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Shear thinning fluid mixing in unbaffled stirred vessels

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  1. Tez No: 403431
  2. Yazar: NİHAT YAVUZ
  3. Danışmanlar: Dr. K. P. SANDEEP
  4. Tez Türü: Doktora
  5. Konular: Gıda Mühendisliği, Food Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2016
  8. Dil: İngilizce
  9. Üniversite: North Carolına State Unıversıty
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: Belirtilmemiş.

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Özet (Çeviri)

Mixing is a common operation used in many process industries. Highly viscous fluids are often mixed in stirred vessels equipped with various impellers. However, certain performance issues arise due to the nature of these fluids. Isolated mixing regions (IMRs) are formed in laminar flow. For impellers located at the center of the vessels, IMRs appear right above and below the impeller and only after impractically long times, they dissipate by diffusion. Increasing rotational speed or using large diameter impellers such as an anchor or helical ribbon minimize IMRs at the expense of increased power consumption. Several strategies have been developed to eliminate IMRs. Insertion of baffles, variable rotational speed/direction protocols, and multiple impellers are proven to be effective in improving mixing under laminar flow conditions. The presence of extra surface area resulting in possible cleaning problems and the need for advanced control systems necessitate simpler solutions. Therefore, mixing of 1% carboxymethyl cellulose (CMC) solution in an unbaffled stirred vessel was studied in two parts. In the first part, the objective was to investigate the effects of eccentricity (locating impeller away from the center of the vessel) and modification of a standard pitched blade turbine (PBT) on laminar mixing and power consumption. Numerical analysis was also conducted to replicate the same experimental conditions. Laminar mixing was evaluated by the evolution of a tracer dye within the vessel. The measurements were made using the planar laser induced fluorescence (PLIF) method. Increasing eccentricity reduced the size of IMRs and increased concentration homogeneity without increasing power consumption. A Dual-flow PBT, which is a modified PBT generates simultaneous upward and downward flow, did not perform better than PBT for all the cases studied. The Metzner-Otto constant (Ks) was also calculated for both impellers to determine Power curves for further use in the second part of the study. The objective of the second part was to scale-up the results obtained in the first part based on equal values of Reynolds (Re) number, tip speed, and power per volume. For PBT, similar % area coverage and concentration distribution values were achieved with all three scale-up criteria. Time to reach the same end point and power consumption were the main differences between each criteria. Scale-up with equal power per volume resulted in achieving the end point faster than that with the other two methods while power consumption was the lowest for scale-up based on equal Re. For Dual-flow PBT, only equal Re number was successful in achieving good mixing, thereby suggesting that the dynamic behavior of the impeller may be affected significantly by scale-up.

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