A comparison of the heat transfer behaviors of biogas–H2 diffusion and premixed flames

• Stability of biogas premixed flame is improved by hydrogen addition.
• Temperature of biogas premixed flame is enhanced by hydrogen addition.
• Hydrogen addition also leads to higher burning speed of the fuel mixture.
• Hydrogen addition causes higher heating rate for premixed combustion.
• Hydrogen addition causes lower heating rate for diffusion combustion.

An experimental study was conducted to investigate the influence of hydrogen addition on the heat transfer characteristics of a biogas (60%CH4–40%CO2) flame. Results show improved flame stability and higher flame temperature in the premixed flame upon hydrogen addition. Both temperature and burning speed are increased in 1.0 ≤ Ф ≤ 1.5. Comparison of the premixed and diffusion flames reveals that the former yields higher heat transfer than the latter, due to higher flame temperature and larger volume of hot gas in the premixed flame. The total heat transfer rates of the two flames show opposite trends with increasing level of hydrogen addition, which is explained by the different structures. In the premixed flame, the contact of large cool core with target plate configures the high-temperature flame zone to a radial location with larger distance from the stagnation point than that of the diffusion flame, contributing to its higher heat transfer rate.