粉煤灰傳熱的影響研究

第30卷第7期計算機與應用化學(xué)2013年7月28日Computers and applied ChemistryJuly28,2013粉煤灰傳熱的影響研究楊偉',薛思瀚",張樹(shù)光1.遼寧工程技術(shù)大學(xué)建筑工程學(xué)院,遼寧,阜新,1230002.遼寧工程技術(shù)大學(xué)土木與交通學(xué)院,遼寧,阜新,123000摘要:近年來(lái)國內外競相開(kāi)展利用粉煤灰等制備保溫材料的硏發(fā)工作,以粉煤灰傳熱為硏究對象,對粉煤灰內的流場(chǎng)、溫度玚高溫壁面平均努謝爾數M進(jìn)行硏究:采用整場(chǎng)求解法方法進(jìn)行數值求解,對網(wǎng)格的獨立性和計算過(guò)程進(jìn)行了驗證:得到了粉煤灰傳熱一些基本數據,分析了粉煤灰溫度場(chǎng)和流場(chǎng)隨瑞利數Rα的變化規律。硏究結果表明:隨著(zhù)Rα增加,開(kāi)始流線(xiàn)均勻分布為一個(gè)順時(shí)針大窩,逐漸變化為流線(xiàn)集中分布在流場(chǎng)外側,而在中央基本上保持靜止狀態(tài):當Ra很小時(shí),無(wú)量綱等值線(xiàn)近似于平行高低溫壁面的垂直線(xiàn),隨著(zhù)Ra數逐漸增大,對應的溫度等值線(xiàn)近似呈高溫至低溫的線(xiàn)性變化趨勢;Ra小于105時(shí),高溫壁面Mat基本為2.37~3.31的定值;高溫壁面底部努謝爾數№!數大,最大值為30.8,上部Na數小,最小值為1.19。關(guān)鍵詞:粉煤灰:空氣;傳熱:數值模擬中圖分類(lèi)號:TD985,O414獻標識碼:A文章編號:1001-4160(2013)07-721-724DOl: 10. 11719/com. app. chem201307041引言左表面溫度為高溫壁面T、右表面溫度為低溫壁面T’上下壁面為絕熱邊界條件,粉煤灰骨架與內部空氣近年來(lái)國內外競相開(kāi)展利用無(wú)機非金屬礦物和工業(yè)溫度相等,即采用局部熱平衡假設:空氣密度隨溫度變廢棄物等制備保溫材料的研發(fā)工作。粉煤灰作為一種二化,其他熱物性為常量,采用 Brinkman擴展達西模型描次資源進(jìn)行開(kāi)發(fā)利用具有廣闊的應用前景。由于粉煤灰述流動(dòng),在模型壁面上速度均采用無(wú)滑移邊界條件。假具有孔洞結構、密度低、有較高的表面活性和較大的比設粉煤灰巖體為各向同性、均質(zhì)的多孔介質(zhì),內部充滿(mǎn)表面積等特點(diǎn),非常適合用于保溫材料。徐子芳對空氣。粉煤灰、煤矸石低溫焙燒優(yōu)等保溫磚及性能分析,馮武對充滿(mǎn)空氣的二維不可壓縮層流的穩態(tài)自然對流換威對礦物聚合法制備膨脹珍珠巖保溫材料的實(shí)驗研究。熱,其控制方程的無(wú)量綱形式如下他們對粉煤灰研究都是采用混合物的形式,單獨粉煤灰連續方程為的基礎物理研究較少。粉煤灰獨立存在時(shí)是一種多孔介△·v=0質(zhì)結構,內部充滿(mǎn)飽和空氣或水等流體介質(zhì),采用數值動(dòng)量方程為:計算的方法對他的基礎研究有重要的科學(xué)意義和實(shí)踐價(jià)P物理問(wèn)題及數值方法研究對象可選為二維模型。研究區域選定為長(cháng)寬為(VVv可p,PrVPrHXH的二維平面。物理模型見(jiàn)圖1。traPreoTan=0流體相能量方程為:Vv=V2+H(O2-0)T固體相能量方程為ar/an=0上述方程無(wú)量綱量定義為:無(wú)量綱坐標X=xHFig. I The physical modelYyH,無(wú)量綱速度U=uh,V=wHa,無(wú)量綱壓力圖1物理模型收稿日期:2013-03-15;修回日期:2013-05-23基金項目:國家自然科學(xué)基金項目資助(50804021)作者簡(jiǎn)介:楊偉(1965-),男,遼寧省阜新人,副教授,研究方向為流固耦合傳熱機理,E-mai:Igdyw@163.com聯(lián)系人:薛思瀚,E-mail:14704183608xueshanI63a163.com計算機與應用化學(xué)2013,30(7)P-pH'p'oa2,無(wú)量綱準則普朗特數Pr=w、達西數05021.0.505,從計算結果可以看出,都精確到小數點(diǎn)Da=KH2,瑞利數Ra=gB( T-THV a,無(wú)量綱溫度后2位,后3種網(wǎng)格對計算結果基本沒(méi)有影響,網(wǎng)格已具0=(7-T)(T-T)。其中:C為慣性常數;an為多孔介質(zhì)有獨立性。采用100×100網(wǎng)格進(jìn)行計算。在計算之前首的有效熱擴散系數,a.k()}:k,有效導熱系數先進(jìn)行計算方法和程序的考核。利用 SIMPLER算法進(jìn)行求解,對流項采用QUCK格式12進(jìn)行離散,使計算具有kn(-9+,其中φ為孔隙率,k和k,分別是多孔介較高的精度?？己顺跏贾祵τ嬎憬Y果影響,選擇無(wú)量綱質(zhì)內固體相和流體相的導熱系數。溫度初始值分別為0、0.5和1進(jìn)行計算,計算過(guò)程如圖2(圖無(wú)量綱邊界條件為2僅顯示計算100步)。從如圖2可知方程在收斂很快,說(shuō)Y=0,Y=1,U=V=0,OT/an=0;明本文計算方法和程序的計算結果是可靠的。