Brian T. Fisher

Evaluating the Potential of a Direct-Injection Constant-Volume Combustion Chamber as a Tool to Validate Chemical-Kinetic Models for Liquid Fuels

ABSTRACT Chemical-kinetic mechanisms for low-volatility hydrocarbons and alternative fuels are mostly scarce or not validated, primarily due to the lack of a convenient experimental platform to investigate fundamental ignition and combustion behavior for such fuels. This work reports experimental measurements of ignition delays and related combustion behavior of n-heptane in a direct-injection constant-volume combustion chamber, for pressures ranging from 0.2 MPa to 1.0 MPa and temperatures ranging from approximately 390°C to 500°C while holding the global equivalence ratio constant at 0.62. Results showed clear two-stage ignition and negative temperature coefficient behavior, and data at higher pressures agreed closely with predictions from a perfectly-stirred-reactor model. At lower pressures, impingement of liquid fuel on the chamber wall likely caused experimental data to deviate significantly from model predictions. Overall, this work strongly suggests that this experimental platform can be used to validate chemical kinetics of liquid fuels.

Experimental measurements of n-heptane liquid penetration distance and spray cone angle for steady conditions relevant to early direct-injection low-temperature combustion in diesel engines

Early direct-injection is a diesel-engine combustion strategy in which fuel is injected early in the compression stroke when in-cylinder temperature and density are both much lower than for conventional operation, increasing the potential for impingement and wall wetting due to slower vaporization of fuel. Quantitative understanding of liquid-phase fuel penetration and dispersion, therefore, is necessary for these conditions in order to avoid problems such as impingement that are counterproductive to advanced combustion strategies. This work reports liquid penetration distances and spray cone angles of n-heptane measured using high-speed imaging of elastic light scattering in a constant-pressure flow vessel. Air temperatures in the constant-pressure flow vessel included well below, near, and well above saturation temperatures of n-heptane, which should provide insight into spray behavior of diesel fuel components for early-injection conditions that also potentially range from below to above saturation. Maximum liquid penetration distances and spray angles were found to be essentially independent of injection pressure and injection duration, as well as air flow velocity, but strongly dependent on thermodynamic properties of the flowing air. Maximum liquid penetration trends with respect to temperature and spray angle trends with respect to density were in qualitative agreement with model predictions, but did not agree with expected quantitative values. In addition, trends of maximum liquid penetration versus density and spray angle versus temperature were either non-existent or in direct opposition to model predictions. Results strongly suggest that well-accepted models, based on mixing-limited vaporization of fuel and ignoring heat and mass transfer of individual droplets, are insufficient for these conditions relevant to early direct-injection.

Experimental and Computational Study of n-Heptane Autoignition in a Direct-Injection Constant-Volume Combustion Chamber

The purpose of this study was to characterize experimental n-heptane combustion behavior in a direct-injection constant-volume combustion chamber (DI-CVCC), using chamber pressure to infer ignition delay and heat-release rate. Measurements generally displayed expected trends and indicated entirely premixed combustion with no mixing-controlled phase. A significant finding was the observation of negative temperature coefficient (NTC) behavior. Comparing results with CHEMKIN-PRO simulations, it was found that a homogeneous combustion model was reasonably accurate for ignition delays longer than 5 ms. The combination of NTC behavior and homogeneous fuel-air mixtures suggests that this DI-CVCC can be useful for validation of chemical-kinetic mechanisms.

Measurements of n-Heptane Liquid Length and Spray Cone Angle for Low-Temperature, Low-Density Conditions in an Optically Accessible Flow Vessel

The purpose of this study was to measure global properties of n-heptane sprays using high-speed spray visualization in a newly developed constant-pressure flow vessel. Liquid-phase fuel penetration distance and cone angle were determined for low-density and low-temperature ambient conditions, which are increasingly relevant due to the advent of early direct-injection low-temperature combustion. Results indicated that fuel sprays under these conditions do not behave as predicted by established models. Penetration distances increased steadily throughout and after injection, and liquid-phase fuel persisted long after end of injection. Results suggest that these sprays vaporize extremely slowly and could cause wall-wetting issues.Copyright

Ignition Delay and Heat-Release Rate for n-Heptane in a Direct-Injection Constant-Volume Combustion Chamber: Experiments and Computations

