動(dòng)時(shí)間。為詳細(xì)分析原因,以90 W加熱功率為例,4種工質(zhì)蒸發(fā)段溫度變化曲線如圖3(a)所示。為了更清楚地觀察脈動(dòng)熱管啟動(dòng)時(shí)間,將圖3(a)中啟動(dòng)階段局部放大后為圖3(b)。由圖可以看出,濃度為0.001%的SDBS水溶液的啟動(dòng)時(shí)間最短,為87 s,之后是純水,為98 s,然后是0.005%的SDBS水溶液,為103 s,最后是0.009%的SDBS水溶液,為120 s。0.001%的SDBS水溶液的啟動(dòng)時(shí)間比純水的啟動(dòng)時(shí)間提高了11.2%。分析原因是,0.001%的SDBS水溶液表面張力低,且黏度與水接近,根據(jù)式(1)可知,表面張力越小,蒸發(fā)段工質(zhì)越容易成核,脈動(dòng)熱管更容易啟動(dòng)。同時(shí),表面張力的減小導(dǎo)致管內(nèi)毛細(xì)阻力變小,從而減小了液塞的運(yùn)動(dòng)阻力。但是,隨著SDBS濃度的增加,溶液的黏度會(huì)變大,雖然更容易成核,但是成核后氣泡的脫離和脈動(dòng)速度變得困難,從溫度曲線上表現(xiàn)為啟動(dòng)時(shí)間會(huì)更長。
4.2表面活性劑SDBS對(duì)脈動(dòng)熱管熱阻的影響
圖4所示為4種工質(zhì)脈動(dòng)熱管熱阻隨加熱功率的變化曲線。且在相同工況下,重復(fù)5次實(shí)驗(yàn),所得實(shí)驗(yàn)平均值列于圖4。從圖中可以看出,SDBS對(duì)銅-水脈動(dòng)熱管傳熱熱阻產(chǎn)生了一定的影響,且以SDBS水溶液為工質(zhì)的脈動(dòng)熱管的傳熱熱阻隨著加熱功率的增加而減小,這一特點(diǎn)與水為工質(zhì)的脈動(dòng)熱管一致。
圖4 不同加熱功率的脈動(dòng)熱管傳熱熱阻Fig.4 Thermal resistance of PHPs under different heating power
隨著功率從50W升高到130W,0.001%的SDBS水溶液脈動(dòng)熱管傳熱熱阻從1.08℃·W-1下降到0.569℃·W-1,而0.009%的SDBS水溶液脈動(dòng)熱管傳熱熱阻從1.25℃·W-1下降到0.14℃·W-1。同時(shí),純水的熱阻降低了59.4%,0.001%、0.005%和0.009% SDBS水溶液的熱阻降低了47.3%、59.9%和67.0%。由此可以看出低濃度溶液脈動(dòng)熱管熱阻在低加熱功率條件下熱阻最小,但下降的斜率最緩,而高濃度溶液脈動(dòng)熱管熱阻下降的斜率最大,且在高加熱功率條件下熱阻最小。原因是在低加熱功率條件下,根據(jù)式(1)和Young-Laplace方程可知,表面張力小的工質(zhì)更容易成核,且脈動(dòng)阻力小,因此更容易脈動(dòng)起來,而高濃度的溶液表面張力雖然小,但是其黏度會(huì)變大,由于黏度的增大,會(huì)增大脈動(dòng)阻力。所以在低加熱功率條件下,低濃度的SDBS水溶液脈動(dòng)熱管傳熱熱阻小。而隨著加熱功率的增大,加熱段產(chǎn)生劇烈的沸騰傳熱,高濃度的溶液表面張力更低,更容易沸騰。大量氣泡的產(chǎn)生為脈動(dòng)過程提供了充足的動(dòng)力,克服了黏度較高的問題,從而使得高濃度的SDBS水溶液在高功率時(shí)強(qiáng)化了脈動(dòng)熱管的傳熱。
測(cè)試了3種濃度SDBS水溶液的熱物性,并與去離子水對(duì)比研究了SDBS表面活性劑對(duì)脈動(dòng)熱管啟動(dòng)時(shí)間及傳熱性能的影響。結(jié)論如下。
(1)表面活性劑SDBS的添加對(duì)水的表面張力影響很大,而密度、比熱容、熱導(dǎo)率影響很小,可以忽略。對(duì)于0.001%的SDBS水溶液其黏度與純水相比變化很小,而其他濃度的SDBS水溶液黏度與水相比有一定變化。
(2)表面活性劑對(duì)脈動(dòng)熱管啟動(dòng)時(shí)間有一定影響。在低加熱功率條件下,0.001%SDBS水溶液的啟動(dòng)時(shí)間最快。而當(dāng)加熱功率足夠大之后,4種工質(zhì)的啟動(dòng)時(shí)間基本一樣。
(3)表面活性劑對(duì)脈動(dòng)熱管傳熱熱阻影響很大。在低加熱功率條件下,0.001% SDBS水溶液的熱阻最小。而在高加熱功率條件下,0.009% SDBS水溶液的脈動(dòng)熱管傳熱熱阻最小。
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Influence of SDBS on start time and heat transfer performance of pulsating heat pipe
CAI Jichi, WANG Ruixiang, XU Rongji, ZHANG Yihao, DING Siyuan
(Beijing Engineering Research Centre of Sustainable Energy and Buildings, Beijing University of Civil Engineering and Architecture, Beijing 100044, China)
Surface tension is one of the important factors that influences the behaviors of the working fluid bubbles, such as generation, pulsation split and merge. Furthermore, the heat transfer performance of pulsating heat pipe (PHP) is affected. Three concentration solutions 0.001%, 0.005% and 0.009% were configured using sodium dodecyl benzene sulfonate (SDBS) as solute and deionized water as base fluid. The surface tension of water was decreased by 42.2% after addition of 0.005% SDBS, while little change of the specific heat, density and viscosity. Both the start-up and heat transfer performance of PHP were test with the concentration solutions under different heating power input. The experimental results indicated that SDBS can reduce the start-up time of the pulsating heat pipe. Start-up time of 0.001% solution was the shortest with the heating power input no more than 90 W. Differently, the addition of SDBS has little effect on the start-up time under high heating power input. At steady working station, the heat transfer performance of PHP was improved by adding SDBS.
heat and mass transfer;pulsating heat pipe;surface tension;start-up time;heat transfer performance
date: 2015-07-20.
XU Rongji, xurongji@bucea.edu.cn
supported by the National Natural Science Foundation of China (51506004) and the Scientific Research Project of Beijing Educational Committee (KM201410016001).
TK 172.4
A
0438—1157(2016)05—1852—06
2015-07-20收到初稿,2016-01-19收到修改稿。
聯(lián)系人:徐榮吉。第一作者:蔡驥馳(1990—),男,碩士研究生。
國家自然科學(xué)基金項(xiàng)目(51506004);北京市教委科技計(jì)劃項(xiàng)目(KM201410016001)。