Fuel Injection and Sprays
Investigation of the Temperature and Pressure Dependency of High Pressure Fuel Spray Cavitation
Cavitation vaporizes fuel in the nozzle during injections from high-pressure fuel injectors. The effects and causes of cavitation are not definitively known, although the dependence of the degree of severity of cavitation on various engine operating parameters has been studied. Building on the research of Dr. Payri at CMT Valencia, the dependence of cavitation on fuel temperature and pressure will be investigated. The cavitation of multiple single component fuel sprays will be measured indirectly by measuring the mass flux of the injection and calculating the discharge coefficient. The apparatus for this experiment is shown in the Facilities section of this website, under Rate of Injection Apparatus.
Combustion and Emissions
The Influence of Fuel Injection Rate-Shaping on Combustion and Emissions of Diesel-like Sprays
Increasingly stringent emissions regulations have created a demand for cleaner burning engines. Low-temperature combustion (LTC) strategies have been proposed to meet low soot and nitrogen oxides emissions but LTC strategies suffer from excessive unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. These emissions have been shown to originate from overly fuel-lean mixtures near the nozzle that do not burn to completion. These mixtures are said to be over-mixed beyond a flammability limit and are caused by increased entrainment during end-of-injection. The coupling between end-of-injection entrainment and incomplete combustion of near-nozzle mixtures is not well understood, however, in part due to the large parameter space in engines. Thus, this work aims to develop tools and models to measure end-of-injection combustion observables and predict the likelihood of UHC and CO emissions over a wide range of conditions. A complete description of the work can be found here.
This work seeks to perturb the coupling between end-of-injection entrainment and incomplete combustion of near-nozzle mixtures by systematically varying the ambient thermodynamic conditions, injection parameters, as well as the end-of-injection transient. To this end, three significant developments were made: a novel injection system that can vary the end-of-injection transient on command, a measurement technique to quantify the transient injection rate with high confidence, and characterization of three simultaneous, high-speed optical diagnostics for measurements of end-of-injection combustion observables.
Four distinct behaviors of the spray flame following end-of-injection were identified: soot recession, complete combustion recession, partial combustion recession, and no/weak combustion recession. Combustion recession is the process whereby the initially lifted reaction zone retreats back towards the nozzle immediately following end-of-injection, thus consuming UHC/CO that would otherwise remain near the nozzle. Soot recession spatially and temporally overlaps with combustion recession and is the result of igniting rich mixtures. Regression of a comprehensive dataset indicates that combustion recession is promoted with higher ambient temperatures, higher ambient oxygen concentrations, higher ambient densities, longer end-of-injection transients, lower injection pressures, and larger nozzle orifice diameters. Similar trends are observed for soot recession as well.
Rather than rely solely on regression for predictions of combustion recession, a first-principles based approach was used to develop a scaling law for combustion recession that is applicable to a wider class of injectors and injection strategies than those tested experimentally. Using a definition for the local Damköhler number throughout the jet, a limiting location of ignition was identified and linked to the flame lift-off length to develop both an end-of-injection ignition timescale and a steady injection ignition timescale. The proportionality between the two timescales was used to predict the likelihood of combustion recession and thus UHC/CO emissions. More information on this scaling approach can be found here.
A new reduced-order model, premised on the similarity between diesel sprays and dense turbulent gas-jets, was developed that captures only key physics regarding combustion recession. This model was used to better understand the coupling between end-of-injection mixing and near-nozzle ignition timescales that are thought to control combustion recession. Please feel free to use this model for your own purposes and cite our IJER paper.
Multidimensional Engine Modeling
A Novel Approach to Assess Diesel Spray Models Using Joint Liquid-Phase Extinction and X-Ray Radiography Measurements
Conventional spray modeling validation practices do not guarantee accurate representation of the measured spray. We have shown that two model set-ups can result in matching predictions of global parameters such as liquid and vapor penetration, yet exhibit different spray morphology predictions [Magnotti and Genzale, 2014]. As a result, there is a need for a robust methodology to validate and assess the theoretical basis for spray models.
The two main theories underlying spray models are based either on aerodynamic-induced or liquid-turbulence breakup. The appropriateness of these theories under engine-relevant conditions has not been robustly validated. Due to the optically dense nature of diesel sprays, by and large, aerodynamic-induced spray breakup has only been indirectly validated using global spray behavior measurements, such as spreading angle. On the other hand, liquid-turbulence induced breakup has been extensively validated using holographic imaging; however data has been limited to ambient densities much lower than that of typical diesel engine operating conditions.
To adequately assess current spray models, it is necessary to have experimental data that can quantify spray morphology under engine-relevant conditions. Both visible and x-ray extinction offer insight into the local spray behavior as the experimental signals are functions of spray parameters such as droplet size and number density. The joint x-ray / visible extinction measurements enable unique evaluation of predicted spray morphology. To leverage the information contained in these measurements, we have developed visible and x-ray extinction models and integrated them into our spray model code. This affords us the unique capability to directly compare measurements and model predictions, and assess the predictive capability of CFD spray models.