朱光昱 高 力 于方小稚 田齊偉

氣泡微細(xì)化沸騰傳熱特性

朱光昱1高 力1于方小稚2田齊偉1

1（中國核電工程有限公司 總體所 北京 100084）
2（北京理工大學(xué) 自動(dòng)化學(xué)院 北京 100081）

氣泡微細(xì)化沸騰(Micro-bubble Emission Boiling, MEB)是一種特殊的過冷沸騰現(xiàn)象，當(dāng)其發(fā)生時(shí)加熱面的熱流密度會(huì)遠(yuǎn)高于臨界熱流密度(Critical Heat Flux, CHF)。根據(jù)采集到的可視化沸騰資料，對(duì)MEB的傳熱機(jī)理進(jìn)行了分析。結(jié)果表明，MEB發(fā)生時(shí)，加熱面上不穩(wěn)定氣膜的周期性破裂，破壞了過熱液層，導(dǎo)致了良好的氣液置換對(duì)流換熱?？紤]MEB的特殊傳熱過程，對(duì)Rohsenow關(guān)系式中部分項(xiàng)進(jìn)行修正，并根據(jù)最小二乘法對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合，得到了適用于10 mm銅加熱面上的MEB沸騰關(guān)系式，誤差不超過±15%，可滿足一般的工程計(jì)算要求。

氣泡微細(xì)化沸騰，臨界熱流密度，最小二乘法

近幾十年內(nèi)，很多核能裝置的發(fā)熱元件熱通量已經(jīng)遠(yuǎn)超1 MW·m-2，國際熱核聚變實(shí)驗(yàn)堆(International Thermonuclear Experimental Reactor, ITER)濾偏器上的最大熱負(fù)載達(dá)到了30 MW·m-2；在托卡馬克第一壁上，當(dāng)?shù)入x子體破滅時(shí)，其局部功率也達(dá)到500 kW·m-2。

常見的空冷、水冷技術(shù)已無法滿足這些設(shè)備的制造或改進(jìn)要求。由Suzuki等[1-2]發(fā)現(xiàn)了氣泡微細(xì)化沸騰(Micro-bubble Emission Boiling, MEB)，因其可獲得遠(yuǎn)高于臨界熱流密度(Critical Heat Flux, CHF)的熱流，被認(rèn)為是換熱器設(shè)計(jì)優(yōu)化和提升換熱極限的理想手段之一。

Zhu等[3]繪制了15-85 K過冷度下的池式沸騰特性曲線，分析了加熱面上的氣泡脫離和破裂頻率，總結(jié)了過冷度對(duì)MEB沸騰以及偏離泡核沸騰點(diǎn)(Departure from Nuclear Boiling, DNB)的影響。Shoji等[4]在過冷度高于40 K的鉑加熱絲的池沸騰實(shí)驗(yàn)中實(shí)現(xiàn)了MEB，他們獲得的最高熱流密度達(dá)到了10MW·m-2。Kumagai等[5]研究了15 mm×6 mm矩形管道內(nèi)的過冷流動(dòng)沸騰現(xiàn)象，并根據(jù)實(shí)驗(yàn)條件和現(xiàn)象將MEB區(qū)分為：(1) 壁面過熱度較低、過冷度較高、液體流速較高條件下的Stormy-MEB I；(2) 壁面過熱度較高、過冷度較低、液體流速較高條件下的Stormy-MEB II；(3) 壁面過熱度較高、熱流密度增長緩慢的Calm MEB。本文根據(jù)采集到的可視化資料，分析了MEB的傳熱機(jī)理，給出了MEB沸騰換熱關(guān)聯(lián)式。為MEB的工程化應(yīng)用和發(fā)生機(jī)理研究提供了基礎(chǔ)。

