Mixing with Primary and Secondary Jets of Air

Regardless of the velocity of secondary air, flow rate, or the angle at which air is injected into the fire, secondary air tends to lower the temperature of gases. Researchers have found that injecting secondary air into the side of the flame in a Rocket stove results in most effective mixing.*

The Jet-Flame, on the other hand, blows primary air jets up into the bed of made charcoal below the burning sticks of wood, creating a “mini blast furnace.” The jets of primary air increase the temperature in the charcoal, frequently resulting in higher temperatures in the combustion chamber. The mixing function is up into the fire, not into the side as with secondary air jets.

Boman et al., 2005 report that temperatures of 850C or above are needed for close to complete combustion in short residence times, as in a cookstove. Since excess air lowers temperatures, using the minimal volume of air in secondary air jets to achieve thorough mixing seems preferable. Researchers have recommended that the jets should penetrate into the middle of the flame but not enter into each other. (*Lefebvre and Ballal, 2010; Udesen, 2019; Vanormelingen and Van den Bulck, 1999).

Unfortunately, raising the temperature of pre-heated secondary air by a lot more than ~ 100C seems to be difficult. Cookstove combustion chambers are usually small, limiting the area exposed to high temperatures. The heat transfer efficiency is much lower from degraded temperatures further from flame.

 Residence time and temperature are easily measured. However, “thorough mixing” has not been defined and is not yet measured in our experiments. We infer that the woodgas/air/flame was thoroughly mixed when the emissions of PM2.5 and CO are close to zero as measured with the LEMS emissions hood.