X=1,U=V=0,=01.0局部努謝爾數及平均努謝爾數定義為initial value is 0.5initial value is Iwarl!0.2平均努謝爾數Mu過(guò)局部努謝爾數沿壁面積分獲得,H沿壁面總長(cháng)度。選代次數3網(wǎng)格劃分、數值求解Fig 2 Computation process of dimensionless temperature 0,圖2無(wú)量綱溫度θ計算過(guò)程采用非均勻網(wǎng)格明,網(wǎng)格的單元分別采用60×60、80×80、100×100、120×120共4種網(wǎng)格進(jìn)行計算。為取得4計算結果和討論穩態(tài)解1,假定變量迭代求解滿(mǎn)足在計算中取粉煤灰孔隙率為0.6,比熱比為1,∑叫:g.∑q≤10Pr=0.701,Da數為102。分別針對Ra數為1、10、102、為收斂(為變量,n為迭代次數)?？疾辄c(diǎn)(0.5,0.5)103、104、105、105、107、103、10、101°、10和導熱工無(wú)量綱溫度值,計算結果分別為:0.5006、0.5013,況進(jìn)行計算,部分計算結果見(jiàn)圖3~圖6。0.045-130.005(d)()a=10,(b)Ra10,()R10,(d)Ra=10Fig 3 Stream lines curve of Ra圖3流線(xiàn)數隨Ra變化關(guān)系從圖3流線(xiàn)數隨Ra變化關(guān)系可以看出:隨著(zhù)Ra增靜止狀態(tài)加,粉煤灰內對流增強,由Ra=103時(shí)流函數最大值圖4為無(wú)量綱溫度θ隨Ra變化關(guān)系。從圖中可以看Wmax≈0.045,發(fā)展至Ra=10時(shí)wmax≈13;對流強出:當Ra很小時(shí),當粉煤灰內內對流作用較弱時(shí),熱的度分布愈來(lái)愈不均勻,由開(kāi)始流線(xiàn)均勻分布狀態(tài)Ra=103傳輸主要依靠傳導作用,因此無(wú)量綱等值線(xiàn)近似于平行一個(gè)順時(shí)針大窩,逐漸變化為流線(xiàn)集中分布在流場(chǎng)外側高低溫壁面的垂直線(xiàn),隨著(zhù)Ra數逐漸增大,對應的溫度(Ra=103),即在流場(chǎng)外圍劇烈流動(dòng)而在中央基本上保持等值線(xiàn)近似呈高溫至低溫的線(xiàn)性變化趨勢;而當對流作2013,3楊偉,等:粉煤灰傳熱的影響研究用較強時(shí),它成為熱傳輸的主要動(dòng)力。0.9(a)Ra=103,(b)Ra=105,(e)Ra=10°,(d)Ra=10Fig 4 The variation relationship of 0 with the Ra圖4θ隨Ra變化關(guān)系值為237~3.31:超過(guò)該值后,Nu沿豎向壁面由高向低變小,最大值為30.8,最小值為1.19。分析其物理機理為底部溫差大,Mu數大,上部溫差小,Mt數小結語(yǔ)通過(guò)對粉煤灰傳熱研究,可得到以下結論:(1)隨著(zhù)Ra增加,粉煤灰內對流增強并且對流強度分布愈來(lái)愈不均勻,由開(kāi)始流線(xiàn)均勻分布為一個(gè)順時(shí)針大窩,逐漸變化為流線(xiàn)集中分布在流場(chǎng)外側,而在中央基本上保持靜止狀態(tài)。ig. 5 Nu curve of Ra on high temperature wall.(2)當Ra很小時(shí),熱的傳輸主要依靠傳導作用,無(wú)圖5高溫壁面Nu與Ra變化關(guān)系量綱等值線(xiàn)近似于平行高低溫壁面的垂直線(xiàn),隨著(zhù)Ra數圖5為高溫壁面Na與Ra變化關(guān)系。從圖5可明顯看逐漸增大,對應的溫度等值線(xiàn)近似呈高溫至低溫的線(xiàn)性出:瑞利數對腔內對流換熱過(guò)程的影響是顯著(zhù)的,當瑞變化趨勢;而當對流作用較強時(shí),它成為熱傳輸的主要利數小于103時(shí),粉煤灰內自然對流作用非常微弱,此時(shí)動(dòng)力腔內換熱機理主要依靠導熱進(jìn)行;隨著(zhù)瑞利數逐漸增加,(3)當瑞利數小于10時(shí),腔內換熱主要依靠導熱進(jìn)№u迅速增加,粉煤灰內換熱機理主要依靠對流換熱進(jìn)行;隨著(zhù)瑞利數逐漸增加,№迅速增加,換熱杋理主要依靠對流換熱進(jìn)行。(4)當瑞利數小于103時(shí),高溫壁面M基本為Heat conduction2.37~3.31的定值;超過(guò)該值后,底部Ma數大,最大值為30.8,上部M數小,最小值為1.19。Refer20Ra-105I Luo Daocheng. Yi Pinggui and Liu Junfeng. Development status0°and suggestion for comprehensive utilization of fine coal ash in2002,126):8-10perlite heat-insulating products, Non-metallic Mines, 2000,(9)3 Han Fuxing. Research on utilizing ceramic waste for porousceramics. Ceramic Studies Journal, 2002, 17(1): 24-26Fig 6 Nu curve of Ra on high temperature wallInvestigation of preparation and sintering of Sic porous ceramics圖6高溫壁面M與Ra變化關(guān)系Acta Chimica Sinica, 2003, 61(12): 2002-2007.5 Xu Zifang, Zhang Mingxu, Min Fan. Performance analysis of圖6為高溫壁面M與Ra變化關(guān)系。