何鴻展,宋翀芳,潘武軒,雷勇剛

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基于CFD的防風(fēng)抑塵網(wǎng)非均勻孔隙率的優(yōu)化研究

何鴻展,宋翀芳*,潘武軒,

(太原理工大學(xué)環(huán)境科學(xué)與工程學(xué)院,山西 太原 030024)

孔隙率是影響抑塵網(wǎng)防護(hù)效果的最主要因素,不同孔隙率抑塵網(wǎng)對(duì)料堆表面的顯著作用區(qū)域不同,高孔隙率(30.3)網(wǎng)后料堆中下部揚(yáng)塵得到明顯抑制,低孔隙率(<0.3)網(wǎng)的抑塵作用則于料堆上部突顯.基于均勻孔隙率的抑塵區(qū)域提出不同孔隙率組合的非均勻抑塵網(wǎng),選取6種典型非均勻工況,應(yīng)用Fluent6.3對(duì)網(wǎng)和料堆周圍流場(chǎng)進(jìn)行數(shù)值模擬,結(jié)果顯示:網(wǎng)下部孔隙率(L)相同,上部孔隙率(H)由0增至0.1時(shí),網(wǎng)后氣流擾動(dòng)減弱,基于湍流結(jié)構(gòu)和料堆受力判定H取0.1較好;網(wǎng)上部孔隙率(H)相同,下部孔隙率(L)由0.3增至0.6時(shí),緊貼料堆表面風(fēng)速隨L增大而增大,L為0.3時(shí)最優(yōu).比較非均勻抑塵網(wǎng)最佳工況(H=0.1/L=0.3)與均勻網(wǎng)(=0.1和=0.3)的料堆表面受力顯示:H=0.1/L=0.3非均勻網(wǎng)可使起塵量最大的迎風(fēng)面的各個(gè)區(qū)域剪切力均顯著減小,中下部比=0.1時(shí)減小85.2%,上部比=0.3時(shí)減小84.3%,料堆表面剪切力總和的減少量可達(dá)均勻網(wǎng)時(shí)的50%左右.

防風(fēng)抑塵網(wǎng)；非均勻孔隙率；數(shù)值模擬；剪切力

多個(gè)城市PM2.5和PM10的源解析表明,揚(yáng)塵已成為大氣顆粒物的主要來源[1-4].控制大型堆場(chǎng)和港口的露天揚(yáng)塵是環(huán)境治理的重要工作,營(yíng)建防風(fēng)抑塵網(wǎng)作為削減揚(yáng)塵的有效措施,已成功應(yīng)用在實(shí)際工作中[5-8].

國內(nèi)外學(xué)者在優(yōu)化抑塵網(wǎng)防護(hù)系統(tǒng)的研究領(lǐng)域得出了許多重要的結(jié)論[9-13],普遍認(rèn)為孔隙率是影響抑塵網(wǎng)防護(hù)性能的最主要因素[14-17]宣捷[18]等通過風(fēng)洞實(shí)驗(yàn),分析抑塵網(wǎng)周圍空氣流場(chǎng)和料堆起塵特性,得出抑塵網(wǎng)的抑塵效果取決于網(wǎng)的孔隙率;Subhas等[19]以脈動(dòng)壓力、平均流速和表面平均壓力分布為表征,分析了不同孔隙率時(shí)抑塵網(wǎng)上部空氣的分離狀態(tài),發(fā)現(xiàn)氣流流動(dòng)的平均壓力對(duì)孔隙率具有強(qiáng)烈的敏感性; Lee等[20]驗(yàn)證了數(shù)值模擬結(jié)果與風(fēng)洞實(shí)驗(yàn)數(shù)據(jù)的一致性,指出孔隙率在0.3~0.5范圍內(nèi)能有效削減湍流動(dòng)能和料堆表面平均壓力;林官明等[21]應(yīng)用子波分析研究了孔隙率為0.4的抑塵網(wǎng)后的湍流信號(hào),發(fā)現(xiàn)抑塵網(wǎng)在降低風(fēng)速的同時(shí)也降低了渦旋發(fā)生的頻率;Dong[22]將網(wǎng)后流場(chǎng)劃分為7個(gè)典型區(qū)域,討論了孔隙率與區(qū)域之間的內(nèi)在聯(lián)系,為探究抑塵網(wǎng)的抑塵效果提供了關(guān)于動(dòng)力機(jī)制的重要科學(xué)理論.

