Flat premixed laminar CH4/air flames

Aims and Motivation

Flat premixed laminar flames offer the advantage of well-defined conditions with respect to flow field, gas concentrations, temperature profiles, and spatial uniformity. They are, thus, ideally suited for fundamental investigations or reference cases in various areas of combustion research. In our studies, they are used as calibration source for a laser Raman scattering system.

The gas composition in the post-flame region can be accurately calculated by flame simulation codes and the major species concentrations are very close to chemical equilibrium. A more complicated task is the determination of the temperatures of the gases because the flames are typically not adiabatic due to heat loss to the burner plate. In the investigations described here, Coherent anti-Stokes Raman Scattering (CARS) was used to measure the temperatures of 22 "standard" CH₄/air flames at atmospheric pressure.

Burner and Flames

The burner is a commercially available flat flame burner with a sintered bronze disc (Holthuis & Associates, formerly McKenna Products). The diameter of the matrix is 60 mm, the annular shroud flow was not used in these investigations. The matrix was cooled by water with an inlet temperature of 16°C at a flow rate of 1 liter/s. Methane (purity 99.5%) and dry air were premixed and supplied to the burner at room temperature. The flow rates were adjusted by calibrated mass flow meters (Brooks 5850 and 5851) and are accurate within ±1%. The reference point within the exhaust gas of the burner and, thus, the measuring location for the temperature was on the axis of the burner 15 mm above the burner plate. This location was chosen because it allows good optical access and because temperature gradients are negligible around this point.

Temperature Measurements

The temperatures were measured with the mobile CARS system of our institute employing a USED CARS beam geometry [1,2,3]. The measuring volume had a diameter of approx. 0.15 mm and a length of 2 mm and was located on the burner axis at h=15 mm above the burner plate. For each flame, a series of 1200 single-pulse CARS spectra was measured from which the average temperature was determined. The accuracy of the temperature measurement is ±2.5%.

Exhaust Gas Composition

The exhaust gas composition was calculated using the "Gaseq" chemical equilibrium program [4] which computed the equilibrium composition for the product temperatures determined by the CARS measurements. The calculated major species mole fractions are displayed in the table, minor species, e.g., radicals or NO, are not listed. Therefore, the sum of the mole fraction is not always unity.

Results

The tables show the data of 22 premixed CH₄/air flames at atmospheric pressure. The CH₄ and air flows are given in standard liters per minute (slpm), i.e., volume flow at T=0 °C and p=1013 mbar. Phi is the equivalence ratio determined from the flow rates and T_ad is the adiabatic flame temperature for this equivalence ratio calculated with Gaseq starting from the unburnt gas mixtures (at T=300 K). The measured temperatures, T_CARS, are lower than T_ad because of heat transfer from the flame to the burner matrix. The heat transfer increases with decreasing distance D_h between the burner plate and the flame front. D_h is typically <1mm and increases with flow rate and decreases with the burning velocity which has its maximum value at phi about 1.1. These relationships explain qualitatively the temperature differences between T_ad and T_CARS as a function of flow rate and phi. The table displays further the mole fractions X of the major species calculated for the exhaust gas temperature T_CARS.