Flow boiling heat transfer characteristic of NH3/LiNO3+H2O absorption refrigeration working fluid in horizontal tubes: A comprehensive experimental evaluation and comparison

Abstract

Research on working fluids of refrigerant/absorbent mixture is important for the development of the absorption refrigeration technology. The ternary NH₃/LiNO₃+H₂O working fluid has advantages including good cycle performance, simple system composition and superior heat-transfer performance compared with other binary ammonia-based solutions. In this paper, an experimental investigation is conducted on the flow boiling heat transfer coefficient of the NH₃/LiNO₃+H₂O mixture in horizontal tubes. The effects of heat flux (from 4.6 to 78.5 kW/m²), vapor quality (from 0.044 to 0.133), mass flux (from 164.1 to 389.5 kg/(m²∙s)), and tube diameter (6, 8, 10 mm) are analyzed, and the experimental results are compared with semi-empirical correlations. Besides, this paper provides a comprehensive comparison of the flow boiling heat and mass transfer characteristics of ammonia-based working fluids: NH₃/LiNO₃, NH₃/NaSCN, NH₃/LiNO₃+H₂O with various H₂O mass concentration of 5 %, 10 %, 15 % and 20 %. Experimental results show that heat flux has a positive impact on the intensity of flow boiling heat transfer at relatively low heat fluxes, but the positive correlation becomes less noticeable at the high heat flux range above 50 kW/m² resulting from the concentration boundary layer in the boiling process of highly non-azeotropic mixtures. The flow boiling heat transfer coefficient shows a positive correlation with both the mean vapor quality and the mass flux, reduces with the decrease of tube diameters. The comparison results show that the mean flow boiling heat transfer coefficient of NH₃/NaSCN working fluid is 157.3 % larger than that of NH₃/LiNO₃, and as the various H₂O mass concentration increases, that of four NH₃/LiNO₃+H₂O working fluids are 18.1 %, 67.0 %, 84.5 %, and 149.3 % larger than that of NH₃/LiNO₃, respectively. The present work provides references for the application of ammonia working fluids, and for the design and optimization of finned-tube or tube-in-tube generator in absorption refrigeration systems.