Experimental investigation on flame shapes and radiation properties of turbulent buoyant diffusion flames

Wenbin YAO; Zhijie HUANG; Mingjun XU; Xiao CHEN; Zehua YANG; Jiyun WANG

doi:10.16511/j.cnki.qhdxxb.2025.27.058

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Journal of Tsinghua University(Science and Technology) ›› 2026, Vol. 66 ›› Issue (1) : 83-91. DOI: 10.16511/j.cnki.qhdxxb.2025.27.058

Fire Science

Experimental investigation on flame shapes and radiation properties of turbulent buoyant diffusion flames

Wenbin YAO ¹ ,
Zhijie HUANG ¹ ,
Mingjun XU ²^,* ,
Xiao CHEN ¹ ,
Zehua YANG ¹ ,
Jiyun WANG ³

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Abstract

Objective: In recent years, gas-fueled burners have been widely used to emulate real fires of condensed fuels (e.g., solid polymers and liquid hydrocarbons) in key fields such as fire research, fire safety testing, and fire-fighting training. However, gas-fueled burners differ from real fires in key aspects such as fuel combustion mechanisms, flame stability, and heat release. These differences may result in deviations in simulating flame behaviors and radiation effects. Therefore, exploring the effect of fuel mixing (specifically ethylene-propane blends) on flame shapes and radiation characteristics of buoyant diffusion flames is of great significance. This paper not only clarifies the correlation between fuel composition and flame performance but also establishes a foundation for an equivalent model that matches the burning rate, flame shape, and flame radiation characteristics between gas-fueled burners and real fires, thereby improving the accuracy of fire simulations. Methods: This work examined the combustion characteristics of buoyant turbulent diffusion flames fueled by ethylene-propane gaseous mixtures, including flame shapes, axial temperature distributions, and flame radiation intensities. An experimental burner with a circular nozzle design and a diameter of 0.08 m was adopted to ensure uniform fuel injection. Different ethylene-propane mixing ratios (0%—100% propane by volume, with 10% intervals) were achieved by separately controlling the flow rates of ethylene and propane using mass flow controllers. The total volume flow rate of the mixed fuel was adjusted within the range of 1—6 L/min to vary the heat release rate. Flame shape was analyzed using video footage captured by a high-resolution charge-coupled device camera. Images were converted to grayscale and binarized using the maximum interclass variance method. Subsequently, morphological parameters, including flame height (Lf) and width (Df), were determined using the intermittent rate distribution characteristics of the binary images, referencing a previously proposed flame height definition. In addition, axial flame temperatures were measured using a thermocouple tree, whereas radiant heat flux was measured using a water-cooled Schmidt-Boelter radiant heat flux meter. Results: The experimental results were as follows. 1) The flame height, flame width, flame surface area, and flame volume increased with increasing heat release rate within a total volume flow rate range of 1—6 L/min. 2) The axial temperature of the flames first increased and then decreased with height, and the maximum axial temperature was maintained at 700—900 ℃. 3) For a given propane volume fraction, the average flame radiation power increased with the heat release rate of buoyant turbulent diffusion flames. As the propane volume fractions increased, the flame radiation fractions first increased and then decreased. 4) When the gas-fueled flames were controlled to achieve a heat release similar to that of liquid pool fires, pure propane gas flames better simulated the flame height and radiation characteristics of n-heptane pool fires, whereas pure ethylene gas flames were more suitable for simulating the flame height and radiation characteristics of aviation kerosene pool fires. Conclusions: By integrating theoretical equations and analyzing datasets, this paper established prediction models for the flame shape and radiation fraction of ethylene/propane turbulent diffusion flames and proposed an equivalent method for gaseous flames and liquid pool fires; this equivalent method provides theoretical guidance for real-fire simulation technologies in fire research, fire safety testing, and fire training.

Key words

turbulent buoyant diffusion flames / binary fuel mixture / flame shape / flame radiation

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Wenbin YAO , Zhijie HUANG , Mingjun XU , et al . Experimental investigation on flame shapes and radiation properties of turbulent buoyant diffusion flames[J]. Journal of Tsinghua University(Science and Technology). 2026, 66(1): 83-91 https://doi.org/10.16511/j.cnki.qhdxxb.2025.27.058

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