Experimental study of the water in oil emulsions features by differential scanning calorimetry analysis Ernesto Mura a , Christophe Josset b , Khaled Loubar a , Jérôme Bellettre b,⇑ , Patrizio Massoli c a GEPEA, UMR 6144 CNRS, EMN, 4, rue Kastler, 44307 Nantes, France b LTN, UMR 6607 CNRS, Univ-Nantes, rue Pauc, 44306 Nantes, France c Istituto Motori – CNR, Via Marconi, 8, 80125 Napoli, Italy article info Article history: Received 5 July 2011 Received in revised form 14 December 2011 Accepted 30 December 2011 Available online 24 January 2012 Keywords: W/O emulsion Explosive vaporization Vaporization temperature DSC abstract The present work proposes a differential scanning calorimetric (DSC) analysis approach in order to char- acterize a W/O emulsion in function of the vaporization temperature of the inner water droplets. The cor- relation between the most probable boiling temperatures of the droplets and their radii involves the evaluation of the story of the released energy during the vaporization process. The experimental set- up consists in a heat flow DSC system provided by a temperature programmable oven. A set of seven emulsions, containing the same amount of water, is classified in function of the dispersed water droplet diameters and tested several times. The resulting thermograms are statistically processed to obtain the most probable vaporization temperature for every emulsion in function of the dispersed water droplets size. The results show that the smaller the dispersed water droplet, the higher the vaporization temperature. Moreover, a notable degree of metastability, included in a range temperature from 5 to 10 °C, is pointed out and it is evident that it increases in reducing the dispersed water droplets size. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In the field of multiphase colloidal dispersions, micro-scale emulsion is considered one of the most useful techniques because of the wide range of applications such as in food and energy indus- tries [1,2]. One of the possible application is to emulsify waste oils with water (W/O emulsion) in order to improve the efficiency of combustion process in comparison to neat oil. Several works have pointed out the advantages obtained from the combustion of emul- sions in both boilers and engines systems using various based products: from heavy oils to animal fat [3–7]. As a matter of fact, to burn water in oil emulsions (W/O) is considered as an effective alternative to bring out waste oils. The explosive vaporization (micro-explosion effect) of the inner water droplets during the combustion process is one of the most interesting aspects of this application. When a W/O emulsion is heated in a flame, the vigor- ous expansion of the inner dispersed water droplets leads to the fragmentation of the surrounding oil into several and smaller droplets. The phenomenon of explosive vaporization is strictly related to the features of the emulsions and to the persistence in a metasta- ble state of the water: the greater the level of metastability, the higher the vaporization temperature [6,8]. In terms of measurable quantities, the limit of stability, for a stable or metastable sub- stance at constant pressure, corresponds to the divergence of iso- thermal compressibility (K T ) [9]:  @P @V   T ¼ 0 ) K T !1 ð1Þ where P is the pressure, V the volume and T the temperature (con- stant) of the thermodynamic process. The metastability of a water droplet surrounded by an oil shell could be affected by several variables: (1) Difference in boiling point, (2) the external pressure, (3) the amount of water [10]. This state is broken by nucleation of small embryos of the new phase by homogeneous or heterogeneous process [11,12]. Recent new experimental results, concerning the effect of the features of dispersed water droplets during the heating process of a W/O emulsion droplet [13], drew our attention to the interest- ing aspects of the influence of the droplets size distribution in the vaporization temperature and in the metastable state. It is already proved that it is possible to morphologically char- acterize the dispersed phase of an emulsion with a suitable tech- nique based on the correlation between the most probable freezing temperatures of the droplets and their radii [14,15] by dif- ferential scanning calorimetric (DSC) analysis. The present work proposes a new methodology, still based on the analysis of DSC 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.12.095 ⇑ Corresponding author. Tel.: +33 (0)0240683104. E-mail address: jerome.bellettre@univ-nantes.fr (J. Bellettre). Applied Energy 97 (2012) 834–840 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy