單波長(zhǎng)溶液液膜厚度測(cè)量方法

劉暢，田井輝，丁家琦，郭曉龍，石建偉，楊薈楠，蔡小舒(上海理工大學(xué) 能源與動(dòng)力工程學(xué)院，上海市動(dòng)力工程多相流動(dòng)與傳熱重點(diǎn)實(shí)驗(yàn)室，上海，200093)

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單波長(zhǎng)溶液液膜厚度測(cè)量方法

劉暢，田井輝，丁家琦，郭曉龍，石建偉，楊薈楠，蔡小舒
(上海理工大學(xué) 能源與動(dòng)力工程學(xué)院，上海市動(dòng)力工程多相流動(dòng)與傳熱重點(diǎn)實(shí)驗(yàn)室，上海，200093)

摘要：提出基于朗伯?比爾定律的未知質(zhì)量分?jǐn)?shù)尿素水溶液液膜厚度測(cè)量方法，選取尿素水溶液吸收率對(duì)質(zhì)量分?jǐn)?shù)的變化率為0的波數(shù)位置(6 613.25Cm?1)，研制單波長(zhǎng)半導(dǎo)體激光吸收光譜(DLAS)尿素水溶液液膜厚度測(cè)量系統(tǒng)。通過(guò)溶液液膜厚度可調(diào)(0～1mm)的標(biāo)準(zhǔn)具對(duì)該方法測(cè)量精度進(jìn)行驗(yàn)證。利用研制的單波長(zhǎng)DLAS液膜測(cè)量系統(tǒng)對(duì)常溫下水平透明石英玻璃板上的尿素水溶液液膜蒸發(fā)過(guò)程進(jìn)行研究。研究結(jié)果表明：尿素水溶液液膜厚度測(cè)量平均相對(duì)誤差約為1.50%，3次重復(fù)性實(shí)驗(yàn)厚度標(biāo)準(zhǔn)偏差小于24.68 μm。在液膜蒸發(fā)初期，液膜厚度隨時(shí)間的變化率很大；在蒸發(fā)階段中期，液膜蒸發(fā)速率趨于穩(wěn)定；但在蒸發(fā)階段后期，劇烈的光束轉(zhuǎn)向現(xiàn)象造成了強(qiáng)烈的數(shù)據(jù)波動(dòng)。

關(guān)鍵詞：溶液液膜；測(cè)量；厚度；吸收光譜；蒸發(fā)

