Completely premixed combustion
Complete premixed combustion is developed on the basis of partially premixed combustion. It uniformly mixes gas and air in a certain proportion, and then sprays them through the burner fire hole for combustion. Due to the uniform pre-mixing, the combustible mixture can be burned in an instant as soon as it reaches the combustion zone. The combustion flame is extremely short and does not emit light, and is often invisible, so it is also called flameless combustion.
Due to the uniform premixing, fully premixed combustion can achieve complete combustion with a small excess air coefficient, so the combustion temperature can be very high.
The propagation speed of completely premixed flame is very fast, the flame stability is poor, and backfire easily occurs. In order to prevent backfire, the velocity field of the airflow must be made as uniform as possible to ensure that the airflow velocity at each point is greater than the flame propagation speed under the lowest load. The use of small fire holes increases the heat dissipation of the flame from the fire hole wall, thereby reducing the flame propagation speed, which is an effective measure to prevent backfire. Small flame hole burners are widely used in civilian burning appliances where the heat load is not very large. But for industrial burners with high thermal intensity, a large number of small fire holes will greatly increase the size of the burner head, making it inappropriate. Water can be used to cool the burner head to enhance heat dissipation from the flame, thus reducing the flame propagation speed.
Completely premixed combustion, because a large amount of air is premixed before combustion, the outlet velocity of the premixed airflow is greatly increased. When the load is large, there is also the possibility of misfiring. Industrial fully premixed burners often use an adjacent fire channel to stabilize the flame. When the evenly mixed gas-air mixture enters the fire channel through the fire hole, a high-temperature flue gas return zone is formed at the entrance of the fire channel due to the sudden expansion of the flow cross section. The fire channel is made of refractory materials and is similar to an adiabatic combustion chamber, where combustible gas can reach high combustion temperatures. The recirculating flue gas not only heats the mixture, but also acts as a stable ignition source. The recirculating high-temperature flue gas and the hot fire channel wall both play a very good role in stabilizing the flame.