to make a fire, blaze
燀 centers on creating a strong, bright fire: lighting a blaze or making a fire burn intensely.
燀 combines 火 (fire) with 單, which likely contributes the sound and may relate to the idea of spreading or extending, as a fire spreads into a blaze.
The left side is fire 火, and the right side 單 looks like a complex structure with many openings. Picture a fire spreading through a large, multi-chambered furnace, creating a roaring blaze.
For セン, imagine a single spark (セン sounds like 'sen' in 'sensation') igniting a huge blaze: the spark sensation cues セン, and the fire 火 makes it a blaze.