Current Issue : October - December Volume : 2014 Issue Number : 4 Articles : 5 Articles
The intention of this study is to investigate the effect of surfactant over a water-diesel mixture to make an emulsified fuel. The total surfactant concentration in each of the diesel-water emulsion samples studied (span 20, span 80, tween 20, tween 80) is fixed at 1% w/w. The water content is variable at 5-15% w/w. The non-ionic surfactants used in this study tend to reduce the interfacial tension between water and diesel giving rise to a new fuel with increased burning efficiency as that of diesel alone. It is found that brake power, engine power and also the engine torque have been improved with the emulsified fuels for 15% of water content in the water diesel emulsion....
A lifted H2/N2 turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure. The\nconditional velocity (CV) and the conditional scalar dissipation rate (CSDR) submodels are chosen such that they are fully\nconsistent with the moments of the presumed ? probability density function (PDF). The CV is modelled using the PDF-gradient\ndiffusionmodel. Two CSDR submodels based on the double integration of the homogeneous and inhomogeneous mixture fraction\nPDF transport equations are implemented. The effect of CSDR modelling is investigated over a range of coflow temperatures (Tc)\nand the stabilisationmechanism is determined fromthe analysis of the transport budgets and the history of radical build-up ahead\nof the stabilisation height. For all Tc, the balance between chemistry, axial convection, and micromixing, and the absence of axial\ndiffusion upstreamof the stabilisation height indicate that the flame is stabilized by autoignition.This conclusion is confirmed from\nthe rapid build-up ofHO2 ahead ofH,O, andOH.The inhomogeneous CSDRmodelling yields higher dissipation levels at themost\nreactive mixture fraction, which results in longer ignition delays and larger liftoff heights. The effect of the spurious sources arising\nfrom homogeneous modelling is found to be small but nonnegligible, mostly notably within the flame zone....
Accurate modelling of preignition chemical phenomena requires a detailed description of the respective low-temperature oxidative\nreactions.Motivated by the need to simulate a diesel oil spray evaporation device operating in the ââ?¬Å?stabilizedââ?¬Â cool flame regime, a\nââ?¬Å?tabulated chemistryââ?¬Â tool is formulated and evaluated.Thetool is constructed by performing a large number of kinetic simulations,\nusing the perfectly stirred reactor assumption. n-Heptane is used as a surrogate fuel for diesel oil and a detailed n-heptane\nmechanism is utilized. Three independent parameters (temperature, fuel concentration, and residence time) are used, spanning\nboth the low-temperature oxidation and the autoignition regimes. Simulation results for heat release rates, fuel consumption\nand stable or intermediate species production are used to assess the impact of the independent parameters on the systemââ?¬â?¢s\nthermochemical behaviour. Results provide the physical and chemical insight needed to evaluate the performance of the tool\nwhen incorporated in a CFD code. Multidimensional thermochemical behaviour ââ?¬Å?mapsââ?¬Â are created, demonstrating that cool\nflame activity is favoured under fuel-rich conditions and that cool flame temperature boundaries are extended with increasing\nfuel concentration or residence time....
Amethod is presented to significantly improve the convergence behavior of batch nonpremixed counterflow flame simulationswith\nfinite-rate chemistry.Themethod is applicable to simulations with varying pressure or strain rate, as it is, for example, necessary for\nthe creation of flamelet tables or the computation of the extinction point. The improvement is achieved by estimating the solution\nbeforehand. The underlying scaling rules are derived from theory, literature, and empirical observations. The estimate is used as\nan initialization for the actual solver. This enhancement leads to a significantly improved robustness and acceleration of batch\nsimulations.The extinction point can be simulated without cumbersome code extensions.The method is demonstrated on two test\ncases and the impact is discussed....
In this study the hazardous potential of flammable hydrogen-air mixtures with vertical concentration gradients is investigated\nnumerically.The computational model is based on the formulation of a reaction progress variable and accounts for both deflagrative\nflame propagation and autoignition. The model is able to simulate the deflagration-to-detonation transition (DDT) without\nresolving all microscopic details of the flow. It works on relatively coarse grids and shows good agreement with experiments.\nIt is found that a mixture with a vertical concentration gradient can have a much higher tendency to undergo DDT than a\nhomogeneous mixture of the same hydrogen content. In addition, the pressure loads occurring can be much higher. However,\nthe opposite effect can also be observed, with the decisive factor being the geometric boundary conditions.Themodel gives insight\ninto different modes of DDT. Detonations occurring soon after ignition do not necessarily cause the highest pressure loads. In\nmixtures with concentration gradient, the highest loads can occur in regions of very low hydrogen content. These new findings\nshould be considered in future safety studies....
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