Posted on December 5, 2012

Written by John Allport; Talent Development Leader – Asia and Martyn Jupp; Applied Mechanics Development Group Leader

With special thanks to A.Pezouvanis, G.W.Janicki, A.I. Zdunek, A.J.Day, P.Olley, B.Mason and M.K.Ebrahimi, School of Engineering, Design and Technology, University of Bradford

In the turbocharger industry, vibration not only has a direct impact on durability and warranty costs, but also plays an important role in the end user’s perception of vehicle quality. That is why an accurate measurement of blade dynamics is necessary when designing a turbine wheel.

Although strain gauging has been used for this purpose for many years, it has limitations and so an alternative method has been investigated.

Cummins Turbo Technologies and the University of Bradford have collaborated on the Advanced Turbocharger Technology Engineering Project (ATTEP). In this project, a tip-timing method which has been proven in the aero industry but has seen only limited application on small turbines as used in turbochargers, was combined with recently developed finite element simulation techniques to provide a means of assessing the susceptibility of turbine blades to High Cycle Fatigue damage.

The tip-timing method uses a set of optical probes that measure the blades arrival times and uses the time differences between expected and actual arrival times to calculate blade deflections within a time domain. In comparison to the existing strain gauge based testing procedures, the tip-timing represents a non-invasive approach, which is more likely to record real-life turbocharger operation parameters. What is more, tip-timing can provide complete information about the total rotary system, returning a great amount of data for further system analysis, rather than the limited data related to a single blade obtained through the more conventional strain gauging approach.

The resulting findings can then be used to optimise the design of turbine wheels, making them more resistant to mechanical loads and their corresponding vibrations. Ultimately, more robust, durable and reliable turbochargers could be produced as a result of this work.

Tip-timing method
The tip-timing method used involves a very accurate measurement of the times at which the blades pass by a series of unequally spaced sensors. In this case, the sensor set consists of a series of laser probes mounted in the turbine housing, which reflect a beam of light off the blade as it passes. If the blade is rotating at a constant rate, a particular blade should pass each sensor at a predictable time.

Deflection due to vibration causes the blade to pass the sensor either earlier or later than predicted, hence the difference of these times can be used to calculate the deflection from the initial position for each individual blade. By using a powerful laser with a small point of focus and a very fast data acquisition card, the system is able to detect vibrations of less than 0.2 μm at the working speed of the turbocharger as shown in figure 1.

For synchronous vibration, the order of interest is defined and the software tries to match the vibration measured to that order and gives the amplitude and level related to the matched order. When there is simultaneous vibration at different frequencies (different modes), the system is still able to recognise and separate modes, provided the appropriate number of sensors are used.

By increasing the number of sensors, several modes of vibration can be detected simultaneously, with 2N+2 sensors required to analyse N modes. In this case, eight sensors were used to analyse three modes.

Tip-timing software with output data examples
One of the biggest advantages of the tip-timing system is that it collects data from all the blades simultaneously, so this gives the opportunity to compare their vibration, which was not possible previously. Specific orders and frequencies can be tracked and data can be plotted in a variety of different ways, allowing a more detailed understanding of the underlying physics to be gained.

Synchronous vibration analysis is based on Least Squares Model Fitting (LSMF). The LSMF curve fits sine waves to the data, so the first thing to determine is the order of vibration which is of interest for a given sample. The software checks which order best fits the sample and displays the data in the form of a bar chart (Figure 2). The highest bar is the most probable order of vibration measured in the given data sample.

Fig 2 – Chart of model to test data fit

Comparison of laser probes with other sensors
There are a number of advantages of the optical sensors which are used in the tip-timing method relative to the traditional method using strain gauges. There is always some question about measurements where the sensor needs to be attached to the component being measured – is the measurement affected by the act of measuring?

The use of strain gauges requires the rotor speed to be held at the point of resonance so that a reliable measurement can be taken. It is therefore characterised by poor repeatability as it is difficult to maintain all parameters constant simultaneously.

The best type of proximity sensor currently available for this application is a reflective laser sensor. These are characterized by very high working speed, a small focus point that further improves their accuracy and resistance to extremely high temperatures. They are also immune to any kind of electromagnetic interference. Moreover, these sensors require very little space for installation in the housing since they are smaller than most other sensors. Figure 3 shows the relationship between the results from the strain gauge and tip timing methods. 

Simulation and test results correlation
In order to minimise possible variation caused by model simplifying assumptions such as cyclic symmetry, the CAD turbine models represented a complete wheel, rather than an individual blade. This approach was believed to produce more complete results, providing information about the inter-coupling effects of the blades and giving a more realistic frequency distribution than is obtained using single blade models.

At the same time, modal analysis returned calculated natural frequencies of the wheel, together with their complex mode shape visualisations. It may be verified that the maximum number of natural frequencies for an object is the number of FE Nodes.

Nevertheless, due to the characteristics of the turbine forcing function, the effective vibrations diminish with the increasing frequency, or may even be impossible to achieve. That is why the number of natural frequencies that were investigated has been limited to the fifth mode of vibration.

The analysis of the simulation results involved a classification of the frequencies, based on their value, nodal diameters and mode shapes. Validation of the results proved that the new method is capable of a successful prediction of modes 1, 2 and 3 resonant frequencies. The demonstrated accuracy for the new methodology on the initial limited dataset was found to be within the accepted range.

Summary
After two years of research, some significant improvements have been made. They include the application of tip-timing method and advanced simulation techniques for the prediction of vibration of turbine blades.

Knowledge about turbine vibrations has been extended, enabling the fulfilment of the project’s objectives. The effects of various turbocharger operation parameters have been investigated, by both simulation and tests.

With Cummins Turbo Technologies’ new, state of the art laser tip timing approach, the potential failure mode of one of the major causes of turbocharger failure, High Cycle Fatigue, is more easily detected. The additional detail provided by this method allows validation of changes to be made with measurement precision far exceeding that of strain gauging. As such, Holset turbocharger customers are assured that their products have been optimized using the most up to date techniques of turbine vibration reduction.

As a result of this collaborative research, a new method for turbine design has been formulated and validated. The analysis of validation results showed a good match between modeled and experimental data. The aims and objectives of this project have therefore been exceeded. This confirms Cummins Turbo Technologies commitment to innovation and ground breaking reliable technologies.

This article is a summary of a technical paper delivered at the IMechE 10th International Conference on Turbochargers and Turbocharging. The full paper can be accessed at www.cummins.com/turbos