液膜在熱能工程、空調(diào)制冷、航空航天、石油化工等工業(yè)中有著廣泛的應(yīng)用[1?5]，例如液膜流動(dòng)(膜式流動(dòng))，因其具有流量小、溫差小、傳熱傳質(zhì)系數(shù)高、熱流密度高、氣液接觸面積大、表面更新快、結(jié)構(gòu)簡(jiǎn)單且動(dòng)力消耗小等優(yōu)點(diǎn)在化學(xué)工業(yè)過(guò)程中應(yīng)用廣泛[6]，如膜式蒸發(fā)、膜式氣液反應(yīng)等。但在某些工業(yè)過(guò)程中，須避免液膜的形成。例如汽車尾氣脫硝的選擇性催化還原(selectiveCatalytic reduction，SCR)系統(tǒng)[7]中汽車尾氣排放管上形成的尿素水溶液液膜[8]，其形成的三聚氰胺等混合物容易堵塞管路和影響催化反應(yīng)效率，造成安全隱患[9]，因此，應(yīng)盡可能控制尿素溶液液膜的形成?？傊?，對(duì)液膜厚度進(jìn)行研究對(duì)優(yōu)化所涉及的各種工業(yè)過(guò)程具有重要的意義。近年來(lái)，國(guó)內(nèi)外科研人員通過(guò)數(shù)值模擬和無(wú)干擾的測(cè)量方法對(duì)溶液液膜進(jìn)行了研究，如：BIRKHOLD等[9]采用數(shù)值模擬法研究了尿素水溶液液膜的厚度以及質(zhì)量分?jǐn)?shù)；邱慶剛等[10?11]通過(guò)數(shù)值模擬的方法分別對(duì)水平管外降膜流動(dòng)的厚度和水平管降膜蒸發(fā)液膜厚度及傳熱系數(shù)進(jìn)行了研究。溶液液膜厚度的無(wú)干擾測(cè)量方法主要包括電學(xué)法和光學(xué)法。電學(xué)法測(cè)量膜厚是基于不同厚度液膜的電學(xué)性質(zhì)(例如電阻[12]、電容[13])不同，通過(guò)建立這些電學(xué)性質(zhì)與液膜厚度的關(guān)系來(lái)獲取液膜厚度信息，電學(xué)法也須對(duì)測(cè)量結(jié)果進(jìn)行標(biāo)定且測(cè)量精度及其測(cè)量范圍易受到液膜導(dǎo)電性、液膜形態(tài)等因素的影響[14]。而光學(xué)法測(cè)量液膜厚度則主要具有無(wú)干擾、測(cè)量結(jié)果無(wú)需標(biāo)定等優(yōu)點(diǎn)，如：GRESZIK 等[15?16]利用激光誘導(dǎo)熒光技術(shù)和拉曼散射法獲得了透明石英片上純水水膜的二維分布信息，但測(cè)量結(jié)果須進(jìn)行標(biāo)定且設(shè)備昂貴；YANG等[17?19]分別利用吸收光譜法測(cè)量了液膜厚度。本文作者對(duì)不同質(zhì)量分?jǐn)?shù)(10%～50%)的尿素水溶液吸收光譜特性進(jìn)行研究，選取吸收率對(duì)質(zhì)量分?jǐn)?shù)的變化率(dK/dc)為0的波數(shù)，建立基于朗伯?比爾定律的液膜厚度反演模型，研制單波長(zhǎng)半導(dǎo)體激光吸收光譜(DLAS)尿素水溶液液膜厚度測(cè)量系統(tǒng)。利用液膜厚度可調(diào)的標(biāo)準(zhǔn)具對(duì)該系統(tǒng)進(jìn)行驗(yàn)證，通過(guò)比較液膜厚度測(cè)量值與真實(shí)值獲得該系統(tǒng)的測(cè)量誤差。在此基礎(chǔ)上，利用該系統(tǒng)對(duì)常溫下水平透明石英玻璃板上的尿素水溶液液膜蒸發(fā)過(guò)程進(jìn)行研究。

1　理論方法

尿素水溶液的光譜吸收率與溶液質(zhì)量分?jǐn)?shù)相關(guān)，圖1所示為通過(guò)傅里葉變換紅外光譜儀獲得的質(zhì)量分?jǐn)?shù)為10%～50%的尿素水溶液在近紅外區(qū)域(6 000～8000Cm?1)的吸收率K(v,c)[20]。由圖1可見：隨著尿素水溶液質(zhì)量分?jǐn)?shù)的增加，吸收峰向著波數(shù)減小的方向偏移。通過(guò)求取吸收率對(duì)質(zhì)量分?jǐn)?shù)的導(dǎo)數(shù)(dK/dc)可知：當(dāng)波數(shù)為6 613.25Cm?1時(shí)，吸收率隨質(zhì)量分?jǐn)?shù)的變化率為0(這里變化率近似為0)，即質(zhì)量分?jǐn)?shù)變化對(duì)吸收率無(wú)影響。因此，本文選取該波長(zhǎng)，建立不同質(zhì)量分?jǐn)?shù)下的尿素水溶液液膜厚度反演模型。即基于朗伯?比爾定律，假設(shè)波長(zhǎng)為v時(shí)，液膜厚度L可表示為

式中：It和 I0分別為透射光強(qiáng)和入射光強(qiáng)；K 為尿素水溶液在6 613.25Cm?1時(shí)的吸收率，該值為8.03Cm?1。

圖1　質(zhì)量分?jǐn)?shù)為10%～50%的尿素水溶液的FT?IR譜Fig.1　FT?IR spectra of urea-water solutions for mass fractions of10%?50%

