Abstract:

Rotor thermal bow is one common cause of high vibration of rotating turbo machinery during hot restart. After shutdown of gas turbine, the rotor in standstill condition will exchange heat with the surrounding gas through natural convection. Due to the asymmetrical cooling efficiency between the upper and lower surfaces of the rotor disks, the thermal gradient across the shaft section will develop and result in rotor thermal bow. The temperature distribution and thermal difference of the shaft section as well as the thermal bow of the rotor in natural convection cooling during standstill are calculated. The results show that both the temperature differences of shaft section and rotor thermal bow will gradually increase up to the maximum values and then decrease as time increases. However, their maximums occur at different times. If the standing time is long enough, the temperature differences will close to zero and the thermal bending disappear. Thermal bow induced vibration response of the rotor restarted at different standing time is obtained by solving the equation of motion of the thermal bending rotor. And the permitted standing time to directly restarting the thermal bending rotor is suggested which can lead to acceptable vibration of the rotor with no extra turning. Also is confirmed the effectiveness of low speed turning for eliminating the rotor thermal bending when the rotor is restarted with the maximum thermal bending. This result shows practical significance as reference for the normal startup and shutdown of the gas turbine rotor as well as its safe operation.

Key words: natural convection heat transfer, thermal bending, turning, gas turbine