Contact Person: Christos Argiropoulos [], Vasiliki Papadopoulou []

Boiling is encountered in many engineering fields such as energy conversion, environmental applications, food and chemical process industries and space applications. There is consequently a great diversity of situations in which boiling processes are present and must be well understood. Εmpirical laws are so far valid, only when they are used in the range of parameters they were developed for. The main reason for this restriction is the huge number of physical phenomena governing the heat and mass transfer process.

RUBI (Reference mUltiscale Boiling Investigation), a fluid science experiment developed and built by Airbus for the European Space Agency (ESA), addresses the fundamentals of the boiling of innovative coolants. The supply mission (CRS-18, Figure 1) has been launched from Cape Canaveral, Florida and transports a special ‘steam engine’ to the International Space Station (ISS). The fluid experiment is operated and controlled by the Belgian User Support and Operation Centre (B-USOC) in Brussels.

Fig. 1: Launch of the CRS-18 mission

RUBI studies the phenomena of phase transition and heat transfer during the evaporation of fluids in microscopic and macroscopic dimensions. RUBI’s core element is a cell filled with fluid, which can be heated and cooled thermoelectrically. The boiling process is then triggered on a metal-coated glass heater using a laser. The goal of Multiscale Boiling is to investigate the basics of boiling heat transfer phenomena on a heater surface, in a pool boiling configuration.

A multi-scale experimental and analytical approach is adopted including the application of two external forces (electrical field and shear flow, Figure 2). They are two external effects which play a role on the bubble shape and boundary layer near the bubble. The investigation of these two effects will contribute to a better understanding of the heat transfer mechanisms. The aim is to investigate the influence of the application of an electric field on the dynamics of single and multiple bubbles (growth and detachment).

Fig. 2: Schematic of the functionalities to be gathered in the RUBI Experiment Container

High-resolution cameras record the formation and growth of vapour bubbles in both the visible and infrared spectrum. A greyscale (B/W) high-speed (HS) camera is used to observe the bubbles from the side, to measure the evolution of their diameter in time, while an infrared (IR) high speed camera is used to observe the bubble on the heater from below. By taking up to 500 images per second, RUBI’s cameras can create a three-dimensional representation of the bubble shapes and analyse the temperature distribution on the heater, enabling the scientists to precisely determine evaporation conditions and heat flux densities.

The AUTh team in cooperation with a group of software engineers from ITE in Crete (leader Dr. Xenophon Zaboulis) have developed a dedicated software in order to extract various geometrical characteristics of growing bubbles during boiling experiments.

The movement of a vapour bubble growing over a planar substrate during boiling is recorded by two cameras (side and bottom view) providing orthogonal perspectives of the phenomena. Droplet images acquired from top and side views are analyzed independently to extract contour outlines as shown in Figures 3 and 4. The recorded contours are employed to compute 2D geometrical bubble features versus heating time (contact line length, height, horizontal diameter, surface area, right and left contact angles with the heater, location of center of mass, etc) utilizing data from both the side (BW data) and the bottom (IR data) cameras, Figure 5. The combination of 2D contours from side and bottom views avails the 3D reconstruction of the droplet shape presents as shown in Figure 6.

Fig. 3: Indicative bubble time stills from the sideview high speed camera

Fig. 4: Indicative bubble time stills from the bottom view high speed IR camera

Fig. 5: Geometrical features of bubbles and comparisons of sequential bubbles in a single flow boiling experiment