2 實(shí)驗(yàn)研究

2.1實(shí)驗(yàn)驗(yàn)證

本文利用液膜厚度可調(diào)(0～1mm)的標(biāo)準(zhǔn)具對(duì)單波長(zhǎng)尿素水溶液液膜測(cè)量方法的精度進(jìn)行驗(yàn)證。標(biāo)準(zhǔn)具內(nèi)含有2塊平行的石英玻璃板，通過(guò)調(diào)節(jié)石英玻璃板之間的距離可以改變?nèi)芤阂耗さ暮穸?。?shí)驗(yàn)裝置圖如圖2所示。利用激光控制器將分布反饋式半導(dǎo)體激光器的溫度和電流控制在恒定值，以確保輸出激光波數(shù)為 6613.25Cm?1。激光通過(guò)準(zhǔn)直器聚焦后穿過(guò)標(biāo)準(zhǔn)具內(nèi)的尿素水溶液液膜，之后被銦砷化鎵探測(cè)器(Thorlabs,PDA10CS?EC)接收。實(shí)驗(yàn)數(shù)據(jù)的采集(采樣頻率為1kHz)和處理過(guò)程均在LabVIEW中進(jìn)行。

圖2實(shí)驗(yàn)裝置示意圖Fig.2Schematic drawing of experimental setup

分別對(duì)室溫下質(zhì)量分?jǐn)?shù)為10%～50%的尿素水溶液液膜進(jìn)行 3次重復(fù)測(cè)量，其測(cè)量結(jié)果如圖 3所示。由圖3可以看出：隨著石英玻璃板間距的增加，不同質(zhì)量分?jǐn)?shù)的尿素水溶液液膜厚度的測(cè)量值與真實(shí)值的相對(duì)偏差均減小，尿素水溶液液膜厚度測(cè)量平均相對(duì)誤差約為1.50%，3 次重復(fù)性實(shí)驗(yàn)厚度標(biāo)準(zhǔn)偏差小于24.68 μm。

圖3　標(biāo)準(zhǔn)具驗(yàn)證實(shí)驗(yàn)結(jié)果Fig.3　Validation results measured inCalibration tool

2.2水平石英玻璃板上液膜蒸發(fā)研究

利用研制的單波長(zhǎng)尿素水溶液液膜測(cè)量系統(tǒng)對(duì)常溫下水平透明石英玻璃板上的初始質(zhì)量濃度為20%的尿素水溶液液膜蒸發(fā)過(guò)程進(jìn)行研究。在實(shí)驗(yàn)數(shù)據(jù)開始采集前，將透明石英玻璃板上無(wú)液膜時(shí)的激光光強(qiáng)記錄為入射光強(qiáng)。在開始采集實(shí)驗(yàn)數(shù)據(jù)后，利用注射器噴射尿素水溶液，從而在透明石英玻璃板上形成液膜。圖4所示為測(cè)得的液膜厚度隨時(shí)間的變化趨勢(shì)。由圖4 可知：由于噴射過(guò)程中(數(shù)據(jù)采集后 0～11.88 s)劇烈的光束轉(zhuǎn)向現(xiàn)象導(dǎo)致穿過(guò)液膜后的激光光束不能被接收光纖接收，從而造成了強(qiáng)烈的數(shù)據(jù)波動(dòng)；在液膜蒸發(fā)初期(數(shù)據(jù)采集后11.88～606.82 s)，液膜厚度隨時(shí)間的變化率很大，此時(shí)的液膜厚度減小可能是由液膜蒸發(fā)和液膜向外擴(kuò)散共同引起；在蒸發(fā)階段中期(數(shù)據(jù)采集后606.82～5 754.40 s)，液膜蒸發(fā)速率基本趨于穩(wěn)定(0.104 μm/s)，測(cè)量厚度的浮動(dòng)可能是不均勻蒸發(fā)引起液膜重組所致；在蒸發(fā)階段后期(數(shù)據(jù)采集后5 754.40～6000.00 s)，液膜蒸發(fā)速率較大(約0.520 μm/s)。大約在數(shù)據(jù)采集后6 000.00 s，激光測(cè)量區(qū)域附近尿素已基本結(jié)晶析出，因此，劇烈的光束轉(zhuǎn)向現(xiàn)象再次造成強(qiáng)烈的測(cè)量厚度波動(dòng)，這種劇烈的光束轉(zhuǎn)向現(xiàn)象可以在進(jìn)一步工作中通過(guò)在探測(cè)器前放置積分球來(lái)消除。

