An Innovative Method of Adaptive Meshing for Hydrogen Explosion Simulations with the CFD Code REACFLOW

In the past years the Computational Fluid Dynamic (CFD) program REACFLOW has been developed at JRC. With REACFLOW numerical simulations of hydrogen explosions can be performed in order to evaluate the explosion consequences. Such explosions could occur in a severe nuclear accident when large amounts of hydrogen might be produced and released into the reactor containment during the core melt down. A hydrogen explosion could jeopardise the containment integrity, causing the release of radioactivity into the external environment. Simulating such explosions is therefore of particular interest for safety considerations. Such an explosion will develop through many stages starting as a laminar flame after ignition. In the flame front the hot combustion products will expand and drive some flow also in front of the flame. This flow can cause some turbulence. In return this turbulence may enhance the heat and mass transfer into the flame front, which will increase the burning rate causing the combustion products to expand even faster. As a consequence the turbulence might increase even more causing a self-acceleration of the flame. As long as the flame speed is rather slow compared to the speed of sound of the unburned mixture, the pressure increase within a closed volume will be slow as shown in Fig. 1 left and almost equal everywhere in the volume. As soon as the flame reaches a speed close to the speed of sound, pressure waves generated by the flame due to the expanding hot combustion products build up in front of the flame front. A typical pressure profile is shown in Fig. 1 middle. In general the higher the flame speed, the higher and steeper the pressure peak in front of the flame and the shorter the distance between the flame front and the pressure peak. In case the flame accelerates even further, the pressure peak in front of the flame will build up even further and the distance between pressure peak and flame will become smaller. This means that for fast turbulent flames the pressure wave and the flame front are decoupled in space. In case of a rapid geometrical expansion in the path of the flame, the flame speed normally decreases whereas any pressure wave in front of the flame will proceed with its own speed. In such a case the flame and the pressure wave are fully decoupled. Under certain conditions DDT (Deflagration to Detonation Transition) is possible. In detonations the very fast combustion is caused by an abrupt adiabatic compression within the shock wave. In detonations the pressure wave and the flame front propagate coupled together causing an even larger pressure peak. Adaptive meshing is a numerical strategy that is well suitable of describing these different combustion regimes. Adaptive meshing is the capability of performing the refinement of the computational mesh in regions where a high spatial resolution is required and to carry out the de-refinement when the finer mesh is not required anymore.

WILKENING Heinz;  BARALDI Daniele; 

2007-01-19

INFORUM Verlags- und Verwaltungsgesellschaft mbH

JRC33619

https://publications.jrc.ec.europa.eu/repository/handle/JRC33619,