Turbulence Characteristics of Explosion Airflow in Tee Pipes Under Different Opening Conditions

Literature Overview

This paper by Wang Kaimin and colleagues from Changzhou University, published in China Safety Production Science and Technology (2022, Vol. 18, No. 4), investigates the turbulence behavior of explosion airflow within tee pipe configurations under various opening conditions. The study combines numerical simulation with physical analysis to characterize how the geometry of pipe openings influences the propagation and intensity of explosion-induced turbulence.

Technical Context

Tee pipe configurations are ubiquitous in industrial piping systems, particularly in process plants handling flammable gases and vapors. When an explosion occurs within such a system, the resulting pressure wave and turbulent flow interact with the tee geometry in complex ways. Understanding these interactions is essential for explosion protection design, including the placement of explosion vents, suppression systems, and deflagration venting arrangements.

Numerical Simulation Approach

The study employed computational fluid dynamics (CFD) simulation to model the explosion propagation within tee pipe geometries. The turbulence characteristics were analyzed in terms of turbulent kinetic energy (TKE), which is a key parameter in characterizing the intensity and structure of the flow field.

Configuration Variants Studied

Key Findings

Turbulent Kinetic Energy Distribution

The maximum TKE peak consistently occurs within the vertical branch pipe across all configurations. This is attributed to the flow separation and reattachment that occurs when the explosion-driven flow encounters the tee junction, creating intense shear layers and vortical structures in the branch.

The opening conditions have a dramatic effect on the TKE magnitude:

• Opening the vertical branch increases the TKE peak by approximately 30%, likely due to the enhanced flow acceleration as the explosion pressure drives gas into the open branch.
• Opening one horizontal end reduces the TKE peak by about 10%, as the pressure relief reduces the overall driving force for turbulence generation.
• Opening both horizontal ends dramatically increases the TKE peak by nearly 178%, indicating that the bidirectional flow interaction creates far more intense turbulence than any single opening configuration.

Shock Wave-Turbulence Interaction

A critical finding is the interaction between the explosion-driven shock wave and the turbulent flow:

• When the shock wave propagates in the same direction as the turbulent flow, it accelerates the gas and increases TKE, creating a positive feedback loop between shock propagation and turbulence intensification.
• When the shock wave propagates against the turbulent flow, it impedes the development of turbulence, effectively acting as a barrier to turbulent growth.

This directional dependence has important implications for explosion vent design: vents that allow the shock wave to escape in the same direction as the primary flow will experience more intense loading than vents that oppose the flow direction.

Flame Front-Turbulence Interaction

The study reveals a positive feedback mechanism between combustion and turbulence: the flame front is more likely to contact turbulent eddies carrying unburned gas, which enhances the combustion rate and generates more heat and pressure, which in turn intensifies the turbulence. This self-reinforcing cycle is a key mechanism in the escalation of explosion severity within confined piping systems.

External Flow Structures

When the explosion-driven flow exits the pipe, the turbulence characteristics change dramatically. The study identifies a dual-vortex structure forming outside the pipe opening, where the high-velocity jet interacts with the ambient atmosphere. This external turbulence can pose a hazard to nearby equipment and personnel, and should be considered in the siting of explosion vents.

Engineering Practice Implications

Explosion Protection Design

The findings have direct implications for the design of explosion protection systems in industrial piping:

1. Vent sizing and placement: The TKE data indicate that opening configurations that allow bidirectional flow (both ends open) create the most severe turbulence conditions. Explosion vents should be designed to accommodate the highest expected TKE loading, which may correspond to the most open configuration.
2. Piping layout optimization: The tee geometry is identified as a turbulence amplification location. In piping layouts where explosion risk exists, the number and configuration of tees should be minimized or designed with features that reduce turbulence generation, such as increased corner radii.
3. Pressure relief system design: The shock wave-turbulence interaction data inform the design of pressure relief valves and rupture disks, which must be sized to handle the peak loads that occur when shock and turbulence interact constructively.

Safety Assessment

For existing installations, the study provides a basis for reassessing explosion hazards in piping systems with tee configurations. Sites where both ends of a tee are open to the atmosphere or to other process sections should be identified as high-risk locations for explosion escalation.

Study Insights and Reflections

This paper provides valuable quantitative data on the turbulence behavior of explosion airflow in tee pipes, filling a gap in the understanding of explosion propagation in branched piping systems. The numerical simulation approach, while validated against physical principles, should be interpreted with awareness of the inherent limitations of turbulence modeling in explosive flow conditions.

The finding that the vertical branch consistently experiences the highest TKE is particularly important for practical design, as it identifies a specific location where explosion protection measures should be concentrated. The dramatic increase in TKE when both horizontal ends are open (178%) is a striking result that underscores the nonlinear nature of explosion-turbulence interactions.

The identification of the positive feedback mechanism between flame front and turbulence is a critical insight for explosion severity assessment. This mechanism means that small changes in initial conditions or geometry can lead to disproportionately large changes in explosion severity, which has implications for the margin of safety required in explosion protection design.

The dual-vortex structure observed outside the pipe opening is a relatively novel finding that highlights the need to consider external flow effects in explosion vent design. The turbulence generated outside the vent can interact with nearby structures and potentially cause secondary hazards.