Numerical Simulation of Gas Explosion with Non-uniform Concentration Distribution by Using OpenFOAM

Gas explosions in coal mines have occurred occasionally, which may cause casualties and economic losses. In the actual mine roadway, the gas concentration distribution is uneven because the gas density is lower than that of air. Gas explosion characteristics of uneven gas distribution with the concentration gradient in mine roadways were analyzed by using the open-source computational fluid dynamic code OpenFOAM. The flame and pressure characteristics were calculated, and the flame and shock wave propagation laws of the non-uniform gas-air mixture explosion with different concentration gradients were analyzed and compared with the uniform gas-air mixture gas. The results show that when the overall gas concentration is the same, the flame velocity and the pressure growth rate of the uniform gas explosion are lower than those of the non-uniform, but the pressure peaks of both are similar. At the same time, when the initial volume concentration is 10%, the non-uniform gas explosion has the highest flame propagation velocity and peak value. The peak explosion pressure of different concentration gradients is proportional to the initial concentration. The above studies clarified the characteristics of gas-air mixture explosions with concentration gradients and provided theoretical support for the prevention and control of gas explosion disasters.

Propagation laws of discontinuous gas supply in the excavation roadway

An explosion with a discontinuous gas supply (DGS-explosion) is more complicated than a common secondary explosion. We present the results of a study on the propagation laws of the DGS-explosion induced by a gas explosion in excavation roadways. A rectangular tube was established using ANSYS, similar to an excavation roadway in an underground coal mine. The gas, flame, and shock wave propagation laws were determined by analyzing the explosive gas as it exited the excavation roadway. The results show that the initial explosion caused the flame generated in the DGS-explosion to be significantly stretched. Moreover, the shock wave was reflected by the end of the tube, which resulted in the reverse migration of the local gas after the DGS-explosion. Meanwhile, with the increase in local gas concentrations, the pressure peak and the entire explosion system can increase after the DGS-explosion. The flame region, temperature peak, and flame irregularity in the tube positively correlate with the concentration. These results can provide theoretical support and an experimental basis for preventing and responding to accidents caused by gas explosion accidents.

Numerical Simulations of DDT Limits in Hydrogen-Air Mixtures in Obstacle Laden Channel

The main aim of this study was to perform numerical simulations of deflagration to detonation transition process (DDT) in hydrogen–air mixtures and assess the capabilities of freeware open-source ddtFoam code to simulate and capture DDT limits. The numerical geometry was based on the real 0.08 × 0.11 × 4 m (H × W × L), rectangular cross-section detonation channel previously used to experimentally investigate DDT limits in obstacle-filled channel. The constant blockage ratio (BR) equal to 0.5 was kept for three obstacle spacing configurations: S = H, 2H, 3H. The results showed that hydrogen concentration limits for successful DDT from simulations are close to the experimental values, however, the simulated DDT limits range is wider than the experimental one and depends on the obstacles spacing. The numerical results were analyzed by means of propagation velocities, overpressures, and run-up distances. The best match between numerical and experimental DDT limits was observed for obstacles spacing L = 3H and the lowest match for spacing L = H. The comparison between experimental and numerical results points at the possible application of ddtFoam in geometry with a relatively low level of congestion. This work results proved that simulations in such geometry provide numerical flame acceleration velocity profiles, run-up distance, and recorded overpressures very close to experimentally measured.

Laminar Burning Velocity Model Based on Deep Neural Network for Hydrogen and Propane with Air

The aim of the study was to develop deep neural network models for laminar burning velocity (LBV) calculations. The present study resulted in models for hydrogen–air and propane–air mixtures. An original data-preparation/data-generation algorithm was also developed in order to obtain the datasets sufficient in quality and quantity for models training. The discussion about the current analytical models highlighted issues with both experimental data and methodology of creating those analytical models. It was concluded that there is a need for models that can capture data from multiple experimental techniques with ease and automate the model design and training process. We presented a full machine learning based approach that fulfills these requirements. Not only model development, but also data preparation was described in detail as it is crucial in obtaining good results. Resulting models calculations were compared with popular analytical models and experimental data gathered from literature. The calculations comparison showed that the models developed were characterized by the smallest error with regards to the experiments and behaved equally well for variable pressure, temperature, and equivalence ratio. The source code of ready-to-use models has been provided and can be easily integrated in, for example, CFD software.

Computational and Microstructural Stability Analysis of Shock Wave Interaction with NbB2-B4C-Based Nanostructured Ceramics

Despite extensive research on developing different transition metal boride composites for aero-thermostructural applications, the understanding of the shockwave interaction using high pressure shock testing facilities and computational simulation of such interactions are much less explored. This aspect is even more important for much less explored ceramics, like NbB₂-based materials. While addressing this aspect, the present investigation reports the thermostructural stability of spark plasma sintered NbB₂-(0-40) mol % B₄C composites under the hypersonic aero-thermodynamic conditions using a miniature detonation-driven shock tube facility. All the ceramic discs underwent mild surface oxidation, as a consequence to impulsive load together with the thermomechanical shock. Using the in situ recorded pressure pulse data together with conjugate heat transfer analysis, spatiotemporal evolution of ceramic surface temperature was computationally analyzed for the given test conditions. Importantly, the NbB₂-(0 and 20) mol % B₄C composite retained structural integrity even after exposure to 10 shock pulses with maximum reflected shock temperature and pressure of 5000 K and 37.5 MPa, respectively. In contrast, NbB₂-40 mol % B₄C underwent structural failure by shattering to pieces. An attempt has been made to rationalize such results on the basis of thermal shock resistance parameters, estimated using the Kingery and Hasselman model. It is observed that NbB₂-(0 and 20) mol % B₄C shows higher crack propagation resistance, that is, 20 and 30%, respectively, under thermal shock (R″) than NbB₂-40 mol % B₄C. Interestingly, all the shock exposed NbB₂-B₄C ceramics show a measurable increase in hardness, which is attributed to transient melting and solidification of constituent phases due to interaction with shock heated gas, for a short duration of ∼5 ms. Taken together, the present study establishes the potential of NbB₂-B₄C composites for aero-thermostructural applications.

Computational Fluid Dynamics Simulation of Deflagration-to-Detonation Transition in a Full-Scale Konvoi-Type Pressurized Water Reactor

For the purpose of nuclear safety analysis, a reactive flow solver has been developed to determine the hazardous potential of large-scale hydrogen explosions. Without using empirical transition criteria, the whole combustion process including deflagration-to-detonation transition (DDT) is computed within a single solver framework. In this paper, we present massively parallelized three-dimensional explosion simulations in a full-scale pressurized water reactor (PWR) of the Konvoi type. Several generic DDT scenarios in globally lean hydrogen–air mixtures are examined to assess the importance of different input parameters. It is demonstrated that the explosion process is highly sensitive to mixture composition, ignition location, and thermodynamic initial conditions. Pressure loads on the confining structure show a profoundly dynamic behavior depending on the position in the containment. Computational cost can effectively be reduced through adaptive mesh refinement (AMR).