收稿日期:2023-05-22
基金項(xiàng)目:湖南省戰(zhàn)略性新興產(chǎn)業(yè)項(xiàng)目(2019GK4021)
第一作者:彭德其(1972-),男,教授,博士,研究方向?yàn)檫^(guò)程強(qiáng)化與節(jié)能環(huán)保。E-mail:pengshuaike@163.com。
通信作者:吳淑英(1981-),女,副教授,博士,研究方向?yàn)閺?qiáng)化傳熱和儲(chǔ)能技術(shù)。E-mail:wushuying5876@126.com。
文章編號(hào):1673-5005(2024)02-0170-09""" doi:10.3969/j.issn.1673-5005.2024.02.019
摘要:為提高鈦橢圓低肋橫槽管外降膜蒸發(fā)器傳熱效率,在試驗(yàn)驗(yàn)證基礎(chǔ)上對(duì)橢圓系數(shù)E在1.0~2.0內(nèi)管外液膜流熱特性進(jìn)行多相流數(shù)值模擬。結(jié)果表明:管外液膜分布均存在不同程度干斑,且液膜在臨近干斑處較其他區(qū)域增厚明顯,當(dāng)E為1.2時(shí)液膜覆蓋率最高;液膜沿軸/周向均減速鋪展,當(dāng)E為1.0和1.8時(shí)管上端液膜速度在無(wú)量綱軸向長(zhǎng)度Z*為-0.25~-0.375及0.25~0.375之間驟減,液膜厚度沿軸向從噴淋口正下方位置向噴淋口遠(yuǎn)端變化趨勢(shì)為先減小后增大;最大傳熱系數(shù)隨著E增大而增大,管壁在液膜與干斑交界區(qū)域附近傳熱系數(shù)最高;平均傳熱系數(shù)先增大后減小,相較圓管,E為1.2和2.0時(shí)平均傳熱系數(shù)分別增大34.5%和15.7%;槽內(nèi)汽含率分布與液膜分布呈相反規(guī)律,液膜整體汽含率隨E的增大先增大后減小,槽內(nèi)/外平均汽含率差值隨E增加而增大。
關(guān)鍵詞:鈦橢圓低肋橫槽管; 降膜蒸發(fā); 橢圓系數(shù); 液膜分布; 汽含率
中圖分類號(hào):TK 124""" 文獻(xiàn)標(biāo)志碼:A
引用格式:彭德其,謝禹凡,蔣衛(wèi)東,等.鈦橢圓低肋橫槽管外降膜蒸發(fā)流熱特性[J].中國(guó)石油大學(xué)學(xué)報(bào)(自然科學(xué)版),2024,48(2):170-178.
PENG Deqi, XIE Yufan, JIANG Weidong, et al. Heat flow characteristics of" Titanium elliptical low rib groove tube with outer falling film evaporation[J]. Journal of China University of Petroleum (Edition of Natural Science),2024,48(2):170-178.
Heat flow characteristics of Titanium elliptical low rib groove tube with outer falling film evaporation
PENG Deqi1, XIE Yufan1, JIANG Weidong2, YU Tianlan3,WU Shuying1, TAN Zhuowei1, WANG Zhiqi1
(1.School of Mechanical Engineering and Mechanics, Xiangtan University, Xiangtan 411105, China;
2.Hunan Changxing Energy Environmental Protection Technology Company Limited, Changsha 410000, China;
3.School of Mechanical Engineering, Hunan University of Technology, Zhuzhou 412007, China)
Abstract: In order to improve the heat transfer efficiency of the falling film evaporator with a titanium elliptical low rib transverse groove tube, the heat flow characteristics of the liquid film flow outside the tube were numerically simulated with the ellipse coefficient E being in the range of 1.0 to 2.0. The results show that, there are different degrees of the dry patches outside the tube. The liquid film is significantly thicker near the dry patches than in other areas, and the liquid film coverage is the highest when the value of E is 1.2. The liquid film spreads slower in both of the axial and circumferential directions. The liquid films velocity at the upper of the tube decreases abruptly between Z*=-0.25~-0.375 and 0.25~0.375 when E is 1.0 and 1.8, and the changing trend of liquid films thickness along the axial direction from the position directly below the spray nozzle to the far end of the spray nozzle decreases firstly and then increases. The maximum heat transfer coefficient increases with the increase of E. The tube wall has the highest heat transfer coefficient at the junction between the liquid film and the dry patches, and the average heat transfer coefficient increases and then decreases. Compared to the round tube, the average heat transfer coefficient increases by 34.5% and 15.7% when E is 1.2 and 2.0, respectively. The distribution of vapor-holdup in the tank is opposite to that of the liquid film. The overall vapor-holdup of the liquid film firstly increases and then decreases with the increase of E, and the difference between the average vapor-holdup inside and outside the tank increases with the increase of E.
