Development of a Rapid Compression Controlled-Expansion Machine for Chemical Ignition Studies

The ability to accurately model fuel combustion processes is essential to the development of transportation, power generation, and manufacturing technology. Models describing the kinetics of chemical oxidation are readily available and highly refined for a wide range of test fuels. However, these models still suffer from high levels of uncertainty under engine-relevant conditions, largely due to a lack of consistency between published validation data. An experimental testing apparatus, known as the Rapid Compression Controlled-Expansion Machine (RCCEM) has been designed and fabricated to conduct chemical kinetic studies. The RCCEM features a pneumatically-driven, custom-designed cam, which governs the volumetric compression and expansion of the combustion chamber. This machine has been designed to test various compression ratios, compressed pressures, and compressed temperatures. Central to the operation of the RCCEM, the cam assembly is modular with the ability to incorporate different cams with unique compression and expansion profiles. This capability is intended to control heat loss rates in experiments via volumetric expansion, and as a result, increase understanding of its influence on the interpretation of validation data. Performance characterization of the RCCEM, using iso-octane and hexane, has shown that the machine is capable of testing a wide range of conditions with exceptional repeatability. Ignition delay times for iso-octane are reported for compressed temperatures of 630-700 K. Additionally, two computational fluid dynamics (CFD) studies have been conducted to investigate the role of non-uniform boundary temperatures as a potential cause of discrepancies among data in the literature. The effect of these boundary conditions on ignition delay time predictions and compressed-gas temperature field development has been investigated for heated RCM experiments that use either creviced or flat pistons. Three unique boundary temperature cases for non-reactive simulations showed that a large temperature gradient forms over the crown of the piston due to heterogeneities present in the initial temperature fields. Subsequently, five boundary temperature cases were investigated for reactive simulations and demonstrated the effect of these non-uniformities on ignition delay time predictions. Through this work, it was determined that the flat piston is susceptible to these non-uniform conditions causing discrepancies in ignition delay times, whereas the creviced piston data was only minimally influenced

Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Application of a Multi-Zone Model for the Prediction of Species Concentrations in Rapid Compression Machine Experiments

Accurate chemical kinetic models, which predict species evolution and heat release rates in chemically reactive systems, are essential for further advancements in fuel and combustion technology. An experimental facility that is widely used for evaluating the accuracy of kinetic models is a rapid compression machine (RCM), which creates a well-defined reaction environment by compressing a reactive mixture inside a chamber. Generally, RCM experiments are conducted in order to obtain ignition delay data. However, chemical speciation data provides greater insight into reaction pathways, and is therefore a more rigorous benchmark for validating kinetic models. In order for a chemical kinetic model to be evaluated using RCM data, the kinetic model must be coupled with a thermodynamic model that can predict the temporally varying conditions that evolve during an RCM experiment. The most common approach is to utilize a thermally and compositionally homogeneous 0-dimensional reactor model (HRM), which predicts conditions inside the hot core region of the main combustion chamber of an RCM, where a significant portion of the chemical reaction in an RCM takes place. This approach requires an effective volume profile, which is derived from the pressure profile of either a non-reactive experiment with similar transport properties as the condition of interest, or a separate multi-zone model (MZM), via the relationship between pressure and volume for an isentropic process. While HRMs have been shown to yield adequate ignition delay predictions, they cannot be used to predict average speciation data, since the conditions in the core region vary considerably from the average conditions of the total reaction chamber. This work introduces a modified MZM, which simulates chemical reaction throughout the entire temperature-varying main combustion chamber of an RCM, in addition to boundary work, conduction, and crevice flows as the traditional MZM approach. Simulating chemistry in the MZM allows for average speciation predictions, and eliminates the need for an HRM. The new approach is shown to yield similar average speciation data as CFD simulations (within 15% difference) for the combustion of primary reference fuels at various conditions

Measured and Modeled Performance of a Spring Dominant Free Piston Engine Generator

