Application of LED-based thermographic phosphorescent technique to diesel combustion chamber walls in a pre-burn-type optical constant-volume vessel

Abstract

This study presents a method for direct measurement of wall surface temperature realising two-dimensional, thermographic phosphorescence imaging. The experiments were performed in a constant-volume combustion chamber (CVCC) wherein the ambient gas and fuel injection conditions closely mimic those found in a typical diesel engine. The new method successfully made use of a high-power light emitting diode (LED) system and intensified charge-coupled device (ICCD) camera—which offers practical advantages such as much lower cost and reduced safety concerns in comparison to laser-based phosphorescence diagnostics. Previous studies have successfully demonstrated measurement strategies using LEDs (Atakan et al., Exp Measur Strat. https://doi.org/10.1088/0957-0233/20/7/075304 , 2009; Salem et al., Exp Fluids 49:797–807, 2010); however, most have implemented a lifetime-based approach. Aizawa and Kosaka (Int J Fuels Lubr 1:549–558, 2008) demonstrated the applicability of the absolute intensity method as a viable alternative, using a laser source for excitation. This paper combines Aizawa and Kosaka’s intensity-ratio methodology with LED excitation, and implements the technique to a diesel flame/wall interaction case in a pre-burn-type optical CVCC. Europium-doped lanthanum oxysulphide (La2O2S:Eu) was selected as a suitable phosphor material due to its high temperature sensitivity at this engine relevant condition. The present study first investigates how phosphorescence signals are influenced by the coating thickness. The thickness of the phosphor-binder layer was measured by a scanning electron microscope (SEM) in the 11.9–19.0 µm range. The optimal layer thickness was chosen as 14.3 µm, the thinnest possible layer capable of achieving the highest emission signal, as thicker coatings are more prone to thermal gradients—especially at a transient flame/wall interaction condition. Calibration images were obtained in the 294–523 K range with ± 2 K uncertainty in an electric furnace, thereby allowing two-dimensional temperature measurements in the CVCC application. The results obtained for various timings after the start of fuel injection, namely before and after the flame impingement on the wall, indicate effective wall temperature measurements with low spatial (pixel-by-pixel) and shot-to-shot variations of < 7.5 K (95% confidence).