Keywords: Titanium elliptical low rib transverse groove tube; falling film evaporation; ellipse coefficient; liquid film distribution; vapor-holdup
降膜蒸發(fā)器在海水淡化[1]、制冷[2]等領(lǐng)域應(yīng)用廣泛,但傳統(tǒng)吸收式制冷系統(tǒng)降膜蒸發(fā)器中的銅管易被溴化鋰工質(zhì)腐蝕,為緩解腐蝕問(wèn)題選用鈦管[3]。然而鈦材導(dǎo)熱率較低限制了蒸發(fā)器的換熱性能,需深入研究鈦管降膜蒸發(fā)流熱特性,優(yōu)化其換熱性能。降膜蒸發(fā)器中常采用異型管或?qū)懿谋砻孢M(jìn)行結(jié)構(gòu)優(yōu)化來(lái)達(dá)到強(qiáng)化換熱的目的,對(duì)此國(guó)內(nèi)外學(xué)者做了大量研究[4-6]。Sun等[7-9]對(duì)橢圓管、半橢圓管、扭曲管等異型管研究較多,各管型通過(guò)改變管外液膜流動(dòng)特性、減小液膜厚度、增加液膜速度等方法實(shí)現(xiàn)強(qiáng)化傳熱。橢圓管及半橢圓較圓管有更薄的液膜厚度及熱邊界層,有更大的傳熱系數(shù),傳熱性能更佳[10-12]。扭曲管降膜蒸發(fā)器在管側(cè)、殼側(cè)傳熱膜系數(shù)及整體壓降均優(yōu)于直圓管,且有更佳的管外濕潤(rùn)率及傳熱效果[13-14]。表面結(jié)構(gòu)優(yōu)化管通過(guò)開槽、加肋、制造多孔表面等方法增加換熱面積、增強(qiáng)管外液膜擾動(dòng)、增多汽化核心數(shù)目來(lái)強(qiáng)化傳熱[15-17]。橫槽管傳熱性能優(yōu)于光管,且各流型轉(zhuǎn)換對(duì)應(yīng)雷諾數(shù)均要小于光管[18-19]。微肋管對(duì)傳熱及濕潤(rùn)率較光管有明顯優(yōu)化效果[20-21]。多孔表面結(jié)構(gòu)優(yōu)化能增加管壁親水性來(lái)抑制降膜的部分干化,增強(qiáng)降膜中核沸騰效果[22-23]。為了更好地強(qiáng)化鈦管降膜蒸發(fā)器中換熱性能,筆者綜合這兩種優(yōu)化技術(shù)提出橢圓低肋橫槽管,研究在不同管結(jié)構(gòu)參數(shù)及流動(dòng)工況下的流熱特性。
1" 研究方法
1.1" 物理模型及邊界條件設(shè)置
選用常見(jiàn)鈦管,管外徑D為20.5 mm,管壁厚S為0.75 mm。依據(jù)某企業(yè)制造能力確定橢圓系數(shù)E為1.0~2.0,并根據(jù)等周長(zhǎng)原則確定橢圓管長(zhǎng)短軸數(shù)據(jù)。在管外壁開設(shè)等間距環(huán)形凹槽,經(jīng)試算得出運(yùn)行工況下最佳尺寸如圖 1 所示。槽間距a為1 mm, 槽寬b為0.3 mm,槽深h為0.3 mm,槽底部截面為圓弧。根據(jù)管模型對(duì)稱性,將計(jì)算模型簡(jiǎn)化為完整模型二分之一(圖 2(a))。管內(nèi)為熱水,管外為冷水及水蒸氣,物性參數(shù)見(jiàn)表1。
圖3為管橫截面示意圖(θ為周向角,由于管頂點(diǎn)和最低點(diǎn)處液柱波動(dòng)劇烈,無(wú)明顯規(guī)律,研究時(shí)忽略該區(qū)域,選取周向角為22.5°~157.5°內(nèi)7個(gè)角度研究液膜周向流動(dòng)分布規(guī)律。定義橢圓系數(shù)E為
