Battistella, Alessandro (PhD)
Despite the widespread use in industry of gas-liquid contactors, such as bubble columns, the complex interplay between hydrodynamics, mass & heat transfer and chemical reaction still represents a major challenge to a complete fundamental understanding. An additional difficulty arises when the heterogeneous regime is considered, where a chaotic movement of both the gas and the liquid appears. This regime is the most common in industrial application, thus the understanding of the principles represents a relevant issue for engineers.
With the increasing computer power, the use of Computational Fluid Dynamics (CFD) to develop detailed models is growing. The use of computer models presents the considerable advantage of reducing costs and time consumption, compared to experiments, which could be expensive or even difficult to perform in some cases. A different level of fundamental understanding can be gained at different length scales using a multi-scale modeling approach. In this project, the focus is on large scale and two different models will be used: an Euler-Lagrange (E-L) and an Euler-Euler (E-E) model.
The Euler-Lagrange model is the Discrete Bubble Model (DBM). In the DBM, at an intermediate scale, each bubble is tracked individually, allowing to have information on bubbles number, size, position and velocity as part of the solution. Collisions, as well as coalescence and breakups, are easily implemented, returning relevant information on bubble binary interactions. In addition, this model represents a valuable tool to study large scale bubble behaviors and to eventually develop and validate the larger scales models, such as E-E or phenomenological.
At a larger scale, the Euler-Euler model considers both fluids as a continuum. This approximation permits to limit the computational effort to resolve the bubble tracking and thus allows simulations at a larger scale. This model, which will be developed from an open source software (OpenFOAM), can be easily adapted to columns of different shapes or with internals. Indeed, this is the preferred simulations tool in industry, where CFD is applied, due to its versatility. On the other hand, to have information on bubbles, it needs to include additional equations such as population balance equations (PBE) which often include closure relations that are mostly empirical and holding for limited cases, giving less fundamental information on the bubbly flow.
The links between the two scales is also very important: not only the DBM can be used as a development and validation tool for the E-E model, but also the comparison of the two can give valuable information about scale effects in bubbly flows.
The objectives of the project are thus:
- Study heat/mass transport in dense bubbly flows at both (intermediate and large) scales
- Study phase transition (either due to boiling or reaction) at both scales
- Understand the interplay of those phenomena with the hydrodynamics of the system
- Study scale effects and compare the two models