目前,對(duì)孔隙率的研究大多是針對(duì)抑塵網(wǎng)均勻孔隙率時(shí)的抑塵效果,即在同一孔隙率下,以料堆表面風(fēng)速最小或散塵量之和最小為判定最佳孔隙率的依據(jù).但是,通過料堆表面微觀動(dòng)力學(xué)研究發(fā)現(xiàn):不同孔隙率的抑塵作用隨料堆表面位置而異[9,20,23-25].目前尚無一個(gè)均勻孔隙率可使料堆表面所有位置散塵量均為最小.本文基于均勻抑塵網(wǎng)研究結(jié)果,選擇多種孔隙率組合的非均勻抑塵網(wǎng),對(duì)料堆周圍風(fēng)場(chǎng)的微觀特性進(jìn)行研究,以料堆不同位置剪切力均達(dá)到最小為判定依據(jù),為探究孔隙率作用提供了新的認(rèn)識(shí),同時(shí)也為優(yōu)化抑塵網(wǎng)防護(hù)系統(tǒng)提供了依據(jù).

1 數(shù)值模擬

隨著計(jì)算機(jī)技術(shù)的迅猛發(fā)展,CFD (computational fluid dynamics)數(shù)值模擬以其設(shè)計(jì)周期短、細(xì)節(jié)數(shù)據(jù)充足和便于測(cè)量等優(yōu)點(diǎn)成為研究流體流動(dòng)的有力工具[26-28],本文應(yīng)用Fluent6.3軟件,對(duì)防風(fēng)抑塵網(wǎng)前后和料堆周圍的流場(chǎng)進(jìn)行模擬,探究不同孔隙率抑塵網(wǎng)時(shí)空氣的流動(dòng)特性.

1.1 幾何模型

以三維棱臺(tái)為料堆模型,設(shè)計(jì)參數(shù)為:下表面長(zhǎng)154m,寬51m;上表面長(zhǎng)113m,寬10m;堆高17m.研究表明[23-24,29-30]:當(dāng)抑塵網(wǎng)高度在料堆高度的1.5倍以上時(shí),隨網(wǎng)高的增加,抑塵效果不再發(fā)生明顯變化,抑塵網(wǎng)最佳高度為料堆高度的1.1~1.5倍;網(wǎng)與料堆間距離宜控制在1.0~1.5倍堆高范圍內(nèi).故取網(wǎng)高為1.3倍堆高(即22m),網(wǎng)長(zhǎng)為1倍堆長(zhǎng)(154m),網(wǎng)與料堆間距取1倍堆高(17m).計(jì)算區(qū)域的選擇對(duì)研究至關(guān)重要,區(qū)域過小會(huì)導(dǎo)致計(jì)算結(jié)果不準(zhǔn)確,區(qū)域過大則會(huì)增加計(jì)算量.相關(guān)研究表明[31],當(dāng)計(jì)算區(qū)域的長(zhǎng)、寬、高分別設(shè)為14倍堆寬、2倍堆長(zhǎng)和7倍堆高時(shí),風(fēng)速和料堆表面剪切力均不再發(fā)生變化,因此計(jì)算區(qū)域取為714m′308m′119m,如圖1所示,坐標(biāo)原點(diǎn)設(shè)置在料堆下表面中心位置,以來流空氣方向?yàn)檩S正向,沿堆高方向?yàn)檩S正向,沿堆長(zhǎng)方向?yàn)檩S正向.