圖4水平透明石英板上尿素水溶液液厚度隨時(shí)間的變化Fig.4Liquid film thickness variation of urea?water solutions on transparent quartz plate

3 結(jié)論

1)提出了基于朗伯?比爾定律的不同質(zhì)量分?jǐn)?shù)的尿素水溶液液膜厚度測(cè)量方法，選取尿素水溶液吸收率對(duì)質(zhì)量分?jǐn)?shù)的變化率為0的波數(shù)位置(6 613.25Cm?1)，研制了單波長(zhǎng)半導(dǎo)體激光吸收光譜(DLAS)尿素水溶液液膜厚度測(cè)量系統(tǒng)。

2)利用液膜厚度可調(diào)(0～1mm)的標(biāo)準(zhǔn)具對(duì)該系統(tǒng)進(jìn)行驗(yàn)證可知，隨著標(biāo)準(zhǔn)具內(nèi)液膜厚度的增加，不同質(zhì)量分?jǐn)?shù)的尿素水溶液液膜厚度的測(cè)量值與真實(shí)值的相對(duì)偏差均減小，測(cè)量平均相對(duì)誤差約為1.50%。

3)劇烈的光束轉(zhuǎn)向造成了液膜形成和蒸發(fā)后期強(qiáng)烈的數(shù)據(jù)波動(dòng)，但是這種劇烈的光束轉(zhuǎn)向現(xiàn)象可以在進(jìn)一步工作中通過(guò)在探測(cè)器前放置積分球來(lái)消除。

4)不同溫度下尿素水溶液在近紅外區(qū)域的吸收光譜會(huì)出現(xiàn)偏移，下一步工作中將對(duì)本文提出新方法進(jìn)行優(yōu)化，以實(shí)現(xiàn)非常溫下的液膜多參數(shù)研究。

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(編輯 劉錦偉)

Film thickness measurement of aqueous solution based on single diode laser absorption spectroscopy

LIUChang,TIAN Jinghui,DING Jiaqi,GUO Xiaolong,SHI Jianwei,YANG Huinan,CAI Xiaoshu
(Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering,School of Energy and Power Engineering,University of Shanghai for Science and Technology,Shanghai 200093,China)

Abstract:A novel method was proposed to determine liquid film thickness of urea-water solutions at different mass fractions.The wavenumber position at6613.25Cm?1wasChosen,where the derivative of the absorptionCoefficient of urea-water solutions with respect to massConcentration was zero.A single diode laser absorption spectroscopy(DLAS)sensor was developed to obtain liquid film thickness of urea-water solutions.The measurement accuracy of this method was validated with aCalibration tool with which liquid film thicknessCan be adjusted from0to1mm.Furthermore,the evaporation process of liquid film of urea-water solutions on transparent quartz plate at room temperature was also studied.The results show that average relative error between the measured thickness and known liquid film thickness is about1.50%，and the standard deviation of film thickness is less than 24.68 μm for three repeated experiments.The evaporation rate of film thickness is large at the beginning of film evaporation process,and it keepsConstant in the middle of the film evaporation process.However,at the end of the evaporation process,strong fluctuation of the film thickness occurs because of the serious beam steering effect.

Key words:liquid film of aqueous solution? measurement? thickness? absorption spectroscopy? evaporation

中圖分類號(hào)：TK31

文獻(xiàn)標(biāo)志碼：A

文章編號(hào)：1672?7207(2016)01?0286?04

DOI:10.11817/j.issn.1672-7207.2016.01.039

收稿日期：2015?01?31；修回日期：2015?03?20

基金項(xiàng)目(Foundation item)：國(guó)家自然科學(xué)基金資助項(xiàng)目(51306123)；博士點(diǎn)基金聯(lián)合資助課題(20133120120008)(Project(51306123)supported by the National Natural Science Foundation ofChina? Project(20133120120008)supported by the Joint Specialized Research Found for the Doctoral Program of Higher Education)

通信作者：楊薈楠，博士，副教授，從事激光光譜測(cè)量方法與應(yīng)用的研究；E-mail: yanghuinan@usst.edu.cn