Transient thermal stress FE-analysis method development

Abstrakti

This research paper addresses the challenges of thermomechanically loaded components in four-stroke medium-speed engines, focusing on exhaust pipe failures due to low-cycle thermal fatigue. Wärtsilä's shift towards 100 % renewable energy has altered engine operating conditions, leading to new challenges in exhaust components subjected to fluctuating thermal conditions. The study focuses on the transient method's ability to detect phenomena during heating and cooling in stress and temperature histories, optimizing the transient analysis definitions and providing some principles for design modifications in thermal stress problems. A case study of nodular cast iron exhaust manifold is used as an example. Traditional methods using cyclic steady-state temperatures have been found insufficient, prompting the development of a more accurate transient method that uses measured temperatures during the engine's thermal cycle. The paper compares conventional steady state heat transfer analysis and two transient heat transfer analyses for defining thermal boundary conditions. The temperatures in the first transient analysis are defined accurately from the measurements, leading to more realistic results and long calculation time. The second transient analysis is improved to offer a balanced method between accurate thermal boundary definitions and shorter calculation time. The transient method reveals higher stress amplitudes in previously low-stress zones, identifying the actual critical points on the exhaust pipe.