TAILIEUCHUNG - Two Phase Flow Phase Change and Numerical Modeling Part 12

Tham khảo tài liệu 'two phase flow phase change and numerical modeling part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 320 Two Phase Flow Phase Change and Numerical Modeling Two-phase heat transfer Evaporation Evaporation is one of major two-phase heat transfer mechanisms. In an evaporation process a mass transfer occurs which means liquid meniscus including a triple contact line TCL has a motion. Therefore we need to consider a dynamic contact angle advancing and receding contact angles as shown in Fig. 3. Generally the advancing contact angle will tend to toward a lower value during evaporation Picknett Bexon 1977 . Most of studies for wettability effects on the evaporation fundamentally are focused on an evaporation of a sessile drop. The evaporation process of the droplet can be classified to few steps as shown in Step 1 saturation of atmosphere Step 2 constant contact radius with a decreasing drop height and contact angle Step 3 a constant contact angle with a decreasing a contact radius and Step 4 final drop disappearance . In most previous studies focused on step 2 3 and 4. Chandra et al. 1996 studied on the contact angle effect on the droplet evaporation. Three kinds of droplets of pure water surfactant 100 ppm and 1000 ppm on a stainless steel surface were visualized. Their results indicate that a reduced contact angle makes a droplet thickness thinner and a contact area larger. Thus an increased heat transfer area and a decreased conductive resistance enhance the droplet evaporation Fig. 11 . Takata et al. 2004 2005 measured an evaporation time a wetting limit and Leidenfrost temperatures on stainless steel copper and aluminum surfaces. They used a plasma-irradiation to increase a wetting property of those surfaces. Their results indicate that the evaporation time decreases and the wetting limit and the Leidenfrost temperatures increase in hydrophilic surfaces. Therefore the hydrophilic surface has potentials for the enhancement of evaporation. Fig. 10. Evaporation process for water on ETFE with initial drop volume of 5 pL Diameter Height and Angle .

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