Current Issue : July - September Volume : 2013 Issue Number : 3 Articles : 5 Articles
The performance of algebraic flame surface density (FSD) models has been assessed for flames with nonunity Lewis number (Le)\r\nin the thin reaction zones regime, using a direct numerical simulation (DNS) database of freely propagating turbulent premixed\r\nflames with Le ranging from 0.34 to 1.2. The focus is on algebraic FSD models based on a power-law approach, and the effects\r\nof Lewis number on the fractal dimension D and inner cut-off scale ?i have been studied in detail. It has been found that D is\r\nstrongly affected by Lewis number and increases significantly with decreasing Le. By contrast, ?i remains close to the laminar flame\r\nthermal thickness for all values of Le considered here. A parameterisation of D is proposed such that the effects of Lewis number\r\nare explicitly accounted for. The new parameterisation is used to propose a new algebraic model for FSD. The performance of the\r\nnew model is assessed with respect to results for the generalised FSD obtained from explicitly LES-filtered DNS data. It has been\r\nfound that the performance of the most existing models deteriorates with decreasing Lewis number, while the newly proposed\r\nmodel is found to perform as well or better than the most existing algebraic models for FSD....
Reduced-gravity experiments were performed on combustion of droplets composed of n-heptane mixed with methanol or ethanol.\r\nThe initial alcohol mass fraction in a droplet was 0% (pure heptane) or 5%. The experiments were performed at 0.1MPa and 25�°C\r\nwith air or with ambients of oxygen and helium with oxygen mole fractions of 0.3 or 0.4. Initial droplet diameters were in the\r\nrange 0.67mmto 0.92 mm. After consideringmeasurement uncertainties, burning rates decreased appreciably as the initial droplet\r\ndiameter increased for combustion in air but not for combustion in the oxygen/heliumenvironments. It was also found that addition\r\nof either methanol or ethanol did not influence burning rates appreciably and that burning rates were larger for the oxygen/helium\r\nenvironments than for air if initial droplet diameter dependences were accounted for....
First test flights using blends with algae oil are already carried out and expectations by the aviation and other industries are high.\r\nOn the other hand technical data about performance of cultivation systems, downstream processing, and suitability of algae oil as\r\nfuel are still limited. The existing microalgae growing industry mainly produces for the food and feed market. Energy efficiency is\r\nso far out of scope but needs to be taken into account if the product changes to biofuel. Energy and CO2 balances are used to estimate\r\nthe potential of algae oil to fulfil the EU sustainability criteria for biofuels. The analysis is supported by lab tests as well as data\r\ngained by a pilot scale demonstrator combined with published data for well-known established processes. The algae oil composition\r\nis indicator of suitability as fuel as well as for economic viability. Approaches attaining high value fractions are therefore of\r\ngreat importance and will be discussed in order to determine the most intended market....
A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of\r\ndifferent turbulent Reynolds numbers has been used to assess the performance of algebraic flame surface density (FSD) models\r\nbased on a fractal representation of the flame wrinkling factor. The turbulent Reynolds number Ret has been varied by modifying\r\nthe Karlovitz number Ka and the Damk�¨ohler number Da independently of each other in such a way that the flames remain\r\nwithin the thin reaction zones regime. It has been found that the turbulent Reynolds number and the Karlovitz number both\r\nhave a significant influence on the fractal dimension, which is found to increase with increasing Ret and Ka before reaching an\r\nasymptotic value for large values of Ret and Ka. A parameterisation of the fractal dimension is presented in which the effects of\r\nthe Reynolds and the Karlovitz numbers are explicitly taken into account. By contrast, the inner cut-off scale normalised by the\r\nZelâ��dovich flame thickness ?i/dz does not exhibit any significant dependence on Ret for the cases considered here. The performance\r\nof several algebraic FSD models has been assessed based on various criteria. Most of the algebraic models show a deterioration in\r\nperformance with increasing the LES filter width....
A Dynamic model of Homogeneous Charge Compression Ignition (HCCI), based on chemical kinetics principles and artificial\r\nintelligence, is developed. The model can rapidly predict the combustion probability, thermochemistry properties, and exact\r\ntiming of the Start of Combustion (SOC). A realization function is developed on the basis of the Sandia National Laboratory\r\nchemical kinetics model, and GRI3.0 methane chemical mechanism. The inlet conditions are optimized by Genetic Algorithm\r\n(GA), so that combustion initiates and SOC timing posits in the desired crank angle. The best SOC timing to achieve higher\r\nperformance and efficiency in HCCI engines is between 5 and 15 degrees crank angle (CAD) after top dead center (TDC). To\r\nachieve this SOC timing, in the first case, the inlet temperature and equivalence ratio are optimized simultaneously and in the\r\nsecond case, compression ratio is optimized by GA. The model�s results are validated with previous works. The SOC timing can be\r\npredicted in less than 0.01 second and the CPU time savings are encouraging. This model can successfully be used for real engine\r\ncontrol applications....
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