1.2 數(shù)學(xué)模型

采用標(biāo)準(zhǔn)紊流模型進(jìn)行模擬,空氣為不可壓縮流體且與周圍物體無質(zhì)量和熱量交換,認(rèn)為流動(dòng)是穩(wěn)態(tài)絕熱,控制方程組如下:

連續(xù)性方程:

動(dòng)量方程:

(2)

方程:

(4)

式(2)中:S為源項(xiàng),由粘性損失和慣性損失兩部分組成,表達(dá)式如式(5)、(6)所示;式(3)中:μ為湍流粘性系數(shù),N·S/m2,如式(7)所示,G為湍流動(dòng)能生成項(xiàng),kg/(m3·s),如式(8)所示;方程組中常數(shù)C、C、C和分別取1.44、1.92、0.09、1.0和1.3[32-33],空氣動(dòng)力粘性系數(shù)取1.79× 10-5N·S/m2.各項(xiàng)具體表達(dá)式:

式(5)中代表多孔介質(zhì)的滲透性,m?s;2為慣性阻力因子,m-1;式(6)中f為網(wǎng)孔總面積,m2;p為網(wǎng)板總面積m2;為網(wǎng)厚度,取0.002m;近似等于0.98.

1.3 網(wǎng)格劃分與邊界條件

應(yīng)用Gambit軟件對(duì)模型進(jìn)行網(wǎng)格劃分,考慮計(jì)算區(qū)域空氣流動(dòng)特性,在靠近防風(fēng)抑塵網(wǎng)以及料堆表面區(qū)域進(jìn)行加密處理,使此區(qū)域網(wǎng)格分布較密集,而計(jì)算區(qū)域的遠(yuǎn)端網(wǎng)格較稀疏.采用三角形網(wǎng)格均勻劃分料堆表面,四邊形網(wǎng)格劃分地面,六面體網(wǎng)格劃分控制體.為設(shè)置抑塵網(wǎng)非均勻孔隙率及分析料堆表面剪切力的微觀分布,對(duì)網(wǎng)和料堆表面進(jìn)行細(xì)致劃分,用間距1m的平行平面分別截取抑塵網(wǎng)和料堆表面,使抑塵網(wǎng)、堆頂、料堆迎風(fēng)面和背風(fēng)面分別由22、10、17和17個(gè)平面組成.由于流場(chǎng)具有對(duì)稱性,選取大于零的區(qū)域進(jìn)行計(jì)算,網(wǎng)格劃分如圖2所示.

考核網(wǎng)格獨(dú)立性是兼顧計(jì)算結(jié)果準(zhǔn)確和最大限度降低網(wǎng)格劃分工作量的必要工作[34],本文以迎風(fēng)面剪切力為判據(jù)對(duì)計(jì)算區(qū)域網(wǎng)格進(jìn)行獨(dú)立性考核,具體結(jié)果如圖3所示.

圖3 網(wǎng)格獨(dú)立性考核
Fig.3 Assessment of grid independence

由圖3可見,當(dāng)網(wǎng)格數(shù)大于1876359時(shí),迎風(fēng)面剪切力曲線趨于平緩,變化率僅為0.29%,故取1876359為有效計(jì)算網(wǎng)格數(shù).

邊界條件設(shè)置:防風(fēng)抑塵網(wǎng)為多孔介質(zhì)跳躍模型(porous-jump);入口邊界為速度入口,設(shè)入口風(fēng)速為5m/s;出口截面法向方向的速度梯度為零,出口邊界設(shè)為自由壓力出口;料堆表面和地面采用無滑移壁面;計(jì)算域前后及上表面為對(duì)稱邊界.本文中壓力、動(dòng)量、湍流動(dòng)能和耗散項(xiàng)均采用二階迎風(fēng)格式,并用SIMPLE(Semi-Implicit Method for Pressure Linked Equations)算法處理壓力與速度耦合項(xiàng),收斂誤差為10-5.

