Investigation into flow boiling heat transfer in a minichannel with enhanced heating surface

Abstract

The paper presents results of flow boiling in a minichannel of 1.0 mm depth. The heating element for the working fluid (FC-72) that flows along the minichannel is a single-sided enhanced alloy foil made from Haynes-230. Micro- recesses were formed on the selected area of the heating foil by laser technology. The observations of the flow structure were carried out through a piece of glass. Simultaneously, owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to measure temperature distribution on the heating wall through another piece of glass. The experimental research has been focused on the transition from single phase forced convection to nucleate boiling, i.e. the zone of boiling incipience and further development of boiling. The objective of the paper is determining of the void fraction for some cross-sections of selected images for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed in Corel graphics software and binarized. The analysis of phase volumes was developed in Techystem Globe software. Boiling is a very efficient heat transfer process used in power engineering, chemical engineering and nuclear engineering. Mini heat exchangers are used in the interest of providing higher cooling capability for new technologies. It means a reduction of their size and cost, for an identical power. Owing to the change of state, which accompanies flow boiling in minichannels, it is possible to meet contradictory demands simultaneously, i.e. to obtain a heat flux as large as possible at small temperature difference between the heating surface and the saturated liquid and, at the same time, retain small dimensions of heat transfer systems. The study presented here was conducted on a modernized experimental stand. The modernization of the previous stand described in (1-4,6,7) aimed to simplify and miniaturize the setup, update the data and image acquisition system, and expand the system to enable accurate deaeration. The new setup was adapted to environmental protection requirements and prepared for carrying out a wide spectrum of experiments. The main raw data available in minichannel boiling heat transfer experiment with the application of thermosensitive liquid crystal technique included the hue distribution along the heating foil and the volumetric heat flux generated inside the foil. The new setup also enabled simultaneous observation of two-phase flow structures being generated in flow boiling in minichannels.