High-Pressure Hydrogen Release Auto-Ignition in Tee Pipes Numerical Simulation Study Note

Literature Overview

The paper by Zhang Shaogang and colleagues, published in the Journal of Safety and Environment (2025, Vol. 25, No. 10, pp. 3775-3783), investigates the auto-ignition characteristics of high-pressure hydrogen released into tee pipe geometries using computational fluid dynamics (CFD) simulation with the Fluent software. The research is supported by the National Natural Science Foundation of China Youth Fund (52404230), the Shanghai Science and Technology Innovation Action Plan Soft Science Project (24692105900), the China Transportation Education Research Association Project (JT2022ZD034), and Shanghai Social Development Science and Technology Projects (22dz1201102, 22dz1201103). The authors model a shock tube hydrogen release scenario and study the auto-ignition process in three tee pipe configurations: T-shape tee, beveled T-shape tee, and human-shaped tee (arc tee).

Technical Background and Safety Context

Hydrogen is increasingly used as an energy carrier in transportation, storage, and industrial applications. The release of high-pressure hydrogen into downstream piping systems can trigger auto-ignition through shock-heating mechanisms, leading to explosions and fires. This is a critical safety concern for hydrogen infrastructure, including refueling stations, storage facilities, and transport systems.

The auto-ignition of hydrogen occurs when the temperature of the hydrogen-air mixture reaches or exceeds the auto-ignition temperature of hydrogen, which is approximately 500-571 degrees Celsius depending on pressure and mixture composition. In high-pressure release scenarios, the rapid expansion of hydrogen generates shock waves that can locally heat the mixture to temperatures sufficient for auto-ignition without any external ignition source.

Simulation Methodology

Model Configuration

The authors establish a shock tube high-pressure hydrogen release model in Fluent, with the hydrogen source at high pressure (up to 8 MPa) releasing into a tee pipe connected to a downstream pipe containing air. Three tee geometries are studied:

Tee Type	Description	Geometry Characteristic
T-shape tee	Standard 90-degree branch	Sharp corner at branch junction
Beveled T-shape tee	T-shape with beveled branch edges	Rounded transition at branch junction
Human-shaped tee	Arc-shaped branch	Smooth curved transition

Physical Models

The simulation incorporates multiple physical models to capture the complex phenomena involved in hydrogen auto-ignition:

Shock wave propagation: The rapid expansion of hydrogen generates shock waves that propagate through the tee geometry and downstream pipe.
Multi-dimensional shock reflection: Shock waves reflect off the tee walls and branch junction, creating complex wave patterns that can focus energy and locally heat the mixture.
Turbulent mixing: The interaction between the hydrogen jet and the surrounding air creates turbulent mixing zones where the hydrogen-air mixture can reach stoichiometric or near-stoichiometric concentrations.
Boundary layer heating: The shock waves heat the gas near the pipe walls, creating high-temperature boundary layers that can serve as ignition sites.
Combustion chemistry: The simulation includes chemical reaction mechanisms that model the hydrogen oxidation process and flame propagation.

Key Findings

Effect of Release Pressure

The release pressure is a critical parameter governing auto-ignition behavior. As the release pressure increases:

The shock wave intensity increases, providing more energy for heating the mixture.
The hydrogen jet velocity increases, enhancing turbulent mixing with air.
The auto-ignition probability increases, and the flame propagates more stably once ignited.

At 8 MPa release pressure, the human-shaped tee exhibits a distinctive extinction-reignition phenomenon where the auto-ignition flame initially extinguishes but then reignites. This complex behavior is attributed to the interplay between shock heating effects and the dynamic behavior of the hydrogen jet.

