對液滴蒸發之可變溫度風洞紊流特徵實驗研究

Abstract

本研究主要以實驗的方式探討可供人為控制且相當均勻的高溫及低溫紊流流場，以提供液滴蒸發實驗之所需，並再深入瞭解流場中特徵尺寸尺度彼此間相互影響的關係。在實驗方法上，採用雷射都卜勒測速儀來量測流場的基本性質，找出流場中具均質性(Homogeneous)的位置，以供液滴的置放參考。 本實驗主要探討吾人所設計之風洞的特性，對不同紊流強度的高低溫紊流流場的速度分佈剖面、速度擾動剖面、能量頻譜、速度擾動之概率密度函數分析（PDF，Probability Density Function）等，做一個全面性的探討，並採用 Isotropic Turbulence 理論，找出流場中的特徵尺寸尺度與時間尺度。

An open-air vertical wind tunnel is constructed to generate nearly uniform high- and low-turbulence flow field with variable temperatures, ranging 300-700K, for future droplet evaporation and ignition experiments. Mean speed ranges from 0.5 to 2 m/s and turbulence intensity ranges from 1.5 to 20 %. Laser Doppler Velocimetry is used to characterize the mean and turbulence properties, including mean streamwise velocity, turbulence intensity, probability density function and energy power spectrum. Integral length scale and Komogoroff scale are computed using Taylor hypothesis and assumming isotrpic turbulence theory. A relatively homogeneous region about 10-20 mm in the middle of the tunnel exit is indetified, which is wide enough for future droplet experiments.