炽 is mostly literary; in modern use it appears in compounds like 炽热 (chìrè, 'red-hot'), 炽烈 (chìliè, 'blazing'), 白炽灯 (báichìdēng, 'incandescent lamp'). On its own it's poetic.
memory
The 火 (fire) radical on the left tells you everything: 炽 means fire so intense it's almost showy — 'splendid' and 'illustrious' come from that same blaze metaphor.
