非對(duì)稱噴嘴噴霧模擬網(wǎng)格適用性研究

楊小龍　張澤坪　陳征　徐光輝

摘 要：離散油滴模型（DDM）和拉格朗日油滴歐拉氣體（LDEF）算法的噴霧數(shù)值模擬對(duì)網(wǎng)格具有很強(qiáng)的依賴性.對(duì)某非對(duì)稱三噴孔噴嘴的噴霧過程進(jìn)行了網(wǎng)格參數(shù)適用性研究，分析了網(wǎng)格依賴性的來源，探討了網(wǎng)格形式以及徑向、軸向、周向網(wǎng)格尺寸變化對(duì)噴霧的影響，與實(shí)驗(yàn)數(shù)據(jù)對(duì)比了噴霧形態(tài)和貫穿距離，并研究了對(duì)索特直徑的影響.分析了不同網(wǎng)格長(zhǎng)寬比的影響.結(jié)果表明，非對(duì)稱三噴孔噴嘴的噴霧數(shù)值仿真中，極坐標(biāo)類型的網(wǎng)格具有更好的適用性.隨著網(wǎng)格尺寸減小，貫穿距離和索特直徑基本都呈現(xiàn)增大的趨勢(shì)，但徑向尺寸對(duì)貫穿距離的影響稍大于軸向尺寸.網(wǎng)格尺寸和長(zhǎng)寬比存在一個(gè)最優(yōu)值使得模擬的貫穿距離與實(shí)驗(yàn)值擬合得最好.

關(guān)鍵詞：噴霧特性；網(wǎng)格適用性；數(shù)值模擬；離散油滴模型；拉格朗日油滴歐拉氣體算法

中圖分類號(hào)：TK427 文獻(xiàn)標(biāo)識(shí)碼：A

Abstract：Spray simulation using Lagrangian-Droplet and Eulerian-Fluid （LDEF） method and the Discrete Droplet Model （DDM） has been proved to be of grid dependency. The grid effects on the spray simulation of an asymmetric three-hole nozzle were studied. First， the sources of grid-dependency were analyzed. The effects of grid types and different grid sizes of radial， azimuthal and axial directions on the spray process were investigated by comparing the spray shape and liquid penetration distance with experiment data. And the grid effect on the Sauter Mean Diameter （SMD） was studied. Finally， the aspect ratio between the radial and the axial directions of the grid was optimized. The result has shown that， for the asymmetric three-hole nozzle spray simulation， the polar mesh has better performance among the different mesh forms. With the decrease of the grid size on three directions， the spray tip penetration and SMD basically present the tendency of increase， while the radial grid size shows greater influence than the axial size. The spray tip penetration fits best with experimental data when the grid size and the aspect ratio are optimized.

Key words：spray characteristic；grid dependency；numerical simulation；Discrete Droplet Model（DDM）；LDEF method

液體燃料在高速射流過程中的噴霧破碎對(duì)發(fā)動(dòng)機(jī)噴霧的發(fā)展以及混合氣的形成都起著主導(dǎo)作用，因此燃料噴霧效果的好壞對(duì)發(fā)動(dòng)機(jī)的燃燒效率的提高、尾氣排放的降低和運(yùn)行穩(wěn)定性的保持都有很大影響.近些年來，隨著計(jì)算流體動(dòng)力學(xué)（CFD）的發(fā)展，通過對(duì)液滴的湍流擴(kuò)散、蒸發(fā)、碰撞和破碎等過程建立一系列的子模來對(duì)發(fā)動(dòng)機(jī)噴霧過程進(jìn)行數(shù)值模擬的方法已經(jīng)獲得廣泛的應(yīng)用.其中大多都在離散油滴模型（DDM）基礎(chǔ)上，利用拉格朗日油滴歐拉氣體的方法（LDEF）來對(duì)噴霧兩相流動(dòng)進(jìn)行數(shù)值模擬[1].然而，許多研究表明，拉格朗日油滴歐拉氣體方法對(duì)網(wǎng)格具有很強(qiáng)的依賴性[2-4].Abani和Post等[5-6]指出了LDEF方法網(wǎng)格依賴性的兩個(gè)主要來源：噴霧的破碎模型和氣液動(dòng)量耦合.

目前，由于傳統(tǒng)發(fā)動(dòng)機(jī)多采用單噴油器中間布置結(jié)構(gòu)，其噴油器噴孔大多都是對(duì)稱的，所以過去對(duì)噴霧過程網(wǎng)格依賴性的研究主要集中在單孔或具有對(duì)稱性的噴嘴上，而對(duì)置活塞對(duì)置氣缸發(fā)動(dòng)機(jī)由于其雙噴油器側(cè)面布置、相對(duì)噴射的結(jié)構(gòu)特點(diǎn)，通常采用非對(duì)稱三噴孔噴嘴的特殊結(jié)構(gòu)[7-9]，網(wǎng)格對(duì)其噴霧模擬的影響有待進(jìn)一步驗(yàn)證.本文通過仿真與實(shí)驗(yàn)相結(jié)合的方法，對(duì)某非對(duì)稱三噴孔噴嘴的噴霧數(shù)值仿真過程進(jìn)行了網(wǎng)格類型以及網(wǎng)格尺寸的適用性研究.

1 燃油噴霧實(shí)驗(yàn)

為了研究非對(duì)稱三噴孔噴嘴的噴霧特性并對(duì)其進(jìn)行標(biāo)定，本文進(jìn)行了噴霧可視化試驗(yàn)，所研究的非對(duì)稱三噴孔（圖中黑點(diǎn)）噴嘴幾何位置如圖1所示.

測(cè)試系統(tǒng)如圖2所示.實(shí)驗(yàn)裝置主要包括定容燃燒彈、紋影系統(tǒng)、高壓共軌供油系統(tǒng)、CCD攝像機(jī)及其控制器等.試驗(yàn)中采用燃油單次噴射測(cè)量?jī)x對(duì)噴油量進(jìn)行測(cè)量，測(cè)量結(jié)果采用多次測(cè)量取平均值的方法[10].

實(shí)驗(yàn)中采用的非對(duì)稱噴嘴其噴孔直徑為0.2 mm.供油系統(tǒng)為博世的高壓共軌供油系統(tǒng)，系統(tǒng)提供最大可達(dá)160 MPa軌壓.噴射燃油為5#柴油，噴射燃油溫度為60 ℃.考慮到單次噴霧之間的誤差，實(shí)驗(yàn)對(duì)同一工況和時(shí)間點(diǎn)下5張照片的貫穿距離進(jìn)行平均處理.為了得到精準(zhǔn)的尺寸參照，拍照前使用尺寸標(biāo)定板對(duì)噴霧照片的尺寸進(jìn)行精確標(biāo)定.