If you want to know grown - up's name , you should change 이름 to 성합 or 존함 this is more respectful express.

We can say in many different way to say "What's your name?" but the most formal expression is "이름이 뭐예요?" That's not kind of humble expression but more casual :) '이름' which means "name" '이' is like postpositional word. I think that's not easily understandable for foreigners. Korean has more several postpositional word like '이'. They are '는', '가', '를' etc.. It's little bit hard to explaining about it but simply it's almost same like 'is' or 'are' of English. '뭐예요?' = 'What?'

"이름이 뭐에요?" (Yi-Reum-Yi Mueo-E-Yo?)