High-Pressure Hydrogen Release Auto-Ignition Characteristics in Tee Tubes

Literature Overview

This research by Zhang Shaogang and colleagues from Shanghai Maritime University, COSCO Shipping Petroleum Transportation Co., Ltd., and Tongji University Shanghai Institute of Disaster Prevention and Mitigation was published in the Journal of Safety and Environment Engineering (2025, Vol. 25, No. 10, pp. 3775-3783). The study investigates the auto-ignition characteristics of high-pressure hydrogen release into tee tubes using Fluent-based numerical simulation, examining three tee geometries: T-tube, beveled T-tube, and human-shaped tube. The work is supported by multiple funding sources including the National Natural Science Foundation Youth Fund (52404230) and Shanghai Science and Technology Innovation Action Plan projects.

Safety Context and Research Significance

High-pressure hydrogen systems are increasingly deployed in energy storage, fuel cell vehicles, and industrial processes. When hydrogen escapes from pressurized vessels or pipelines, it may auto-ignite upon contact with air, leading to fire and explosion incidents. The tee tube geometry, commonly used in piping systems for flow splitting and merging, presents unique challenges for hydrogen safety due to its complex internal flow patterns and potential for shock wave generation.

Understanding auto-ignition mechanisms in tee geometries is critical for:

Designing safe hydrogen release and ventilation systems.
Establishing safe operating pressure limits.
Selecting appropriate tee geometries for hydrogen service.
Developing emergency response protocols for hydrogen incidents.

Simulation Methodology and Parameters

The researchers established a shock tube high-pressure hydrogen release model using Fluent software, simulating hydrogen release into three different tee tube geometries:

Tee Geometry	Description	Structural Feature
T-tube	Standard perpendicular tee	Sharp 90-degree branch junction
Beveled T-tube	T-tube with beveled branch inlet	Reduced flow disruption
Human-shaped tube	Curved branch transition	Smooth flow path

The simulation examines shock wave propagation, hydrogen-air mixing, temperature field evolution, and auto-ignition phenomena across different release pressures.

Key Findings

Shock Wave Behavior

The pipe structure significantly influences shock wave propagation and auto-ignition characteristics. Compared to the standard T-tube, curved tee tubes (human-shaped and beveled T-tube) effectively reduce shock wave intensity. The smoother geometry reduces flow disruption and consequently diminishes the energy available for auto-ignition initiation.

Auto-Ignition Mechanism

Multi-dimensional reflected shock waves combined with turbulent mixing promote hydrogen-air mixture formation. The critical factor for auto-ignition is the high temperature at the boundary layer within the pipe combined with the hydrogen-air mixture. Ignition points first appear near the high-temperature zones adjacent to the diverging branch inner walls.

Geometry-Specific Behavior

T-tube: Longest auto-ignition delay time, highest safety margin.
Human-shaped tube: Branch structure facilitates auto-ignition initiation but does not sustain initial flames effectively.
Beveled T-tube: Intermediate behavior between the other two geometries.

Pressure Effects

Increasing release pressure promotes both auto-ignition occurrence and flame stable propagation. At 8 MPa release pressure, the human-shaped tube exhibits a flame extinction-reignition phenomenon due to the combined effects of shock heating and dynamic hydrogen jet behavior.

Engineering Practice Implications

For hydrogen pipeline and equipment design, this research provides several actionable recommendations:

Tee geometry selection: Standard T-tubes offer the highest safety margin for high-pressure hydrogen service due to their longer auto-ignition delay times, despite generating stronger shock waves.
Pressure management: Release pressures should be limited where possible, as higher pressures significantly increase auto-ignition probability and flame propagation stability.
Ventilation design: The boundary layer near diverging branch walls represents the critical ignition zone, suggesting that ventilation strategies should focus on cooling and diluting hydrogen concentrations in these specific locations.
Material considerations: While not directly addressed in this study, the high-temperature boundary layer conditions identified suggest that material selection should account for potential thermal exposure during hydrogen release events.

Study Insights and Implications

This research highlights the complex interplay between geometry, pressure, and combustion physics in hydrogen safety engineering. The finding that curved tees reduce shock intensity but may actually facilitate auto-ignition initiation demonstrates that reducing one hazard parameter does not necessarily improve overall safety. The T-tube's superior safety performance, despite its harsher shock wave environment, illustrates that auto-ignition delay time is a more critical safety metric than shock wave intensity alone. For engineers designing hydrogen piping systems, this study underscores the importance of holistic safety evaluation rather than optimizing individual parameters in isolation. The identification of boundary layer high-temperature zones as the primary ignition locations provides a clear target for protective design measures, including localized cooling, inert gas blanketing, or enhanced ventilation at diverging junctions. As hydrogen infrastructure expands globally, understanding these fundamental auto-ignition mechanisms in common pipe fittings becomes increasingly important for ensuring safe operation of hydrogen systems across industrial, transportation, and energy storage applications.