- 學位論文
流體與多孔介質雙層流域中奈米流體的熱對流穩定性分析
Thermal stability of nanofluids in superimposed fluid-porous domain
摘要
本論文建立一套奈米流體充滿於疊加的流體層與多孔介質層之系統中，利用奈米流體的布朗運動與熱對流耦合關係而產生的雙擴散對流效應，讓奈米流體雙層系統成為影響熱傳量的自然機制。此機制對熱傳量的放大效果，與諸多物理性質和幾何條件相關，而這些物理參數條件均能用來設計增強電子套件散熱機構，對提升CPU運算能力與DRAM存取速度，均有實質效果。對於目前全面發展的電動車馬達散熱，更有諸多的啟發與應用，但須將本論文之分析從直角座標轉換成圓柱座標，以符合輪軸形狀之設計。 本研究旨在以時間的線性穩定性理論，並探討厚度比及濃度差對此系統的不穩定性影響。整個系統當厚度比較小時，長波對流比短波對流不穩定，對流主要發生在多孔介質層；隨著厚度比變大，短波對流比長波對流不穩定，對流主要發生在流體層，即為系統發生的模態轉換，而渦漩的個數也變多，使得渦漩形狀為指狀，即發生所謂指狀對流(finger convection)。除了模態轉換外，亦探討流體層的雷利數及多孔介質層的雷利數隨著厚度比變化趨勢；隨著厚度比增加，流體層的雷利數大幅增加 ，而多孔介質層雷利數則是隨著厚度比增加而下降。除此之外，根據雙擴散理論，只有當下板濃度高時，才會有振盪頻率，而在轉換模態時，振盪頻率特別低，當厚度比增加時，振盪頻率會大幅增加。 在工業應用上，不同的振盪頻率應對熱傳量亦有不同影響；除此之外，為了設計適當的尺寸，我們固定總長度去探討不同的厚度比需要多少溫差才可產生紊流，得到厚度比越大越容易產生紊流，但是主要發生對流處在流體層，但是對流胞比較小即對流面積比較小，整體散熱效果不一定會較佳，因此需要達到雙層系統都有散熱且最小溫差應選轉換模態時的厚度較好。
並列摘要
In this paper, a system of nanofluid is used to fill the fluid layer and the porous layer. By using the double diffusion convection effect of Brownian motion and thermal convection coupling of nanofluid, the double layer system is the natural mechanism that affects the heat transfer. This mechanism is related to the physical properties and geometric conditions of the heat transfer, and these physical parameters can be used to design the enhanced electronic kit cooling mechanism, to enhance the CPU computing power and DRAM access speed have substantial effect. For the current full development of the electric car motor cooling, but the analysis of this paper from the right angle coordinates into cylindrical coordinates to meet the axle shape of the design. We study the linear stability theory and discuss the influence of thickness ratio and concentration difference on the instability of this system. When the thickness is relatively small, the long wave is unstable than the short wave, and the convection mainly occurs in the porous medium layer. As the depth ratio becomes larger, the short wave is unstable than the long wave , and the convection mainly occurs in the fluid layer, so-called modal conversion. The number of vortex also become more, so that the vortex become shape as finger, that is, the so-called finger convection . In addition to the modal conversion, the t the Rayleigh number of the fluid layer and of the porous media layer are changed with the thickness ratio. With the increase of the thickness ratio, the Rayleigh number of the fluid layer is greatly increased. In addition, according to the double diffusion theory, only when the board concentration is high, there will be oscillation frequency, and in the conversion mode, the oscillation frequency is particularly low, when the thickness ratio increases, the oscillation frequency will increase significantly. To design the appropriate size, we fixed the total length to explore the different thickness than the temperature difference required to produce turbulence. To get the depth of the heat transfer in the industrial application, the different oscillation frequency should also have different effects. The larger thickness ratio more prone to turbulence , but the main occurrence of convection in the fluid layer. The relatively small flow of convection area is relatively small, the overall cooling effect is not necessarily better, so the need to achieve double-layer system has a heat dissipation and the minimum temperature difference should be converted to modal thickness it is good.