Signal Reconstruction of Two-Color Pyrometry Technique Using CFD and a Detailed Spectral Radiation Model in a Marine Diesel Engine Setup

Optical soot pyrometry is a mature experimental technique that has been applied to a broad range of combustion systems for measuring soot temperature and concentration. Even though the method is widely used and well documented, the line of sight nature of the technique makes the interpretation of its results challenging. Notably, gradients in temperature and soot concentration along the line of sight or across the field of view can introduce significant levels of uncertainty in the results. This paper presents a numerical study where the signal from the experimental two-color pyrometry technique in a marine diesel engine reference experiment is reconstructed employing computational fluid dynamics (CFD) and a detailed Line-by-Line (LBL) spectral radiation model. The analysis is aimed at qualitatively supporting interpretability of experimental observations. Further, the role of soot and temperature gradients along and normal to the line of sight are assessed in two canonical configurations in which the signals are reconstructed numerically. In addition, the impact of different optical thicknesses is analyzed parametrically. The results obtained with the numerical pyrometry showed that gradients in temperature along the line of sight significantly affect the pyrometry temperature, leading to an overestimation of the effective soot averaged temperature in the range of 50 K to 250 K (i.e. 2-12%), depending on the soot concentration. Further, a saturation effect in the pyrometry KL-factor along the line of sight was observed at higher soot concentrations, resulting in an underestimation of the average soot volume by a factor of four to six in the canonical configurations. Gradients across the field of view were found to have a lower impact on the pyrometry results and to introduce much smaller biases than gradients along the line of sight.