Study of Marangoni convection and liquid film breakdown along a heated tube using thermography

点击翻译按钮获取中文摘要

To understasnd the Marangoni effects on wave structure and liquid film breakdown, theoretical and experimental investigations ware conducted on a subcooled liquid film flowing a heated plane and tube. Thermography technique was employed for determing the temperatur distribution at a liquid surface. The characteristics such as emissivity, absorption coefficient and liquid layr thickness were examined for the three kinds of liquid, water, silicone-oil and 1-propanol at the stationary state conditions. Then, measurements were performed to the liquid film flowing down a heated plane. Based on the experimental results, a simplefied approach to analyze the wave structure was used by adding the shear stress due to the Marangoni effects to the isothermal film flow equation. The following results are deduced :1) Infrared image analysis reveals the surface temperature of a liquid film sharply increases behind a disturbance wave, then a dry patch occurs at the hot spot on a liquid film surface. The observation results suggest that the Marangoni effects may trigger the dry patch and evaporation may accelerate the liquid film breakdown.2) Theoretical analysis shows the existence of two critical flow rates for instability occur at a fixed heat flux of the tube surface. The instability for a higher flow rate is due to fluid dynamic effects of the suraace tension force. On the other hand for a lower flow rate the instability is caused by thermal effects due to the Marangoni force3) The critical Marangoni number obtained from instability analysis is the constant value of 0.23, while that experimenatally obtained for liquid film breakdown is 6*10^3 for q_w>0.4W/cm^2. Theoretical film thicness for Ma_c=0.23 is estima approximately 1/2 to that for 6*10^3. This may suggest the minimum film thickness just before film breakdown occur is at least than half to the average film thickness.