Quantifying Statistical Measures of Diesel Spray Soot Characteristics using Laser-Induced Incandescence

Increased understanding is needed with respect to soot formation from high pressure diesel sprays as emissions standards become increasingly stringent and require complex methods for its reduction. Understanding soot formation and its spatial distributions is necessary to advance fundamental spray combustion knowledge. Given that the underlying nature of high pressure combusting diesel sprays results in turbulent mixing and combustion, significant variation in the location of soot and its structure is observed even when the conditions of the test are closely controlled. In this work, diesel spray soot characteristics are studied in a constant volume optically accessible combustion vessel through the application of laser-induced incandescence (LII). Studies are performed using a piezoelectric high pressure common rail injector with a high cetane (CN=56.5) diesel fuel at an injection pressure of 620 bar. Combusting sprays are examined at a part load charge-gas condition of 11.6 kg/m 3 density, 1300 K temperature, and in 21% oxygen and 15% oxygen (simulating 38% exhaust gas recirculation) environments. Images are acquired at 1.0 ms after start of injection in 21% oxygen and 1.5 ms after start of injection in 15% oxygen. Tests are repeated 14 times at each condition to provide a statistically significant sample. Images are compared, with average images and images of the ratio of the local standard deviation to the average, or local coefficient of variation, also considered to understand structure variations test to test. The results are quantified for total soot intensity and location of first soot. From this the required number of samples for a 95% confidence interval and 5% allowed error are determined. Introduction Diesel combustion is largely spray and mixing controlled, and understanding the spatial soot formation within the combustion chamber is imperative to provide an improved understanding of soot formation and emissions as a function of charge-gas conditions. Laser-induced incandescence (LII) is an optical laser diagnostic that enables qualitative visualization of soot concentrations. This methodology has been used by various investigators to provide visual information on soot formation, and, when used in conjunction with laser extinction, quantitative indication of soot levels can be determined [1-4]. In this diagnostic, a high power pulsed laser beam at 532 nm is used to excite the soot particles by depositing energy from the laser beam, which heats the particles, and their subsequent incandescence is measured with an intensified camera [5-6]. Due to the turbulent and transient nature of the combusting spray process, there are large shot to shot variations in the observed LII signal. The goal of this paper is to characterize this variability through visual and quantitative comparisons. This consists of characterization of statistics including number of samples for a given allowed error, consideration of standard deviation, coefficient of variation, and average value for quantified values of intensity and location of first soot. Additionally, a visual comparison of images is undertaken, along with discussion on average and local coefficient of variation images. Experimental Setup Experiments were conducted in the optically accessible constant volume chamber shown in Figure 1. The vessel has an approximately 1 liter internal volume with six face-ports housing three sapphire windows, a spark plug – dual fan port, a diesel fuel injector port (Figure 1), and one blank port. Additionally, there are eight access ports on the combustion vessel (CV) cube vertices containing a pressure transducer, inlet and exhaust valves, and blank ports. A preburn procedure is used to produce representative diesel engine conditions in the combustion chamber including oxygen level (simulating air – 21% O2 or 38% EGR – 15% O2), pressure, temperature, and density [7-9]. This process involves spark igniting a fuel-lean acetylene, hydrogen, oxygen and nitrogen gaseous mixture which yields a precombustion event and pressure rise, followed by a subsequent cool-down. Pressure is monitored during the cool-down and at the desired pressure (temperature) condition, fuel injection is triggered and combustion of the diesel liquid fuel ensues. Full details on this combustion vessel and its operation and use are provided in [10-12]. *Corresponding author: [email protected] ICLASS 2012, 12 th Triennial International Conference on Liquid Atomization and Spray Systems, Heidelberg, Germany, September 2-6, 2012 Figure 1: Optically accessible combustion vessel with gas panels for mixture creation (left). Internal view of combustion chamber (center) and external view of diesel injector window (right). The injector used in this study is a Bosch Generation III piezoelectric common rail fuel injector (external view of mounting in CV shown in Figure 1). The injector is equipped with a sac-type nozzle with a single hole oriented 15° off axis to replicate positioning in the production nozzle. The hole is nominally 1.0 mm long and 0.145 mm in diameter, with a length to diameter (L/D) ratio of 6.9. This injector is driven by an EFS IPoD piezoelectric injector driver in multi-peak regulation mode, which requires setting peak current, open and close voltage, and current slope levels, with drive characteristics being set to match production operation. The electronic trigger injection duration was set to 0.4 ms, and the resulting spray (liquid fuel exiting the injector) was 0.7 ms in duration. The fuel supply system is a high pressure system capable of injection pressures to 4140 bar, compatible with multiple fuels including diesel, biodiesel, gasoline, ethanol and others, with a high-cetane diesel fuel used in the current study, with properties provided in Table 1. High cetane fuel was utilized to reduce the ignition delay to ensure ignition before the fuel jet has reached the window. Table 1: Fuel properties for the high-cetane diesel fuel used in the current study. Property Value Specific Gravity at 15.6°C 0.8303 Viscosity at 40°C 2.819 mm /sec Cetane Number 56.5 Net Heating Value 43.182 MJ/kg