Effect of Tee Geometry

The tee geometry significantly influences shock wave propagation and auto-ignition characteristics:

Parameter	T-shape tee	Beveled T-shape tee	Human-shaped tee
Shock wave intensity	Highest	Reduced	Lowest
Auto-ignition ease	Moderate	Moderate	Easier
Flame stability	Best	Moderate	Poorer
Ignition delay time	Longest	Intermediate	Shortest
Overall safety	Highest	Moderate	Lower

Ignition Mechanism

The study identifies two key factors that promote hydrogen auto-ignition in tee pipes:

Multi-dimensional reflected shocks and turbulent mixing: The complex shock wave patterns generated by the tee geometry create localized high-temperature zones where hydrogen and air are mixed. These conditions are favorable for auto-ignition.
High-temperature boundary layer: The shock waves heat the gas adjacent to the pipe walls, creating a boundary layer with temperatures approaching or exceeding the hydrogen auto-ignition temperature. The ignition point first appears near the branch section wall surface where the high-temperature boundary layer coincides with the hydrogen-air mixture.

Geometry-Specific Behavior

The human-shaped tee and beveled T-shape tee reduce shock wave intensity compared to the standard T-shape tee due to their smoother geometric transitions. However, this reduction in shock intensity comes with a trade-off: the arc-shaped branch structure of the human-shaped tee facilitates auto-ignition but does not support sustained flame propagation. The T-shape tee, despite having stronger shock waves, has the longest ignition delay time and the most stable flame propagation, making it the safest configuration overall.

Engineering Practice Implications

Tee Geometry Selection for Hydrogen Systems

The study provides clear guidance for the selection of tee geometries in hydrogen piping systems. The T-shape tee, despite its higher shock wave intensity, offers the best overall safety performance due to its longer ignition delay time and more stable flame behavior. This finding has direct implications for the design of hydrogen refueling stations, storage systems, and transport infrastructure.

FMEA Integration

The findings can be integrated into a Failure Mode and Effects Analysis (FMEA) for hydrogen piping systems:

Failure Mode	Cause	Effect	Severity	Occurrence	Detection	RPN
Auto-ignition at tee	Shock heating + mixing	Fire/explosion	High	Medium	Low	High
Flame propagation	Stable flame front	Damage spread	High	Low-Medium	Low	Medium
Extinction-reignition	Dynamic jet behavior	Unpredictable ignition	High	Low	Very Low	High

Material and Design Considerations

From a materials engineering perspective, the tee pipe materials used in hydrogen systems must be selected with consideration of hydrogen embrittlement resistance, temperature resistance under shock loading, and long-term durability under cyclic hydrogen exposure. The high temperatures generated by shock waves can also affect the mechanical properties of pipe materials, particularly at elevated temperatures where strength degradation occurs.

Study Insights and Reflections

This study makes an important contribution to hydrogen safety engineering by quantifying the effects of tee geometry on auto-ignition behavior. The finding that the T-shape tee is the safest configuration, despite having the strongest shock waves, challenges the intuitive assumption that smoother geometries always reduce safety risks. The key insight is that safety is not solely determined by shock intensity but by the complex interplay between shock heating, mixing, and flame dynamics.

The extinction-reignition phenomenon observed in the human-shaped tee at 8 MPa is particularly noteworthy. This behavior introduces unpredictability into the ignition process, which is a significant safety concern. Systems where ignition timing and location are unpredictable are inherently more dangerous than systems where the ignition behavior is more deterministic.

The boundary layer heating mechanism identified in this study suggests that the internal surface condition of tee pipes is a critical factor in auto-ignition. Surface roughness, contamination, and material properties that affect heat transfer to the wall can all influence the boundary layer temperature and, consequently, the ignition probability. This finding has implications for surface finish specifications and material selection in hydrogen piping design.

Summary

The numerical simulation study demonstrates that tee pipe geometry significantly influences high-pressure hydrogen auto-ignition behavior, with the standard T-shape tee providing the highest overall safety through its longest ignition delay time and most stable flame propagation characteristics, while smoother arc-shaped tees facilitate ignition but compromise flame stability, leading to unpredictable extinction-reignition phenomena at high release pressures.