Itaru Murakami

A Correlation Between Vibration Stresses and Flow Features of Steam Turbine Long Blades in Low Load Conditions

The vibration stress of long steam turbine blades during low load operating conditions is examined in this paper. A series of experiments has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressure fluctuation. It is found that a steady pressure distribution over the blade tip is much to do with the unsteady pressure and fluctuation of the vibration stress. A precise investigation of unsteady wall pressure near blade tip explains the relationship between pressure fluctuation and the vibration stress, and reveals the existence of particular frequency which affects blade axial modes. Blade to blade flow mechanisms and aerodynamic force and properties during low load operating condition were investigated by a steady CFD simulation. FFT of aerodynamic force by another steady full arc CFD simulation provides various pattern of harmonic excitation which account for the behavior of vibration stresses well. The mechanism of the rapid stress increase and a step drop were examined by considering CFD results and measured unsteady pressure data together.Copyright

An Experimental Investigation of the Influence of Flash-Back Flow on Last Three Stages of Low Pressure Steam Turbines

A shutdown operation of a large size steam turbine could possibly cause flashing phenomena of the pooled drain water in low-pressure heaters. The boiled steam is sometimes in the same amount as the main flow in the case where shutdown is executed during low load conditions, and returns to the steam flow path through the extraction lines. A series of experimental work with a subscale model turbine facility has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressures under the flashing back (FB) conditions. It was observed that the blades of the two stages before the last stage (L-2) and a stage before the last stage (L-1) presented their peak vibration stresses immediately after the flash-back flow reached the turbine. In the meantime, the vibration stresses of the last stage (L-0) blades were reduced for a few tens of seconds. It can be thought that the flash-back flow pushed out the reverse flow region around the L-0 blades and allow the blades to be more stable. A detailed examination with measured data of the L-2 blade explained that, as long as the flash-back flow has small wetness, the blade is excited in its specific vibration modes in larger than eighth harmonic of rotational speed, but once the flash-back flow carries water droplets, the fluid force in random frequencies remarkably increases and excites the blade in less than seventh harmonic range.

Development of 60Hz Titanium 48-Inch Last Stage Blade for Steam Turbine

The titanium 48-inch last stage blade that has world’s largest class exhaust annulus area and tip speed for 60Hz steam turbines has been developed. Concept of this blade is to achieve high performance and compact design of steam turbine for 1000MW thermal power plant and 300MW combined cycle plant. In the design of this blade, the optimization design has been done by using the recent analysis technologies, three dimensional CFD in aerodynamic design and FEA in mechanical design. The blade has curved axial fir-tree dovetail, snubber cover both at the tip and at the mid-span. To achieve superior vibration characteristics, continuously coupled structure was adopted for blade connection. To confirm the validity of design, first, sub-scale model blades were provided and tested in model steam turbine test facilities. Second, one row of actual size blades were assembled on the wheel of test rotor and were exposed rotating vibration test in a wheel box. Finally, these blades were tested at actual steam conditions in a full scale steam turbine test facility. In this paper, aerodynamic and mechanical design features will be introduced, and the test results of both sub-scale and actual size blades under real steam turbine operating conditions will be presented.Copyright

Analysis of Multifactor Damage for Heat Resistant Alloy. II. Multifactor Damage Simulation Analysis of High Temperature Low Cycle Fatigue for Superalloy Based on the Statistical Information of Crack Distribution.

Actual components used under severe conditions often suffer from the complicated material damage affected by multiple influencing factors. “Multifactor damage simulation analysis” was proposed to solve such complex damage problems. In this paper, a discrete cluster model of material and the associated damage state matrix [DN] are introduced to solve a high temperature low cycle fatigue damage problem of Co-base superalloy FSX414.The damage state at Nth cycle is determined by the damage state at (N-1)th cycle expressed as follows.[DN]=[Λ][DN-1]As this equation is similar to the discrete dynamical system and operator matrix [Λ] is non-linear, the chaotic behavior, of damage evolution may occur. Instead of solving the matrix equation directly, simulation was conducted using 2-dimensional model of 2500 clusters at 5×5mm area for the dendrite and grain boundary structure of FSX414. Damage of crack initiation and growth was calculated deterministically and the randomness was only introduced at the initial condition of the material.High temperature low cycle fatigue test was conducted to follow up the damage process at the total strain range of 1% and at the temperature of 1123K. Surface crack morphologies were investigated by the replication technique at 20% and 40% of the failure life.The trends of crack numbers, maximum crack length and mean crack length against fatigue cycles were obtained by the simulation. These trends were complicated like chaos due to the interaction of cracks and material structure. The crack length distribution by the simulations agreed well with the experimental results and better agreement was obtained by using intermittent inspection informations. These results suggested that the method would be effective for damage prediction of actual components based on inspection informations.