Hydrogen flames propagating through pipelines present unique challenges due to hydrogen’s distinct combustion properties. Understanding flame behavior is critical for safety and risk assessment in hydrogen transportation systems. The propagation dynamics depend on multiple factors, including pipe diameter, pressure, mixture composition, and the presence of obstacles. Additionally, the transition from deflagration to detonation poses significant hazards that require careful evaluation.

Flame propagation in pipelines begins with the ignition of a hydrogen-air or hydrogen-oxygen mixture. Hydrogen has a wide flammability range (4% to 75% in air), enabling combustion even in lean or rich conditions. Once ignited, the flame front travels at speeds influenced by the mixture’s composition and the surrounding environment. In smooth, unobstructed pipelines, laminar flame speeds for stoichiometric hydrogen-air mixtures are approximately 2.5 to 3.0 m/s at standard conditions. However, turbulence can drastically increase flame speeds, reaching hundreds of meters per second under certain conditions.

Pipe diameter plays a crucial role in flame acceleration. Narrow pipes restrict flow and increase turbulence, which can enhance flame speed. Experiments show that flame acceleration is more pronounced in smaller-diameter pipes due to higher flow resistance and boundary layer effects. For example, in pipes with diameters below 50 mm, flame speeds can escalate rapidly, especially if the pipeline is long and unobstructed. Conversely, larger-diameter pipes (above 100 mm) tend to exhibit slower flame acceleration due to reduced turbulence and lower velocity gradients.

Pressure within the pipeline also significantly affects flame propagation. Higher initial pressures increase the energy density of the hydrogen mixture, leading to faster combustion rates. Studies indicate that flame speeds scale with pressure according to a power-law relationship, where doubling the pressure can increase flame speeds by a factor of 1.5 to 2.0, depending on the mixture composition. Elevated pressures also raise the likelihood of deflagration-to-detonation transition (DDT), a hazardous phenomenon where a subsonic flame abruptly transitions into a supersonic detonation wave.

The presence of obstacles inside pipelines further complicates flame behavior. Obstacles such as valves, bends, or protrusions generate turbulence, which can dramatically accelerate flames. Turbulence increases the flame surface area, enhancing heat and mass transfer and leading to faster combustion. Experimental data reveal that obstacles can increase flame speeds by an order of magnitude compared to smooth pipes. For instance, repeated obstacles spaced at regular intervals can cause flame speeds to exceed 500 m/s, significantly raising the risk of DDT.

Deflagration-to-detonation transition is a critical concern in hydrogen pipeline safety. DDT occurs when a flame accelerates sufficiently to produce shock waves that compress and heat the unburned mixture ahead of the flame front. If conditions are favorable, these shock waves can trigger detonation, characterized by supersonic propagation (typically 2000 m/s or more for hydrogen-air mixtures) and extreme overpressures (up to 20 times the initial pressure). The likelihood of DDT depends on several factors, including mixture composition, pipe geometry, and obstacle density. Research indicates that DDT is more likely in hydrogen-air mixtures with equivalence ratios close to stoichiometric (around 30% to 50% hydrogen by volume) and in pipelines with high obstacle densities.

Experimental studies have provided valuable insights into hydrogen flame propagation and DDT. Large-scale tests conducted in pipelines with diameters ranging from 100 mm to 500 mm demonstrate that flame acceleration follows predictable patterns under controlled conditions. For example, tests in smooth pipes show that flame speeds plateau after a certain distance, whereas obstructed pipes exhibit continuous acceleration until DDT occurs. Measurements also confirm that detonation overpressures can exceed 1.5 MPa in confined spaces, posing severe risks to infrastructure.

Mitigating flame acceleration and DDT requires careful pipeline design and operational strategies. Smooth inner surfaces, minimal obstructions, and pressure relief systems can reduce flame speeds and prevent detonation. Additionally, maintaining hydrogen concentrations outside the most reactive ranges (e.g., below 10% or above 50% in air) can lower the risk of rapid flame acceleration. Computational fluid dynamics (CFD) models and empirical correlations help predict flame behavior under various scenarios, aiding in the development of safer pipeline systems.

The following table summarizes key experimental findings on hydrogen flame propagation:

| Parameter | Effect on Flame Propagation | Experimental Observations |
|-------------------------|-----------------------------------------------------|---------------------------------------------------|
| Pipe Diameter | Smaller diameters increase turbulence and flame speed | Flame speeds > 500 m/s in pipes < 50 mm |
| Initial Pressure | Higher pressure increases flame speed and DDT risk | Doubling pressure increases flame speed by 1.5-2x |
| Obstacle Density | More obstacles enhance turbulence and acceleration | DDT observed with obstacle spacing < 10 pipe diameters |
| Mixture Composition | Stoichiometric mixtures most reactive | Peak flame speeds at 30-50% hydrogen in air |

In conclusion, hydrogen flame propagation in pipelines is a complex phenomenon influenced by pipe geometry, pressure, and obstacles. The risk of deflagration-to-detonation transition remains a critical safety concern, necessitating rigorous design and operational measures. Experimental research continues to refine predictive models, enabling safer hydrogen transportation infrastructure. Understanding these dynamics is essential for minimizing hazards and ensuring the reliable deployment of hydrogen as an energy carrier.