tailieunhanh - Computational Fluid Dynamics 2011 Part 4

Tham khảo tài liệu 'computational fluid dynamics 2011 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 84 Computational Fluid Dynamics boundary equals the fluid yield stress and th e predominant motion of the fluid within the cavern is tangential. Elson et al. 1986 1988 mdified that for a more appropriate cylindrical cavern. A cylindrical cavern model was also proposed by Hirata Aoshima 1994 1996 . Amanullah et al. 1998 developed a mathematical model for a torus-shaped cavern. This model considers the total momentum imparted by the impeller as the sum of both tangential and axial shear components transported to the cavern boundary by the pumping action of the impeller. Wilkens et al. 2005 empirically developed an elliptical torus model neglecting the axial force to predict the cavern diameter and height and using the yield stress of the fluid to define the cavern boundary . Table 2 lists these cavern models. Model Reference Witchterle and Wein 1975 1981 F8 I Solomon et al. 1981 ZM2D2 Ekion etal. 1986 Djf ZJ Re Hcíbc n 3 ũđ Elson 1988 w Hirata Aoshima 1995 1996 Amanullah et al. 1998 Wilkens et a 2q 5 s Table 2. Various models proposed for cavern size The contours of velocity magnitude generated CFD simulations can be used to analyze the formation of cavern around the impeller Pakzad et al. 2008a Saeed et al. 2008 Pakzad et al 2008b Ein-Mozaffari Upreti 2009 . The boundary velocity for the cavern is considered to be where rtip is the impeller tip velocity. Thus the fluid velocity is greater than within the cavern and less than in the surrounding. Pakzad et al. 2008a employed CFD to measure the size of the cavern generated around the Scaba 6SRGT impeller in the mixing of xanthan gum soluti on. Figure 4 shows the formation of cavern around the Scaba 6SRGT impeller for Re equato 30 and 110 Pakzad 2007 . To validate the CFD results for the cavern size Pakzad et al. 2008a used the Electrical Resistance Tomography ERT to measure the cavern diameter. ERT is a non-intrusive technique to interrogate the three dimensional concentration fields inside .