E=A/B.(1)
式中,A和B分別為橢圓長(zhǎng)軸和短軸,m。
圖4為無(wú)量綱軸向長(zhǎng)度示意圖。為便于分析管外液膜分布規(guī)律,定義無(wú)量綱軸向長(zhǎng)度,噴淋口中心正下方為無(wú)量綱軸向長(zhǎng)度Z*,表示為
Z*=Z/L.(2)
式中,Z為從噴淋口中心點(diǎn)到計(jì)算域邊緣的軸向距離,m;L為計(jì)算域軸向距離,m。
圖5為槽序號(hào)示意圖。根據(jù)最佳槽間距開設(shè)22個(gè)橫槽,定義噴淋口正下方為0、1槽,遠(yuǎn)離噴淋口的兩端槽序號(hào)為0~-10、1~11。
考慮到模型復(fù)雜程度及計(jì)算穩(wěn)定性采用VOF模型追蹤液膜界面[24],管外液膜相變采用蒸發(fā)-冷凝Lee模型[25],當(dāng)流動(dòng)雷諾數(shù)小于160時(shí),可以選用Realizable k-ω湍流模型[1,26]。采用CSF模型模擬表面張力對(duì)液膜的作用[27]。
VOF各項(xiàng)滿足連續(xù)性方程:
·u=0.(3)
動(dòng)量方程:
ut+(u·)u=1ρ·Σ+f.(4)
式中,u為流體速度矢量,m/s;f為流體所受合外力,N。
圖6為液膜分區(qū)示意圖。根據(jù)液膜鋪展規(guī)律將液膜分為沖刷區(qū)、延展區(qū)、交匯區(qū),其中實(shí)線為液膜邊緣線,虛線為管壁。
由于管外流體換熱蒸發(fā),控制方程中增加能量方程:
ρDDte+12
u·u=·(Σ·u)+ρu·f-·q.(5)
式中,e為流體能量,J;q為流體熱通量,W/m2;ρ為流體密度,kg/m3;t為時(shí)間,s。
通過(guò)Fluent軟件對(duì)降膜蒸發(fā)過(guò)程進(jìn)行模擬,邊界條件設(shè)置為:
①VOF模型中設(shè)置三相,主相為水蒸氣,次相為管內(nèi)水與管外水;②入口條件,管內(nèi)外均為速度進(jìn)口(velocity-inlet),管內(nèi)入口處和管外噴淋入口處的水體積分?jǐn)?shù)均設(shè)置為1;③出口條件,管內(nèi)/外均為壓力出口(pressure-outlet),表壓設(shè)置為0;④管內(nèi)/外壁設(shè)置為無(wú)滑移壁面,采用Couple進(jìn)行耦合傳熱;⑤動(dòng)量、湍流動(dòng)能、湍流耗散率、能量均采用二階迎風(fēng)格式進(jìn)行離散。
1.2" 網(wǎng)格無(wú)關(guān)性驗(yàn)證
根據(jù)某企業(yè)提供的運(yùn)行工況為:管外冷水水溫Tin1為4.8 ℃,頂部噴淋口間距為22 mm,入口質(zhì)量流量Γ為0.0818 kg/ s(噴淋口入口流速為1.8 m/s),管外空間絕對(duì)壓力為970 Pa(對(duì)應(yīng)飽和水溫度Tsat為7 ℃),管內(nèi)水溫Tin2為14 ℃,管內(nèi)水流速vin為0.5 m/s。
采用ANSYS ICEM 對(duì)計(jì)算域進(jìn)行結(jié)構(gòu)化網(wǎng)格劃分,E為1.2時(shí)采用5種網(wǎng)格數(shù)量在Z*=0.5、θ=90°點(diǎn)處液膜厚度及管外平均傳熱系數(shù)模擬結(jié)果見(jiàn)圖7。當(dāng)網(wǎng)格超過(guò)200×104后液膜厚度及平均傳熱系數(shù)變化不超過(guò)2%,因此采用數(shù)量為2236371的網(wǎng)格模型。
