Deflagration-to-Detonation Transition (DDT) is a critical phenomenon relevant to the development of next-generation detonation-based propulsion systems for hypersonic flight. However, the exact mechanisms initiating DDT have remained complex and often appeared random. Researchers at Shanghai Jiao Tong University have made significant strides in understanding this unpredictable and dangerous process. Their research, systematically identifies four distinct DDT initiation pathways and the essential characteristics of DDT process.
Led by Professor Bo Zhang, the team conducted experiments in a specially designed rectangular tube equipped with state-of-the-art high-speed schlieren photography (capturing 200,000 frames per second) and ion probes. They tested three different fuel-oxidizer mixtures (2H2+O2+40%Ar, C2H4+3O2+40%Ar, and CH4+2O2), known for producing detonations ranging from highly regular to highly irregular cellular patterns. The key breakthrough was observing and categorizing four distinct physical interactions that consistently led to the formation of a DDT-initiating "hot spot":
1. Flame-Boundary-Layer Interaction: In mixtures like C2H4+3O2+40%Ar, the accelerating flame front interacting with the slower-moving gas near the tube wall generated a localized explosion point. This hot spot rapidly developed into a detonation wave that overtook the precursor shock.
2. Flame-Shock Interaction: Particularly evident in CH4+2O2 mixtures, the turbulent flame front interacted with weak pressure waves or bifurcated precursor shocks ahead of it. Coupling occurred at the triple point of a shock bifurcation, triggering a local detonation traveling along the shock slip line.
3. Shock-Wall Interaction: Using 2H2+O2+40%Ar mixtures, the team showed how precursor shock waves reflecting off the end wall of the tube could create high-pressure, high-temperature regions. While a single reflection might not suffice, subsequent reflections or the interaction of the reflected shock with the incoming flame front generated hot spots initiating detonation.
4. Shock-Shock Interaction: By replacing the flat end wall with a concave surface, the researchers demonstrated that colliding precursor shock waves, focusing their energy, could directly generate a hot spot and initiate detonation, even without the flame front being directly involved at the initiation point. This occurred below the usual critical pressure threshold for DDT in that mixture.
"Despite the diverse appearances of these initiation events," explained Professor Zhang, "the fundamental physics is consistent: a local hot spot is created through energy focusing. This hot spot then rapidly transitions into a detonation wave."
The research didn't stop at initiation. The team also investigated the behavior of the detonation wave immediately after DDT. They consistently observed that the initial detonation bubble, created by the hot spot, collides with the tube walls. This collision initially causes a regular reflection, but quickly transitions into a Mach reflection.
"The Mach reflection generates powerful bow shocks," said Zezhong Yang, the paper's first author. "Unlike the weaker, multiple transverse waves characteristic of established detonations, these initial strong transverse shocks persist behind the detonation front for a significant time."
Crucially, the team identified two distinct propagation modes for these strong transverse shocks: 1. Opposite Double-Wave Mode: Two strong transverse shocks propagate in opposite directions. 2. Single-Wave Mode: Only one dominant strong transverse shock is present. Numerical simulations using detailed chemical kinetics confirmed that the location where the initiating hot spot forms dictates which mode emerges. Hot spots near the tube center favored the opposite double-wave mode, while hot spots forming near a wall led to the single-wave mode. The simulations' predicted cellular patterns matched long-exposure experimental images, validating the models.
The research was supported by the National Natural Science Foundation of China, the Innovation Program of Shanghai Municipal Education Commission, the Natural Science Foundation of Shanghai, and the Shanghai Science and Technology Planning Project.
The team published their work in the Chinese Journal of Aeronautics on May 31, 2025.
Original Source
Zezhong YANG, Bo ZHANG. Typical onset modes of DDT and behavior of strong transverse shocks [J]. Chinese Journal of Aeronautics, 2025, https://doi.org/10.1016/j.cja.2025.103602.
About Chinese Journal of Aeronautics
Chinese Journal of Aeronautics (CJA) is an open access, peer-reviewed international journal covering all aspects of aerospace engineering, monthly published by Elsevier. The Journal reports the scientific and technological achievements and frontiers in aeronautic engineering and astronautic engineering, in both theory and practice. CJA is indexed in SCI (IF = 5.7, Q1), EI, IAA, AJ, CSA, Scopus.
