張建偉,張志剛,馮穎,施博文
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撞擊流反應(yīng)器內(nèi)流場特性研究進(jìn)展
張建偉,張志剛,馮穎,施博文
(沈陽化工大學(xué)能源與動力工程學(xué)院,遼寧沈陽 110142)
綜述了國內(nèi)外撞擊流反應(yīng)器內(nèi)流場速度和脈動振蕩特性的研究進(jìn)展。目前,對非限制撞擊流反應(yīng)器內(nèi)撞擊流體的徑向速度發(fā)展及軸向速度與撞擊駐點(diǎn)的脈動特性都有了系統(tǒng)研究,對撞擊駐點(diǎn)的振蕩模式進(jìn)行了劃分,并得出大量適用于不同噴嘴間距的速度關(guān)聯(lián)式,但對駐點(diǎn)振蕩模式的產(chǎn)生機(jī)理還沒有明確解釋。層流狀態(tài)下隨著雷諾數(shù)增大,眾多學(xué)者對受限撞擊流反應(yīng)器內(nèi)流型的流動模式進(jìn)行了劃分,提出了出現(xiàn)吞噬流模式的臨界參數(shù)關(guān)聯(lián)式,由于結(jié)構(gòu)等參數(shù)的變化當(dāng)前還沒有普遍適用控制流型模式變化的關(guān)聯(lián)式。在浸沒撞擊流反應(yīng)器內(nèi)用非線性分析法確定撞擊區(qū)并劃分了流場區(qū)域,但目前尚不能揭示湍流狀態(tài)下流場能量分布與速度信號等的變化規(guī)律。最后作者對撞擊流反應(yīng)器內(nèi)部流場結(jié)構(gòu)的研究前景進(jìn)行了展望。
撞擊流反應(yīng)器;流場特性;脈動振蕩;速度場
撞擊流技術(shù)在相間傳遞方面具有獨(dú)特性質(zhì),現(xiàn)已成為工業(yè)應(yīng)用上的一種重要流動形式,不乏國內(nèi)外學(xué)者對其進(jìn)行研究[1-4],已廣泛應(yīng)用于混合[5]、干燥[6]、吸收[7]、燃燒[8]、結(jié)晶[9]、超細(xì)粉體制備[10]等領(lǐng)域。但受撞擊流場流動的復(fù)雜性影響,目前對撞擊流的研究還是以關(guān)注工業(yè)應(yīng)用中的湍流規(guī)律為主,而對于其基本流場流動結(jié)構(gòu)研究很少。因此,很有必要把對撞擊流的研究重點(diǎn)從工業(yè)應(yīng)用轉(zhuǎn)移到基本流動的實驗和數(shù)值模擬研究上來[11]。本文作者對撞擊流反應(yīng)器流場流動規(guī)律的研究進(jìn)展進(jìn)行了 綜述。
經(jīng)過幾十年的發(fā)展,撞擊流的結(jié)構(gòu)形式不斷完善,尤其在進(jìn)入21世紀(jì)以來,流體力學(xué)實驗和CFD模擬手段日新月異,對撞擊流的研究發(fā)展起到了巨大的推動作用,其分類和研究領(lǐng)域也趨于多元 化[12]。其中屠功毅[13]對撞擊流的發(fā)展形式進(jìn)行了分類,如表1。
表1 撞擊流分類
眾所周知,推動力、相介面積和傳遞系數(shù)是決定熱質(zhì)傳遞的3要素,提高推動力和增加相介面積受材質(zhì)、過程特性和適用場合的限制,而傳遞系數(shù)與相對速度的1/3~1/4次冪成正比。因此,提高相間相對速度是強(qiáng)化傳遞過程最有效的途徑[14]。
1.1 徑向速度特性及其擴(kuò)展率
在撞擊流反應(yīng)器中,撞擊后流體的徑向流動形態(tài)及擴(kuò)展對大尺度渦結(jié)構(gòu)的產(chǎn)生和發(fā)展有顯著影響,而這些大尺度渦結(jié)構(gòu)直接影響化工過程中的混合單元。因此,通過研究反應(yīng)器內(nèi)徑向流動特性來優(yōu)化其幾何結(jié)構(gòu)和操作參數(shù)對提高混合效率具有重要意義。