1.3" 模型驗(yàn)證
圖8為自主搭建的水平鈦管降膜蒸發(fā)試驗(yàn)平臺(tái),主要由管外制冷劑循環(huán)回路、管內(nèi)水循環(huán)回路、抽真空系統(tǒng)組成。試驗(yàn)過(guò)程:①檢查裝置氣密性,打開真空泵調(diào)節(jié)系統(tǒng)內(nèi)氣壓為970 Pa;②啟動(dòng)冷水機(jī)組,調(diào)節(jié)管外水制冷劑溫度至4.8 ℃后開啟冷水泵,調(diào)節(jié)閥門使噴淋流量為設(shè)定值,開啟負(fù)壓泵開始管外循環(huán);③啟動(dòng)水蒸氣鍋爐換熱器,水溫達(dá)到設(shè)定值后開啟熱水泵,讀取管內(nèi)熱水進(jìn)口流
量qc,使入口水流速達(dá)到設(shè)定值,開始管內(nèi)循環(huán);④待試驗(yàn)系統(tǒng)穩(wěn)定后,讀取各熱電偶測(cè)量值,記錄管內(nèi)進(jìn)口溫度Tin和出口溫度Tout及管外壁各區(qū)域溫度,求得管壁平均溫度Tw。為減少誤差,每個(gè)數(shù)據(jù)均測(cè)量3次并取平均值。
平均傳熱系數(shù)Kmean為
Kmean=qccp(Tout-Tin)A0(Tw-Td) .(6)
式中,A0為管壁面積,m2;TD為管內(nèi)水平均溫度,℃。
圖9為運(yùn)行工況下不同E下試驗(yàn)與模擬所得管外平均傳熱系數(shù)對(duì)比。由圖9可知,試驗(yàn)及模擬結(jié)果相差3.88%~6.34%,誤差在可接受范圍內(nèi),認(rèn)為計(jì)算模型可靠。
2" 結(jié)果及分析
2.1" 橢圓系數(shù)對(duì)管外降膜流動(dòng)影響
圖10為不同E時(shí)液膜流動(dòng)形態(tài)及速度。液柱以速度為1.8" m/s噴出后與管壁碰撞形成液膜,
液膜在重力及慣性力作用下沿周向鋪展,在管壁阻力作用下沿軸向向噴淋口遠(yuǎn)端做減速運(yùn)動(dòng)。結(jié)合表2可知:①干斑占比隨E增大呈現(xiàn)先減小后增大趨勢(shì),其中E為1.2時(shí)干斑占比最小為6.92%;②E為1.0時(shí)圓管外干斑占比最大達(dá)33.92%,出現(xiàn)液膜未在軸向延展完全且積聚增厚現(xiàn)象,形成較大范圍干斑;③E在1.2~1.4時(shí),由于液膜延展阻力減小,水充分潤(rùn)濕延展區(qū)及交匯區(qū)上部,周向出現(xiàn)不同程度液膜增厚;④沖刷區(qū)液膜速度最大,且E為1.2、1.4、1.6時(shí),液膜沿軸向充分延展,在交匯區(qū)進(jìn)行交匯作用使得液膜速度降低;而E為1.0、1.8、2.0時(shí)液膜未能延展至交匯區(qū),在表面張力作用下向噴淋口中間收縮聚攏,在延展區(qū)速度減小、停滯形成干斑。
為探究管外液膜內(nèi)部流動(dòng)及形態(tài)情況,選取E為1.0、1.2和1.8(E=1.0為圓管,E=1.2~1.6液膜形態(tài)類似,E=1.8~2.0液膜形態(tài)類似)的典型管外液膜形態(tài)及速度剖面(灰色代表管壁)見(jiàn)圖11。
結(jié)合表2可知:①噴淋口沖刷區(qū)液膜速度最大為1.8 m/s,之后沿周向及軸向減速鋪展;3種E的液膜速度分布在Z*為-0.25~0.25間類似,其中E為1.2時(shí)管上端液膜速度衰減均勻,使液膜在軸向上能充分延展至交匯區(qū),E為1.0和1.8時(shí)管上端液膜速度在Z*為-0.25~-0.375及0.25~0.375之間從1 m/s左右驟減