1 實(shí)驗(yàn)裝置

圖1所示為過冷池沸騰實(shí)驗(yàn)裝置，詳見文獻(xiàn)[6]。加熱元件為4個(gè)硅碳棒，導(dǎo)熱銅芯上部為直徑10mm的加熱面。導(dǎo)熱銅芯中軸上裝有3個(gè)直徑1mm的T分度號(hào)銅-康銅鎧裝熱電偶，距離加熱面分別為3 mm、5.75 mm和8 mm，并依次間隔30o，以減少熱電偶對(duì)銅芯導(dǎo)熱的影響。加熱面的壁溫?cái)?shù)據(jù)可以根據(jù)銅芯中軸的溫度通過Fourier導(dǎo)熱定律求得。水箱中的水溫由在加熱面上方5 mm處的熱電偶測(cè)量，水的過冷度由一個(gè)電加熱器和一個(gè)冷卻器共同控制。

2 結(jié)果和分析

圖2為30 K和50 K過冷度下熱流密度為6.3MW·m-2和5.7 MW·m-2時(shí)的MEB沸騰現(xiàn)象。當(dāng)MEB發(fā)生時(shí)，加熱面上周期性的生成不穩(wěn)定氣膜，在氣膜完成生長后，由于氣液界面不穩(wěn)定而發(fā)生劇烈破碎。如圖2(a)中2.75 ms和圖2(b)中1.2 ms處所示，氣膜破損產(chǎn)生的沖擊橫掃加熱面，使加熱面可以直接與過冷水接觸，并迅速生成新的氣膜。整個(gè)變化周期約在3 ms，這種由氣膜破碎導(dǎo)致的氣液置換是MEB可以達(dá)到極高熱流密度的原因。而當(dāng)加熱面低于隔熱陶瓷0.5 mm時(shí)，氣液置換過程受到阻礙，MEB不會(huì)發(fā)生[3]。

圖1 過冷池沸騰實(shí)驗(yàn)裝置Fig.1 Experimental setup for subcooled pool boiling.

圖2 30 K (a)和50 K (b)過冷度下熱流密度為6.3 MW·m-2(a)和5.7 MW·m-2(b)時(shí)的MEB 沸騰現(xiàn)象Fig.2 Bubble behavior of MEB at liquid subcooling of 30 K (a) and 50 K (b) with q=6.3 MW·m-2(a), 5.7 MW·m-2(b).

式中，cpf為水的比熱容，J·kg-1·K-1；ΔTsat為壁面過熱度，K；hfg為水的汽化潛熱，J·kg-1；Csf為流體加熱面組合的特性函數(shù)；μf為水的動(dòng)力粘度系數(shù)，N·s·m-2；σ為水的表面張力系數(shù)，N·m-1；g為重力加速度，m·s-2；ρf和ρg為水和蒸汽的密度，kg·m-3；Pr為普朗特?cái)?shù)。

在大容積飽和沸騰過程中，水溫為飽和溫度，為防止達(dá)到CHF，壁溫通常也不會(huì)太高。而當(dāng)MEB發(fā)生時(shí)，水的過冷度都在20 K以上，加熱面的過熱度可以超過180 K[1]。所以式(1)中等號(hào)左側(cè)項(xiàng)需要改寫將過冷水加熱至飽和，以及將蒸汽加熱至過熱兩個(gè)部分：

式中，ΔTsub為水的過冷度；cpg為水蒸汽的比熱容，J·kg-1·K-1。

在式(1)中，工質(zhì)為水時(shí)Pr數(shù)的系數(shù)m=1。而在MEB沸騰過程，需要考慮蒸汽過熱度和水的過冷度對(duì)Pr數(shù)的影響，所以需對(duì)Pr數(shù)進(jìn)行如式(3)修正并重新擬合其系數(shù)m。