在開放性露天堆場(chǎng)中,料堆的散塵特性很大程度上依賴于近壁邊界層的流動(dòng)特性和壁面剪切力,對(duì)N-S方程擴(kuò)散項(xiàng)的計(jì)算需確定壁面剪切力的求解方法[35].本文在粘性支層中采用半經(jīng)驗(yàn)公式將自由流中的湍流與壁面附近的流動(dòng)連接起來,近壁區(qū)流動(dòng)的計(jì)算采用壁面函數(shù)法[36-37].

porous-jump邊界條件是指抑塵網(wǎng)按孔隙率不同在動(dòng)量方程中設(shè)為不為0的源項(xiàng),是一種將實(shí)際物體阻礙流體運(yùn)動(dòng)的作用處理成動(dòng)量損失的方法.鑒于數(shù)值模擬中未將物體的全部形狀信息采集,為確認(rèn)模擬結(jié)果的準(zhǔn)確度,本文以抑塵網(wǎng)的壓力損失系數(shù)為驗(yàn)證指標(biāo),將模擬結(jié)果與Park等[25]風(fēng)洞實(shí)驗(yàn)結(jié)果進(jìn)行對(duì)比:模擬值與實(shí)驗(yàn)測(cè)試值的變化規(guī)律相符,相對(duì)誤差僅為7.17%,誤差值在允許范圍內(nèi).可見porous-jump邊界條件具有可行性,且對(duì)孔隙率不同的抑塵網(wǎng)均適用.

壓力系數(shù)的定義式為[19,25,38-39]:

式(9)中:為料堆表面的壓力,Pa;0為參考?jí)毫?Pa,數(shù)值模擬與風(fēng)洞實(shí)驗(yàn)保持一致,取0.4m高度處的壓力為參考?jí)毫?為空氣密度,kg/m3;in為來流風(fēng)速,m/s.

2 結(jié)果與討論

2.1 均勻孔隙率料堆表面微觀動(dòng)力學(xué)分布

對(duì)不同孔隙率(0,0.2,0.25,0.3,0.4,0.6,1)抑塵網(wǎng)后的空氣流場(chǎng)進(jìn)行數(shù)值模擬,計(jì)算料堆表面受力.料堆迎風(fēng)面剪切力如圖4所示,以沿坡面向上為正向.

由圖4可見,高孔隙率(30.3)時(shí):剪切力沿整個(gè)坡面均為正值,即方向向上,沿坡面高度變化趨勢(shì)與無網(wǎng)工況(=1)相似,隨高度增加剪切力逐漸增大,最大散塵點(diǎn)在17m處(坡頂),孔隙率由0.6減小到0.3,剪切力亦減小.15m以上剪切力陡增,這是因?yàn)橥ㄟ^抑塵網(wǎng)的滲流空氣和繞流空氣在堆頂匯合,氣流繞流料堆,風(fēng)速增加,剪切力梯度增大,揚(yáng)塵驟增,抑塵效果隨孔隙率的減小而增強(qiáng).相比無網(wǎng)工況,高孔隙率抑塵作用主要體現(xiàn)為減小剪切力的大小,剪切力方向不變,且對(duì)中下部揚(yáng)塵抑制較為明顯.

