Numerical simulation of and experimental study on effect of ring obstacles on detonation initiation and propagation

Shortening the distance and time of DDT (deflagration to detonation transition) stage is crucial in the development of PDE (pulse detonation engine). Shchelkin spiral has attracted much interest as an obstacle that disturbs free flow of combustible liquid/gaseous fuel mixture and consequently shortens the distance and time of DDT stage. But the 3D geometry of Shchelkin spiral makes numerical simulation very difficult. We use ring obstacles so that numerical simulation and experimental study can go hand in hand. In numerical simulation, unsteady 2D axisymmetric N-S (Navier-Stokes) equations and finite rate model for chemical reaction are used. Near the thrust wall, high pressure and high temperature ignition zone initiates the detonation of mixture. Experiments are performed on the platform of a 60 mm I. D. (inside diameter) detonation tube. The simulations show that; (1) before detonation, reflection and focusing of shockwave produce local high temperature and high pressure zones in which chemical reactions become very fast and energy is released immediately; such zones can cause local constant volume explosion; (2) ring obstacles inside detonation tube can shorten time and distance of detonation initiation as compared with those in smooth detonation tube; when spacing interval of ring obstacles is increased from 40 mm to 60 mm, initiation time and distance become longer. Both simulations and experiments show that; (1) sudden increase of pressure during DDT process will not occur until flame has spread to a certain region upstream of the position where detonation wave forms; meanwhile two compression waves, which propagate in opposite directions, come into being, one is strengthened by combustion and detonates rapidly, but the other is weakened as a result of fuel lack; (2) detonation wave forms before detonation wave, and can combust the unburned mixture behind obstacles in deflagration zone.