D. Chatzikyriakou e-mail: d.chatzikiriakou@imperial.ac.uk S. P. Walker B. Belhouachi Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK C. Narayanan D. Lakehal ASCOMP GmbH, Technoparkstrasse 1, 8005 Zurich, Switzerland G. F. Hewitt Department of Chemical Engineering and Chemical Technology, Imperial College, Prince Consort Road, London SW7 2BY, UK Three Dimensional Modeling of the Hydrodynamics of Oblique Droplet-Hot Wall Interactions During the Reflood Phase After a LOCA During the reflood phase, following a loss-of-coolant-accident (LOCA), the main mecha- nism for the precursory cooling of the fuel is by convective heat transfer to the vapor, with the vapor being cooled by the evaporation of the entrained saturated droplets. However, it is believed that the droplets that reach the rod could have an effect on this cooling process. Despite the fact that those droplets do not actually wet the fuel rod due to the formation of a vapor film that sustains them and prevents them from touching the wall, the temperature drop caused by the impingement of such water droplets on a very hot solid surface (whose temperature is beyond the Leidenfrost temperature (1966, “A Track About Some Qualities of Common Water,” Int. J. Heat Mass Transfer, 9, pp. 1153–1166)) is of the order of 30–150°C (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liquid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The associated heat flux is of the order of 10 5 –10 7 W / m 2 and the heat extracted is in the range of 0.05 J over the time period of the interaction (a few ms) (2008, The Role of Entrained Droplets in Precursory Cooling During PWR Post-LOCA Reflood, TOPSAFE, Dubrovnik, Croatia, 1995, “Heat Transfer During Liq- uid Contact on Superheated Surfaces,” ASME J. Heat Transfer, 117, pp. 693–697). The hydrodynamic behavior of the droplets upon impingement is reported to affect the heat transfer effectiveness of the droplets. In the dispersed flow regime the droplets are more likely to impinge on the hot surface at a very small angle sliding along the solid wall, still without actually touching it, and remaining in a close proximity for a much larger time period. This changes the heat transfer behavior of the droplet. Here, we investigate numerically the hydrodynamics of the impingement of such droplets on a hot solid surface at various incident angles and various velocities of approach. For our simulations, we use a computational fluid dynamics (CFD), finite-volume computational algorithm (TransAT © ). The level set method is used for the tracking of the interface. We present three-dimensional results of those impinging droplets. The validation of our simulation is done against experimental data already available in the literature. Then, we compare the findings of those results with previous correlations. DOI: 10.1115/1.4000867 1 Introduction The phenomenon of the impingement of liquid droplets onto superheated surfaces is of great importance in many industrial applications. One occasion when the cooling of a hot surface by impingement of water droplets is important is the recovery pro- cess “reflood” following a postulated loss-of-coolant-accident LOCA in a pressurized water nuclear reactor PWR. Then, the fuel elements have risen in temperature 600–900°C. Water is introduced from the bottom emergency core cooling system and a two-phase mixture of water and vapor starts rising up the fuel rods. Above this rewetting front, liquid is present in the form of a liquid core, swept upwards by the vapor flow, which breaks up in a complex way to form drops. Cooling of the fuel by this droplet- steam mixture above the rewetting front “precursory cooling” is vitally important in the reflood process. In this region, the condi- tions are characterized by the flow of superheated vapor between even hotter metal surfaces, with a population of small droplets entrained in the vapor flow. It is important to understand the mechanisms by which droplets interact with hot surfaces, and this is the focus of the work described here. Ultimately, the question that both this and the subsequent stud- ies would like to answer is whether those droplets could provide a significant augmentation of the cooling process. More specifically, when a droplet bounces from a hot solid surface, heat is trans- ferred from the solid to the liquid and vapor phases. This both increases the droplet mean temperature if it is subcooled and evaporates liquid from the droplet. If the heat transfer rate is large enough during the impact, liquid vaporized from the droplet forms a vapor layer between the liquid and the solid surface 1, which prevents direct contact of the droplet with the surface. In this case, heat transfer is obstructed significantly. In the dispersed flow regime characterizing reflood, the droplets are likely to impinge on the hot surface at a very small angle, sliding along the solid wall, still without actually touching it, and remaining in a close proximity for a much larger time period than in the case of a perpendicular approach. The processes accompanying the interaction of a droplet with a Contributed by the Nuclear Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 29, 2009; final manuscript received August 4, 2009; published online July 7, 2010. Editor: Dilip R. Ballal. Journal of Engineering for Gas Turbines and Power OCTOBER 2010, Vol. 132 / 102914-1 Copyright © 2010 by ASME Downloaded 12 Aug 2010 to 18.51.1.222. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm