In this study, the fundamental problem of the ignition of a kerosene single droplet in a quiescent medium at engine high-altitude relight conditions is investigated using numerical simulations. The ...main objective is to improve the understanding of ignition phenomena with a focus on the effect of droplet evaporation in determining the growth of the ignition kernel and flame establishment. Results show that when the droplet is fully immersed in a high temperature region, ignition occurs when the scalar dissipation rate associated with evaporation decreases enough to allow the initiation of a flame. The ignition time depends on the droplet diameter and the far field temperature, i.e. the position of the droplet with respect to the spark location. As the fuel is consumed, the flame is found to move closer to the droplet surface until the flame cannot sustain itself any more due to increasing scalar dissipation rates. Furthermore, results show that at very low temperatures typical of high-altitude relight conditions no flammable mixture is available around the droplet. Therefore, the success of an ignition event mainly depends on the energy released by the spark and the rate at which this energy is diffused toward the droplet surface to enhance the evaporation rate and create a flammable mixture. The findings are analysed from the perspective of gas turbine applications.
The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an ...igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.
The cryptomonad Pyrenomonas salina presumably has arisen from a symbiotic event involving a flagellated phagotrophic host cell and a photosynthetic eukaryote as the symbiont. Correspondingly, in this ...unicellular alga there are four different genomes, e.g., the nuclear and the mitochondrial genomes of the host cell as well as the plastid genome and the genome contained in the vestigial nucleus of the endocytobiont (nucleomorph). To analyze the origin of one of the symbiotic partners the small subunit rRNA gene sequence of the host cell nucleus was determined, and a secondary structure model has been constructed. This sequence is compared to those of 40 other eukaryotes. A phylogenetic tree constructed using the neighborliness method revealed a close relationship between the host cell of P. salina and the chlorophytes, whereas the rhodophytes diverge more deeply in the tree.
Ripe, deep-red fruits of Palisota barteri contain tubulous chromoplasts which develop from unpigmented leucoplasts. These plastids contain, besides large spherical inclusion bodies, numerous ...osmiophilic globules which, in the course of ripening, frequently show transition states to tubular structures. The tubules contain, in all stages of their development, acylated β-citraurin, which is also the main pigment of Citrus fruits. The tubular structures have been isolated, fragmented by French-pressure treatment, and separated into three fractions on sucrose gradients. The lightest fraction (1.044 g·cm-3) contained thick fragments ('saccules') with diameters of 50—60 nm, whereas the heaviest (1.083 g·cm-3) consisted of tubules 20—25 nm in diameter. The relative amounts of polar lipids, proteins, and carotenoids of the different fractions are consistent with a molecular structure model of tubules and saccules, according to which a wick of longitudinally oriented carotenoid molecules of variable thickness is coated by a monolayer of polar lipids and proteins. High-resolution 'negative-stainings' showed the surface of the tubules to be covered with globular particles of about 6 nm diameter. The main protein of all fractions is a 30-kDa polypeptide; it is assumed that the particles are oligomers of this specific protein.
The size, frequency and distribution of the nucleoids of chloroplasts (cl-nucleoids) and chromoplasts (cr-nucleoids) of the daffodil have been investigated in situ using the DNA-specific fluorochrome ...4′6-diamidino-2-phenylindole. Chromoplasts contain fewer nucleoids (approx. 4) than chloroplasts (> 10), and larger chromoplasts (cultivated form, approx. 4) contain more than smaller ones (wild type, approx. 2). During chromoplast development the nucleoid number decreases in parallel with the chlorophyll content. Each nucleoid contains 2—3 plastome copies on average. In chloroplasts the nucleoids are evenly distributed, whereas they are peripherally located in chromoplasts. The fine structure of isolated cl- and crnucleoids, purified either by Sepharose 4B-CL columns or by metrizamide gradients, was investigated electron microscopically. The cl-nucleoids consist of a central protein-rich core with 'naked' DNA-loops protruding from it. In cr-nucleoids, on the other hand, the total DNA is tightly packed within the proteinaceous core. The protein-containing core region of the nucleoids is made up of knotty and fibrillar sub-structures with diameters of 18 and 37 nm, respectively. After proteinase treatment, or increasing ion concentration, most of the proteins are removed and the DNA is exposed even in the case of cr-nucleoids, the stability of which proved to be greater than that of cl-nucleoids. The chemical composition of isolated plastid nucleoids has been determined qualitatively and quantitatively. Chromoplast-nucleoids contain, relative to the same DNA quantity, about six times as much protein as cl-nucleoids. Accordingly the buoyant density of cr-nucleoids in metrizamide gradients is higher than that of cl-nucleoids. In addition to DNA and protein, RNA could be found in the nucleoid fraction. No pigments were present. The cr- and cl-nucleoids have many identical proteins. There are, however, also characteristic differences in their protein pattern which are possibly related to the different expression of the genomes of chloroplasts and chromoplasts. Nucleoids of both plastid types contain some proteins which also occur in isolated envelope membranes (probably partly in the outer membrane) and thus possibly take part in binding the DNA to membranes.
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