Browsing by Author "Materego, Myeji C."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Auto-ignition characterisation of synthetic fuels via Rapid Compression Machine
    (2015) Materego, Myeji C.
    Availability and sustainability of fuels for road and air transport is essential for economic development and growth of any nation. New alternative fuels provide an opportunity to limit the use of ever declining conventional petroleum oil reserves as well as offsetting CO2 generation from their use. Liquid fuels have the highest energy density for transportation applications and synthetic liquid fuels, which can be produced from renewable non-food bio feedstock offer an exciting opportunity for partial or even total substitution of remaining fossil fuel supplies. It is therefore of great interest to study the fundamental combustion characteristics of these fuels if they are to be used commercially. This work is aiming at characterising the auto-ignition properties of individual fuel components representative of the chemical families present in the synthetic fuels which in this case are toluene, iso-octane, n-heptane, and bio-alcohols; ethanol and n-butanol. The auto-ignition characterisation was made by measurements of ignition delay times, τ. The time τ for these fuels and their blends were measured after rapidly compressed to an elevated pressure and temperature using a Rapid Compression Machine (RCM). RCM provides good platform to study the fuel auto-ignition process without complicated physical effects in engines which are continually changing. However, they are not without problems, practical applications are usually not within the ideal conditions. Different machines have different extent of deviation from ideal conditions, making comparison of results between rigs difficult. In the present study, a dedicated work was conducted to study the difference between the measurements originated from these rigs and were characterised against their deviations from ideal conditions. These cover chemical reaction during the finite compression time, the effects of heat loss during the ignition delay period, the effects of piston displacement (piston bounce), and non-homogeneous auto-ignition. An interesting aspect of the study is that a plot of the measured different delay times at a given temperature, on the separate machines, against the corresponding degrees of reaction during compression, when extrapolated to zero reaction, yield a more accurate delay time for that condition. As the temperature is increased, so also are the oscillatory pressure amplitudes generated at the auto-igniting hot spots. This is in line with other studies of hot spot auto-ignition. Measurements of ignition delay times of different chemical groups separately and when blended with each other were made. They provided an understanding of how their interaction influences the overall ignition delay times. When blended the change of their τ values do not vary linearly especially when the blended components have large difference in reactivities. Toluene for example, which is commonly known for its long ignition delay times, was made extremely reactive when blended with n-butanol. Comparison of addition of bio-alcohols (ethanol and n-butanol) on gasoline surrogate fuel (TRF) showed that at lower temperatures, they both increased the ignition delay times of TRF, while at high temperatures they reduced TRF delay times to almost the same value. n-butanol started to reduce TRF delay times at lower temperatures compared to ethanol. Development of auto-ignition blending laws offers an opportunity to enable quick methods for choosing an appropriate blend for a particular application. In this work, a Linear by Mole (LbM) auto-ignition blending law was proposed, it uses the measured ignition delay times of individual components in the blend and varies them linearly with the fractional concentration of each component. This was found to be satisfactory only for blends of chemical families without NTC behaviour such as CH4/H2, for fuels with NTC behaviour an empirical based law was generated for the conditions studied. Overall, this study has broadened our understanding in auto-ignition behaviour of selected individual fuel components and their blends at varying conditions of pressure, temperature and concentration. It has also enabled substantial development of Leeds RCM to achieve fast compression with good piston damping.
  • Thumbnail Image
    Item
    Interpretation of Auto-ignition Delay Times Measured in Different Rapid Compression Machines
    (White Rose University Consortium, 2015) Bradley, Derek; Lawes, Malcolm; Materego, Myeji C.
    An international collaboration was initiated by thirteen different research groups to understand and explain the differences in auto-ignition delay times, measured on different rapid compression machines, RCMs, of different design and size [1,3]. The Consortium measured delay times, k, for ioctane under the same conditions: fixed oxygen content of 21%, pressure at the end of compression, Po, 2.0 MPa, and compression temperatures, To, in the range 650-950K. Figure 1 gives the experimental auto-ignition delay times, ke , from seven different RCMs plotted against 1000/To. Each point is identified by a number unique to each participating group. There is significant scatter in ke , particularly at the intermediate and low temperatures
  • Thumbnail Image
    Item
    Low Temperature Ignition Properties of N-Butanol: Key Uncertainties and Constraints
    (Laboratory of Heterogeneous Mixtures and Combustion Systems, 2015) Agbro, E.; Materego, Myeji C.; Lawes, Malcolm; Tomlin, A. S.
    A recent kinetic mechanism (Sarathy et al., 2012) describing the low temperature oxidation of n-butanol was investigated using both local and global sensitivity/uncertainty analysis methods with ignition delays as predictive targets over temperature ranges of 678-898 K and equivalence ratios ranging from 0.5-2.0 at 15 bar. The study incorporates the effects of uncertainties in forward rate constants on the predicted outputs, providing information on the robustness of the mechanism over a range of operating conditions. A global sampling technique was employed for the determination of predictive error bars, and a high dimensional model representation (HDMR) method was further utilised for the calculation of global sensitivity indices following the application of a linear screening method. Predicted ignition delay distributions spanning up to an order of magnitude indicate the need for better quantification of the most dominant reaction rate parameters. The calculated first-order sensitivities from the HDMR study show the main fuel hydrogen abstraction pathways via OH as the major contributors to the predicted uncertainties. Sensitivities indicate that no individual rate constant dominates uncertainties under any of the conditions studied, but that strong constraints on the branching ratio for H abstraction by OH at the α and γ sites are provided by the measurements.