Caracterização dos padrões de escoamento na ebulição do isobutano em um micro canal

Abstract

New applications for refrigeration systems are emerging all the time, as industrial manufacturing processes, nuclear, aerospace and other systems that generate heat during their use are always being developed, due to this need, compact heat exchangers need to be improved, requiring studies related to thermal exchange and improvements in energy efficiency. Com pact systems are studied to meet these demands, so combining the fluid phase change process with the development of compact mini and micro channel systems becomes increasingly nec essary. The present work analyzed data on the boiling of the isobutane refrigerant (R600a) in a horizontal micro channel (internal diameter of 1 mm), which makes up an experimental bench made up of three sections (pre-heating, testing and visualization), as well as several sensors for measuring temperature, flow, pressure and an optical sensor. Tests were carried out with variations in mass velocities (520 m kg 2s ∼ 1220 m kg 2s ) and heat flow (10 kW m2 ∼ 60 kW m2 ) and the acquisition of temperature, pressure and flow data for further treatment. Flow images were also recorded under different conditions and the signals from an optical sensor associated with these were recorded. Thus, the flow patterns were assessed by identifying the images and also by treating the optical sensor data with the Fast Fourier Transform (FFT) methodology combined with Power Spectral Density (PSD), as well as treating the images of the patterns to determine the longitudinal area occupied by the steam in the tube. For the analysis of the characteristic frequencies, the PSD method was used, the void fraction, flow velocities and vapor title were calculated and the images treated with an python algorithm developed in this work. For the lowest values of mass velocity (G), the highest vapor title were achieved, and all patterns of isolated, elongated, slug, churn and annular bubbles were observed. While for the higher G, even when a greater heat flow was applied to the test section (TS), the vapor title were reduced and the predominantly observed patterns were intermittent (slug and churn) and annular. In the frequency analysis, similar values of frequencies (where the highest power spectral densities are concentrated) were found between the same patterns for different tests performed, for the pattern of isolated bubbles the highest value occurs at the frequency of 15 Hz and for the churn patterns and wavy annular for values below 5 Hz, going up to the frequency of 10 Hz. The image processing through the developed algorithm was able to identify the longitudinal área occupied for the patterns of isolated bubbles and elongated bubbles.