低孔隙率(<0.3)時(shí):剪切力分布特性與高孔隙率(30.3)截然不同,剪切力絕對(duì)值沿坡面高度出現(xiàn)“增大-減小-再增大”的波動(dòng),且方向不全為正.這是由于低孔隙率時(shí),空氣通過抑塵網(wǎng)以繞流為主,網(wǎng)后壓力驟降,而網(wǎng)頂上方氣壓增強(qiáng),在垂直方向的壓差作用下,網(wǎng)和料堆迎風(fēng)面間形成順時(shí)針渦旋.當(dāng)=0即擋風(fēng)墻時(shí),空氣全部繞流抑塵網(wǎng),渦旋中心位于料堆上方,整個(gè)迎風(fēng)面處于渦旋中,剪切力均為負(fù)值,以13m(迎風(fēng)面3/4處)為界,在此高度以下,剪切力隨高度增加而增大,13m以上的剪切力有所降低,但坡頂處剪切力突增,揚(yáng)塵達(dá)到最大.=0.2時(shí),由于通過網(wǎng)的滲流增強(qiáng),渦旋中心下降,剪切力方向發(fā)生變化,15m(迎風(fēng)面7/8處)以下區(qū)域剪切力為負(fù)值,揚(yáng)塵在迎風(fēng)面1/2處達(dá)到最大,此后有較大幅度減少,當(dāng)高度大于15m時(shí),受繞流空氣影響,剪切力再次增大,方向變?yōu)檠仄旅嫦蛏?當(dāng)=0.25時(shí),渦旋中心高度繼續(xù)下降,渦旋強(qiáng)度削減,整個(gè)迎風(fēng)面揚(yáng)塵較少,剪切力方向變化點(diǎn)移至12m(迎風(fēng)面2/3)處.相比無網(wǎng)工況,低孔隙率時(shí)抑塵作用主要體現(xiàn)在減小料堆上部表面剪切力,從而抑制揚(yáng)塵.

以上分析可知,不同孔隙率抑塵網(wǎng)的顯著抑塵位置不同,高孔隙率(30.3)時(shí),料堆中下部剪切力明顯降低,抑塵作用在料堆中下部體現(xiàn)明顯;低孔隙(<0.3)時(shí),抑塵網(wǎng)和料堆間存在渦旋,在渦旋作用下料堆中下部氣流擾動(dòng)反而增強(qiáng),抑塵作用主要體現(xiàn)為減小料堆上部剪切力.

現(xiàn)有對(duì)均勻孔隙率的研究大多是針對(duì)其宏觀的抑塵效果[8,21,29],通過上述分析發(fā)現(xiàn),在均勻孔隙率下,料堆坡面不同高度的揚(yáng)塵變化不同,即使在最佳孔隙率=0.25時(shí),也并非在料堆表面所有位置起塵均為最小.據(jù)此,本文通過調(diào)整孔隙率組合,將抑塵網(wǎng)劃分不同區(qū)域,以期使料堆各個(gè)區(qū)域的揚(yáng)塵均達(dá)到最小.

2.2 非均勻孔隙率料堆周圍湍流結(jié)構(gòu)及料堆表面動(dòng)力學(xué)分布

高孔隙率(30.3)時(shí),抑塵作用體現(xiàn)為削減迎風(fēng)面中下部剪切力;低孔隙率(<0.3)時(shí),迎風(fēng)面上部抑制效果較好,這為設(shè)置不同孔隙率組合以優(yōu)化抑塵效果提供了可能,即抑塵網(wǎng)下部選取高孔隙率,上部分選取低孔隙率.

2.2.1 非均勻孔隙率料堆周圍速度矢量場(chǎng) 將抑塵網(wǎng)均分為上下2個(gè)區(qū)域,設(shè)置上部(H)為低孔隙率(<0.3),下部(L)為高孔隙率(30.3),表示為H/L,并從大量非均勻組合的計(jì)算結(jié)果中選取6種典型工況加以分析,料堆前后的速度矢量分布如圖5所示.

