Trip from USA to Sri Lanka, Layover in Shanghai

Dear Experts,

I'm studying at USA, But my nationality is Sri Lanakan. I booked a ticket for a round trip from USA to Sri Lanka. In my departing flight, I have a 7.30 hours layover and my return flight I have 15 hours layover in Shanghai. If I don't want to leave the airport, Do I need to get a transit visa?

Besides, Is there any chance that I can see the city while I will be in 15 hours layover ?
I will be thankful for a reply.