There is airport shuttle bus from Beijing Capital Airport to Tianjin, which takes only 2 hours and 45 minutes. This is the most convenient way. If you want to take train from Beijing to Tianjin, you need to get to the Beijign South Railway Station. The airport has no bus or metro to this Railway Station. So, if your time is just enough, firstly take the Airport Express to the terminal Dongzhimen and then transfer to city-bus of No.106 to the South Railway Station directly. Or, you could take the Airport shuttle of Line 3 to Beijing Railway Station where the buses of 20, 122, 692 running to South Station. This one takes a little longer time but less money (about 10yuan).

If your train arrives Tianjin Railway Station, you could take the No.8 and 832 buses and get off at Qilitai or NO.59 and get off at Xihucun where the Tianjin University is just located.