Author's personal copy A molecular dynamics simulation investigation of fuel droplet in evolving ambient conditions Hiromichi Yanagihara a,b , Igor Stankovic ´ a,c,⇑ , Fredrik Blomgren d , Arne Rosén d , Ichiro Sakata a a Research & Development, Toyota Motor Europe NV/SA, B-1930 Zaventem, Belgium b ODY Co. Ltd., Musashino, Tokyo, Japan c Scientific Computing Laboratory, Institute of Physics Belgrade, University of Belgrade, 11080 Belgrade, Serbia d Chalmers Industrial Technologies, Chalmers Science Park, SE-412 88 Göteborg, Sweden article info Article history: Received 31 January 2013 Received in revised form 22 May 2013 Accepted 5 September 2013 Available online 15 October 2013 Keywords: Spray Evaporation Combustion chemistry Molecular dynamics abstract Molecular dynamics simulations are applied to model fuel droplet surrounded by air in a spatially and temporally evolving environment. A numerical procedure is developed to include chemical reactions into molecular dynamics. The model reaction is chosen to allow investigation of the position of chemical reac- tions (gas phase, surface, liquid phase) and the behavior of typical products (alcohols and aldehydes). A liquid droplet at molecular scale is seen as a network of fuel molecules interacting with oxygen, nitrogen, and products of chemical fuel breakdown. A molecule is evaporating when it loosens from the network and diffuses into the air. Naturally, fuel molecules from the gas phase, oxygen and nitrogen molecules can also be adsorbed in the reverse process into the liquid phase. Thus, in the presented simulations the time and length scales of transport processes – oxygen adsorption, diffusion, and fuel evaporation are directly determined by molecular level processes and not by model constants. In addition, using ab initio calcu- lations it is proven that the reaction barriers in liquid and gas phases are similar. Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Modeling of spray evaporation attracts much interest due to its significance for spray and combustion engineering applications. The theories of evaporation have been developed and improved over the last 100 years following the Maxwell paper [1]. Even to- day, the issue of primary interest in theoretical approaches is mass transfer of vapor quantized through evaporation and mass trans- port rates of the vapor molecules. There are two reasons for this. First, theory is relatively simple, and omits effects on the level of a single spray droplet, which are known to be important but hard to estimate. Second, experiments have mostly been restricted to observations of a spray as a whole, since it is hard to measure fea- tures important for the evaporation of a single droplet under real- istic conditions. As a result, studies are usually focused on the evaluation of effects as an integral part of a wider problem of the spray dynamics [2–8]. A disadvantage of this approach is that com- plex interactions at the surface of an evaporating droplet that in- clude heat transfer and chemical reactions are studied indirectly. In contrast to the articles referenced above, evaporation was re- cently explored at level of a single droplet in a systematic way in several experimental and modeling studies. Fang and Ward, achieved a breakthrough in understanding heat transfer influence, with a series of very precise measurements of temperature distri- bution near an evaporating surface, see Ref. [9]. In Refs. [10–13] models for droplet heating and evaporation have been developed. These models include convective and radiative heating of single droplets, and effects of the recirculation inside droplets. Another line of research is dedicated to convective burning of droplets. Raghavan et al. [14] have made experimental and numerical inves- tigations of a droplet burning in a convective environment. Wu and Sirignano [15] have analyzed transient behavior of an isolated con- vecting burning droplet. They have considered effects of droplet surface regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, non-uniform surface tem- perature, and effect of surface tension. An initial envelope flame was found to persist in time, and an initial wake flame was always transitioned into the envelope flame at a later time, with the nor- malized transition delay controlled by the initial Reynolds number and the initial Damköhler number [15,16]. The moment of transi- tion is postponed further into the lifetime for smaller initial droplet radius, greater initial Reynolds number, or smaller initial Damköh- ler number. Localized ignition of droplet-laden flows is important for direct- injection internal combustion engines. An example of combustion concepts currently under development is homogeneous charged 0010-2180/$ - see front matter Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2013.09.002 ⇑ Corresponding author at: Scientific Computing Laboratory, Institute of Physics Belgrade, University of Belgrade, 11080 Belgrade, Serbia. E-mail address: igor.stankovic@ipb.ac.rs (I. Stankovic ´). Combustion and Flame 161 (2014) 541–550 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame