hf. J. Heoz M,,ss Transfer. Vol. 30, No.9,pp. 177~1785, 1987 0417-9310/87 f3.00+0.00 zyxwvu Printed in Great zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Britam Pergamonlournals Ltd. Vaporization of a liquid by direct contact in another immiscible liquid Part I: vaporization of a single droplet Part II: vaporization of rising multidroplets L. TADRIST, I. SHEHU DISO, R. SANTINI and J. PANTALONI Laboratoire des Syst&mes tnergttiques et Transferts Thermiques, Universitt: de Provence, C.N.R.S. UA 1168, Centre St-J&me, rue H. PoincarC, 13397 Marseille Cedex 13, France zyxwvutsrqponmlkj (Received 17 March 1986 and injinalform 5 September 1986) Abstract-The present work is an experimental and theoretical study of the vaporization by direct contact of refrigerant R113 and n-pentane dispersed into a column of water flowing countercurrently. The vapor- ization of a single droplet in a stagnant liquid medium, and the evaporation of a multidroplet flowing system are studied. A formalism has been developed to determine the effective exchange surface for the bubble&oplet during its rise in an immiscible liquid. The numerical results are in good agreement with the experiments. The mechanical equilibrium of a bubble-droplet was studied when considering only the surface tension forces. The results obtained in the precedent analysis about the liquid-liquid area estimation were explained. Experiments were Performed to investigate the influence of the different parameters on the behaviour of the direct contact vapour generator. A dimensional analysis based on characteristic transfer times was done. From this point of view correlations were established for determining the volumetric heat transfer coefficient and the exchange efficiency. For a multidroplet flowing system an analytical model was proposed giving the evolution of the void fraction and the temperature of each fluid along the exchange column. Experimental and numerical results were compared. zyxwvutsrqponmlkjihgfedcbaZYX INTRODUCTION TEE RECENT developments on heat transfer by direct contact between two fluids associated with the phase change of one of the fluids have shown many advan- tages. Due to the higher effective heat transfer coefficient, relative simplicity of design and absence of the scaling surfaces associated with direct contact evaporators and condensers, this type of heat ex- changer is particularly suitable for the vaporization of thermal energy at low and moderately high tem- peratures. Applications include geothermal heat recovery, sea water desalination, waste heat recovery and energy storage systems [l, 21. They can also be found in many pilot electric plants [3,4]. The experi- ence acquired with direct contact heat exchangers where one of the liquids changes phase shows that the best performance is obtained when the dispersed liquid is the phase change fluid (solidification or vaporization) [ 1, 2, 51. A good understanding of the various mechanisms implemented in such heat exchangers is necessary for sizing direct-contact boilers and condensers and for determining the correlations between the fundamental characteristic parameters. Due to lack of data for sizing these exchangers, Honegger [7] and Wilke et al. [8] used correlations relative to liquid-liquid systems. Wilke et al. [8] studied experimentally the variations of the volumetric heat transfer coefficient vs dispersed and continuous phase flow rates for the sea water- aroclor system. A similar study was carried out by Sideman and Gat [9] with the pentane-water couple for (Tc- T,,,) < 1.5”C. Smith et al. [lo] attempted to calculate the volumetric heat transfer coefficient for direct contact evaporation from an analytical model. This paper constitutes a study of the fundamental mechanisms governing the heat and momentum trans- fers of a boiling drop in another immiscible liquid, and an experimental study of direct contact evaporation in spray columns. The experiments are conducted with Freon 113-water and n-pentane-water couples in which the first fluid is evaporated in the water. Exper- imental observations show that the n-pentane and Freon 113 drops obtained by dispersion vaporize with a constant mass, i.e. the vapour bubble remains attached to the liquid sheath. In the absence of bubble-forming nuclei ; drops, suspended or rising in another immiscible liquid, can be highly superheated [ll-131. In this case, evap- oration occurs explosively and is characterized by a ‘ping’ sound. This phenomenon corresponds to the metastable states of the phase change. To avoid this superheating, several authors [ 14, 151 favoured nucleation by injection of tiny bubbles of air. Much research has been done to study the phenomenon of a single moving drop vaporizing in a stagnant column of immiscible liquid. Experimental investigations of this problem have been reported for a number of couples of liquids and conditions : n-pentane-water, butane-water, pentane-glycerol [12, 14-171. Expres- sions of the heat transfer coefficient of a vaporizing drop were determined from theoretical and experi- mental studies [14, 18-211. Much work has also been carried out to study the evolution (velocity and HHT 30:9-r 1773