(104ap) Experimental and Computational Fluid Dynamics (CFD) Studies of Deflagration to Detonation Transition (DDT)

Deflagration to Detonation Transition (DDT)

Camilo Rosas, S. Nayak, E. Petersen and M. Sam Mannan

Mary Kay O’Connor Process Safety Center

Artie McFerrin Department of Chemical Engineering

Texas A&M University

College Station, Texas 77843-3122

The increment in the burning speed form subsonic to greater velocities than the speed of sound is known as the transition from deflagration to detonation. Originally, the flame and the precursor wave will be decoupled (deflagration). However, when the transition occurs, these two will be coupled together and form the detonation wave (detonation). When this coupling occurs, very high overpressures can be realized. To achieve a rapid acceleration of the flame, a certain level of confinement and obstruction is recommended. Also, the substance properties are important, e.g. flammability limits and reactivity, among others. Most of the industry processes present appropriate conditions where deflagration to detonation transition (DDT) is possible. Therefore, it is of great importance to understand this transition mechanism, and parameters that influence DDT, to reduce the hazards associated with flammable substances. 

Accordingly, in this study we conducted experiments and developed theoretical models to understand and quantify the DDT. The experiments were performed in a 2.75-m long detonation tube, with an internal diameter of 0.0386 m. The inside volume of the detonation tube was varied using different obstructions, and at one end of the tube there is a flame expansion zone. Acetylene is a very flammable compound with a flammability range of 2.4% to 83% in air [1]. One of its main characteristics is its high reactivity [2]; hence this substance has high likelihood to undergo a transition from deflagration to detonation when ignited. Hence, in this study acetylene was used as a probe molecule.  Initially, the tube is filled with a rich acetylene-air mixture, and ignited at the end opposite to flame expansion zone. Two distinct methods are used to determine if transition of the shock wave from deflagration to detonation is possible.  In the first method, pressure transducers are used.  Here the pressure data are reported in 13 different positions, five along the tube and eight in the flame expansion zone. The second method utilizes the principle of the soot foil technique, where an aluminum sheet is evenly coated with a fine layer of soot –obtained from the burning of a kerosene-soaked rag. This method allows one to observe the cell structure of the detonation and its pattern. The theoretical calculations of both the flame dynamics and the overpressure are performed using Computational Fluid Dynamics (CFD) models. Even though CFD models do not represent the transition from deflagration to detonation, they do resolve either a fast deflagration or a detonation; however, various CFD models have shown good agreement with experimental data when high overpressures are achieved. The simulated results are compared with the experimental data obtained from the detonation tube to determine the predictive capability of the developed CFD models.   References

1.            Design Institute for Physical Properties, S.b.A., DIPPR Project 801 - Full Version, Design Institute for Physical Property Research/AIChE.

2.            Mine Safety and Health Administration, M.S.A.H. Special Hazards of Acetylene.  10-10-2011; Available from: http://www.msha.gov/alerts/hazardsofacetylene.htm.