Two-Colour Pyrometry Measurements of Low-Temperature Combustion using Borescopic Imaging

Abstract

Low temperature combustion (LTC) of diesel fuel offers a path to low engine emissions of nitrogen oxides (NOx) and particulate matter (PM), especially at low loads. Borescopic optical imaging offers insight into key aspects of the combustion process without significantly disrupting the engine geometry. To assess LTC combustion, two-colour pyrometry can be used to quantify local temperatures and soot concentrations (KL factor). High sensitivity photo-multiplier tubes (PMTs) can resolve natural luminosity down to low temperatures with adequate signal-to-noise ratios. In this work the authors present the calibration and implementation of a borescope-based system for evaluating low luminosity LTC using spatially resolved visible flame imaging and high-sensitivity PMT data to quantify the luminous-area average temperature and soot concentration for temperatures from 1350-2600 K. The visible flame area is used to adjust the PMT measurements to account for sooting area (i.e., the fraction of the region of interest that contains discernible flame/natural luminosity). The validity of the approach is assessed using spatially resolved temperature and soot concentration data collected from a full optical single-cylinder engine operating using non-premixed natural gas combustion, with soot concentrations similar to those seen in the LTC data. The high sensitivity and low noise of the PMTs, combined with the broad field of view of the borescope, and the proposed sooting area correction method, provide robust measurement of flame temperature and KL factor in low temperature and low-sooting combustion conditions. Whilst the sooting area correction is found to have only a small effect on the measured temperature, it is shown to be important to provide soot concentrations that are representative of spatially resolved results. This work provides new insights into the use of relatively low-cost, high sensitivity PMTs to assess advanced and low-emission combustion systems.