Electromagnetic–thermal–mechanical coupling analysis of bent rotor straightening via electromagnetic induction heating

Rotors of steam turbines in power plants can be locally deformed by undesired situations, such as rubbing between the rotors and stationary parts. A straightening process is required to correct bending without causing additional damage because a rotor bending displacement of ∼0.15 mm can stop turbine unit operation. In this study, a numerical framework was established to simulate the straightening process using electromagnetic induction heating, which is straightforward and economical among the methods for straightening bent rotors. The straightening process involves complex coupling of electromagnetic, thermal, and mechanical phenomena. For efficiency, sequential coupling was used in the simulations, dividing the multiphysics phenomena into electromagnetic–thermal and thermal–mechanical fields. The temperature distributions resulting from electromagnetic induction heating were calculated through two-way coupling of the electromagnetic–thermal analysis. The thermal deformations of the rotors were obtained by solving the coupled equations for the thermal field obtained from the electromagnetic–thermal analysis and the mechanical field. Using the established numerical framework, the thermal–mechanical behaviors and straightening mechanisms of bent rotors were investigated. Furthermore, the effects of process parameters, including the direction of gravity and heating and cooling conditions, on the straightening performance were determined. Appropriate parameters were identified to achieve the desired straightening performance with final bending displacements of <0.1 mm for bent rotors with initial bending displacements of 0.15–0.3 mm. For a rotor made of A182 F11 Class 2, the best straightening performance was obtained by heating the rotor to a maximum temperature of 650 °C for 20 h under insulation, followed by natural cooling. The simulation results revealed that the straightening performance can be improved when the rotor is rapidly heated to a high maximum temperature and cooled immediately, as long as the temperature conditions do not cause phase transformation or unintended plastic deformation of the bent rotors.