FLOW BOILING HEAT TRANSFER IN ENHANCED TUBES WITH COMPOSITE STRUCTURE

To fill the research gaps in the study of flow boiling on composite surfaces, four heat exchange tubes with different structures were investigated: an enhanced tube with herringbone teeth (EHT_HB), a tube with a dimple structure (EHT_DIM), a tube with a composite herringbone tooth/dimple structure (EHT_HB/DIM), and a smooth tube as a baseline for comparison purposes. The experimental conditions were set as follows: saturation temperature 6°C, mass flux 50-205 kg/(m2.s), and vapor quality 0.2-0.8. After confirming the validity of the experimental results, the effects of the mass flux on the flow boiling heat transfer and pressure drop characteristics were examined. Results showed that the flow boiling heat transfer coefficient and pressure drop increase with the increase of the mass flux and the vapor quality in the tested tubes. The EHT_HB/DIM tube was found to combine the advantages of the two structures and to have the highest heat transfer coefficient. The average value of the heat transfer coefficient was found to be 43.75% higher than that of the smooth tube. Also, the average flow boiling heat transfer coefficients of the EHT_HB and the EHT_DIM tubes were found to be 15% and 35% higher that of the smooth tube, respectively. From the perspective of the frictional pressure drop, it was found that the EHT_HB/DIM tube exhibited the maximum frictional pressure drop. The optimum working conditions of the enhanced tubes were determined by introducing a performance factor that captures the combined effects of heat transfer and pressure drop.