由上可見,研究者獲得了不同研究體系下的徑向速度關(guān)聯(lián)式,這表明徑向射流發(fā)展受徑向距離、入口初速度、噴嘴間距和直徑的影響也不相同;并且四噴嘴結(jié)構(gòu)流場的速度特性也不是兩噴嘴結(jié)構(gòu)流場的簡單疊加。目前學(xué)者們對撞擊流反應(yīng)器的研究主要集中在兩噴嘴和四噴嘴結(jié)構(gòu)形式,而工業(yè)生產(chǎn)中存在因兩不等量物料對撞造成“噴嘴堵塞”等問題,為此重視開發(fā)研究三噴嘴結(jié)構(gòu)撞擊流反應(yīng)器或可解決這一問題。
有學(xué)者認(rèn)為彎曲壁面內(nèi)兩流體的初始動量比決定了流體撞擊位置和撞擊后的流動方向,其徑向射流擴(kuò)展率為0.15;而平面壁面射流撞擊流的徑向射流擴(kuò)展率在=20時為0.2、=240時為0.3,約為自由圓射流的3倍[21-23]。以上受實驗條件影響得出不同結(jié)論,而徑向射流擴(kuò)展率的大小不僅反映流場特性還直接影響混合效果,為對擴(kuò)展率有更全面系統(tǒng)的認(rèn)識,李偉鋒等[18]在同一體系內(nèi)(= 0.5~100)發(fā)現(xiàn)擴(kuò)展率在0.5<<8時緩慢增長到0.15左右,在8<<20時快速增至0.3左右,是自由圓射流的1.5~3倍。
綜上,撞擊流徑向射流擴(kuò)展率隨噴嘴間距增大而增大,且比自由圓射流的徑向射流擴(kuò)展更快。另外在模擬預(yù)測實驗時,受撞擊流駐點(diǎn)不穩(wěn)定性影 響[24-25],采用穩(wěn)態(tài)數(shù)值模擬方法存在一定誤差,而選取RNG模型較Standard和RNG模型可獲得精度更高的預(yù)報擴(kuò)展率。
1.2 軸向速度特性
另外,在數(shù)值計算時模型的選擇和網(wǎng)格的劃分都對計算精度有很大影響[30],因此尋找一個高精度的預(yù)測模型對研究復(fù)雜的撞擊流場是很有必要的。但眾多研究表明,DNS僅適用于低雷諾數(shù)下,而RANS對預(yù)測不夠準(zhǔn)確,相對而言LES精度更高并逐漸成為模擬撞擊流場的主流模型[31-33]。ABDEL-FATTAH[34]嘗試用v2-湍流模型模擬撞擊駐點(diǎn)處速度、壓力等分布情況,相比Standard-和RNG 模型與實驗結(jié)果更接近。
撞擊流反應(yīng)器內(nèi)流場流動結(jié)構(gòu)是極其復(fù)雜的,其中撞擊面的不穩(wěn)定性和駐點(diǎn)偏移是撞擊流反應(yīng)器中不可忽視的重要現(xiàn)象,認(rèn)識并控制這種往復(fù)振蕩不僅能確保整個流場的均勻發(fā)展,還可以增加裝置的穩(wěn)定性和使用壽命。
2.1 振蕩理論的提出與發(fā)展
表2 撞擊流反應(yīng)器的徑向速度經(jīng)驗關(guān)聯(lián)式
2.2 操作參數(shù)對駐點(diǎn)振蕩特性的影響
在以上振蕩理論的基礎(chǔ)上,研究者對駐點(diǎn)振蕩區(qū)間及其影響參數(shù)進(jìn)行了系統(tǒng)分析,發(fā)現(xiàn)在不同噴嘴間距范圍內(nèi)駐點(diǎn)偏移量受氣速比、噴嘴直徑等因素的影響有所不同,下文按噴嘴間距由小到大順序進(jìn)行綜述。
小噴嘴間距時,低雷諾數(shù)下兩對稱噴嘴產(chǎn)生的流場和駐點(diǎn)位置關(guān)于噴嘴間距中心面對稱,但改變氣速比導(dǎo)致撞擊面彎曲、駐點(diǎn)偏移并有回流產(chǎn)生,同時鋒面變化隨雷諾數(shù)的增大滯后現(xiàn)象明顯[40];而且噴嘴間距(=2)微小偏差能使撞擊面偏移中心0.15[26]。另外,李偉鋒等[29]考察噴嘴間距、氣速比和出口速度分布發(fā)現(xiàn),駐點(diǎn)偏移量隨噴嘴間距 (<2)增大或氣速比減小而增大,邊界層厚度大的“禮帽”分布對駐點(diǎn)偏移量更大。
中等噴嘴間距時,李偉鋒等[24]總結(jié)OGAWA 等[21,23,41]研究,針對不同間距下駐點(diǎn)位置是否由動量比決定的問題研究了不同氣速比下(2<<10)的駐點(diǎn)偏移規(guī)律,認(rèn)為2<<8時,駐點(diǎn)對氣速比的變化反應(yīng)靈敏且近似呈線性變 化[42],<2或>8時駐點(diǎn)隨氣速比變化不明顯且與氣速比有非線性關(guān)系。其中,文獻(xiàn)[41]實驗條件=4.3,處于駐點(diǎn)受氣速比影響敏感范圍;而文獻(xiàn)[21]實驗條件=240處于不敏感范圍。由以上可見不同間距下駐點(diǎn)受動量比影響不同。另外,有報道三角形噴嘴撞擊流的駐點(diǎn)偏移量與氣速比呈正比[43]。
