Correlation of Flame Speed With Stretch in Turbulent Premixed Methane/Air Flames

Direct numerical simulations of two-dimensional unsteady premixed methane/air flames are performed to determine the correlation of flame speed with stretch over a wide range of curvatures and strain rates generated by intense two-dimensional turbulence. Lean and stoichiometric premixtures are considered with a detailed C{sub 1}-mechanism for methane oxidation. The computed correlation shows the existence of two distinct stable branches. It further shows that exceedingly large negative values of stretch can be obtained solely through curvature effects which give rise to an overall nonlinear correlation of the flame speed with stretch. Over a narrower stretch range, {minus}1 {le} Ka {le} 1, which includes 90% of the sample, the correlation is approximately linear, and hence, the asymptotic theory for stretch is practically applicable. Overall, one-third of the sample has negative stretch. In this linear range, the Markstein number associated with the positive branch is determined and is consistent with values obtained from comparable steady counterflow computations. In addition to this conventional positive branch, a negative branch is identified. This negative branch occurs when a flame cusp, with a center of curvature in the burnt gases, is subjected to intense compressive strain, resulting in a negative displacement speed. Negative flame speeds are also encountered for extensive tangential strain rates exceeding a Karlovitz number of unity, a value consistent with steady counterflow computations

Ignition of hydrogen/air mixing layer in turbulent flows

Autoignition of a scalar hydrogen/air mixing layer in homogeneous turbulence is studied using direct numerical simulation. An initial counterflow of unmixed nitrogen-diluted hydrogen and heated air is perturbed by two-dimensional homogeneous turbulence. The temperature of the heated air stream is chosen to be 1,100 K which is substantially higher than the crossover temperature at which the rates of the chain branching and termination reactions become equal. Three different turbulence intensities are tested in order to assess the effect of the characteristic flow time on the ignition delay. For each condition, a simulation without heat release is also performed. The ignition delay determined with and without heat release is shown to be almost identical up to the point of ignition for all of the turbulence intensities tested, and the predicted ignition delays agree well within a consistent error band. It is also observed that the ignition kernel always occurs where hydrogen is focused, and the peak concentration of HO{sub 2} is aligned well with the scalar dissipation rate. The dependence of the ignition delay on turbulence intensity is found to be nonmonotonic. For weak to moderate turbulence the ignition is facilitated by turbulence via enhanced mixing, while for stronger turbulence, whose timescale is substantially smaller than the ignition delay, the ignition is retarded due to excessive scalar dissipation, and hence diffusive loss, at the ignition location. However, for the wide range of initial turbulence fields studied, the variation in ignition delay due to the corresponding variation in turbulence intensity appears to be quite small

Characteristic Boundary Conditions for Direct Simulations of Turbulent Counterflow Flames

The direct numerical simulation of counterflow flames using a compressible flow formulation has encountered many numerical difficulties associated with inflow and outflow boundary conditions. In this study, one of the most critical issues is found to be the treatment of the transverse terms in the standard local one-dimensional inviscid (LODI) relations. While this issue applies to the general Navier-Stokes characteristic boundary conditions (NSCBC), the counterflow configuration is one of the first showcases to reveal that the transverse terms can no longer be ignored in the LODI relations. In this study, a modified formulation of LODI including the transverse terms is derived for both inflow and outflow boundary conditions that can prevent spurious pressure and velocity behavior at the boundaries. Test simulations demonstrate that retaining the transverse terms in LODI relations is critical in ensuring correct pressure and velocity fields. In analogy with the pressure damping used in previous studies, an additional transverse term damping is proposed, for which optimal damping coefficients are identified by considering the steady state limit. Applications to nonreacting and reacting counterflow simulations demonstrate the robustness of the improved boundary conditions and their successful implementation

Göttinger Bodenkundliche Berichte 44

Trotz jahrzehntelanger methodischer Forschung auf dem Gebiet der Bodenfeuchte-Bestimmung sind viele damit zusammenhängende Probleme bislang als technisch nicht gelöst zu betrachten. Wandlungen und Fortschritte im Bereich der boden-hydrologischen und boden-ökologischen Forschung stellen neue Anforderungen an die Meß-Technik. So verlangen z.B. Untersuchungen der Bodenwasser-Bilanz im Felde, die auf der kontinuierlichen Bestimmung hydraulischer Gradienten und der Ermittlung von spezifischen W asser-Leitfähigkelten der einzelnen Boden-Schichten basieren, exakte und kontinuierliche Bestimmungen der Boden-Wassergehalte möglichst dUnnen Boden-Schichten. Dies erweist sich als umso notwendiger, als neuere Untersuchungen zur Saugspannungs-W asser- gehalts-Hysterese bei Wasser-Aufnahme und -Abgabe im Boden gezeigt haben, daß die zur Berechnung erforderlichen Wasser- gehalts-Angaben durch direkte Messungen ermit telt werden sollten und nicht als abgeleitete Größen, z.B. aus der Saugspannung Uber die Wassergehalts- Saugspannungs-Charakteristik, verwendet werden sollten.researc