An experimental study of the evaporation characteristics of emulsified liquid droplets M. Abu-Zaid Abstract This paper presents the results of an experi- mental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualita- tively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kero- sene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial con- centration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical condi- tions. List of Symbols h fg latent heat of vaporization, KJ/kg m mass of the droplet, gm T b boiling temperature, °C T c critical temperature, °C T L Leidenfrost temperature, °C T s initial surface temperature of the hot surface, °C 1 Introduction The emphases on energy conservation and environmental aspects have brought resurgence to interest in mixing water with fuel for combustion. A water/oil-emulsified fuel has been demonstrated to decrease the harmful exhaust emissions of internal combustion engines. The emulsion fuels tested on the Millbrook London Transport Bus (MLTB) achieved significant reduction in nitrogen oxides (NO x ), particulates, and unregulated emissions [1]. Com- bustion characteristics of the emulsified diesel fuels were investigated in a Rapid Compression and Expansion Machine [2]. They concluded that among the test cases studied, the 40 water to diesel ratio by volume has shown the best performance, with respect to the efficiency and NO x and soot emissions. Evaporation of emulsion fuel droplets on hot surfaces is of current technological importance for its relevance to droplet combustion in many practical applications. For instance, in C.I. engines, vaporization of fuel droplets takes place during the physical ignition delay period, which is defined as the time required to form a stoichiometric mixture in the vapor film surrounding the liquid droplet. Hence, knowledge of the behavior of an evaporative emulsion fuel droplet, is an essential pre-requisite for understanding and prediction, the performance of com- bustion process of emulsion fuels. The physical properties of emulsified fuel, was studied by Sawa et al. [3]. They concluded that, as the water content increases, the viscosity of emulsified fuel increases. Cho et al. [4] studied experimentally the micro- explosion behavior of water/oil emulsion droplets over a hot place. They showed that, for a droplet originally in contact with the plate, with increasing plate temperature the droplet life time will first decrease, then increase rap- idly as the droplet attempts to levitate itself by its outg- asing vapor, and finally decrease gradually when it is in the totally levitated mode. Avedisian and Fatehi [5] performed an experimental study, of the liedenfrost evaporation characteristics of water in heptane, and water in decane emulsions on stainless-steel surface at 565, 620, 680 k. Results show that the droplet evaporation rate decreases, and total droplet evaporation time increases, with increasing water content. Sheffield et al. [6] focused attention on the micro-explosion characteristics of emulsion at hot sur- faces. They observed that droplets in film evaporation, often exploded violently when water was mixed with several high boiling point hydrocarbons (e.g. hexade- cane). The present work, addresses the problem of evapo- ration of water-diesel and water-kerosene droplets, softly deposited and subsequently evaporated on a horizontal stainless steel and aluminum surfaces, at temperature ranging from 100 to 460 °C. the purpose is to study experimentally, the evaporation characteristics of Heat and Mass Transfer 40 (2004) 737–741 DOI 10.1007/s00231-003-0473-5 737 Received: 26 September 2002 Published online: 7 April 2004 Ó Springer-Verlag 2004 M. Abu-Zaid Department of Mechanical Engineering, Faculty of Engineering, Mu’tah University, Al-Karak, Jordan E-mail: Abuzaid1958@yahoo.com