Free Piston Engine Generators (FPEG) directly convert the reciprocating piston motion into electricity by using a linear alternator. Unlike conventional engines with piston motion restricted by a crankshaft mechanism, the FPEG piston motion is constrained by the energy available in the system. When stiff springs are considered in the design, the FPEG system attains high frequency with high power and efficiency. The main objective of this research was to model stiff spring-assisted FPEG system dynamics and performance accurately, and to apply the modeling results to the development of a 1kW, spark ignited, natural gas fueled, FPEG experimental prototype. The experimental data was further utilized to refine and improve the existing model. First, a MATLAB®/Simulink based multi-cycle numerical model was developed for single and dual cylinder FPEG systems to study the effects of major design parameters on FPEG dynamics and performance. When stiff springs were added, the dynamics became more sinusoidal and symmetric with respect to the initial starting position. For a total displacement of 34 cc, trapped compression ratio of 8.25, and assumed combustion efficiency of 95%, the modeled frequency and electric power varied from 72.3 Hz to 80.8 Hz and 0.81 kW to 0.88 kW for a single cylinder FPEG as the spring stiffness changed from 372 kN/m to 744 kN/m. For a dual cylinder FPEG with the same conditions, these modeled values changed from 76.8 Hz to 84.1 Hz and 1.7 kW to 1.8 kW with increasing spring stiffness. The numerical model was then expanded for sensitivity studies of major design parameters. When FPEG operating conditions were considered, the effective stroke length was found to have a dominant effect on efficiency followed by compression ratio, cylinder bore, and spring stiffness respectively. The experimental FPEG prototype generating 550 W of electricity with indicated efficiencies exceeding 13.8% was used for model validation. Finally, the stable FPEG system requires a control strategy to match the power generated by the engine to the power demanded by the alternator. A model-based control strategy was developed in Stateflow® for alternator mode switching, calibration maps, energy management, ignition and fuel injection timings. With the proposed control strategy and stiff spring dominance, the modeled and experimental FPEG system maintained stable operation with cycle-to-cycle variations less than 5%

Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines

University of Minnesota Ph.D. dissertation. July 2019. Major: Mechanical Engineering. Advisor: William Northrop. 1 computer file (PDF); xvii, 114 pages.There is a growing effort to reduce carbon dioxide (CO2) emissions produced by internal combustion (IC) engines as an effort to curb anthropogenic climate change. The transportation sector accounts for 28% of anthropogenic CO2, motivating fundamental combustion research to understand and develop more efficient advanced combustion modes. Study of ignition delay time, autoignition pressure and temperature, the chemistry of fuel mixtures, and speciation of combustion products provide important insights into phenomena like pre-ignition (knock) and pollutants (CO2, oxides of nitrogen, soot, etc.) from modern-day IC engines. This body of work investigates novel speciation methods for studying combustion products from IC engines and unique piston geometries for rapid compression machines (RCMs). Quantifying combustion products is an important step in creating accurate numerical models for engine combustion. Many groups have used various instruments in conjunction to characterize a range of combustion generated hydrocarbons but few have used instruments in tandem to improve speciation methods during unconventional combustion modes and address the issues associated with off-line speciation. The first part of this thesis presents an investigation that quantified light unburned hydrocarbons (UHC) using a combination of Fourier transform infrared (FT-IR) spectroscopy and gas chromatography-mass spectroscopy (GC-MS). A light-duty diesel engine is used to generate hydrocarbons at various exhaust gas recirculation (EGR) levels and partially premixed low-temperature combustion (LTC) modes. Exhaust samples are extracted with a novel fixed-volume sampling system and sent into a gas chromatograph (GC) while minimizing unknown dilution, light unburned hydrocarbons (LHC) losses, and removing heavy unburned hydrocarbons (HHC). Along with the wide range of LHCs quantified in this study, focus is directed towards the problem of misidentification of propane by the FT-IR during LTC modes. In the region commonly identified as the absorption spectra of propane (2700 and 3100 cm-1), analysis of the FT-IR spectra indicates absorption band interference caused by components found in unburnt diesel fuel. One of the primary findings of this work is that GC-MS can aid in FT-IR spectral analysis to further refine FT-IR methods for real-time measurement of unconventional combustion mode exhaust species. Rapid compression machines (RCMs) and rapid compression and expansion machines (RCEMs) are apparatuses that have the ability to operate at engine-relevant conditions to study fuel autoignition and pollutant formation. These machines are currently limited for use in speciation studies due to thermal and mixture inhomogeneities caused by heat transfer and gas motion during compression. Studies have shown the disadvantages of using common flat and enlarged piston crevice designs for sampling reaction chamber gases during and after combustion. For instance, computer fluid dynamics (CFD) simulations performed by numerous groups, including collaborators on this work, have confirmed that unburnt fuel mixture emerges from the enlarged crevice after compression then subsequently mixes with reaction chamber gases during RCM and RCEM operation. This disadvantage renders whole-cylinder sampling techniques inaccurate for quantifying combustion products and reduces the relevance of RCMs and RCEMs for comparison with IC engines. Complex fast-sampling systems are implemented by a number of research groups to extract small quantities of gas from the center of the chamber before mixing occurs. Drawbacks with this approach include small sample volumes, local composition non-uniformities, and non-uniform progression of chemical kinetics during sampling. Experimental and computational studies emphasize the importance of piston design for the formation of a well-mixed, homogeneous core gas inside RCM and RCEM reaction chambers. In the second part of this thesis, a novel piston containing a bowl-like geometry similar to those used in diesel engines is implemented to overcome thermal and compositional non-uniformities within RCMs/RCEMs. By eliminating the enlarged crevice and introducing squish flow with the bowl piston, CFD studies show increased thermal uniformity for both RCM and RCEM trajectories. Experiments to characterize piston performance includes flat, enlarged crevice, and bowl piston profiles and four fuel mixtures using the University of Minnesota – Twin Cities controlled trajectory RCEM (CT-RCEM). Heat release analysis (HRA) indicates greater combustion efficiencies when using the bowl piston opposed to the standard flat and enlarged creviced pistons. This is indicative of smaller fractions of unburnt fuel left in the combustion chamber after combustion, ideal for dump sampling and the differentiation of unburnt fuel from combustion products during speciation. Ignition analysis for the bowl piston derived stronger ignition characteristics than the enlarged crevice and flat piston designs. As a result of stronger ignition and better uniform burning, the amount of fuel converted to products of combustion is increased

