Corporan, E., DeWitt, M., and Klingshirn, C., "Evaluation of Downstream Dilution Techniques for the Sampling of Turbine Engine Particulate Matter," SAE Technical Paper 2013-01-2130, 2013, doi:10.4271/2013-01-2130.
Measurement of turbine engine particulate matter (PM) requires diligent handling of the sample to maintain integrity and minimize any alteration due to sampling or transport artifacts. PM sample dilution at the probe tip is a common and widely used technique to condition the sample in order to reduce PM losses and potentially “freeze” chemical reactions that may occur throughout the sampling train to the instruments. Diluting the PM sample at a location downstream in the sample line is preferred by engine manufacturers as probes used for gas emissions can be used for PM; however, implications on PM characteristics and comparisons against probe-tip dilution are unknown. The present study compares the characteristics of turbine engine PM diluted at the probe tip and at a location downstream in the sampling train. Downstream dilution was accomplished by injecting nitrogen through a commercial ejector (operated both as a diluter and pump) and a through simple concentric tube arrangement. The tests were conducted using the exhaust of a T63 turboshaft engine as the source of PM. Particle number and size distribution data were collected for a wide range of dilution ratios and sample residence times for the different dilution schemes. Results show that significant particle loss and change in mean particle diameters occur for the downstream dilution relative to probe-tip dilution under specific conditions. It was observed that very good agreement between tip and downstream dilution techniques can be obtained with the ejector; however, this agreement is dependent on engine conditions and is highly influenced by the residence time of the raw and diluted samples in the sample lines. Correcting for particle losses based on particle size, flow and sampling system characteristics may be necessary to improve the accuracy of the measurement for both dilution techniques. In summary, it has been demonstrated that downstream dilution of turbine engine PM is a viable technique that can provide a sample with “probe-tip dilution” characteristics; however, care must be taken to ensure proper sampling conditions. Details on the impact of engine condition, sample residence times and dilution ratio on particle characteristics are discussed.