綜上,大噴嘴間距撞擊流駐點(diǎn)偏移量受噴嘴直徑、間距及氣速比影響較大,而對中、小噴嘴間距情況研究卻未考察噴嘴直徑對其變化,僅得出間距及氣速比對駐點(diǎn)偏移的影響規(guī)律,可在今后研究中綜合考察參數(shù)變化對流場不穩(wěn)定性的影響。
2.3 外界干擾對脈動及振蕩頻率的影響
學(xué)者在考察參數(shù)變化對流場脈動影響方面已做了大量工作,但影響流場脈動的不只是一些人為可控的參數(shù)因素,還有實驗設(shè)備自身產(chǎn)生的不可消除的干擾因素等。
如單束射流撞擊平板時產(chǎn)生的低頻主頻分量和高頻分量分別是由射流管路的機(jī)械振動和渦流湍動造成的[45]。而且實驗裝置自身產(chǎn)生的擾動影響了流場振蕩頻率,特別是在脈動振幅達(dá)到臨界值時對流場有調(diào)制作用[46]。另外,王亭杰等[45,47]認(rèn)為低頻區(qū)的射流脈動頻率等同于往復(fù)泵的脈動頻率,且脈動頻率范圍是由高壓泵的幾何結(jié)構(gòu)決定,且在高氣速時平面撞擊流的偏轉(zhuǎn)振蕩頻率可達(dá)226Hz。
可見射流脈動頻率不僅受撞擊面不穩(wěn)定性的影響,而且與泵的工作頻率、管路的振動及渦流湍動的強(qiáng)弱都有著密切聯(lián)系。但由于泵及管路等裝置自身因素難以消除,而為進(jìn)一步探究這些因素對振蕩頻率的影響,LI等[48]將激勵因素引入撞擊流反應(yīng)器,發(fā)現(xiàn)激勵引起撞擊面沿軸向周期性振蕩且振蕩頻率等于激勵頻率,并在文獻(xiàn)[49]中報道了聲波激勵對撞擊流的振蕩影響,發(fā)現(xiàn)水平振蕩區(qū)(≤4)的振蕩頻率等于聲波激勵頻率。以上實驗驗證了外界激勵在一定條件下能對振蕩振幅產(chǎn)生明顯影響,這為研究流場的脈動規(guī)律拓寬分析方向但同時也為揭示脈動本質(zhì)帶來更大挑戰(zhàn)。
2.4 受限空間撞擊流反應(yīng)器的流場振蕩特性
隨撞擊流反應(yīng)器的多元化發(fā)展,學(xué)者們設(shè)計了一類限制撞擊流體在反應(yīng)腔內(nèi)自由擴(kuò)散的反應(yīng)器,這類反應(yīng)器多用于快速微觀混合的單元過程,在尺寸上基本都采用微型結(jié)構(gòu)。其中,受限撞擊流反應(yīng)器(CIJR)和T型撞擊流反應(yīng)器是目前最常見的。
2.4.1 受限撞擊流反應(yīng)器(CIJR)流場振蕩特性
對于CIJR,低雷諾數(shù)時鋒面擺動頻率受撞擊面的沖擊反饋作用而保持穩(wěn)定,并與平均速度03/2成正比,較非受限撞擊流反應(yīng)器高;受空間限制撞擊破碎產(chǎn)生的漩渦增強(qiáng)了流場混沌和湍流效應(yīng),尤其在脈動頻率接近撞擊流自身振蕩頻率時,脈動對流場影響作用最為顯著[52-50]。而LIU等[53]則對流場速度進(jìn)行分析并用直接測量結(jié)果驗證CFD模型的微尺度湍流流動,發(fā)現(xiàn)-模型對湍流預(yù)測不準(zhǔn)確。
另外為深入認(rèn)識流場流動形態(tài),研究者對不同雷諾數(shù)下撞擊后流場進(jìn)行了相應(yīng)的流型劃分,并模擬發(fā)現(xiàn)二維、三維撞擊流場經(jīng)歷的流型有所差別。