(a)H=0 /L=0.3 (b)H=0/L=0.4

(c)H=0/L=0.6 (d)H=0.1/L=0.3

(e)H=0.1/L=0.4 (f)H=0.1/L=0.6

圖5 不同孔隙率組合的料堆周圍速度矢量場(chǎng)
Fig.5 Velocity vector fields over the pile with deferent porosity combinations

由圖5(a)可知,網(wǎng)上半部無氣流通過,空氣躍過網(wǎng)頂,在網(wǎng)上方形成高速剪切層,網(wǎng)后垂直方向壓力懸殊,受壓差作用影響,網(wǎng)下半部通過的部分滲流空氣在網(wǎng)和料堆間形成逆時(shí)針渦旋,渦旋中心距地13m(迎風(fēng)面3/4處);另一部分滲流空氣遇到料堆阻擋,在粘性力作用下沿迎風(fēng)面貼附向上運(yùn)動(dòng).料堆平頂面處于渦旋回流區(qū),平頂面物料顆粒受到渦旋卷吸作用逆向來流方向被揚(yáng)起.

對(duì)比圖5(a),(b)可知,在H相同情況下,增大L為0.4時(shí),網(wǎng)上半部氣流運(yùn)動(dòng)與圖5(a)工況相似,渦旋中心高度幾乎不變,為13.5m;通過網(wǎng)下半部的滲流增加,貼附迎風(fēng)面的氣流流速增大.由于滲流引射作用增強(qiáng),平頂面氣流受其影響沿來流方向運(yùn)動(dòng),料堆頂部顆粒沿來流方向被吹起.當(dāng)L增大至0.6時(shí),如圖5(c)所示,網(wǎng)上下孔隙率差值較大,垂直壓差加劇,網(wǎng)頂后上方形成順時(shí)針渦旋,渦旋中心距地24.6m,高出網(wǎng)2.6m,抑塵網(wǎng)和料堆間出現(xiàn)雙渦旋,湍流強(qiáng)度驟增,且強(qiáng)滲流空氣流經(jīng)料堆,使迎風(fēng)面和平頂面風(fēng)速增大.對(duì)比圖5(d), (e),(f)工況可知,隨L增大迎風(fēng)面和平頂面風(fēng)速增加,但比H=0時(shí)網(wǎng)后上部氣流的流線趨于平緩.

對(duì)比圖5(a),(d)可知,在L相同情況下,增大H至0.1時(shí),網(wǎng)后壓差減小,湍流強(qiáng)度減弱,氣流擾動(dòng)削減.如圖5(d)所示,緊貼迎風(fēng)面13.4m處產(chǎn)生強(qiáng)度較小的渦旋,這是因?yàn)橄虏靠諝庀蛏吓郎^程中遇到上部低速氣流,二者產(chǎn)生混合,在壓差作用下產(chǎn)生渦旋,因?yàn)閴翰钶^小渦旋強(qiáng)度較弱.當(dāng)H增大至0.2時(shí),迎風(fēng)面渦旋消失.工況(b),(e)和工況(c),(f)變化情況與上述相似,且迎風(fēng)面近壁滲流空氣沿坡面貼附流動(dòng).

綜上所述,當(dāng)H=0時(shí),網(wǎng)上部空氣全部繞流抑塵網(wǎng),網(wǎng)后上下部氣流的壓力懸殊,在強(qiáng)壓差作用下氣流擾動(dòng)劇烈,抑塵網(wǎng)和料堆迎風(fēng)面間形成大尺度渦旋,抑塵作用較弱;而當(dāng)H略增至0.1時(shí),網(wǎng)上部有持續(xù)滲流通過,網(wǎng)后壓差減小,湍流強(qiáng)度削減,氣流流線變緩,抑塵作用增強(qiáng).對(duì)比不同L工況,可以看出L=0.3時(shí),迎風(fēng)面貼附流的速度較小,隨L增大,滲流作用增強(qiáng),當(dāng)L為0.4和0.6時(shí),料堆迎風(fēng)面和平頂面的風(fēng)速明顯增大,揚(yáng)塵加劇.

上述的矢量分析可定性看出:H取0.1時(shí),網(wǎng)后氣流擾動(dòng)較弱,且L取0.3時(shí),迎風(fēng)面和平頂面的風(fēng)速最小,綜合以上因素,選取=0.1/=0.3工況為非均勻孔隙率抑塵網(wǎng)的最佳工況.

