An experimental investigation of the onset of detonation

An experimental configuration is devised in the present investigation whereby the condition at the final phase of the deflagration to detonation transition (DDT) process can be generated reproducibly by reflecting a CJ detonation from a perforated plate. The detonation products are transmitted downstream through the plate, generating a turbulent reaction front that mixes with the unburned mixture and that drives a precursor shock ahead of it at a strength of about M = 3. The gasdynamic condition that is generated downstream of the perforated plate closely corresponds to that just prior to the onset of detonation in the DDT process. The turbulence parameters can be controlled by varying the geometry of the perforated plate; thus, the condition leading to the onset of detonation can be experimentally investigated. A one-dimensional theoretical analysis of the steady wave processes was first performed, and the experimental results show good agreement, indicating that the present experimental condition can be theoretically described. Two different detonation tube geometries (one with a square cross-section of 300 mm by 300 mm and the other with a circular cross-section of 150 mm) are used to demonstrate the independence of the tube diameter at the critical condition for DDT. Perforated plates with different hole diameters (d = 8, 15, and 25 mm) were tested, and the hole spacing to hole diameter ratio was maintained at 0.5. Different hydrogen–air mixtures were tested at normal temperature and pressure. For the plate with 8 mm holes, the onset of detonation is never observed. For the plate with 15 mm holes, successful initiation of a detonation is achieved for 0.8 < ϕ < 1.75 in both detonation tubes. For the plate with 25 mm holes, detonation initiation is observed for 0.7 < ϕ < 2.1 in the square detonation tube and for 0.8 < ϕ < 1.6 in the smaller circular detonation tube.