學(xué)者發(fā)現(xiàn)<100時反應(yīng)器內(nèi)兩股流體經(jīng)撞擊壓縮變形后各自穩(wěn)定地呈分離流模式;增大至120左右,駐點(diǎn)難以穩(wěn)定在對置噴嘴間的幾何中心,撞擊面呈“S”形擺動,出現(xiàn)動態(tài)混沌模式;并認(rèn)為撞擊面邊緣的擺動與撞擊駐點(diǎn)的振蕩存在關(guān) 聯(lián)[54-57]。之后LI等[58]細(xì)分完善了撞擊流振蕩區(qū),發(fā)現(xiàn)在>300時自持振蕩模式變?yōu)榱藷o規(guī)則振蕩模式。在層流狀態(tài)下的二維受限反應(yīng)器內(nèi)隨雷諾數(shù)和噴嘴間距比的增大,流場依次經(jīng)歷了穩(wěn)定區(qū)、周期性碰撞區(qū)、隨機(jī)碰撞區(qū);而三維圓撞擊流場只有穩(wěn)定區(qū)和隨機(jī)碰撞區(qū),且撞擊面穩(wěn)定性主要受雷諾數(shù)影響[59-60]。
2.4.2 T型撞擊流反應(yīng)器的流場振蕩特性
隨受限反應(yīng)器結(jié)構(gòu)的多樣化發(fā)展,眾多學(xué)者對用于快速微觀混合的T型受限撞擊流反應(yīng)器內(nèi)流動形態(tài)進(jìn)行研究,并將流場依次劃分為比較穩(wěn)定的分離流模式(<50)、渦流模式、吞噬流模式和振蕩模式等[61-65]。其中,渦流模式處于分離流模式和吞噬流模式之間,在實驗中難以辨別而僅在模擬中發(fā)現(xiàn)。吞噬流模式由ENGLER等[61]在T型反應(yīng)器中發(fā)現(xiàn)并經(jīng)BOTHE等[65-66]驗證。不穩(wěn)定的振蕩模式約出現(xiàn)在≥195時,后經(jīng)學(xué)者細(xì)分為周期性振蕩(240<<400)、擬周期振蕩(400<<500)和混亂振蕩(>500)[62-64];而屠功毅[13]則發(fā)現(xiàn)230<<400時出現(xiàn)周期性的非對稱振蕩模式,400<<480為對稱與非對稱振蕩周期的轉(zhuǎn)化區(qū)。另外研究表明,未充分發(fā)展的入口速度將導(dǎo)致出現(xiàn)吞噬流模式及振蕩模式的雷諾數(shù)增大[13,67]。
目前學(xué)者對流場結(jié)構(gòu)的研究不斷深入細(xì)化,受實驗體系、流場穩(wěn)定性、數(shù)據(jù)采集等因素影響,在流型劃分方面存在部分差異,但在影響流型發(fā)展轉(zhuǎn)變的認(rèn)識上基本是一致的。
為探究撞擊區(qū)上部空間、射流間距和寬高比對流場振蕩影響,SULTAN等[68-69]改進(jìn)T型反應(yīng)器(圖1),發(fā)現(xiàn)在撞擊點(diǎn)下游形成旋轉(zhuǎn)的渦街并將其命名為“自持混沌流模式”,但這種自持振蕩與之前T型反應(yīng)器的振蕩模式不同;同時GAO等[33,70]在圓形T型反應(yīng)器內(nèi)也發(fā)現(xiàn)的變化影響了流場駐點(diǎn)的偏移。
圖1 T型撞擊流反應(yīng)器
通過對T型反應(yīng)器流場結(jié)構(gòu)的研究認(rèn)識,學(xué)者們發(fā)現(xiàn)當(dāng)流體處于吞噬流模式時具有良好的微觀混合效果,為使流體處于該模式而獲得較好混合效果,ENGLER等[61,71]提出了渦流模式轉(zhuǎn)向吞噬流模式的臨界參數(shù)的關(guān)聯(lián)式(表3)。但在后續(xù)研究中,CHERLO等[72]發(fā)現(xiàn)反應(yīng)器入口寬度或?qū)挾刃∮谏疃葧r,需要增大雷諾數(shù)才有可能出現(xiàn)吞噬流模式,同時POOLE等[68,73]也發(fā)現(xiàn)文獻(xiàn)[71]中的關(guān)聯(lián)式在許多工況下也不適用。
由上述可見,反應(yīng)器結(jié)構(gòu)對流型模式變化有著顯著影響,而且隨反應(yīng)器結(jié)構(gòu)的多元化發(fā)展,要想得出一個普遍適用控制流型模式變化的關(guān)聯(lián)式是十分困難的。
以上研究者對受限空間撞擊流反應(yīng)器內(nèi)流場流型和脈動振蕩模式進(jìn)行研究劃分,得出對應(yīng)流型轉(zhuǎn)變的臨界條件及關(guān)聯(lián)式,并對在工業(yè)中應(yīng)用較好的流型進(jìn)行著重研究,已初步形成該類反應(yīng)器的流場理論體系,為今后研究及轉(zhuǎn)化應(yīng)用提供參考和理論指導(dǎo)。但以上研究多局限于層流狀態(tài),因湍流結(jié)構(gòu)的復(fù)雜性,目前對湍流狀態(tài)下流型結(jié)構(gòu)的研究報道較少。