至0.1 m/s以下,液膜延展受阻積聚增厚;②E為1.0和1.8時(shí)液膜未能延展至Z*為-0.5和0.5處,E為1.2時(shí)Z*=-0.5和0.5處兩噴淋口液膜交匯使得液膜最厚。
圖12為液膜分布規(guī)律。由圖12可知,θ為22.5°~112.5°時(shí),從沖刷區(qū)至交匯區(qū)液膜厚度先減小后增大,在Z*為-0.25和0.25處分別達(dá)到液膜覆蓋區(qū)域的厚度最小值,在軸向液膜覆蓋良好情況下在Z*=-0.5和0.5處液膜厚度達(dá)到最大值。而在管下側(cè)半段θ為135°~ 57°時(shí),干斑占比較大,液膜波動(dòng)劇烈,無(wú)明顯變化規(guī)律。
由圖12結(jié)合圖10可知,干斑主要發(fā)生在部分延展區(qū)及交匯區(qū),且與干斑區(qū)臨近的液膜覆蓋區(qū)局部液膜顯著增厚。以E=1.2為例,在Z*=0.375、θ=135°處出現(xiàn)干斑時(shí),θ=90°到θ=112.5°的液膜厚度從0.512 mm增至0.919 mm;以E=1.8為例,當(dāng)Z*=0.5、θ=22.5°出現(xiàn)干斑時(shí),Z*=0.25到Z*=0.375液膜厚度從0.199 mm增至0.47 mm,液膜在不同方向上延展受阻是干斑產(chǎn)生主要原因之一。
2.2" 橢圓系數(shù)對(duì)傳熱性能影響
圖13為管外最大傳熱系數(shù)Kmax及平均傳熱系數(shù)Kmean隨E變化。由圖13可知:①Kmax隨E增大呈現(xiàn)逐漸增大趨勢(shì),最大可達(dá)到27805.2 W·m-2·K-1
,相對(duì)圓管外最大傳熱系數(shù)25113.6 W·m-2·K-1提高了10.72%,說(shuō)明橢圓低肋橫槽管可以提高管外最大傳熱系數(shù);②進(jìn)一步分析整體傳熱性能,E從1.0增加到1.2,Kmean迅速?gòu)?675.9 W·m-2·K-1增加到4942.4 W·m-2·K-1,增大34.5%;E繼續(xù)增大時(shí)Kmean緩慢變小,E為2.0時(shí)Kmean降至4253.4 W·m-2·K-1,仍較圓管增大15.7%;③當(dāng)增大E時(shí),管外液膜周向速度不斷增大,形成更薄的速度、溫度邊界層,實(shí)現(xiàn)強(qiáng)化換熱;但是E較大時(shí)管外局部液膜增厚及局部區(qū)域的干斑比例增大導(dǎo)致傳熱惡化,Kmean反而減小。
圖14為管外壁面局部傳熱系數(shù)分布云圖,藍(lán)色部分表示傳熱系數(shù)低的干斑區(qū)域。由圖14可知,槽內(nèi)局部傳熱系數(shù)優(yōu)于槽外,傳熱性能最好處集中在液膜與干斑交界、過(guò)渡區(qū)域(云圖中紅色區(qū)域),此處液膜鋪展受阻,在交界處之前增厚并在交界處迅速減小至接近0,極薄的液膜獲得高效蒸發(fā)、局部傳熱系數(shù)提升,之后由于干斑導(dǎo)致傳熱性能快速降低。
