72 Lomitas Road

Danville, CA, United States

72 Lomitas Road

Danville, CA, United States
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Kerstein A.R.,72 Lomitas Road | Movaghar A.,Chalmers University of Technology | Oevermann M.,Chalmers University of Technology
Journal of Fluid Mechanics | Year: 2017

Previous studies have predicted We-2/5 dependence of the streamwise location at which primary breakup of turbulent liquid jets begins and We-3/5 dependence of the Sauter mean diameter (SMD) of droplets released at that location, where We is the jet Weber number. Measured deviations from these predictions were attributed to measurement uncertainties and to the simplicity of the analysis, which invoked turbulence inertial-range phenomenology. Here, it is proposed that breakup onset is instead controlled by the residual presence of the boundary-layer structure of the nozzle flow in the near field of the jet. Assuming that the size of the breakup-inducing eddy is within the scale range of the log-law region, We-1 dependence of both the onset location and the SMD at onset is predicted. These dependences agree with the available measurements more closely than those previously predicted. To predict the dependences on the Reynolds number Re, either the friction velocity in conjunction with the Blasius friction law or the bulk velocity can be used, where the former yields Re3/8 and Re1/4 dependence of the onset location and the SMD at onset respectively, while the latter implies no Re dependence of either. The latter result is consistent with the available measurements, but the boundary-layer analysis indicates that the velocity scaling should be based on the friction velocity rather than the bulk velocity, so the origin of the measured lack of Re dependence merits further investigation. A plausible hypothesis is that pressure effects associated with the transition from wall-bounded nozzle flow to jet free-slip boundary conditions induce a transient large-scale flow modification that counteracts the Re dependence of the nozzle flow while preserving the logarithmic flow structure near the jet surface. Notwithstanding the absence of direct evidence supporting this hypothesis, the new analysis and comparisons of its predictions with measurements suggest that transient effects such as the residual influence of the nozzle-flow structure are the likely explanations of the observed parameter dependences. © 2016 Cambridge University Press.


Kolla H.,Sandia National Laboratories | Hawkes E.R.,University of New South Wales | Kerstein A.R.,72 Lomitas Road | Chen J.H.,Sandia National Laboratories
Notes on Numerical Fluid Mechanics and Multidisciplinary Design | Year: 2014

One dimensional auto and cross correlation functions, and corresponding spectra, of reactive scalars in turbulent premixed flames are studied using fully compressible three dimensional direct numerical simulations (DNS) with detailed chemical kinetics and transport properties. The configuration considered is a temporally evolving rectangular slot-jet premixed flame interacting with intense shear driven turbulence. Three simulations were performed by keeping the jet Reynolds number fixed at 10000 and varying the Damköhler number by a factor of four. The fuel is lean premixed hydrogen at an equivalence ratio of 0.7 pre-heated to 700 K and the pressure is 1 atm. Spectra of velocities and scalars, both major species and intermediate radicals, resemble the classical shape with at least one decade of k− 5/3 scaling denoting the inertial range. Qualitative differences arise only in the high wave number reactive-diffusive range with a spike observed in the hydrogen spectrum. However the spectral coherence between various species, which is highest at low wave numbers, begins to drop before suddenly rising significantly at the high wave numbers corresponding to laminar flame scales. This is very likely due to the non-negligible influence of finite rate chemistry effects at high wave numbers. © Springer-Verlag Berlin Heidelberg 2014.


Kolla H.,Sandia National Laboratories | Hawkes E.R.,University of New South Wales | Kerstein A.R.,72 Lomitas Road | Swaminathan N.,University of Cambridge | Chen J.H.,Sandia National Laboratories
Journal of Fluid Mechanics | Year: 2014

Kinetic energy and reactive scalar spectra in turbulent premixed flames are studied from compressible three-dimensional direct numerical simulations (DNS) of a temporally evolving rectangular slot-jet premixed flame, a statistically one-dimensional configuration. The flames correspond to a lean premixed hydrogen-air mixture at an equivalence ratio of 0.7, preheated to 700 K and at 1 atm, and three DNS are considered with a fixed jet Reynolds number of 10 000 and a jet Damköhler number varying between 0.13 and 0.54. For the study of spectra, motivated by the need to account for density change, which can be locally strong in premixed flames, a new density-weighted definition for two-point velocity/scalar correlations is proposed. The density-weighted two-point correlation tensor retains the essential properties of its constant-density (incompressible) counterpart and recovers the density-weighted Reynolds stress tensor in the limit of zero separation. The density weighting also allows the derivation of balance equations for velocity and scalar spectrum functions in the wavenumber space that illuminate physics unique to combusting flows. Pressure-dilatation correlation is a source of kinetic energy at high wavenumbers and, analogously, reaction rate-scalar fluctuation correlation is a high-wavenumber source of scalar energy. These results are verified by the spectra constructed from the DNS data. The kinetic energy spectra show a distinct inertial range with a -5/3 scaling followed by a 'diffusive-reactive' range at higher wavenumbers. The exponential drop-off in this range shows a distinct inflection in the vicinity of the wavenumber corresponding to a laminar flame thickness, δL, and this is attributed to the contribution from the pressure-dilatation term in the energy balance in wavenumber space. Likewise, a clear spike in spectra of major reactant species (hydrogen) arising from the reaction-rate term is observed at wavenumbers close to δL. It appears that in the inertial range classical scaling laws for the spectra involving the Kolmogorov scale are applicable, but in the high-wavenumber range where chemical reactions have a strong signature the laminar flame thickness produces a better collapse. It is suggested that a full scaling should perhaps involve the Kolmogorov scale, laminar flame thickness, Damköhler number and Karlovitz number. © © 2014 Cambridge University Press.