2.5 浸沒撞擊流反應(yīng)器的流場振蕩特性
我國伍沅教授等[74-75]為研究液體介質(zhì)在撞擊流反應(yīng)器中的應(yīng)用提出了液體連續(xù)相撞擊流反應(yīng)器,設(shè)計了浸沒循環(huán)撞擊流反應(yīng)器(SCISR)(圖2)和立式循環(huán)撞擊流反應(yīng)器(VCISR),并考察了其在微觀混合及超細(xì)粉體制備方面的性能;之后楊俠等[76]對VCISR的混合性能進(jìn)行了大量研究工作。
圖2 浸沒循環(huán)撞擊流反應(yīng)器
張建偉等[77-78]則用希爾伯特-黃變換、小波變換等非線性方法對SCISR撞擊區(qū)流場的非線性和混沌特性進(jìn)行研究,分析流場信號發(fā)現(xiàn)流體粒子產(chǎn)生的能量集中在低頻區(qū),找到能量分布與流型轉(zhuǎn)變之間的對應(yīng)關(guān)系,把流場劃分為中心區(qū)、渦旋區(qū)和回流區(qū);在分析撞擊流壓力波動信號時發(fā)現(xiàn)撞擊區(qū)流體具有混沌特性、壓力波動信號具有多重分形特性,得到了撞擊區(qū)的徑向范圍約為0.33倍導(dǎo)流筒直徑,并認(rèn)為是撞擊區(qū)粒子脈動產(chǎn)生的不同尺度漩渦造成了混沌[79-82]。SUN等[83]從功率譜上發(fā)現(xiàn)浸沒循環(huán)撞擊流反應(yīng)器內(nèi)壓力波動主要集中在1000Hz以下,且波動強(qiáng)度隨撞擊速度增大而增大。
由于這種靠導(dǎo)流筒內(nèi)螺旋槳旋轉(zhuǎn)提供的推動力無法滿足較大撞擊強(qiáng)度要求,對此張建偉等[84-85]設(shè)計了對置撞擊流反應(yīng)器(圖3),發(fā)現(xiàn)撞擊后流體含能大尺度結(jié)構(gòu)基本集中在非穩(wěn)定徑向射流附近的低階模態(tài),而駐點(diǎn)振蕩幅值集中在0.1~0.范圍內(nèi)且沒有固定周期,并建立了振幅與噴嘴間距的聯(lián)系。
表3 渦流模式轉(zhuǎn)向吞噬流模式的臨界參數(shù)關(guān)聯(lián)式
圖3 浸沒對置撞擊流反應(yīng)器
通過利用以上非線性等方法的研究分析,目前對液體介質(zhì)在撞擊流反應(yīng)器中的流場能量分布、流型轉(zhuǎn)變和壓力波動、駐點(diǎn)振蕩等特性有了整體了解,但對劇烈湍動的撞擊區(qū)仍缺乏全面認(rèn)識。另外,針對3種物料反應(yīng)及兩不等量物料撞擊造成的“噴嘴堵塞”問題,作者對水平三向撞擊流反應(yīng)器進(jìn)行研究,發(fā)現(xiàn)一種新的流型——漏斗徑向射流[86],但受噴嘴結(jié)構(gòu)水平布置的影響,目前的實驗條件很難全面收集這種流型信息。
綜上,隨實驗手段及模擬方法的不斷發(fā)展,人們對撞擊流反應(yīng)器內(nèi)流場流動規(guī)律有了深入認(rèn)識。由于現(xiàn)有的湍流理論體系還不能解釋撞擊區(qū)復(fù)雜無序的流動機(jī)理,限制了撞擊流技術(shù)的發(fā)展應(yīng)用,對此可嘗試建立非線性分析等方法完善湍流理論來揭示這種復(fù)雜湍流的規(guī)律。而且,對流場結(jié)構(gòu)的研究不能只局限于改變實驗條件,應(yīng)全面分析影響流場的外界參數(shù),尤其應(yīng)重視實驗裝置自身振動等因素對流型變化的影響。其次,目前對撞擊流反應(yīng)器的研究多以單組對置撞擊結(jié)構(gòu)為主,而對多組同時撞擊時流場擴(kuò)展的運(yùn)動規(guī)律鮮有報道。針對工業(yè)應(yīng)用反饋提出的噴嘴堵塞問題以及多物料反應(yīng)需求,迫切需要開發(fā)研究多噴嘴排列或協(xié)同其他元件的新型裝置。另外,僅依靠當(dāng)前實驗手段檢測、收集的流場信息并不完整,將計算流體力學(xué)與先進(jìn)PIV、PLIF技術(shù)有機(jī)結(jié)合,使實驗與模擬互相驗證,不斷促進(jìn)流場規(guī)律的研究。
