1-Sentence-Summary: Measurement techniques for laminar flame thickness and preheat zone thickness are explained, and the effect of temperature and pressure are explored.
Authors: H.M. Heravi, A. Azarinfar, S.I. Kwon, P.J. Bowen, and N. Syred
Read in: Three Minutes
Favorite quote from the paper:
The Chemkin Premix package is used to simulate laminar methane flames with a detailed reaction mechanism (GRI-Mech2.1). Burning velocity, laminar flame thickness, and preheat zone thickness are explored for equivalence ratios between 0.5 ad 1.6. Furthermore, the simulation results are compared to the experimental results of Andrews and Bradley, 1972 and Gottgens et al., 1992. Lastly, the effect of pressure and temperature on flame thickness is investigated.
Three of the main findings from this paper are:
- Two approaches for measuring flame thickness may produce significantly different results.
- For both measurement approaches the preheat zone is approximately 60% of the reaction zone thickness.
- Pressure decreases burning velocity and flame thickness while temperature increases burning velocity but decreases flame thickness.
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: Two approaches for measuring flame thickness are presented
Laminar flames generally consists of two regions, referred to as the preheat zone and reaction zone, respectively. The reaction zone can be further broken into an area of fast reactions and an area of slower, latter time reactions. For methane flames, the slow reactions may extend centimeters into the flame. The term “flame thickness” generally includes the preheat zone and fast reaction zone.
Two methods for determining flame thickness are presented by the authors. The first approach is called the direct method while the second approach is the calculated method.
For the direct method a tangent line is drawn at the inflection point of the temperature distance curve. The flame thickness is then defined as the distance from where the extended tangent meets the unburned fuel temperature, and the combustion products temperature. The preheat zone is defined as the the distance from the unburned meeting point to the inflection point.
In the calculated method, the flame thickness is determined from the burning velocity and fuel characteristics assuming the thermal theory of flame propagation. The final equation for total flame thickness (\(X\)) and preheat zone thickness (\(X1\)) are:
\(X = \frac{\Gamma}{\rho_{u}C_{P}}\frac{T_{b}- T_{u}}{T^{o} – T_{u}}\frac{1}{S_{L}}\)
\(X1 = \frac{\Gamma}{\rho_{u}C_{P}S_{L}}\)
where \(\Gamma\) is thermal conductivity, \(\rho_{u}\) is density of the unburned mixture, \(C_{p}\) is specific heat, \(S_{L}\) is the laminar burning velocity, \(T_{u}\) is the unburned fuel temperature, and \(T_{b}\) is the burned products temperature. \(T^{o}\) is the temperature at the point of the maximum fuel consumption and is generally the same as the inflection point from the direct method.
Finding #2: Preheat zone thickness is approximately 60% of the flame thickness
The authors compare simulation results to the experimental data of Andrews and Bradley, 1972 and Gottgens et al., 1992. In general the flame thickness determined by Andrews and Bradley, 1972 is about twice the size of the other measurements.
For the measurements other than Andrews and Bradley, 1972, the methane laminar flame thickness varied between 0.1 and 3 mm for equivalence ratios between 0.5 ad 1.6. The smallest flame thickness occurred near the stoichiometric mixture.
From the experiential and numerical measurements, the preheat zone was found to be 0.05mm to 2 mm throughout the concentration range studied. In general the preheat zone was 57 to 62% of the determined flame thickness.
Finding #3: Both temperature and pressure alter laminar flame thickness
The authors use the simulation model to investigate the impact of pressure and temperature on laminar burning velocity and flame thickness. Increasing the ambient temperature increases the burning velocity and decreases the flame thickness. The increased temperature increases the combustion reaction and the faster propagation results in a thinner flame structure.
Pressure is found to decrease both burning velocity and flame thickness. The authors propose that flame expansion is decreased from the pressure change and its impact on density. A relation is given that correlates flame thickness with ambient temperature and pressure.
My Personal Take-Aways From
“Determination of Laminar Flame Thickness and Burning Velocity of Methane-Air Mixture”
This paper and it’s references provide useful validation data for experimental testing and computer simulation. Furthermore, the correlation proposed is interesting to estimate flame thickness for those working with natural gas or other methane based fuels. Unfortunately, an electronic version of the paper cannot be found online. Please post a link in the comments if you are able to provide one.
Full Citation: [bibtex file=references.bib key=Heravi2007]
