Comparing Global Spray Combustion Characteristics and Local Shot-to-Shot Variations in a Reacting n-Heptane Spray

Abstract

In support of efforts to develop improved models of turbulent spray behavior and combustion in diesel engines, experimental data and analysis must be obtained for guidance and validation. For RANS-based CFD modeling approaches, representative ensemble average experimental results are important. For high-fidelity models such as LES-based CFD simulations, precise individual experimental results are desirable. However, making comparisons between a given experiment and LES simulation is a challenge since local parameters cannot be directly compared. In this work, an optically accessible constant pressure flow rig (CPFR) is utilized to acquire diesel-like fuel injection and reaction behavior simultaneously with three optical diagnostic techniques: rainbow schlieren deflectometry (RSD), OH* chemiluminescence (OH*), and two-color pyrometry (2CP). The CPFR allows a large number of repeated injection experiments to be performed for statistical analysis and convergence using ensemble-averaging techniques, while maintaining highly repeatable test conditions. Even for stable test conditions, variations in local turbulent fuel-air mixing introduce variability which manifests as significant differences in OH* and 2CP results. Experimental measurements of characteristic parameters including liquid and vapor jet penetration, lift-off length, soot temperature and concentration, and turbulent flame speed, along with the shot-to-shot variability of each data set, are presented and discussed. A statistical method is utilized to analyze the extent of this variability, and to identify superlative injections within the data set for discussion and analysis of shot-to-shot variations.

Robust two-colour pyrometry uncertainty analysis to acquire spatially-resolved measurements

“…The methodology presented in this work was applied to a fuel spray experiment conducted in a CPFR operated at elevated pressures and temperatures to achieve autoignition and combustion of n-heptane fuel within a few milliseconds. The CPFR has been detailed in [4,11] but in brief, preheated compressed air flows through a diffuser/flow conditioner and then into the chamber. Fuel is injected into the chamber from a single 104 µm axial orifice.…”

“…First, measured radiances are substituted into equation ( 1) and solved for temperature, which yields the apparent temperature at each wavelength (T a λ ). Uncertainty in radiance is therefore propagated to uncertainty in apparent temperature by equation (11). The uncertainties in apparent temperatures are propagated to uncertainty in temperature (equation ( 12)) and then to uncertainty in KL (equation ( 13)).…”

“…While some values of T and KL are highly uncertain, the presence of soot incandescence data in these regions could still be used to determine other parameters such as soot lift-off length or flame boundaries [11]. In this study, rejection criteria for uncertain measurements of T and KL were established.…”

“…The 2CP diagnostic benefits from a relatively simple optical configuration and requires a single optical access point, making it practical for implementation in ICE relevant combustion experiments with limited optical access (e.g. [8][9][10][11]). The 2CP technique has been used for several decades as an optical technique to measure temperature and soot concentration in flames, and it is based on radiant emissions from the soot particles in the flame detected simultaneously at two different wavelengths, and then the two signals are correlated to temperature (T) and optical density (KL).…”

Representative Phenomena of Cyclic Turbulent Combustion in High-Pressure Fuel Sprays

“…Hundreds of transient spray combustion experiments are performed in quick succession in a high-pressure, high-temperature environment, and fuel injection, mixing, and combustion processes are probed at high spatial and temporal resolutions simultaneously by three optical diagnostics, each focused on different aspects of the jet behavior. The diagnostics include Rainbow Schlieren Deflectometry (RSD) to visualize the early, noncombusting jet Reggeti et al (2021); Parker et al (2021), OH*CL to detect ignition and reaction zones Blocquet et al (2013), and 2CP to quantify produced soot mass Xuan et al (2019); Reggeti et al (2021). The data-set is of sufficient size to accurately discern injection-to-injection variations Donkerbroek et al (2011); Xuan et al (2019); Reggeti et al (2021); Parker et al (2021); Li et al (2017).…”