# Influence of Coal Dust on Premixed Turbulent Methane-Air Flames – Summary

1-Sentence-Summary: A tube based multiphase burner called the Hybrid Flame Analyzer is used to investigate turbulent burning velocity in methane and coal dust mixtures.

Authors: S.R. Rockwell and A.S. Rangwala

Favorite quote from the paper:

These authors develop a flame tube burner to investigate turbulent burning velocity of hybrid mixtures. The burner chamber is 44 cm tall and 18 cm wide, and takes in dust and gas feed lines. The authors call the device the Hybrid Flame Analyzer (HFA).

Turbulent burning velocities are measured at methane equivalence ratios of 0.8, 1.0, and 1.2. Coal dust concentrations are varied between 25 and 75 g/m3. The coal dust particles have two size ranges: 76 – 90 µm and 106-120 µm. Turbulent velocity fluctuations and turbulent length scale are varied between 0.18 m/s and 0.53 m/s, and 1.6 mm an 1.1 mm, respectively.

Three of the main findings from this paper are:

1. The shadowgraph technique can accurately determine turbulent burning velocity for gas, dust, and hybrid mixtures.
2. Hybrid turbulent velocities are generally higher than gas and dust alone, even for fuel rich mixtures.
3. Traditional turbulent burning velocity correlations can be applied to hybrid mixtures.

The following sections outline the main findings in more detail. The interested reader is encouraged to view the complete article at the link provided below.

## Finding #1: Shadowgraphs can be used to measure dust, gas, and hybrid burning velocities

Due to high flame luminosity from the burning dust, standard photographs cannot be used to determine burning velocities. The authors demonstrate how the shadowgraph technique can be used instead. This approach is based on the work of Grover, 1963 using a similar technique.

Shadowgraph images are used to trace the outer flame surface. These images show density variations in the flow field and allow the flame to be easily outlined. Due to the random nature of turbulence, multiple flame surfaces need to be captured. These are laid on top of each other for the same experimental conditions, and an “average” flame surface can be estimated. Lastly, a flame cone is predicted from the average surface area and used to calculate the burning velocity.

The turbulent burning velocities for pure methane are compared to experimental results from Kobayashi et al., 1996. The comparison demonstrates that the experimental data is generally within the experimental error using the shadowgraph approach.

## Finding #2: Hybrid burning velocities are typically higher than gas or dust alone

The hybrid results showed turbulent burning velocities that are typically higher than both dust and gas flames alone. For the smaller range of particle sizes, the hybrid burning velocities are 2.4 to 3.6 times larger than the laminar burning velocities of the dust alone (reported in Xie et al., 2012) and 10-20% higher than the gas alone. For the larger particle size, the results are more scattered with some hybrid mixtures reporting slightly lower burning velocities than gas alone.

It is particularly interesting that turbulent flame speeds for all fuel rich hybrid mixtures are larger than the dust and gas flames alone. This suggests that particles are doing more than just adding fuel to the system. The authors follow on the logic of Gore and Crowe, 1989 and Crowe, 2000 to discuss how particles can augment and increase turbulence in the flow field.

## Finding #3: Turbulent burning velocity correlations can be applied to hybrid mixtures

Turbulent burning velocity correlations are useful for demonstrating the impact of different system parameters and can be used in numerical simulation. The authors correlate their burning velocity data to the relation

$\frac{S_{T}}{S_{L}} = 1 + C\left(\frac{u^{'}_{rms}}{S_{L}}\right)^{n}$

where $n$ is known as the bending exponent, $C$ contains the influence of the scale of turbulence, $S_{L}$ is the laminar burning velocity, $S_{T}$ is the turbulent burning velocity, and $u^{'}_{rms}$ is the root-mean-square of the turbulent velocity fluctuations.

The authors find two sets of parameters for the smaller particle size range, depending on the mixture being fuel lean or fuel rich. For fuel lean mixtures $C$ and $n$ are determined as 2.2 and 0.2, respectively. For fuel rich mixtures $C$ is adjusted to 1.7 to best fit the data.

## My Personal Take-Aways From “Influence of Coal Dust on Premixed Turbulent Methane-Air Flames”

The tube burner method presented in this paper and used by others in the literature (e.g., Kobayashi et al., 1996 and Julien et al., 2016) is an interesting technique that has the potential to greatly enhance our understanding of turbulent dust and hybrid flame propagation. The measurements presented here are useful for scaling flame speeds between different turbulent conditions. They are also useful as validation test cases for computer simulation, and as input for model development.

The fact that burning velocity of hybrid mixtures is higher than both the dust and gas alone under fuel rich conditions is also an interesting finding. Other experiments using a 20-L bomb generally report a decrease in flame speed as dust is added to a fuel rich gas (e.g., see Denkevits, 2007, Dufaud et al., 2008, and Dufaud et al., 2009). The interaction between combustion, turbulence, and multiphase flow is a complex topic that still remains to be fully understood today.

Full Citation:

• S. R. Rockwell and A. S. Rangwala, “Influence of coal dust on premixed turbulent methane-air flames,” Combustion and flame, vol. 160, pp. 635-640, 2013.
[Bibtex]
@ARTICLE{Rockwell2013,
title={Influence of Coal Dust on Premixed Turbulent Methane-Air Flames},
author={Rockwell, S.R. and Rangwala, A.S.},
journal={Combustion and Flame},
volume={160},
pages={635--640},
year={2013},
}