圖15為槽內(nèi)汽含率隨槽序數(shù)變化及其局部放大曲線。結(jié)合圖15、10可知:①液膜在沖刷區(qū)下端匯聚成液柱,此處周向截面平均液膜厚度最大,導(dǎo)致0、1槽內(nèi)或鄰近槽內(nèi)汽含率最低;②槽內(nèi)汽含率與液膜厚度變化規(guī)律呈現(xiàn)相反趨勢(shì),液膜從沖刷區(qū)向交匯區(qū)鋪展過(guò)程中,即沿槽序數(shù)0~-10、1~11方向槽內(nèi)汽含率先增大后減小再增大;③沖刷區(qū)向延展區(qū)過(guò)渡時(shí)液膜逐漸變薄,熱阻降低使得汽含率上升;產(chǎn)生干斑后液膜急劇降低至0,極小的熱阻導(dǎo)致管端部槽內(nèi)汽含率急劇升高;④延展區(qū)與交匯區(qū)槽內(nèi)汽含率較高,較多汽相逸出阻礙液膜延展,使得此處液膜涌動(dòng)增厚,進(jìn)一步加速干斑產(chǎn)生。
圖16為液膜總汽含率及液膜平均汽含率隨E變化。
不同E時(shí)液膜總汽含率在11%~16%內(nèi)變化,液膜總汽含率在圓管到橢圓管變化時(shí)從11.8%激增至14.5%,之后隨E增大而平緩減小。而槽內(nèi)汽含率隨E的增大而增大,槽外液膜汽含率規(guī)律與總汽含率變化規(guī)律相同。在E較小時(shí),槽內(nèi)/外有更加均勻的蒸發(fā)效果,隨著E增大,槽內(nèi)/外蒸發(fā)性能差別變大。
為進(jìn)一步探究槽內(nèi)/外汽含率產(chǎn)生差異原因,取相同像素下,噴淋口中心Z*=0,θ=90°處槽外液膜及槽1內(nèi)液膜溫度分布,如圖17所示。由圖17可知,槽內(nèi)/外溫度邊界層厚度均隨著E增大逐漸變薄、溫度梯度逐漸變大。在液體飽和溫度為7 ℃時(shí),槽內(nèi)近壁面液膜溫度均在7 ℃以上,而槽外液膜溫度均約為6 ℃,說(shuō)明槽外液膜的傳熱方式主要為強(qiáng)制對(duì)流傳熱,槽內(nèi)是核沸騰換熱與強(qiáng)制對(duì)流換熱并存,造成槽內(nèi)換熱性能、汽含率大于槽外。隨著E增加槽內(nèi)近壁面液膜溫度增高,液體過(guò)熱度增大導(dǎo)致槽內(nèi)汽化核心數(shù)目增加、槽內(nèi)汽含率增大;而槽外近壁面液膜溫度基本不變,使得圖16中槽內(nèi)/外平均汽含率差值隨之逐漸增大。
3" 結(jié)" 論
(1)隨著E增大,液膜干斑占比呈現(xiàn)先減小后增大趨勢(shì),E為1.2時(shí)管外濕潤(rùn)面積最佳;液膜在軸/周向均減速鋪展,E為1.2時(shí)管外液膜速度均勻降低,E為1.0和1.8時(shí)管上端液膜速度在Z*為-0.25~-0.375及0.25~0.375之間驟減,液膜聚積增厚,液膜鋪展受阻為干斑產(chǎn)生主要原因之一。
(2)E小于等于1.2時(shí),Kmax隨著E增大而增大,局部最大傳熱系數(shù)出現(xiàn)在液膜與干斑交界區(qū)域附近;E為1.2時(shí)Kmean最大,較圓管增大34.5%;E大于等于1.2時(shí),Kmean隨E增大而減小,E為2.0時(shí)Kmean仍較圓管增大15.7%。
(3)液膜從沖刷區(qū)向交匯區(qū)鋪展過(guò)程中,槽內(nèi)汽含率先增大后減小再增大,延展區(qū)及交匯區(qū)槽內(nèi)汽含率增大明顯,汽相逸出阻礙液膜鋪展;隨著E增大,液膜總汽含率均呈現(xiàn)先增大后減小規(guī)律,槽內(nèi)外汽含率差值逐漸增大。
(4)隨著E增大,槽內(nèi)/外溫度邊界層逐漸變薄,槽內(nèi)外溫度梯度逐漸增大;槽外液膜換熱方式主要為強(qiáng)制對(duì)流換熱,而槽內(nèi)為核沸騰換熱與強(qiáng)制對(duì)流換熱并存。
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(編輯" 沈玉英)