Análise experimental da ebulição em canais de diâmetro reduzido: efeitos do diâmetro, do fluido e da temperatura

Abstract

The use of heat exchangers with reduced diameter channels has been growing due to the demand for compact heat exchangers which allow high rates of heat transfer, low pressure drop and reduction of refrigerant fluid charge and material cost. The present work analyzes the convective boiling through small diameter channels and the influence of some parameters on the heat transfer coefficient and the pressure drop. The results of three experimental studies are presented to evaluate the influence of the working fluid, the channel diameter and the saturation temperature. In the first analysis, the boiling of isobutane, R600a, and propane, R290, in a channel with a 1 mm internal diameter, with mass velocity from 240 to 480 kg m-2 s-1, heat flux from 5 to 60 kW m-2 and saturation temperature of 25°C. In the second analysis, it’s studied the boiling of R600a in channels with diameters of 1.0 and 2.6 mm, saturation temperature of 22°C, with mass velocity from 188 to 377 kg m-2 s-1 and heat flux from 28 to 56 kW m-2. Finally, the boiling of R600a in a 1.0 mm diameter channel was studied with two saturation temperatures, 20 and 30°C, with a mass velocity from 240 to 480 kg m-2 s-1 and heat flux from 20 to 60 kW m-2. The influence of mass flow, heat flux and vapor quality on the behavior of the heat transfer coefficient and the frictional pressure drop are evaluated. The analysis of the working fluid showed that, in general, R600a presents the highest coefficients of heat transfer and frictional pressure drop. From the analysis of the influence of the channel diameter, it was observed that, for all experimental conditions, the heat transfer coefficient was higher in the 1.0 mm channel. The results also showed that the higher heat transfer coefficients were obtained with the saturation temperature of 30°C. Complementing the analyzes, the flow patterns were evaluated through the recorded images, and the flow pattern maps, comparing the two fluids. The map that best fit the curves were those of Revellin and Thome (2007b) and Ong and Thome (2011). Some proposed heat transfer correlations for mini and micro channels were evaluated with the experimental data, from which it was observed that, in general, the correlation of Kim and Mudawar (2013b) presented a better adjustment, among the evaluated correlations.