energies Article Numerical Simulations of DDT Limits in Hydrogen-Air Mixtures in Obstacle Laden Channel Wojciech Rudy * and Andrzej Teodorczyk   Citation: Rudy, W.; Teodorczyk, A. Numerical Simulations of DDT Lim- its in Hydrogen-Air Mixtures in Ob- stacle Laden Channel. Energies 2021, 14, 24. https://dx.doi.org/10.3390/ en14010024 Received: 23 November 2020 Accepted: 21 December 2020 Published: 23 December 2020 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2020 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Institute of Heat Engineering, Faculty of Power and Aeronautical Engineering, Warsaw University of Technology, Nowowiejska 21/25, 00-665 Warsaw, Poland; andrzej.teodorczyk@pw.edu.pl * Correspondence: wojciech.rudy@pw.edu.pl Abstract: 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. Keywords: DDT; hydrogen-air; ddtFoam; DDT limits; obstacle laden tube 1. Introduction Hydrogen as a carbonless and environmentally friendly fuel is considered as the future energy carrier. Its properties such as wide flammability limits, high laminar burning velocity (in air ~2.5 m/s), low ignition energy (in air ~0.02 mJ), and high lower heating value (~120 MJ/kg) point at a variety of possible hydrogen applications. However, the listed advantages of hydrogen properties might be also considered as disadvantages from the safety point of view. In the case of unintended leakage, hydrogen is easier to ignite under wider concentration conditions, and after ignition, flame will propagate faster. Additionally, the hydrogen–air flame is inherently unstable and if the conditions are favorable the hydrogen flame might go through the deflagration to detonation transition (DDT) process. As the DDT process depends on a variety of parameters (fuel concentration, congestion presence, and its geometrical configuration), it is crucial to investigate these parameters’ influence to prevent DDT in common use. As the experimental investigation is expensive in terms of time and cost, Computational Fluid Dynamics (CFD) codes have been developed to solve complex numerical problems. Nowadays, CFD analysis is applied in many branches of industry such as aerospace, automotive, power generation, manufacturing, petrochemical, process safety, turbomachinery, etc. The main advantage of using CFD simulations is to reduce the time of the designing process and, simultaneously, detailed numerical models might be used to get a better insight into the process that is difficult to investigate experimentally. DDT is one of such processes as it combines a wide range of flame propagation velocities (0–2000 m/s). Therefore, depending on the flame velocity, Energies 2021, 14, 24. https://dx.doi.org/10.3390/en14010024 https://www.mdpi.com/journal/energies