2.2.2 非均勻孔隙率料堆表面微觀動(dòng)力學(xué)分布 為進(jìn)一步量化非均勻孔隙率抑制網(wǎng)的抑塵效果,以速度矢量分布最佳的H=0.1/L=0.3工況為例,探討非均勻孔隙率下料堆各表面剪切力的變化情況,并與均勻孔隙率=0.1和=0.3工況對(duì)比,料堆整體表面剪切力微觀分布如圖6所示.

如圖6所示,3種工況下迎風(fēng)面剪切力的變化曲線形成鮮明對(duì)比.非均勻孔隙率H=0.1/L=0.3曲線夾在均勻孔隙率=0.3和=0.1之間,“迎風(fēng)面-平頂面-背風(fēng)面”整體的剪切力變化平緩,迎風(fēng)坡面上的剪切力最大波動(dòng)范圍不超過±5N,坡面整體揚(yáng)塵較少.=0.3時(shí)迎風(fēng)面剪切力隨高度增加而遞增,且料堆中上部的剪切力梯度明顯增大,而H=0.1/L=0.3的剪切力在料堆迎風(fēng)面5m以上高度有顯著降低,這與預(yù)期的“低孔隙率減小迎風(fēng)面上部剪切力” 相吻合.均勻孔隙率=0.1時(shí),迎風(fēng)面中下部在強(qiáng)烈負(fù)壓差作用下,高強(qiáng)度渦旋使坡面中下部剪切力相當(dāng)大,而非均勻孔隙率H=0.1/L=0.3下部的0.3孔隙率削弱了這一渦旋作用,中下部剪切力明顯減小,這與“高孔隙率削減迎風(fēng)面中下部剪切力”的設(shè)想相符.對(duì)比料堆平頂面剪切力,=0.3時(shí)平頂面受迎風(fēng)面高速氣流的引射作用,剪切力值最大,H=0.1/L=0.3的剪切力與=0.1工況相差無幾.3種工況背風(fēng)面的剪切力變化基本一致,不再另作討論.

表1 不同孔隙率下料堆各表面剪切力值(N)Table 1 Shear force values on each surface of the pile with deferent porosities (N)

由表1可見,H=0.1/L=0.3抑制迎風(fēng)面揚(yáng)塵作用明顯,其料堆中下部剪切力為19.0N,比=0.1時(shí)減少85.2 %,料堆上部剪切力為8.2N,比=0.3減少84.3%;H=0.1/L=0.3的平頂面剪切力之和較=0.3略大,但比=0.1工況有近56N的削減;在抑制料堆表面整體揚(yáng)塵上,非均勻孔隙率的作用效果最佳,其料堆表面剪切力絕對(duì)值總和僅為=0.1的40.4％和=0.3的48.9％,可使揚(yáng)塵減少50%以上.綜合以上分析可知,與均勻孔隙率= 0.1和=0.3工況相比,非均勻孔隙率H=0.1/L=0.3的抑塵效果最為出色.

3 結(jié)論

3.1 高孔隙率(30.3)與低孔隙率(<0.3)抑塵網(wǎng)對(duì)料堆坡面的顯著抑塵區(qū)域不同:高孔隙率抑塵網(wǎng)后料堆表面剪切力隨坡面高度增加而增加,中上部增加為甚,抑塵作用突出表現(xiàn)在減少料堆中下部揚(yáng)塵;低孔隙率(<0.3)網(wǎng)后的渦旋作用削弱了上部剪切力,抑塵網(wǎng)的主要作用體現(xiàn)在抑制料堆上部揚(yáng)塵.

