Two phase flow and phase change heat transfer in small structures

Abstract

New production techniques, which became available since the 1980’s, allowed the mass production of small sized fluidic systems. Reducing the size of a technical system alters the performance due to scaling effects. Physical phenomena which operate on volumes, such as gravity and inertia, become less important with respect to physical phenomena which act on surfaces or lines, such as pressure differences and surface tension. This relatively new research area on small sized fluidic systems is called “micro-fluidics”. An illustrative example on the size reduction of an evaporator system shows a possible advantage of scaling down a physical process. To characterise micro-fluidic flow dimensionless numbers can be used, such as the Reynolds number (Re), Weber number (We), Knudsen number (Kn) and Bond number (Bo). Gas-liquid two-phase flow systems are regarded to be small (or confined) if the Bond number is smaller than 4, or its square root inverse (i.e. the confinement number, Co) should be larger than 0.5. The present work investigated the interaction of phase transition heat transfer and two-phase flow behaviour in small geometries. In order to get insight into the various interactions the research is split into two parts: 1. interaction between two-phase flow and small geometries, 2. interaction between phase transition heat transfer and small geometries. The typical size of the “small geometries” regarded here range from about 5 mm down to about 0.1 mm. Furthermore, in general the liquid phase is water and the gaseous phase is either air or water vapour. In general standard conditions (i.e. pressure ~1 bar and ambient temperature ~293 K) are considered. As a theoretical background the most commonly used two-phase flow concepts are introduced, such as the void fraction, superficial velocity, and two-phase flow pattern map. In literature an alternation in two-phase flow pattern map is observed on reducing the channel diameter. For example no horizontally stratified flow occurs in concurrent two-phase flow in small diameter channels. Only three anchor flow regimes are identified: bubble flow, slug flow, and annular flow. Textbook knowledge and the state of the art insights are presented on boiling bubble behaviour, nucleate pool boiling, and flow boiling. Various heat transfer mechanisms during nucleate boiling are described, as well as their interactions and relative importance. An overview is given in literature readily available two-phase flow pattern maps, which are measured within small diameter channels (0.5