The force balance analysis on the boiling bubble under microgravity conditions

Mehurčkasto vrenje je eden izmed najbolj učinkovitih in zanesljivih načinov za prenos toplote pri majhnih temperaturnih razlikah med površino in delovno tekočino. Proces je zanimiv tudi za prenos toplote in snovi v pogojih mikrogravitacije, zato so bili nedavno izvedeni tudi preizkusi vrenja na Mednarodni vesoljski postaji. V tem delu so analizirani posnetki mehurčkastega vrenja v bazenu v mikrogravitaciji ob prisotnosti visokonapetostnega električnega polja, ki zaradi zagotavljanja elektrostatičnih sil pospešuje odcepitev mehurčkov in s tem izboljšuje prenos toplote. Posnetki hitro tekoče video kamere so analizirani v programu MATLAB, implementirani so algoritmi za avtomatizirano obdelavo slik in detekcijo robov, izračunani so geometrijski parametri parnega mehurčka tekom rasti in sile, ki delujejo nanj. Ugotovljeno je, da v pogojih mikrogravitacije pri vrenju brez prisotnosti zunanjih sil prevladuje sila površinske napetosti, ki drži parni mehurček na grelni površini, v primeru vrenja ob prisotnosti električnega polja pa se njen vpliv zmanjša in se poveča vpliv hidrodinamične sile, ki pripomore k odlepljanju parnega mehurčka od grelne površine.Bubble nucleate boiling is one of the most effective and reliable processes for heat transfer between a heating surface and working fluid at low temperature difference. The process of nucleate boiling is also interesting for heat and mass transfer in microgravity, thus an array of experiments have been recently carried out on the International space station. In this work microgravity pool boiling in presence of high voltage electric field videos are analysed. The high voltage electric provides a constant electric force which promotes bubble departure and enhances heat transfer. The videos taken with a high-speed video camera are analysed in MATLAB. Automatic image processing algorithms and edge detection methods are implemented, as well as calculation of geometric parameters and forces acting on a bubble during bubble growth. It was determined that in pool boiling microgravity conditions with no external forces, the surface tension force is the dominant one and keeps the bubble attached to the surface. In case of boiling with an electric field, the role of dominant surface tension force diminishes and hydrodynamic forces take a bigger role, which leads to bubble departure from the heated wall