微流道中溫度效應對不可壓縮流體流力與熱傳特性之影響

一些研究者比較了已發表的有關微流道摩擦及熱傳的實驗數據後，發現實驗數據與傳統大管道理論所預測的有相當差距，不同的實驗所得的結果差異很大，實驗結果的趨勢也不盡相同。有些研究者認為實驗數據與傳統大管道理論的差異可能是由下列的微尺寸效應所造成：溫度效應、壓縮效應、稀薄效應、黏滯消散、電滲效應、壁面粗糙度及實驗誤差等因素。此外，有些學者為了簡化分析與計算，皆假設熱導係數與黏滯係數為常數，而忽略了溫度變化對這些參數的影響，這也是造成差異的原因。本研究主要是針對三維不可壓縮層流長微流道中，溫度效應對液態流體流力與熱傳現象影響的探討，因此將研究重心放在摩擦與熱傳特性上。本研究發現受到溫度效應的影響，使得較不考慮溫度效應時小，且黏滯係數改變進而造成雷諾數的變化較傳統大尺寸流道來得大，因此過渡區提早發生於雷諾數1200~1500之間，而增加管壁溫度或熱通量的輸入皆會使得磨擦因子降低。在熱傳方面，同樣受到溫度效應的影響，其熱傳效果較不考慮溫度效應時好，而深寬比越大，熱傳效果也越好。此外在溫度效應影響下，等溫壁面條件的Nusselt number會隨著雷諾數的加大而跟著增加，但固定熱通量條件的Nusselt number卻不會改變。而本研究利用簡化的Navier-Stokes方程式，來計算長微流道裡的摩擦與熱傳特性。本數值計算的優點，在於它的統御方程式是一個具有拋物線型特性的方程式，在求解速度上至少比完全 Navier-Stokes 方程式快上百倍到千倍。而此一快速及準確的研究工具將可幫助我們詳細的研究微流道中，因溫度效應造成流體性質變化所引起的種種現象。此外也可讓我們了解更多微流道中的現象及解釋實驗數據與傳統大管道理論的差異。

關鍵字

微流道；溫度效應；黏滯消散；摩擦；熱傳

並列摘要

Several researchers have conducted careful reviews on the published experimental friction and heat transfer data in microchannels. They found that in many cases the experimental data of the friction factor and of the Nusselt number in Microchannels disagree with the conventional theories but they also appear to be inconsistent with one another. Also, the information in the literature does not point to unequivocal trends of variation or reasons of such trends. Some researchers proposed that the deviations between measured data and conventional predictions may be explained by the micro-scale effects. The following scaling effects in micro-flow devices can be identified: temperature effects, compressibility effects, rarefaction effects, viscous dissipation effects, electro-osmotic effects（EDL）, channel surface conditions and experimental errors. The objective of this research is to investigate numerically the effect of temperature on friction and heat transfer characteristics of incompressible flow in microchannels. It was found that the temperature effect reduces . The reduction in viscosity due to rising temperature increases Reynolds number, so that transition was initiated at Re=1200~1500. Increasing wall temperature or heat flux reduces friction factor. In the heat transfer characteristics, the Nusselt number is increased by taking the temperature effect into account. Nusselt number also increases with increasing channel aspect ratio. The major advantage of the present numerical procedure is its fast speed due to the parabolic character of the governing equations. It is at least two to three orders of magnitude faster than the full Navier-Stokes simulation. The present numerical procedure can be a fast and accurate tool in studying long microchannels flows. It also provides some useful explanations on the differences between published experimental data and conventional theories and among experimental data themselves.

並列關鍵字

Microchannel flow ； Friction ； Heat transfer

參考文獻

3.Wu, P.Y. and Little, W.A., “Measurement of friction factor for the flow of gases in very fine channels”, microminiature Joule Thompson refrigerators, Cryogenics, vol. 23, no. 5, pp. 273-277, 1983.

4.Wu, P.Y. and Little, W.A., “Measurement of the heat transfer characteristics of gas flow in fine channel heat exchangers”, microminiature refrigerators, Cryogenics, vol. 24, pp. 415-420, 1984.

8.Peng, X.F., Peterson, G.P., “Forced convection heat transfer of single-phase binary mixtures through microchannels”, Experimental Thermal and Fluid Science, Vol. 12, pp. 98-104,1996.

9.Peng, X.F., Peterson, G.P., and Wang, B.X., “Frictional flow characteristics of water flowing through microchannels”, Experimental Heat Transfer, vol. 7, pp. 249-264, 1994.

10.Peng, X.F., and Peterson, G.P., “Convective heat transfer and flow friction for water flow in microchannel structures”, Int. J. Heat Mass Transfer, vol. 39, pp. 2599-2608, 1996.

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微流道中溫度效應對不可壓縮流體流力與熱傳特性之影響

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