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Research progress of flow field characteristics in impinging stream reactor
ZHANG Jianwei,ZHANG Zhigang,F(xiàn)ENG Ying,SHI Bowen
(College of Energy and Power Engineering,Shenyang University of Chemical Technology,Shenyang 110142,Liaoning,China)
The features of velocity field and pressure fluctuation in impinging stream reactor were summarized in the paper. The radial velocity, axial velocity and oscillation behavior in the unconfined impinging stream reactor were studied systematically. The oscillatory modes of the stagnation points were divided. And a large number of velocity correlation formulas suitable for different nozzle distance were obtained, but the mechanism of oscillation was not clearly explained. With the increase of the Reynolds number in the laminar flow state, many scholars had divided the flow type in the confined impinging stream reactor, and proposed the critical parameter correlation of engulfment flow. Due to the change of the structure parameters, there was no universally applicable correlation. The impinging zone and the flow field pattern were determined by non-linear analysis in the submerged impinging stream reactor. However, the mechanism of energy distribution and velocity signal in the turbulent flow field cannot be revealed. The trends of research on flow field in impinging stream reactor were also pointed out.
impinging stream reactor;flow characteristics; fluctuation and oscillation;velocity field
TQ052
A
1000–6613(2017)10–3540–09
10.16085/j.issn.1000-6613.2016-2101
2016-11-15;
2017-03-14。
國家自然科學(xué)基金項目(21476141)。
張建偉(1964—),男,博士,教授。E-mail:zhangjianwei64@163.com。