3.2 抑塵網(wǎng)下部孔隙率(L)相同情況下,上部孔隙率(H)由0增至0.1時(shí),網(wǎng)后垂直壓差減小,氣流擾動(dòng)減弱,氣流流線減緩,網(wǎng)與料堆間渦旋的范圍和強(qiáng)度皆有衰減,考慮湍流結(jié)構(gòu)和料堆受力H取0.1較好;網(wǎng)上部孔隙率(H)相同情況下,當(dāng)L由0.3增至0.6時(shí),滲流作用隨之增強(qiáng),緊貼迎風(fēng)面和平頂面的風(fēng)速增加,揚(yáng)塵加劇,L取0.3時(shí)最優(yōu),綜合考慮網(wǎng)上下部孔隙率影響,確定H=0.1/L= 0.3為非均勻孔隙率抑塵網(wǎng)速度矢量分布的最佳工況.

3.3 比較非均勻抑塵網(wǎng)最佳工況(H=0.1/L=0.3)與均勻網(wǎng)(=0.1和=0.3)的料堆表面受力,結(jié)果顯示:H=0.1/L=0.3的迎風(fēng)面剪切力比=0.1時(shí)減少85.7 %,且剪切力在5m以上高度比=0.3時(shí)有明顯削減,迎風(fēng)面剪切力之和比=0.3減少62.6%;H=0.1/L=0.3時(shí)平頂面剪切力比=0.1略大,但僅為=0.3工況的46.9%;背風(fēng)面三者差別不大.非均勻抑塵網(wǎng)H=0.1/L=0.3對(duì)料堆表面剪切力總和的減少量可達(dá)均勻網(wǎng)=0.1和=0.3的一半以上,抑塵效果最佳.

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* 責(zé)任作者, 副教授,

Non-uniform porosity design optimization based on CFD simulation for porous fences

HE Hong-zhan, SONG Chong-fang*, PAN Wu-xuan, LEI Yong-gang

(College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China)., 2016,36(6)：1697~1704

Porosity is the most important parameter affecting the efficient of fences, however, the distinct shelter regions varied with different porosity. At high porosities (30.3), the dust emission in the middle and lower parts of the pile over the fence decreased significantly, while the shelter mainly reflected in the upper part at low porosities (<0.3). Obtaining effective protection on each site, this research introduced, based on the reduction region of the uniform porosity, a new combination of non-uniform fence with different porosities for the upper and lower halves. The flow flied behind a porous fence was numerically simulated by software Fluent6.3with six typical combinations of the non-uniform fence. Results showed that when the lower fence porosity of the fence (L) kept consistent and the upper fence porosity (H) transformed from 0to 0.1, the airflow turbulence weakened distinctly. Considering turbulence structure and stress of the pile, the fence with the upper porosityH= 0.1was more accepted. Meanwhile, when the upper porosity (H) remained identical, and the lower porosity (L) increased from 0.3to 0.6,the speed of attached flow along the surface increased with the increasing porosity, therefore, the optimum porosity of the lower half fence (L) was set to 0.3. The shelter effect of non-uniform fence was estimated by comparing the preferred combination (H=0.1/L=0.3)with uniform fence porosity=0.1 and=0.3. The analysis indicated the non-uniform porous fence (H=0.1/L=0.3) seemed to be the most effective in abating the dust emission, especially in reducing the shear stress of the windward which aroused the maximum dust emission. The shear stress of the non-uniform porous fence, in the middle and lower part, decreased by 85.2% for the uniform fence with porosity=0.1, and 84.3% in the upper part for the fence with the porosity=0.3, respectively. Besides, the non-uniform porous fence (H=0.1/L=0.3) could reduce the surface shear force on the pile around 50% for the two uniform fences.

porous fence；non-uniform porosity；numerical simulation；shear stress

X513

A

1000-6923(2016)06-1697-08

何鴻展(1991-),女,河北石家莊人,太原理工大學(xué)碩士研究生,主要研究方向?yàn)榭諝馕廴疚锟刂?發(fā)表論文2篇.

2015-11-24

國家自然科學(xué)基金資助項(xiàng)目(51108295)