The purpose of this study was to characterize combustion behavior for n-heptane using experimental measurements in a direct-injection constant-volume combustion chamber (CVCC) to validate chemical-kinetic mechanisms. This work is focused on compression-ignition (i.e., diesel) combustion, primarily because mechanisms for larger-chain diesel-relevant species are not well developed and require significant attention.The CVCC used in this work can be pressurized and heated to create engine-relevant conditions that enable study of autoignition behavior. In addition, the chamber is equipped with a high-pressure, common-rail diesel injector, making the study of autoignition and combustion in this system highly relevant to modern diesel engines. By varying injection pressure and duration, it is possible to control global equivalence ratio as well. Chamber pressure during injection and combustion is measured using a piezoelectric transducer, and can be subsequently used to infer heat-release rates. Experimental measurements for n-heptane mostly displayed expected trends. As initial chamber pressure increased, ignition delay decreased and peak pressure increased. As injection duration increased, ignition delay decreased due to faster ignition of richer mixtures, and peak pressure increased due to higher total heat release. The effect of temperature on ignition delay, however, was more complex and suggested some amount of NTC (negative temperature coefficient) behavior. For all conditions, heat-release rates indicated entirely premixed combustion with no hint of mixing-controlled combustion.Experimental data were compared with results from CHEMKIN-PRO simulations. The model simulated zero-dimensional combustion using a detailed n-heptane mechanism developed at Lawrence Livermore National Laboratory. These computations were used to infer local equivalence ratio information, based on equivalence ratio required in the model to match experimental ignition delay. For most test cases, the model required an equivalence ratio that was at least ∼2× richer than the global value. In addition, equivalence ratios in the model ranged a full order of magnitude, from ∼0.6 to 6, suggesting that local mixture equivalence ratios varied considerably as experimental conditions were varied. Results suggest that improved models that include details of spray physics are required in order to properly predict local equivalence ratios and resulting autoignition characteristics.Copyright

Development of a High-Pressure, High-Temperature, Optically Accessible Continuous-Flow Vessel for Fuel-Injection Experiments

The focus of this work was to develop a continuous-flow vessel with extensive optical access for characterization of engine-relevant fuel-injection and spray processes. The spray chamber was designed for non-reacting experiments at pressures up to 1380 kPa (200 psi) and temperatures up to 200°C. Continuous flow of inert “sweep gas” enables acquisition of large statistical data samples and thus potentially enables characterization of stochastic spray processes. A custom flange was designed to hold a common-rail diesel injector, with significant flexibility to accommodate other injectors and injector types in the future. This flexibility, combined with the continuous flow through the chamber, may enable studies of gas-turbine direct-injection spray processes in the future. Overall, the user can control and vary: injection duration, injection pressure, sweep-gas temperature, sweep-gas pressure, and sweep-gas flow rate. The user also can control frequency of replicate injections.There are four flat windows installed orthogonally on the vessel for optical access. Optical data, at present, include global spray properties such as liquid-phase fuel penetration and cone angle. These measurements are made using a high-speed spray-visualization system (up to 100 kHz) consisting of a fast-pulsed LED (light emitting diode) source and a high-speed camera. Experimental control and data acquisition have been set up and synchronized using custom LabVIEW programs. The culmination of this development effort was an initial demonstration experiment to capture high-speed spray-visualization movies of n-heptane injections to determine liquid-phase fuel penetration length (i.e., liquid length) and spray cone angle. In this initial experiment, fuel-injection pressure was ∼120 MPa (1200 bar) and the injection command-pulse duration was 800 μs. At room conditions, liquid length and nominal spray cone angle were ∼170 mm and ∼14.5°, respectively. In contrast, with air flow in the chamber at 100 psi and 100°C, liquid length was considerably shorter at ∼92 mm and spray cone angle was wider at ∼16.5°. Future experiments will include the continuation of these measurements for a wider range of conditions and fuels, extension of high-speed imaging to vapor-phase fuel penetration using schlieren imaging techniques, and detailed characterization of spray properties near the injector nozzle and near the liquid length.Copyright

Liquid and Vapor Phase Water Measurements in an Optically Dense Environment

Water concentrations were determined using tunable diode laser absorption and optical density measurements. These measurements were made in a cup burner apparatus to aid in the determination of fire suppression effectiveness of water mist. ©2006 Optical Society of America OCIS codes: (120.1740 Combustion diagnostics); (120.7000 Transmission); (140.2020 Diode lasers); (300.1030 Absorption)