@OrbiSana I think the giant top one is "kyun." It loosely translates to "heart stopping" and it means Hiura felt strong emotions. Granted they literally wrote "gyuuuuun" but I think that is just to give an exaggerated effect.
The smaller bottom one is "shun" and it means "steam escaping" ala a teapot. The joke is that Midou is so hot he fried Hiura's brain.
@mirewitch "gyu" mean something squeezing or being squeezed, the sfx is similar to kyu too. "Shun" in this case I think it means more like he hung his head or dropped his shoulders out of the embarassment.
@MavB_Ver I see your point on gyu making more sense, i.e. Hiura's heart is being squeezed. But regarding shun there's metaphorical steam rising from Hiura in the panel. I agree Hiura's embarrassed though.
@MavB_Ver Huh. Fair enough - I hadn't seen shun used like that before, but that secondary definition does fit this situation really well, and as a non-native speaker I won't argue with the dictionary lol.