式中，Prf和Prg分別為過冷水和過熱蒸汽在物性溫度下的Pr數(shù)。

對(duì)于核態(tài)沸騰來說，Csf一般在0.0027-0.013。對(duì)于MEB而言，流體與加熱面的相互作用方式與核態(tài)沸騰有一定差別，所以Csf需重新擬合。據(jù)實(shí)驗(yàn)數(shù)據(jù)使用最小二乘法擬合，可以得到Csf=0.0385，m=0.25，n=1.42。MEB換熱關(guān)聯(lián)式為：

圖3為實(shí)驗(yàn)獲得的40 K、50 K和60 K過冷度下的沸騰特性曲線。圖4(a)、(b)、(c)分別為40 K、50 K、60 K過冷度下，熱流密度的實(shí)驗(yàn)值和計(jì)算值的對(duì)比?？梢钥闯?，該關(guān)系式與實(shí)驗(yàn)值吻合較好，誤差不超過±15%。

圖3 過冷度對(duì)MEB的影響Fig.3 Effect of subcooling on MEB.

圖4 40 K (a)、50 K (b)和60 K (c)過冷度下實(shí)驗(yàn)結(jié)果與計(jì)算結(jié)果對(duì)比Fig.4 Comparison between experimental values and calculated values at liquid subcooling of 40 K (a), 50 K (b) and 60 K (c).

3 結(jié)論

本文研究了40-60 K過冷度下，10 mm銅加熱面上除氧水的MEB沸騰傳熱特性，根據(jù)MEB的特殊傳熱過程，修改了Rohsenow關(guān)系式中部分項(xiàng)，并根據(jù)最小二乘法對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合，得到了適用于10 mm銅加熱面上的MEB沸騰關(guān)系式，誤差不超過±15%。

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Heat transfer property of micro-bubble emission boiling

ZHU Guangyu1GAO Li1YU Fangxiaozhi2TIAN Qiwei1
1(General Design Division, China Nuclear Power Engineering Co., Ltd., Beijing 100084, China) 2(School of Automation, Beijing Institute of Technology, Beijing 100081, China)

Background:Micro-bubble Emission Boiling (MEB) is a special subcooling boiling phenomenon that the heat flux increases more highly than critical heat flux (CHF). Due to its extremely high heat transfer capability, many researchers have shown interest in it.Purpose:In this paper, we fit the heat transfer correlation of experimental data collected in visualized boiling experiments after deeply analyzing the heat transfer mechanism of MEB. Methods: Four Si-C heaters were employed for heating the copper block, which has a round heating surface with diameter of 10mm on its upper. Temperature data were measured by T-type sheathed thermocouples. The temperature of the heating surface was obtained by extrapolating the temperature distribution. Based on the heating surface temperature date in different subcoolings, least square method was used to fit Rohsenow relation to MEB. Bubble behaviors were captured by high-speed video camera with light system.Results:The experimental results showed that, when the subcooling exceeded 40 K, disturbance emerged at the liquid-vapor interface and the micro-bubble emission boiling occurred after the CHF was attained, thereafter the heat flux increased rapidly with the superheat increasing like that in typical nucleate boiling region. Based on Rohsenow relation, MEB heat transfer correlation is fitted according to the measured temperature data for heating surface of 10-mm copper in different subcoolings. The error of the relation is less than ±15%, which meets the requirement of general engineering. Conclusion: The efficient convective heat transfer of vapor-liquid replacement caused by periodic damage of the unsteady vapor film on the heating surface is the heat transfer mechanism of MEB.

MEB, CHF, Least squares

ZHU Guangyu, male, born in 1989, graduated from Harbin Engineering University in 2015, research areas is nuclear power plants operation and test technology

TL334

10.11889/j.0253-3219.2015.hjs.38.120602

國家自然科學(xué)基金(No.51376052)資助

朱光昱，男，1989年出生，2015年畢業(yè)于哈爾濱工程大學(xué)，研究領(lǐng)域?yàn)楹穗娺\(yùn)行與調(diào)試技術(shù)

Supported by the National Natural Science Foundation of China (No.51376052)

2015-09-28，

2015-11-02

CLCTL334