The Heat Flux Method explained
The adiabatic burning velocity of a given fuel/oxidizer mixture is a key parameter governing a lot of properties of combustion, for example the shape of a premixed flame. Therefore, a lot of effort has been undertaken in the past to measure this parameter accurately for several fuel/oxidizer mixtures.
For measuring burning velocities, a method is needed that does not need any extrapolation due to either stretch or heat loss effects and produces a flame that can practically be investigated in a laboratory. The heat flux method by van Maaren provides such a method. To view an animation on the working principle, please click here.
In Figure 1 a schematic overview of the heat flux method is depicted. The unburnt gas is flowing through a perforated flat burner plate. This burner plate is kept at temperatures typically 60 K above the unburnt gas temperature. The burner plate has a heat loss to the gas mixture causing the gas mixture to increase in temperature. A flame is stabilized on top of this burner plate with a typical temperature of around 2000 K.
Figure 1 – Schematic overview of the Heat Flux method
On the right hand side of Figure 1 the heat flows are depicted between the burner plate, the unburnt gas and the flame. The smaller arrows indicate the heat gain of the unburnt gas mixture from the burner plate, while the larger arrows indicate the heat loss from the flame to the burner plate. When the gas inlet velocity is lower than the laminar burning velocity, the flame is stabilised on the burner. As a result the heat loss of the flame to the burner plate is larger than the heat gain of the gas mixture from the burner plate. When the unburnt gas velocity is larger than the laminar burning velocity the heat gain of the gas from the burner plate is larger than the heat loss of the flame. An adiabatic situation is found when there is no net heat loss to the burner, meaning heat loss and gain level each other off. In this case the laminar adiabatic burning velocity equals the inlet velocity of the gas mixture.
Practically however, it is difficult to adjust the gas flow to represent the exact velocity wherethe gas velocity equals the laminar burning velocity. This practical inconvenience is circumvented by interpolation of the gas velocity towards a zero heat flux.
The idea of this method is based on ideas of Botha and Spalding  who improved the burner of Powling .Originally, Botha and Spalding determined the burning velocity by measuring the heat loss necessary to stabilise a flame on a porous plug burner. They measured the heat loss of the flame to the burner by determining the temperature increase of the cooling water of the burner. However, the temperature increase of the water flow in this setup was rather small resulting in significant uncertainties . Additionally when the unburnt gas velocity gets close to the laminar burning velocity the flame becomes instable and will eventually blow off. To avoid these practical problems de Goey et al.  introduced the heat flux method. Van Maaren  analysed the methodology in detail. The next step was a significant improvement of the method by Bosschaart  who also performed a detailed analysis of the accuracy of the heat flux method. In the same time Evertsen  used the heat flux burner to measure species concentrations by Cavity Ringdown Spectroscopy and planar Laser induced fluorescence.
- J.P. Botha and D.B. Spalding, The laminar flame speeds of propane/air mixtures with heat extraction from the flame. Proc. R. Soc. Lon. ser-A 255(1160):71–95 (1954).
- J. Powling, A new burner method for the determination of low burning velocities and limits of inflammability, Fuel 28(2):25-28 (1949).
- K.J. Bosschaart, Analysis of the Heat Flux Method for Measuring Burning Velocities. Ph.D. thesis, Technische Universiteit Eindhoven, 2002.
Available at: http://alexandria.tue.nl/extra2/200513271.pdf
- L.P.H. de Goey, A. van Maaren, and R.M. Quax, Stabilization of adiabatic premixed laminar flames on a flat flame burner. Combust. Sci. Technol. 92:201–207 (1993).
- A. van Maaren, One-step chemical reaction parameters for premixed laminar flames. Ph.D. thesis, Technische Universiteit Eindhoven, 1994.
Available at: http://alexandria.tue.nl/extra3/proefschrift/PRF10A/9401473.pdf
- Rogier Evertsen, Cavity Ring-Down Spectroscopy in Combustion Environments, Ph.D. thesis, Katholieke Universiteit Nijmegen, 2002.
Available at: http://dare.ubn.kun.nl/bitstream/2066/19193/1/19193_pulscaris.pdf