Blade-Strength Assessment of a Marine Turbocharger under Development

In response to the requirement for higher output of diesel engines in recent years, IHI has recognized that a turbocharger with higher pressure-ratio and volume-flow rate will be required, and has commenced the development of such a turbocharger. An important consideration in the design is the relatively high failure rate of blades presently used in the market. The new turbocharger will need to rotate at a faster speed than similarly-sized turbochargers of the past. Therefore the turbine and compressor blades will be subjected to excessive centrifugal force and it is forecast that strength problems will become increasingly severe. Under such severe operating conditions, damage to the turbine blades and compressor blades is possible as a result of prolonged resonance due to the impact of nozzle wake and pressure distribution. The design life of a loaded blade may be predicted from the centrifugal force and resonance stress to which it is subjected. Centrifugal force may be calculated using a finite element model. However, the prediction of resonance is unreliable as the value of the stimulus (refer to Eq. (1)) of the blade at resonance is not necessarily clear. The stimulus value becomes more difficult to measure as the rotor speed becomes higher due to damage to the sensor attached to the rotating body and problems related to axial vibration. Therefore, only a small amount of data has been obtained in the past. In this report, the turbocharger under development was tested to determine the blade vibration and logarithmic decrement during resonance of the turbine and compressor blades, as well as, to estimate the order of the stimulus value and to subject these values to review. DISCRIPTION OF THE TURBOCHARGER Structure of the Turbocharger Figure 1 shows the cross-section of the turbocharger that is under development. The rotor is comprised of a turbine and a compressor. The load in the axial direction is supported by a thrust bearing and the load in the radial direction is supported by two journal bearings (floating bush). The distance between the journal bearings is 65 mm and the length of the shaft is 294.5 mm. The shape of the floating bush is formed from triple arcs on the inner circumference. A feature of this structure is greater stability in terms of vibration compared to a structure employing a true circle. The compressor wheel is a blisk type and is comprised of 8 main blades and 8 splitter blades for a total of 16 blades that have been formed by cutting aluminum. The maximum diameter of the wheel is 130.8 mm. Moreover, 17 diffusers are equipped at the discharge of the compressor wheel. The maximum design pressure ratio of the compressor is 4.0 and the maximum rotational speed of the turbocharger is 540 m/s.