Z-Pinch Pulsed Plasma Propulsion Technology Development

Fusion-based propulsion can enable fast interplanetary transportation. Magneto-inertial fusion (MIF) is an approach which has been shown to potentially lead to a low cost, small reactor for fusion break even. The Z-Pinch/dense plasma focus method is an MIF concept in which a column of gas is compressed to thermonuclear conditions by an axial current (I approximates 100 MA). Recent advancements in experiments and the theoretical understanding of this concept suggest favorable scaling of fusion power output yield as I(sup 4). This document presents a conceptual design of a Z-Pinch fusion propulsion system and a vehicle for human exploration. The purpose of this study is to apply Z-Pinch fusion principles to the design of a propulsion system for an interplanetary spacecraft. This study took four steps in service of that objective; these steps are identified below. 1. Z-Pinch Modeling and Analysis: There is a wealth of literature characterizing Z-Pinch physics and existing Z-Pinch physics models. In order to be useful in engineering analysis, simplified Z-Pinch fusion thermodynamic models are required to give propulsion engineers the quantity of plasma, plasma temperature, rate of expansion, etc. The study team developed these models in this study. 2. Propulsion Modeling and Analysis: While the Z-Pinch models characterize the fusion process itself, propulsion models calculate the parameters that characterize the propulsion system (thrust, specific impulse, etc.) The study team developed a Z-Pinch propulsion model and used it to determine the best values for pulse rate, amount of propellant per pulse, and mixture ratio of the D-T and liner materials as well as the resulting thrust and specific impulse of the system. 3. Mission Analysis: Several potential missions were studied. Trajectory analysis using data from the propulsion model was used to determine the duration of the propulsion burns, the amount of propellant expended to complete each mission considered. 4. Vehicle Design: To understand the applicability of Z-Pinch propulsion to interplanetary travel, it is necessary to design a concept vehicle that uses it -- the propulsion system significantly impacts the design of the electrical, thermal control, avionics and structural subsystems of a vehicle. The study team developed a conceptual design of an interplanetary vehicle that transports crew and cargo to Mars and back and can be reused for other missions. Several aspects of this vehicle are based on a previous crewed fusion vehicle study -- the Human Outer Planet Exploration (HOPE) Magnetized Target Fusion (MTF) vehicle. Portions of the vehicle design were used outright and others were modified from the MTF design in order to maintain comparability

Optimal piston crevice study in a rapid compression machine

Multi-dimensional effects such as vortex generation and heat losses from the gas to the wall of the reactor chamber have been an issue to obtaining a reliable RCM data. This vortex initiates a flow in the relatively cold boundary layer, which may penetrate the core gas. This resulting non-uniformity of the core region could cause serious discrepancies and give unreliable experimental data. To achieve a homogenous temperature field, an optimised piston crevice was designed using CFD modelling (Ansys fluent). A 2-Dimensional computational moving mesh is assuming an axisymmetric symmetry. The model adopted for this calculation is the laminar flow model and the fluid used was nitrogen. To get the appropriate crevice volume suitable for the present design, an optimisation of the five different crevice volume was modelled which resulted to about 2-10% of the entire chamber volume. The use of creviced piston has shown to reduce the final compressed gas temperature and pressure in the reactor chamber. All the crevice volumes between 2-10% of the chamber volume adequately contained the roll up vortexes, but the crevice volume of 282 mm 3 was chosen to be the best in addition to minimising the end gas pressure and temperature drop. The final pressure trace from experiment shows a reasonable agreement with the CFD model at compression and post compression stage