MAP | CALMAIL |
Combustion Modeling Lab at UC Berkeley

Current Research

Reacting Spray Modeling of Biofuels in an Ignition Quality Tester (IQT)


Anthony DeFilippo, Greg Chin (CML)

In conjuction with the Colorado School of Mines and the National Renewable Energy Laboratory


Introduction

The Ignition Quality Tester (IQT) is a constant-volume combustion apparatus developed to measure ignition qualities of diesel-type fuels. In the current research, the IQT is used to gain enhanced understanding of kinetic pathways to autoignition that have become increasingly important to the development of advanced combustion engines and alternative fuels. Homogeneous charge compression ignition (HCCI) and other forms of low temperature combustion (LTC) strategies strongly depend on fuel autoignition kinetics to control combustion timing. In addition, the autoignition characteristics of renewable biofuels are currently of great interest. Development of accurate chemical kinetic models for the ignition of diesel and biodiesel model compounds relies on well-characterized experiments; however, rigorous experimental validation of these kinetic models has been limited for a variety of reasons.

IQT MeshGrid of IQT.
IQT MeshAlternate Grid of IQT.
For example, shock tubes and rapid compression machines are typically limited to premixed gas phase studies, although there has been some recent progress in studying low vapor pressure fuels with these devices.Nevertheless, research platforms capable of investigating autoignition in a well-characterized, nonhomogeneous combustion environment could accelerate the validation of reduced kinetic models for advanced engine research. Because of their experimental and modeling simplicity relative to engine-based studies, constant volume devices have been used extensively to study the combustion of a wide range of liquid fuels, providing detailed information about fuel spray physics and fuel chemistry effects on the overall combustion event, demonstrating their potential for developing a research platform capable of measuring the complex interactions between spray physics and fuel chemistry of low-volatility fuels.


Abstract

Development of advanced compression ignition and low-temperature combustion engines is increasingly dependent on chemical kinetic ignition models. However, rigorous experimental validation of kinetic models has been limited under engine-like conditions. For example, shock tubes and rapid compression machines are usually restricted to premixed gas-phase studies, precluding the study of heterogeneous combustion and the use of low-volatility surrogates for commercial diesel fuels. The Ignition Quality Tester (IQT) is a constant-volume spray combustion system designed to measure ignition delay of low-volatility fuels, having the potential to validate ignition models. However, a better understanding of the IQTs fuel spray and combustion processes is necessary to enable chemical kinetic studies. As a first step, n-heptane was studied because numerous reduced chemical mechanisms are available in the literature as it is a common diesel fuel surrogate, as well as a calibration fuel for the IQT. A modified version of the KIVA-3V software was utilized to develop a three-dimensional computational fluid dynamics (CFD) model that accurately and efficiently reproduces n-heptane ignition behavior and temporally resolves temperature and equivalence ratio regions inside the IQT. Measured fuel spray characteristics (e.g., spray-tip velocity, spray cone-angle, and flow oscillation) for n-heptane were programmed into the CFD model. Sensitivity analyses of fuel droplet size and velocity showed that their effects on ignition delay were small compared to the large chemical effects of increased chain branching in the isomers 2-methylhexane and 2,4-dimethylpentane. CFD model predictions of ignition delay using reduced/skeletal chemical mechanisms for n-heptane (60-, 42-, 22- and 33-species, and one-step chemistry) were compared, again indicating that chemical kinetics control the ignition process.

IQT SimulationSimulation of reacting liquid spray in IQT.