Study Note on Shock Wave Entrainment of Aluminum Powder Causing Secondary Explosion in Tee Pipe and Explosion Suppression Research

Literature Overview

The paper by Bi Haipu, Xie Xiaolong, Lei Weigang, Wang Kairan, Shao Hui, Yu Rui, and Zhang Jiangtao, published in Fire Science and Technology in 2019, investigates the phenomenon of secondary explosions caused by shock wave entrainment of aluminum powder in tee pipe configurations, along with explosion suppression strategies. The research was funded by the Jiangsu Provincial University Natural Science Research Project (16KJB620001) and the Jiangsu Provincial Graduate Research Innovation Program (KYCX182625). This study is of direct relevance to engineers working in process safety, industrial dust handling systems, and explosion protection design.

Core Technical Findings and Experimental Results

The study designed a comprehensive dust explosion test platform to investigate the secondary explosion phenomenon and its suppression characteristics. The following table summarizes the key experimental findings:

The experimental results demonstrate that the branching structure of a closed tee pipe enhances the explosion pressure and flame propagation speed at the junction of the main pipe, while simultaneously weakening these parameters in the perpendicular branch pipe. This asymmetry in explosion behavior is attributed to the interaction between the shock wave and the dust cloud entrained by the shock front.

Explosion Mechanism and Physical Interpretation

The secondary explosion phenomenon occurs when a primary explosion generates a shock wave that propagates through a dust-laden pipe system. The shock wave entrains settled dust particles, creating a secondary dust cloud that can be ignited by the transmitted flame front. In a tee pipe configuration, the branching geometry creates complex flow and pressure interactions that influence the severity and directionality of the secondary explosion.

Tee Pipe Geometry Effects

The enhancement of explosion pressure at the main pipe branch junction can be explained by the reflection and focusing of the shock wave at the tee junction. When the shock wave encounters the tee junction, part of the wave is reflected back into the main pipe, creating a constructive interference that increases the local pressure. Additionally, the sudden expansion of the flow area at the junction can cause flow separation and turbulence, which promotes better mixing of the entrained dust with the flame front, leading to more vigorous combustion.

Conversely, the weakening of explosion pressure and flame propagation speed in the perpendicular branch is attributed to the diversion of energy and momentum into the main pipe direction. The shock wave and flame front preferentially propagate along the path of least resistance, which in a tee configuration is typically the main pipe. The perpendicular branch receives only a fraction of the total energy, resulting in reduced explosion severity.

Explosion Suppression Strategies

The use of ammonium dihydrogen phosphate (ADP) as an explosion suppression agent is a well-established approach in dust explosion protection. ADP functions through multiple mechanisms:

1. Thermal inhibition: The decomposition of ADP absorbs heat, reducing the temperature of the flame front and slowing the combustion reaction rate.
2. Chemical inhibition: The decomposition products of ADP, including ammonia and phosphoric acid, can interfere with the radical chain reactions that sustain combustion.
3. Physical dilution: The addition of ADP particles dilutes the fuel concentration, reducing the overall reactivity of the dust-air mixture.

The finding that a 10% mass fraction of ADP completely suppresses aluminum powder explosion provides a clear design criterion for explosion protection systems. However, engineers should note that this result is specific to the experimental conditions, including the aluminum powder particle size distribution, concentration, and the tee pipe geometry. In practice, the required suppression agent concentration may vary depending on the specific application.

Suppression System Design Considerations

Integration with Engineering Practice

For engineers involved in the design of dust handling systems, explosion protection, and process safety, this study offers several important insights:

1. Tee pipe configurations in dust handling systems should be carefully evaluated for their potential to enhance secondary explosion severity. The branching geometry can create localized pressure peaks that may exceed design limits, leading to equipment failure and escalation of the explosion.
2. Explosion suppression systems should be designed to account for the specific geometry of the piping system. The effectiveness of suppression agents may vary depending on the location of injection relative to the tee junction and the direction of flame propagation.
3. The particle size distribution of the dust is a critical parameter that influences explosion severity. The study identifies 35 μm as the median particle size for maximum explosion severity, which is consistent with the well-known relationship between particle size and specific surface area. Engineers should ensure that dust sampling and characterization are performed as part of the hazard assessment process.
4. The use of computational fluid dynamics (CFD) simulation can complement experimental studies by providing detailed insights into the flow and combustion processes within complex piping geometries. Engineers should consider developing validated CFD models for their specific systems to support design optimization and safety analysis.

Study Insights and Outlook

This research contributes valuable experimental data to the understanding of secondary explosion phenomena in tee pipe configurations, which are common in industrial dust handling systems. The findings have direct implications for the design of explosion protection systems, particularly in industries where combustible metal dusts are present, such as aluminum processing, aerospace manufacturing, and metal finishing.

A critical observation is that the tee pipe geometry creates an asymmetric explosion behavior, with enhanced severity at the main pipe junction and reduced severity in the perpendicular branch. This asymmetry has important implications for the design of explosion venting and suppression systems. Engineers must ensure that protection devices are installed at locations where they can effectively mitigate the enhanced explosion pressure, rather than assuming uniform explosion severity throughout the system.

Looking forward, several areas of research would be valuable. First, the interaction between explosion suppression agents and tee pipe geometry should be studied in more detail, including the effect of injection location, timing, and distribution pattern on suppression effectiveness. Second, the behavior of different metal dusts in tee pipe configurations should be compared to establish generalizable design guidelines. Third, the integration of explosion detection, suppression, and isolation systems should be investigated to develop comprehensive protection strategies for complex piping networks.

In conclusion, this paper provides essential experimental data and engineering insights for the design of explosion protection systems in dust handling applications. The findings highlight the importance of accounting for piping geometry in explosion hazard assessment and protection system design, and offer actionable guidance for engineers working in process safety and industrial safety engineering.