I need to know the temperature of a fluid passing through a pipe, but the pipe has not a thermowell. Are there empirical equations to estimate the internal temperature of the fluid by measuring the surface temperature of the pipe?
Zoubair has given a very relevant document. However, with high quality thermocouples, care in insulating the walls in the region of the thermocouple and extending it on either side, please remember that it only gives you the local fluid temperature at the wall, not the mixed mean temperature.