Two phase flow and phase change heat transfer in small structures

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 0.5). The present work extends the researched regions of the two-phase flow pattern map to much lower superficial liquid velocities, in which the occurrence of stratified flow might be expected. The present experiments show no stratified flow or wavy flow pattern region. Therefore, in order to theoretically predict the transition lines the stratified flow is most likely not the most adequate initial two-phase flow pattern. Annular flow is suggested to be a more appropriate initial two-phase flow pattern. The present work on confined pool boiling demonstrates an enhancement of the nucleate boiling heat flux when the boiling pot diameter is decreased (i.e. increased confinement). In the nucleate boiling regime the heat flux in confined pool boiling is between 5 to 10 times higher than the heat flux for unconfined pool boiling. The analysis of possible errors and a variation of experimental parameters excludes that the observed enhancement is the result of the chosen boiling liquid, the variation in boiling pot depth, the material of the bounding wall, or the variation in the diameter of the inlet water supply. An explanation is found in the singularisation of the basic process for nucleate pool boiling, leading to a different flow behaviour. The flow pattern is chaotic in the case of unconfined pool boiling, and it becomes directed and circulating as the dimensions of the confinement are reduced. The circulating fluid motion augments the entrainment of liquid by the vapour bubbles, and hence it significantly increases the heat transfer in the nucleate boiling regime. Confined pool boiling could be integrated on a large scale by means of ‘numbering up’ or ‘scaling out’. Our research on flow boiling in small diameter channels gives a better understanding of the origin of the pressure fluctuations and the possible flow reversal. First, the measured length of a slug bubble at detachment agrees with the trend obtained by scaling analysis using a hydrodynamic force balance. The slope of the predicted linear correlation through the measurements corresponds to a 2% film thickness. This relative film thickness is similar to the values given in literature. By applying a similar hydrodynamic force balance on a multiple channel array, a criterion is found predicting vapour flow reversal. Within the flow reversal criterion the bubble cap position as a function of time is an important parameter. High-speed camera recordings allowed us to obtain the vapour bubble length during its explosive growth. Using simple assumptions we found an explicit relation for the vapour bubble length in time. After optimisation of the film thickness, this relation shows reasonable agreement with our explosive bubble length measurements. The explosive bubble growth model combined with the criterion predicting vapour flow reversal allows us to design the inlet restriction.