Correction of Optical Diagnostics for High Optical Density
July 20, 2007
The Research and Technology Directorate Forum was a series of three presentations given by faculty fellows about the research they have been doing this summer. Dr. Mike Cheung gave an interesting talk about the use of optical diagnostics to test engine injectors, and also about the filtering of data he has done to correct for optical diffusion and get more accurate results.
Some types of optical diagnostics include planar Mie scattering (PMIE), planar laser induced fluorescence (PLIF), and chemiluminescence. The PMIE technique for example involves looking at burning fuel droplets and measuring the particle location and size. These techniques are useful for evaluating injector designs and refining simulation code. An important standard simulation is defined by the National Combustion Code (NCC).
However, it is difficult to make optical measurements in a combustion injection system due to the large amount of scattering and absorption of light for measurement, such that both the input and output signals are attenuated. The scattering and absorption are modeled and analyzed using Beer’s Law, which describes the dissipation of light due to the medium’s opacity using a decaying exponential weighting factor.
Dr. Cheung’s software processing uses Beer’s Law and some symmetry assumptions to look at the optical diagnostic data and add correction for the scattering and absorption that occurs. He says the next steps in the field is to add more automation in carrying out these corrections and analyzing the large amounts of data, and is also looking into implementing a multidimensional fit of the data. As a longer term goal, he hopes research in this area can be used to “close the loop” and refine the NCC for better simulation predictions.