Laser-induced incandescence versus photo-acoustics:implications for qualitative soot size diagnostics

Laser-induced incandescence at high-repetition rates can in principle be used to resolve the temporal evolution of soot processes. The intrusive character of this technique, however, requires due care of historical effects associated with multiple exposures of individual soot particles to laser light. On the other hand, repetitive heating and cooling opens up an independent, acoustic detection channel. We illustrate a photo-acoustic soot volume fraction measurement, and show that the comparison to simultaneously recorded laser-induced incandescence provides qualitative information on soot growth. Experiments are performed on a propane-fueled, co-flow stabilized diffusion flame, and signals are collected at varying heights above the burner deck. Results show a clear correlation between the laser-induced incandescence and photo-acoustic signals; small deviations are interpreted as a qualitative indicator for the particle size.</p

Molecular tagging velocimetry in turbulence using biacetyl

We evaluate various molecular tagging velocimetry (MTV) techniques for application in turbulent flows of gases where the smallest length scales must be resolved. We argue that tracer diffusion dictates the use of large complex molecules and discuss a few candidate molecules. The accuracy of MTV is determined by the profile of written lines which widen due to molecular dynamics, including both diffusion and chemical reaction. We evaluate these profiles for tagging with phosphorescing biacetyl molecules, which is a commonly used probe in MTV. For relatively large laser power, these profiles are determined not by molecular diffusion, but by the triplet-triplet annihilation reaction of excited biacetyl molecules. We identify a new reaction pathway, and present a model for the observed line shapes. The rapid widening of tagged lines of biacetyl molecules due to chemical reaction restricts this MTV technique to large-scale turbulent motion in gases of comparable molecular weight

Experimental and numerical analyses of liquid and spray penetration under heavy-duty diesel engine conditions

The modeling of fuel sprays under well-characterized conditions relevant for heavy-duty Diesel engine applications, allows for detailed analyses of individual phenomena aimed at improving emission formation and fuel consumption. However, the complexity of a reacting fuel spray under heavy-duty conditions currently prohibits direct simulation. Using a systematic approach, we extrapolate available spray models to the desired conditions without inclusion of chemical reactions. For validation, experimental techniques are utilized to characterize inert sprays of n-dodecane in a high-pressure, high-temperature (900 K) constant volume vessel with full optical access. The liquid fuel spray is studied using high-speed diffused back-illumination for conditions with different densities (22.8 and 40 kg/m3) and injection pressures (150, 80 and 160 MPa), using a 0.205-mm orifice diameter nozzle. High-speed Schlieren imaging is used to analyze the influence of these boundary conditions on the spray penetration. Simulations of the fuel spray are performed using a dedicated computational mesh with refinements at the known location of the jet to capture the smallest scales of interest. Using a blob injection model refined with a primary atomization and secondary breakup model, correct trends and good agreement are achieved for both liquid and spray penetration. The capability of capturing the trends at largely varying boundary conditions with a single computational approach provides a solid base for future work

Probing the heat during the PCCI beat : determining PCCI engine temperatures using two-line thermometry, Aachen, Germany.

Temperature is a key parameter for reaction progress during combustion, and as such its experimental determination has been a subject of considerable interest for many years. The aim of our present project is to study the 2-D temperature field in a realistic heavy-duty Diesel engine under the conditions of premixed charge compression ignition (PCCI) combustion. Two-line OH Laser Induced Fluorescence (LIF) thermometry will be used, in combination with spontaneous Raman scattering as an independent calibration. Here, we discuss the initial test measurements performed on a high-pressure high-temperature gas cell, and the selection of OH line pairs for thermometry. In addition, we will discuss Raman scattering temperature measurements that were carried out in a realistic engine

Fuel formulation and mixing strategy for rate of heat release control with PCCI combustion

Premixed charge compression ignition (or PCCI) is a new combustion concept that promises very low emissions of nitrogen oxides and of particulate matter by internal combustion engines. In the PCCIcombustion mode fuel, products from previous combustion events and air are mixed and compresseduntil the resulting mixture locally auto-ignites. Auto-ignition at other places in the reacting mixture follows rapidly and combustion takes places across the combustion chamber within a short period. A majorchallenge with PCCI combustion is the accurate control of the start of combustion and of the rate of heatrelease during combustion

Spray growth of regular, synthetic, oxygenated and biodiesels in an optical engine

Spray formation has been studied in an optically accessible heavy-duty diesel engine for regular diesel,synthetic, oxygenated and biofuels using a high-speed digital camera. Images are analyzed with custom madealgorithms to obtain spray penetration length and spray cone angle as function of time. Results from 2 out of the 8 nozzle sprays have been used in the data analysis. Variation in spray equilibrium length and angle is observed between the fuels tested. Modelling of the fuel injection, taking great care to account for individual fuel properties, shows good correspondence with experimental results