In a
premix burner, sometimes provision is made to screen the observed portion of the flame gases from
direct contact with the surrounding air. This may be done either mechanically by placing
a tube on top of the burner around the flame, which produces a zonal separation (separated
flame), or aerodynamically by surrounding the flame with a sheath of
inert gas that emerges from openings at the rim of the burner top (shielded flame).
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