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Numerical Investigation of Surface Wettability Effect on Liquid-Liquid Two-Phase Flow Heat Transfer in Microchannels

Moezzi, Mahsa | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 53000 (58)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Kazemzadeh Hannani, Siamak; Farhanieh, Bijan
  7. Abstract:
  8. It is aimed in this study to numerically investigate the effect of contact angle on the heat transfer coefficient in oil-water two-phase flow. For this purpose, the finite element method (FEM) is used to solve the unsteady Navier-Stokes and energy equations. The level set method is also used to capture the interface between the phases. In the first part of the study, two-phase flow of the water and calibration oil is considered in a T-junction geometry at contact angles of 5°, 40°, and 75°. It is observed that the flow patterns formed in the microchannel depend on the initial flow condition which results in the hysteresis phenomenon. Investigating the effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel provided useful insight into the hysteresis phenomenon. The results show that the hysteresis is significant in superhydrophilic microchannel with the contact angle of 5°, while it disappears at the contact angle of 75°. The role of water to oil flow rate ratio (Qwat/Qoil) on the flow pattern and heat transfer is also studied. Dripping and co-flow regimes are the dominant patterns that form in the investigated range of parameters. The dripping regime mostly occurs at smaller contact angles, while only the co-flow regime is observed at the contact angle of 75°. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Qwat/Qoil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity becomes smaller at larger values of Qwat/Qoil, which reduces the averaged Nu. In dripping regimes, larger flow rate ratios cause a more frequent generation of droplets or slugs with a smaller size. The local Nu experiences a rise during the passage of a slug. Larger slugs produced at smaller Qwat/Qoil create a larger increase in the local Nu than smaller slugs. In the second part of the study, a detailed investigation is conducted to characterize the heat transfer in water-oil flow in a microtube. The main part of the study is devoted to investigating the effect of wettability on the heat transfer coefficient. Four contact angles of 0°, 30°, 150°, and 180° were examined, and it was observed that the contact angle of 150° results in the largest mean Nu. At the contact angle of 30° and 150°, triple points form and the slugs slide on the wall. This results in a larger wall shear, and consequently larger slip velocity on the wall. Larger near-wall velocity enhances the heat removal process and increases the Nu. The curvature of the two-phase interface varies with the contact angle.
    Based on the observed flow configuration and the curvature of the two-phase interface, a novel idea is introduced to apply a non-uniform distribution of contact angle to augment the local Nu. It is observed that changing the wall from hydrophobic to hydrophilic will locally increase the Nu around the transition point. Changing the contact angle modifies the curvature of the interface which disturbs the flow and can increase the local Nu in the fully-developed region. In addition to the contact angle, the slug length, frequency of slug generation, and the film thickness around the slugs affect the Nu. Three Weber numbers (We) at four contact angles are examined by varying the flow rate of the oil phase. The Weber number affects the Nu by changing the frequency of slug generation, and consequently its length. It is observed that the mean Nu increases with the increase in the We. Finally, the effect of film thickness is scrutinized at various capillary numbers (Ca). The film thickness increases with an increase in Ca which reduces the heat removal rate since it behaves as a thermal barrier
  9. Keywords:
  10. Two Phase Flow ; Heat Transfer ; Microchannel ; Wettability ; Contact Angle ; Hysteresis ; Wastes

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