Numerical and Experimental Investigations of the Transient Thermal Behavior of a Steam Bypass Valve at Steam Temperatures Beyond 700 °C

Increasing the efficiency of steam cycle power plants is extremely important for the reduction of their CO2 emissions. Today’s best steam cycle power plants have a net plant efficiency of 46 %. Since the worldwide average efficiency is still in the range of 30 %, there exists a great potential in reduction of CO2 emissions by replacing old power stations with new ones. A further great potential lies in achieving even higher efficiencies by increasing live steam temperatures to more than 700 °C, so that the efficiency of steam power plants is pushed over the 50 % mark. Within a research project funded by the German government the challenges associated with material’s behaviour under elevated temperatures are investigated. In this project, a bypass-valve was installed in an experimental set-up in a real power station and is supplied with over 700 °C steam and investigated under long-term cyclic operation. Thermocouple measurements on reference points on the valve body and thermo graphic camera measurements deliver information about the real transient thermal behaviour of the valve. Numerical investigations aim to accurately model the transient thermal behaviour of the valve during cyclic operation and calculate corresponding three-dimensional temperature distributions, which are essential for conducting reliable mechanical integrity analysis for the applied Nickel-base material. Applying standard FEM thermal analyses that are based on heat transfer boundary conditions is often related with uncertainties regarding the convective heat transfer and corresponding coefficients. The application of a hybrid stepwise frozen conjugate heat transfer calculation approach aims to make use of the advantage of the conjugate heat transfer approach with respect to high accuracy in heat transfer calculation and reduce the calculation effort by freezing the fluid domain at different steps along the loading cycle and coupling it to the transient thermal load calculation in the solid domain. Both the standard FEM thermal analysis method and the hybrid stepwise frozen conjugate heat transfer calculation approach have been applied to calculate the transient thermal load in the valve. A validation of the numerical results has been performed for the reference points on the valve body and shows that the hybrid approach has better accuracy than the standard approach and shows very good agreement with the experimental results.