Flow Boiling of binary mixtures in offset-strip fins

Considerable attention has recently been shown to plate-fin heat exchangers. Known for their high compactness, they have been increasingly used in different applications such as refrigeration and cryogenic systems (such as liquefaction units of AIR LIQUIDE). The fin geometry, in this specific type of heat exchangers, may adopt different shapes. Among them, the offset-strip fins, also called serrated fins. They are characterized by the discontinuity of the flow leading to a consequent heat transfer improvement. Moreover, in the same context of refrigeration, fluid mixtures (as a secondary fluid) are increasingly used since they allow to limit the entropy generation by reducing the pinch between the cold and the hot sides of the heat exchanger. Although flow boiling in offset-strip fins and the use of binary mixtures have been investigated in the literature, they have been studied separately. There is thus a lack of research combining simultaneously offset-strip finned geometries and binary mixtures.The current study focuses on an experimental investigation of upward two-phase flow of binary mixtures in complex geometries composed of offset-strip fins. The flow boiling of a mixture containing R-134a and R-245fa was visualized. On average, the molar composition of the used binary mixture is: 52% mol/mol R-134a and 48% mol/mol R-245fa. The ranges of mass flux and heat flux investigated were 11 – 25 kg.m^(-2).s^(-1) and 9 – 23 kW.m^(-2) respectively. The test section is an offset-strip finned passage whose hydraulic diameter is 1.75 mm. The operating pressure ranges from 2 to 5 bar absolute. The test section is composed of a single finned-passage with offset-strip fins sandwiched between a polycarbonate plate (in order to visualize the two-phase flow) and a heated-aluminum block. A heating resistance, which is an adhesive film, is glued at the back of the vaporizer to supply the heat necessary for the flow boiling. 0.5 mm-T thermocouples are placed at half the width of the finned passage in order to measure the fluid and the wall temperature to calculate the local heat transfer coefficient.Different flow regimes were identified along the test section such as bubbly, slug and annular regimes. The impact of strip-fins on flow maps was analyzed in this study. Moreover, the measured pressure drops were compared to some correlations found in the literature. None of these correlations could fit with the obtained experimental results. Therefore, a new correlation of pressure drop for the used binary mixture and the pure R-245fa within the operating conditions is proposed. Besides, flow reversal is identified in some regions of the test section. Some interesting criteria are introduced in order to prevent this phenomenon. In addition to that, flow instabilities marked by pulsing movements of a liquid front upwards and downwards at the top of the test section were identified and some physical explanations of these flow instabilities were introduced.From heat and mass transfer point of view, local heat transfer coefficient was calculated and presented as a function of vapor quality. It seemed that there is no influence of mass flux on heat transfer coefficients: This suggests that nucleate boiling governs the mechanism of heat transfer in this experiment. Moreover, none of the existing correlations for heat transfer coefficients could fit with the obtained experimental results. A new correlation based on the model of Chen (1966) is introduced with new correlations of the suppression and the intensification factors valid for the used binary mixture.