Passive intensification of the ammonia absorption process with NH 3 /LiNO 3 using carbon nanotubes and advanced surfaces in a tubular bubble absorber Carlos Amaris, Mahmoud Bourouis * , Manel Vallès CREVER e Universitat Rovira i Virgili, Av. Països Catalans No. 26, 43007 Tarragona, Spain article info Article history: Received 8 October 2013 Received in revised form 13 January 2014 Accepted 10 February 2014 Available online 13 March 2014 Keywords: Bubble absorber Carbon nanotubes Advanced surfaces Ammonia Lithium nitrate abstract The present study aims to quantify experimentally the individual and simultaneous effects of CNTs (carbon nanotubes) and advanced surfaces on the performance of an NH 3 /LiNO 3 tubular bubble absorber. Operating conditions are those of interest for use in air-cooled absorption chillers driven by low tem- perature heat sources. Firstly, experimental tests were performed with the tubular absorber fitted with an inner smooth surface to analyze the effect of adding carbon nanotubes (0.01 wt%) to the base mixture NH 3 /LiNO 3 . Then, the tubular absorber was tested using an inner advanced surface tube both with and without adding carbon nanotubes to the base mixture NH 3 /LiNO 3 . The advanced surface tube is made of aluminum and has internal helical micro-fins measuring 0.3 mm in length. Results show that the maximum absorption mass flux achieved with the CNT binary nanofluid and the smooth tube is up to 1.64 and 1.48 times higher than reference values at cooling-water temperatures of 40 and 35  C, respectively. It is also found that simultaneous use of CNT nanoparticles and advanced surfaces resulted in a more pronounced increase in the absorption mass flux and solution heat transfer coefficient with respect to the smooth tube absorber with NH 3 /LiNO 3 as a working pair. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The potential of absorption refrigeration cycles driven by re- sidual heat or solar thermal energy has been recognized for a long time. Over the last years, the interest and necessity to progress in the development of absorption chillers have led to studies of several techniques to improve their performances and make them more competitive than mechanical compression systems. Current research on absorption chillers is essentially focused on the study and development of new cycles and of new working fluids. Well-known drawbacks of the conventional working fluids for absorption refrigeration systems such as crystallization, corrosion and low operating pressures for H 2 O/LiBr cycles and refrigerant vapor rectification at the desorber outlet for NH 3 /H 2 O cycles have increased the interest of researchers in new working mixtures. Investigations with the NH 3 /LiNO 3 mixture have resulted in this mixture being considered a promising alternative working pair for absorption refrigeration cycles driven by low temperature heat sources (Gensch [1], Aggarwal and Agarwal [2] and Infante Ferreira [3]). Refrigeration systems with NH 3 /LiNO 3 as the working fluid can be air-cooled since high condensation temperatures can be ach- ieved without crystallization problems. They also can be operated at low generator temperatures and refrigerant vapor rectification at the generator outlet is not needed (Antonopoulos and Rogdakis [4], Sun [5], Abdulateef et al. [6] and Infante Ferreira [7]). Accordingly, refrigeration systems with NH 3 /LiNO 3 are highly recommended for solar cooling applications. However, previous experimental studies showed that the main drawback of NH 3 /LiNO 3 is its high viscosity, which limits heat and mass transfer processes in the absorber, as compared to the use of NH 3 /H 2 O working fluid (Abdulateef et al. [6], Infante Ferreira [7], Ayala et al. [8], Heard et al. [9]). Regarding the most recent experimental research works with NH 3 /LiNO 3 , studies on the boiling heat transfer in plate heat ex- changers have been reported by Oronel et al. [10], Zacarías et al. [11] and Venegas et al. [12]. Oronel et al. [10] reported an experimental study on the saturated boiling process in a plate heat exchanger for NH 3 /LiNO 3 and NH 3 /(LiNO 3 þ H 2 O) working fluids. The authors analyzed the different boiling regimes (convective, nucleate and film boiling) that take place as well as the effect of operating pa- rameters such as mean vapor quality, heat flux and solution mass flux on the boiling heat transfer coefficient for both NH 3 /LiNO 3 and NH 3 /(LiNO 3 þ H 2 O). Meanwhile, Herrera et al. [13] performed an * Corresponding author. Tel.: þ34 977 55 86 13. E-mail address: mahmoud.bourouis@urv.cat (M. Bourouis). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2014.02.039 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved. Energy 68 (2014) 519e528