Numerical Modeling and Experimental Investigation of Flame Spread over Thin Circular Ducts in Normal Gravity and Microgravity Environments

Core Concepts

The core message of this article is to present a comprehensive numerical investigation and experimental validation of the phenomenon of flame spread over thin circular ducts in normal gravity and microgravity environments. The study focuses on understanding the key parameters affecting the flame spread rate, including fuel radius and opposed flow speed.

Abstract

The article presents a numerical investigation into the phenomenon of flame spread over thin circular ducts in normal gravity and microgravity environments. The study comprises a comprehensive investigation of key parameters affecting flame spread rate, including fuel radius and opposed flow speed.
The authors developed a 2-D axisymmetric flame spread model that accounts for char formation and performed numerical simulations to gain valuable insights into the underlying mechanisms governing flame spread over such geometry. The results from the numerical model were compared with experimentally observed flame spread rates to validate the model.
The key findings from the study are:

As the radius of the circular duct increases, the flame spread rate increases in both normal gravity and microgravity environments. This is attributed to the increased conduction heat feedback and radiation heat gain from the hot char through the gas phase at the inner core region.

The flame spread rate shows a non-monotonic increasing-decreasing trend with the opposed flow speed, ranging from quiescent (0 cm/s) to 30 cm/s, in both normal gravity and microgravity environments. This is due to the competing effects of increased oxygen availability and decreased heat feedback from the flame to the unburnt fuel as the flow speed increases.

The numerical model developed in this study, validated with experimental data, can be used to gain a comprehensive understanding of the underlying physical phenomena governing flame spread over circular duct fuels in various gravitational environments.

Stats

The mass flux from the solid fuel is given by: 𝑚
̇
̅′′
𝑠 = 𝐴̅𝑠𝜌̅𝑠𝑒𝑥𝑝(−𝐸𝑠/𝑇
𝑠).
The non-dimensional mass flux from the solid fuel is given by: 𝑚
̇ ′′
𝑠 = 𝑚
̇
̅′′
𝑠 / (𝜌∗𝑈
̅𝑅).
The non-dimensional mass of char per unit volume is given by: 𝑚
̇ 𝑐ℎ𝑎𝑟
′′′
= 𝑚
̇
̅𝑐ℎ𝑎𝑟
′′′
/ (𝜌∗𝑈
̅𝑅/𝐿
̅𝑅).

Quotes

"As the radius of circular duct increases the flame spread rate increases both in normal gravity and microgravity environments."
"The flame spread rate at different flow ranging from quiescent (0 cm/s) to 30 cm/s is also evaluated and 21 % oxygen and found a non-monotonic increasing decreasing trend of flame spread rate at different opposed flow speed in both normal gravity and microgravity environments."

Key Insights Distilled From

Numerical modelling of flame spread over thin circular ducts

by Vipin Kumar,... at arxiv.org 04-02-2024

https://arxiv.org/pdf/2404.01016.pdf

Deeper Inquiries

How would the flame spread behavior over circular ducts be affected by the composition and properties of the solid fuel material

The flame spread behavior over circular ducts can be significantly affected by the composition and properties of the solid fuel material. The composition of the fuel material determines its combustibility, heat release rate, and the products of combustion. For example, a fuel material with a higher cellulose content may burn more readily and produce different combustion byproducts compared to a fuel with a higher polyamide content. The properties of the fuel material, such as density, thermal conductivity, and heat of combustion, also play a crucial role in determining the flame spread behavior. A fuel material with higher density may lead to slower flame spread rates, while a material with higher thermal conductivity may affect the heat transfer mechanisms during combustion. Additionally, the heat of combustion of the fuel material influences the amount of heat released during combustion, which can impact the flame spread rate and intensity.

What are the potential challenges and limitations in extending the numerical model developed in this study to more complex fuel geometries and flow conditions

Extending the numerical model developed in this study to more complex fuel geometries and flow conditions may present several challenges and limitations. One challenge is the computational complexity involved in simulating intricate fuel geometries, such as irregular shapes or multiple interconnected ducts. The model may need to account for additional parameters and boundary conditions to accurately capture the flame spread behavior in such complex configurations. Another limitation is the need for experimental validation of the extended model to ensure its accuracy and reliability. Complex fuel geometries and flow conditions may introduce uncertainties that require careful calibration and verification against experimental data. Furthermore, the numerical model may need to be optimized for efficiency and scalability to handle the increased computational demands of simulating more complex scenarios.

What are the broader implications of understanding flame spread over circular ducts for fire safety and other practical applications, such as in the design of medical systems or spacecraft

Understanding flame spread over circular ducts has significant implications for fire safety and various practical applications, including the design of medical systems and spacecraft. In fire safety applications, knowledge of flame spread behavior over circular ducts can help in developing effective fire prevention and suppression strategies. By understanding the factors influencing flame spread rates in such geometries, engineers and designers can implement better fire protection measures and materials to enhance safety. In the design of medical systems, where circular ducts are used for flow purposes, insights into flame spread behavior can aid in selecting materials that are less prone to combustion or designing systems with built-in fire safety features. Similarly, in spacecraft design, where fire safety is critical due to the confined and controlled environments, understanding flame spread over circular ducts can lead to improved spacecraft materials and construction techniques to mitigate fire risks and ensure crew safety.