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(2013)Journal ArticleThe autoignition of dimethyl ether (DME) with temperature inhomogeneities is investigated by one-dimensional numerical simulations with detailed chemistry at high pressure and a constant volume. The primary purpose of the study is to provide an understanding of the autoignition of DME in a simplified configuration that is relevant to homogeneous charge compression ignition (HCCI) engines. The ignition structure and the negative temperature coefficient (NTC) behaviour are characterised in a homogeneous domain and one-dimensional domains with thermal stratification, at different initial mean temperatures and length scales. The thermal stratification is shown to strongly affect the spatial structure and temporal progress of ignition. The importance of diffusion and conduction on the ignition progress is assessed. It is shown that the effects of molecular diffusion decay relative to those of chemical reaction as the length-scale increases. This is to be expected, however the present study shows that these characteristics also depend on the mean temperature due to NTC behaviour. For the range of conditions studied here, which encompass a range of stratification length scales expected in HCCI engines, the effects of molecular transport are found to be small compared with chemical reaction effects for mean temperatures within the NTC regime. This is in contrast to previous work with fuels with single-stage ignition behaviour where practically realisable temperature gradients can lead to molecular transport effects becoming important. In addition, thermal stratification is demonstrated to result in significant reductions of the pressure-rise rate (PRR), even for the present fuel with two-stage ignition and NTC behaviour. The reduction of PRR is however strongly dependent on the mean initial temperature. The stratification length-scale is also shown to have an important influence on the pressure oscillations, with large-amplitude oscillations possible for larger length scales typical of integral scales in HCCI engines.
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(2013)Journal ArticleKnowledge of the fractal properties of premixed flame surfaces can potentially be used to help develop turbulent combustion models. Here, direct numerical simulations of low Damköhler number flames are used to analyse the fractal nature of the flames. Two sets of data are considered: (i) thermochemical hydrogen–air turbulent premixed plane-jet flames with detailed chemistry and (ii) thermonuclear flames in type Ia supernovae. A three-dimensional box counting method is used to investigate fractal dimension of the flame surface, characterising the self similarity of flame fronts. In the premixed flames, the fractal dimension is found to vary in time between 2.1 and 2.7. The supernovae flames in distributed combustion regimes yield fractal dimension about 2.7. The results for the maximum fractal dimensions are higher than previously reported. They are explained theoretically by a Reynolds number similarity argument which posits that the high Reynolds number, low Damköhler number limiting value of the fractal dimension is 8/3. Also tested is Mandelbrot’s fractal additive law which relates the fractal dimension determined in two dimensions, which is typical of experimental measurements, to that in three dimensions. The comparison of the fractal dimension in both two-dimensional and three-dimensional spaces supports the additive law, even though the flames considered do not formally satisfy isotropy. Finally, the inner-cut off is extracted from the hydrogen flames and found to be consistent in order of magnitude with Kolmogorov scaling.
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(2010)Conference PaperA recent research program has identified the possibility of using the analysis of casing wall pressures in the indirect measurement of gas turbine rotor blade vibration amplitudes. Analytical modelling of the casing wall pressures and reconstruction of rotor blade vibration amplitudes from the analysis of these simulated pressure signals have shown potential advantages over current non-contact rotor blade vibration measurement methods. However, the modelling made some fundamental assumptions about the casing wall pressure. One of the assumptions made was that the pressure at the blade tip is not significantly different from that measured across the clearance gap at the casing wall. This fluid-structure hypothesis is investigated in this paper. Unsteady computational fluid dynamic modelling of the flow conditions around the blade surface, combined with the blade structural motion, is performed numerically, and the distributions of the pressure across the rotor blade tip and casing clearance gap are investigated and reported.
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(2010)Conference PaperA recent research program has identified the possibility of using the analysis of casing wall pressures in the direct measurement of gas turbine rotor blade vibration amplitudes. Currently the dominant method of non-contact measurement of gas turbine blade vibrations employs the use of a number of proximity probes located around the engine periphery measuring the blade tip (arrival) time (BTT). Despite the increasing ability of this method there still exist some limitations, viz: the requirement of a large number of sensors for each engine stage, sensitivity to sensor location, difficulties in dealing with multiple excitation frequencies and sensors being located in the gas path. Analytical modelling of the casing wall pressures and reconstruction of rotor blade vibration amplitudes from the analysis of these simulated pressure signals has shown significant improvement over current non-contact rotor blade vibration measurement limitations by requiring only a limited number of sensors and providing robust rotor blade vibration amplitude estimates in the presence of simulated measurement noise. However, this modelling was conducted with some fundamental assumptions about the casing wall pressures being made. One of these assumptions presumed that during blade motion the pressure profile around the rotor blades follows the blade’s motion while it oscillates around its equilibrium position. This assumption is investigated in this paper through the numerical modelling of the fully coupled two-way rotor blade motion and fluid pressure interaction.
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(2010)Conference PaperWe demonstrate radio frequency (RF) readout of electrically detected magnetic resonance in phosphorus-doped silicon metal-oxide field-effecttransistors (MOSFETs), operated at liquid helium temperatures. For the first time, the Si:P hyperfine lines have been observed using radio frequency reflectometry, which is promising for high-bandwidth operation and possibly time-resolved detection of spin resonance in donor-based semiconductor devices. Here we present the effect of microwave (MW) power and MOSFET biasing conditions on the EDMR signals.