Thickness dependent variations in surface phosphor thermometry during transient combustion in an HCCI engine

Abstract

Phosphor thermometry is a semi-invasive measurement technique which is commonly used for temperature determination in combustion applications. Surface temperature measurements using thermographic phosphors rely on the assumption that the phosphor layer is thin enough in order to adopt the surface temperature non-intrusively. This work compares the temperature information, recorded from two opposing sides of phosphor films, coated on a translucent part of the combustion chamber wall inside a car engine. The film thickness was varied between 5 and 72 μm and two different phosphors were studied; CdWO₄ and La₂O₂S:Eu. For both phosphors, the results showed no significant differences. Being subject to unsteady heat transfer during fired engine operation, phosphor coatings above 20 μm in thickness experienced a significant temperature gradient between the front- and the backside of the coating, whereas thinner layers did not seem to be affected within the limits of measurement accuracy and precision. Cycle-to-cycle variations of the global gas temperature were not found to correlate with phosphor temperature variations. However, a strong temperature correlation between opposite sides of the phosphor coating was observed for film thicknesses below 20 μm during engine cycle-to-cycle variations.

Introduction

The temperature of combustion chamber walls is an important parameter for the generation and validation of predictive models describing heat transfer in internal combustion engines [1], [2], [3]. Amongst measurement techniques for surface temperatures, laser-induced phosphorescence has drawn increasing attention in the past few years, being a versatile alternative to conventional probing techniques such as thermocouples and pyrometry [4], [5], [6], [7], [8]. Thermographic phosphors allow remote temperature sensing with comparatively high temporal resolution by collecting the commonly red-shifted phosphorescence emission from an optically excited thin film that has been coated onto a target surface. Typical excitation sources are usually pulsed UV lasers, but also laser diodes and light emitting diodes have recently been employed as they continue to improve in terms of both price and output power [9], [10], [11]. In order to determine surface temperatures, the decay time or the relative spectral intensity distribution of the phosphor emission are most commonly exploited, the former one often regarded to allow a higher measurement precision [12], [13].

The phosphorescence emission is generally carrying the temperature information from the coated layer on top of the actual target surface, followed by the implicit assumption that the phosphor film is thin enough to adopt the temperature of the covered surface without perturbation. This assumption, however, is critical in thermal non-equilibrium situations as thermographic phosphors often are described as ceramic materials, which in turn are known to be thermal insulators. As for chemically similar thermal barrier coatings, Gentleman et al. identified temperature differences in the order of 100 K for a 140–170 μm thick surface film of YSZ [14].

In a previous study the authors have shown for the first time that temperature gradients can occur throughout a phosphor film, meaning that the phosphor temperature no longer corresponds to the wall surface temperature below. More precisely, this behavior has been observed for ⩾30 μm thick spray coatings of La₂O₂S:Eu which were applied inside an internal combustion engine [15]. Very recently, a theoretical study by Atakan et al. confirmed these measurement results by thoroughly investigating the optical cross-talk between spatial layers of the phosphor film and by performing a heat transfer analysis based upon our previous experimental results [16]. Another recent study used experimental surface temperatures from thin film thermocouples in an HCCI engine (Homogeneous Charge Compression Ignition) for modeling the influence of combustion chamber deposits (e.g. soot, hydrocarbons) on the wall temperature [17]. Similarly to thermographic phosphors these deposits can have a thermally insulating effect, and as they are globally present, they even influence the combustion process.

In this work a wider range of thicknesses from 5 μm up to 72 μm has been investigated experimentally to remedy a general lack of experimental data needed to quantify this effect and verify theoretical models. Especially, film thicknesses below 30 μm seem a worthwhile target for further investigations in order to study if-, and to which degree, accurate measurements can be obtained by thermographic phosphors in applications with rapidly changing spatial- and temporal temperature gradients. Another question to be addressed in this study is whether and how the temperatures from the front- and backside of phosphor coatings correlate with each other and with cycle-to-cycle variations of the global gas temperature for various film thicknesses. Finally, a second thermographic phosphor (CdWO₄), was investigated along with La₂O₂S:Eu in order to study whether or not temperature gradients for different film thicknesses have a phosphor-specific dependency.