微尺度傳熱課件-peakchina microscale-ncov

1、Microscale and miniscale Natural ConvectionBackgroundThermal design of MEMS, for example, micro convective accelerometer Cooling of micro electronic component Others器件結(jié)構(gòu)研究狀況微尺度與小尺度自然對(duì)流研究的需求不足，研究尚不深入；與大尺度自然對(duì)流的聯(lián)系和區(qū)別沒有得到較好的澄清；實(shí)驗(yàn)研究很少，數(shù)值分析以橢圓型方程為基礎(chǔ)進(jìn)行計(jì)算。FeaturesSmaller Gr, difficult to reach turbulentBoun

2、dary (3-D) effect es strongerBoundary layer equations are not validEffect of variable property should be consideredSmaller velocity corresponds to smaller inertial effect 基本機(jī)制1. 邊界層假設(shè)不再成立，邊緣效應(yīng)增強(qiáng)；2. Gr 越小，導(dǎo)熱作用越強(qiáng)，對(duì)流作用越弱； 數(shù)學(xué)表現(xiàn)： Nu 和 Gr的關(guān)系越來越弱；3. 熱和流動(dòng)區(qū)域分成兩部分：內(nèi)區(qū)為導(dǎo)熱區(qū)，外區(qū)為導(dǎo)熱和對(duì)流共同作用區(qū)；邊界層假設(shè)不再成立，邊緣效應(yīng)增強(qiáng)1.邊緣效應(yīng) 實(shí)

3、驗(yàn)裝置：基底絕 熱的豎窄條邊界層方程不再適用邊緣效應(yīng)增強(qiáng)，換熱發(fā)生了很大變化 對(duì)豎直細(xì)絲的自然對(duì)流來說，等壁溫和等熱流條件的結(jié)果沒有多少差別。其原因就在于邊緣效應(yīng)或三維效應(yīng)的增強(qiáng)。Gr 越小，導(dǎo)熱作用越強(qiáng)，對(duì)流作用越弱表面溫度恒定的球置于靜止流體中, Nu=2 （特征尺度，d)平板間導(dǎo)熱，特征尺度為板的厚度 Nu=1導(dǎo)熱和對(duì)流的對(duì)比發(fā)生了變化從上述小尺寸豎直平板（壁面溫度隨著板高周期變化）的流線圖和等溫圖可以看出： 隨著Gr的增大，導(dǎo)熱的作用變小，對(duì)流 作用增大；相應(yīng)數(shù)學(xué)表達(dá)式為： 點(diǎn)加熱源的Nu 與 Gr數(shù)的關(guān)系3.熱和流動(dòng)存在的分區(qū) 實(shí)線代表流線，虛線代表 等溫線；從圖中可以看出， 接近加

4、熱點(diǎn)源附近的是 導(dǎo)熱區(qū)，而外面則既有導(dǎo) 熱又有對(duì)流； 作用力的相對(duì)重要性可能會(huì)發(fā)生變化 Macroscale Natural ConvectionIsothermal vertical plate Nu(GrPr)1/4 laminar flow Nu(GrPr)1/3 turbulent flow Heat transfer coefficient is independent on height Physical meaning of Gr Inertial force Buoyancy force Viscous forceFor laminar flow, inertial force

5、 and viscous force may be of same order or inertial force may dominate the fluid flow, nevertheless, viscous force may be much larger than inertial force due to very large turbulent viscosity for turbulent flow. Which leads to different relation for Nu to Gr.Effect of viscous force relative to inert

6、ial force will increase with characteristic scale decrease Inertial force L2Viscous force LMicroscale natural convectionMomentum equationFor natural convection around normal-sized object:buoyancy driving forceinertial is dominant over viscous force as ratio of inertial to viscous forceratio of conve

7、ction to conductionFor small sized objectbuoyancy driving forceviscous is dominant over inertial force asratio of inertial to viscous forceratio of convection to conductionRelative importance of forcesNu versus Gr微尺度自然對(duì)流的應(yīng)用范例 熱對(duì)流加速度計(jì)的熱分析 和熱設(shè)計(jì)線性度：量程和靈敏度：結(jié) 論N-S方程雖然依舊成立；邊緣效應(yīng)增強(qiáng)，邊界層假設(shè)不成立；導(dǎo)熱作用隨Gr的減小而增強(qiáng)；粘性力與慣性力的相對(duì)重要性發(fā)生了變