Vapour condensation in boundary layer flows

Zusammenfassung

This thesis has two purposes. Metaphorically we can say that it works in two different scales -which, makes sense, nevertheless-. First, it deals with homogeneous vapour condensation in boundary layer flows. Boundary layer flows, as it is very well known, has the property of ubiquity. Every flow in contact with solid surfaces or even flows in mutual contact become boundary layer flows at some scale. The vapour condensation, and many other phase transition phenomena, develops intensively in that boundary layers. However, the description and modeling of homogeneous condensation has been less treated in the literature if compared with the case of heterogeneous condensation (that is, the condensation in presence of particles). A model of homogeneous vapour condensation in a boundary layer flow has been developed for the special case of stagnation-point incompressible flow near a cold wall with self-similar solution and a monodisperse distribution of resulting droplets. Complete model has been solved numerically and in addition a very good approximation to the model has been obtained by applying perturbative methods. We have extended this model in several directions: other flows admitting self-similar solutions, polydisperse distribution of droplets, mixed homogeneous and heterogeneous condensation and homogeneous condensation in counter ow boundary layers in compressible ows. In the case of mixed condensation we have showed that it is possible to tackle homogeneous and heterogenous condensation independently, in an iterative scheme. Of course, all these new directions have been treated in a less detailed form and keep open to future work. Second, the thesis has intended to bring together closely related themes that has been, however, studied separately. Then, we have widened the initial scope to other aspects a, for instance, coagulation and agglomeration of nanometric particles, thermophoresis and ice formation. Specifically, agglomeration and thermophoresis become essential in the understanding of condensation processes in combustion chambers where a very rich chemical activity is taking place, and ice formation is important if we want to extend the condensation process to atmospheric environments. The essential theme of this thesis is important in many aspects: 1. It deals with phenomena present in a wide variety of natural and industrial situations whose understanding may result in improvements of known processes or in the better forecast of some desirable or not desirable behaviors. 2. We have been forced to gather a lot of normally disperse or not directly connected information and methods that have an e ect in the comprehension and description of those phenomena. The mathematical treatment of some aspects of the problem has been undertaken in parallel with numerical simulation of some others. Therefore, a lot of work is waiting for completion or full develop. This is mainly the case of the stochastic-thermophoretic simulation of the agglomeration process of nanometric particles, that is described in appendix A, or the proposed model for the description of wedge ows near the leading edge, that ends the chapter 2. We have adopted the terms 'boundary layer flow' and 'counter flow boundary layers' for denoting the boundary layers when ocurring close to solid walls and those ocurring at the interface of two flows away from any solid surfaces, respectively. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------