Numerical Simulation On Falling Film Outside The Vertical Tube For [EMIM][DEP]/H₂O Solution

Absorber is the core component of absorption heat pump that always acts as a waste heat recovery plant and consumes low power itself, and its performance optimization is the focus of attention and study for so many researchers. The most significant process must successfully completed by absorber is the absorption of cycle fluid by absorbent, which composes the working pair with the former. Some certain limitations have been found in the practical application for the traditional working pairs, such as strong corrosion and easy to crystallization for LiBr/H2O and insecurity for NH3/H2O respectively. Thus, new-style working pairs with excellent performance must be developed. Moreover, improving the hydrodynamic performance of working pair is also a critical method to enhance the absorber’s ability.Ionic liquid is a kind of green solvents in liquid state at room temperature, and its thermodynamic properties are considered satisfying the performance requirements proposed by absorption heat pump. However, it is still lacking of the falling film performance for ionic liquid, especially the hydrodynamic properties, when applied in the absorber. In addition, some researchers have demonstrated that the vertical tubes with special-shaped wall can efficiently enhance the falling film absorption process, but remained the strengthen mechanism unsolved.The present work carries out more in-depth research in the following respects for the current incomplete study.Primarily, two-dimensional model for falling film of [EMIM][DEP]/H2O solution outside the smooth tube is established to further study its flow characteristics and wavy motions on the surface. The results show that there are inertial waves generated on the film surface as its flowing down outside the tube. These inertial waves strong disturbance to the liquid film, especially the part near the wall, and this would result in an enhancement in the heat and mass transfer process for the film.Then, the influences of solution inlet Reynolds number and concentration to the falling film and surface wave are examined. The simulation results indicate that the film thickness would increase and surface wavy performance weaken as the increase of Re. The wavy motions on the film surface will be in the primary form of capillary waves as the film viscosity will be greatly enhanced when increasing the solution concentration. However, an "abnormal" solitary wave is found nearby the outlet region for certain solution concentration.Finally, physical and mathematical models for [EMIM][DEP]/H20 solution’s flowing outside the spirally grooved tube are both developed in this article for studying on disturbance to the liquid film from the spiral grooves. The influences of operating condition and tube structure are then researched by changing the Re and number of spiral grooves respectively. The results show that the presence of spiral grooves enhance the wettability of liquid film and result in fluid mixing near the wall, because of the separation of boundary layer in the film. Furthermore, falling film for a lower Reynolds number is easier disturbed by the spiral grooves. The single spirally grooved tube exhibited relatively poor wettability for the liquid film when compared with the double and triple one, but eddies were found in the liquid film.