Heat and mass transfer during absorption of ammonia vapour by LiNO 3 –NH 3 solution droplets M. Venegas a,1 , M. Izquierdo b, * ,1 , P. Rodr  ıguez a,1 , A. Lecuona a,1 a Departamento de Ingenier  ıa T ermica y de Fluidos, Universidad Carlos III de Madrid, Avda. Universidad, 30, 28911 Legan es, Madrid, Spain b Instituto de Ciencias de la Construcci on Eduardo Torroja (CSIC), C. Serrano Galvache, s/n, 28033 Madrid, Spain Received 5 June 2003; received in revised form 2 December 2003 Abstract In this paper the simultaneous mass and heat transfer processes taking place during the absorption of ammonia vapour by lithium nitrate–ammonia solution droplets is numerically simulated. The low-pressure and high-pressure absorbers of a double-stage absorption system are considered. Results obtained are compared with Newman model for mass transfer. Simulation shows that droplets analysed, with diameter equal to 60 and 100 lm, follow Newman model for mass transfer. Droplets achieve the equilibrium state in an approximate dimensionless time equal to 0.5, that corresponds to times lower than 1 s. Effect of subcooling on absorption is evaluated. Results obtained are important for the design of spray absorbers of absorption refrigeration systems. Ó 2004 Elsevier Ltd. All rights reserved. 1. Introduction The development of absorption refrigeration tech- nology is basically related with the heat and mass transfer research in the absorption machine compo- nents. Also, it is already known that mass transfer is the main constraint for increasing performance and reduc- ing dimensions of absorption machines. Absorption refrigeration systems commonly use laminar falling film absorbers. However, other absorption methods have shown their potential for reducing the heat and mass transfer area and, as a result, the absorber dimensions. These methods include spray and bubble absorption. For theoretical and experimental studies about spray and bubble absorption the works [1–8] and [9–16] can be consulted, respectively. The simultaneous heat and mass transfer in droplets is a complex process where the mass and energy con- servation equations must be solved simultaneously. To date analytical models that predict the simultaneous variation of the concentration and temperature in droplets, considering their size, velocity, internal circu- lation, flow pattern, etc. do not exist. If all these factors could be accurately considered the characteristics of the heat and mass transfer in individual droplets would correctly predict the global process that takes place in spray absorbers of absorption refrigeration systems. To date several studies have been published that analyse the heat and/or mass transfer in droplets. A classification of these works follows: Analytical models for individual heat or mass transfer. The works [17–25] can be found in the literature. These models have mainly application in chemical engineering and predict the heat or mass transfer in droplets in specific cases. Experimental studies on mass transfer. In addition to the theoretical models for mass transfer in droplets many experimental works are reported. For example, [26–28] for the absorption of CO 2 in water and [29] for the absorption of CO 2 in a solution of carbonate and sodium bicarbonate. Numerical solutions for individual heat or mass transfer. The main limitation of analytical models is given by the simplifications made during the solution * Corresponding author. Fax: +34-91-871-3248. E-mail address: mizquierdo@ietcc.csic.es (M. Izquierdo). 1 Unidad Asociada de Ingenier  ıa T ermica y de Fluidos CSIC-UC3M. 0017-9310/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2003.12.014 International Journal of Heat and Mass Transfer 47 (2004) 2653–2667 www.elsevier.com/locate/ijhmt