從圖5可明顯看preparing high-class insulation brick using fly ash, coal gangue via出:當瑞利數小于10°時(shí),高溫壁面Mt基本為一直線(xiàn)其oasting at low temperature conditions. Non-metallic Mines, 2010,33(4):13-166 Feng Wuwei, Ma Hongwen, Wang Gang, et al. Experiment724計算機與應用化學(xué)2013,30(7)research on preparation of expended perlite heat insulatingmaterial by mineral polymerization. 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School of Civil Engineering and Transportation, Liaoning Technical University, Fuxin 123000, Liaoning Province, China)Abstract: In recent years, researches and development works on making thermal insulation material with fly ash and others are competitivelaunched at home and abroad. Heat transfer of fly ash is the object of study. The flow field, temperature field and average Nusselt numbers nieof the high temperature wall surface in the fly ash are studied. Numerical solution is carried out using whole field solving method. The gridindependence and computation process are verified. Some basic data is obtained and the change of temperature field and flow field of fly ashwith Rayleigh number Ra is analyzed. The results of the study show that: with the increase of Ra, at first the flow lines are uniformlydistributed as a clockwise big nest, gradually the flow lines are concentrated in the outer side of the flow field, while in the centre they remainmainly stationary state: When Ra is very small, non-dimensional contours approximately parallel the vertical line of wall surface of the highand low temperature, with the gradually increase of Ra, the corresponding temperature contours approximately forms as the linear trend ofthe high temperature to the low temperature: When Ra is less than 105, Nu of the high temperature wall surface is a constant value of 2. 37to 3.31: Nu of the bottom of high temperature wall surface is large, the maximum value is 30.8, Nu of the upper part is small, theminimum value is 1.1Keywords: fly ash; air; heat transfer; numerical simulation( Received:2013-03-15; Revised:2013-05-23)