Mayo J.R.,Sandia National Laboratories | Kerstein A.R.,72 Lomitas Road
Proceedings of the 9th Asia-Pacific Conference on Combustion, ASPACC 2013 | Year: 2013

A dynamically passive, randomly advected front propagating at a given speed relative to fluid motion is both a conceptual paradigm of turbulent premixed combustion phenomenology and a starting point for the development of various models that are applied to practical conditions far from flamelet behavior. Progress in the analysis of this fundamental case is summarized, and new results are presented. The weakly turbulent regime is relatively tractable, and its identified behaviors have important implications for the strongly turbulent regime that is more relevant to turbulent premixed combustion. For that regime, the importance of turbulence intermittency is noted. New computational results for intermittency statistics of the 'G field' used in the level-set representation of the flame surface are reported. These results suggest that the G field not only exhibits structure-function saturation but closely approximates the previously hypothesized but heretofore unsubstantiated complete saturation of an ideal advected scalar property. In this regard, the G field is more ideal than the diffusive scalar. Implications for turbulent premixed combustion are noted.


Kerstein A.R.,72 Lomitas Road
Journal of Fluid Mechanics | Year: 2014

A novel concept for simulation of turbulent mixing, termed hierarchical parcel swapping (HiPS), was recently proposed. The method involves either a parameterized representation of the turbulent flow or a more self-contained flow simulation. As a step toward turbulent mixing applications, the latter formulation is used for the first numerical demonstration of model performance. Owing to its suitability for this purpose and its role as a canonical benchmark, channel flow is the target application. Despite its idealized representation of this flow, HiPS is shown to capture salient features of the flow with a notable degree of quantitative accuracy. The implications of this finding with regard to flow physics and with regard to the applicability of HiPS to other problems are discussed. © 2014 Cambridge University Press.


Sannan S.,Sintef | Kerstein A.R.,72 Lomitas Road
Energy Procedia | Year: 2016

Differential molecular diffusion effects are expected to be of particular importance in the combustion of hydrogen and H2-enriched fuels. The mixing between fuel and oxidizer can be significantly influenced, implying changed chemical reaction rates and overall heat release. Here, the LEM3D model based on the Linear Eddy Model (LEM) is employed to simulate differential diffusion effects in a turbulent round jet of H2 and Freon 22 issuing into air. Input to LEM3D, generated from ANSYS Fluent, consists of velocity profiles and profiles for the turbulent diffusivity, the Kolmogorov scale and the integral length scale. In this paper, the LEM3D-Fluent coupling is demonstrated by a coarse steady-state RANS simulation in Fluent with a one-to-one correspondence between the RANS grid cells and the LEM3D control volumes. The LEM3D simulation results are compared with previously obtained measurements of differential molecular diffusion for the given flow configuration.


Kerstein A.R.,72 Lomitas Road
Journal of Statistical Physics | Year: 2013

An economical representation of effects of turbulence on the time-evolving structure of diffusive scalar fields is obtained by introducing a hierarchical (tree) network connecting fluid parcels, with effects of turbulent advection represented by swapping pairs of sub-trees at rates determined by turbulence time scales associated with the sub-trees. The fluid parcels reside at the base of the tree. The tree structure partitions the fluid parcels into adjacent pairs (or more generally, p-tuples). Adjacent parcels intermix at rates governed by diffusion time scales based on molecular diffusivities and parcel sizes. This simple procedure efficiently accomplishes long-standing objectives of turbulent mixing model development, such as generating physically based time histories of fluid-parcel nearest-neighbor encounters and the associated spatial structure of turbulent scalar fields. Correspondences between features of the hierarchical formulation and turbulent mixing phenomenology, both generic and case-specific, are noted. © 2013 The Author(s).


Sannan S.,Sintef | Kerstein A.R.,72 Lomitas Road
Energy Procedia | Year: 2013

The LEM3D stochastic model for numerical simulation of turbulent mixing is used to simulate differential molecular diffusion effects in an isothermal jet of hydrogen and Freon 22 issued into air. The computations are compared with a published experimental study of the flow configuration. Salient features of the measured results are reproduced qualitatively, but the absence of spatial variation of the smallest eddy motion in LEM3D omits the streamwise variation of this length scale in the experimental configuration, resulting in a systematic deviation from the experimental trend. A first-principles basis for incorporating this missing physics into LEM3D is described, indicating the path forward for physically based quantitative prediction of differential diffusion effects, and turbulent combustion phenomenology more generally, using LEM3D. © 2013 Published Elsevier Ltd.


Schulz F.T.,TU Brandenburg | Glawe C.,TU Brandenburg | Schmidt H.,TU Brandenburg | Kerstein A.R.,72 Lomitas Road
Environmental Earth Sciences | Year: 2013

Although pipe transport for storage of CO2 captured from combustion exhaust is most efficient under supercritical conditions, subcritical multi-phase transport might sometimes occur intentionally or unintentionally. To adequately assess the consequences of subcriticality, the fidelity of subcritical multi-phase turbulent transport modeling, including confinement and buoyancy effects, must be improved. For this reason, an extension of one-dimensional turbulence, a stochastic turbulence modeling strategy, for application to this regime is underway. As a step toward this extension, a turbulent liquid jet issuing from a planar channel is simulated and results are compared with liquid jet measurements and channel flow numerical simulations. A previously noted turbulence decay scaling is reproduced, suggesting that the scaling is more robust than might be supposed, given the complicating factors. This work provides a basis for extension to the primary breakup regime of liquid jets and hence to subcritical multi-phase turbulence more generally. © 2013 Springer-Verlag Berlin Heidelberg.

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