Gregory Herbert Evans

A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames

The application of laminar flamelet concepts to turbulent flame propagation requires a detailed understanding of strained laminar flames. Here we use numerical methods, including are-length continuation, to simulate the complex chemical kinetic behavior in premixed methane-air flames that are stabilized between two opposed-flow burners. We predict both the detailed structure and the extinction limits for these flames over a range of fuel-air mixtures. In addition to discussing the flame structure, a sensitivity analysis provides further insight on the chemical behavior near extinction. Finally, we discuss the comparison of the predictions with Laws experimental extinction data. An especially important aspect of this comparison is the recognition that fluid mechanical aspects of the traditional strained-flame analysis are deficient in representing experiments such as Laws. We develop and solve a new system of equations that is able to describe the experiments much more accurately.

Final report on LDRD project: A phenomenological model for multicomponent transport with simultaneous electrochemical reactions in concentrated solutions

A phenomenological model was developed for multicomponent transport of charged species with simultaneous electrochemical reactions in concentrated solutions, and was applied to model processes in a thermal battery cell. A new general framework was formulated and implemented in GOMA (a multidimensional, multiphysics, finite-element computer code developed and being enhanced at Sandia) for modeling multidimensional, multicomponent transport of neutral and charged species in concentrated solutions. The new framework utilizes the Stefan-Maxwell equations that describe multicomponent diffusion of interacting species using composition-insensitive binary diffusion coefficients. The new GOMA capability for modeling multicomponent transport of neutral species was verified and validated using the model problem of ternary gaseous diffusion in a Stefan tube. The new GOMA-based thermal battery computer model was verified using an idealized battery cell in which concentration gradients are absent; the full model was verified by comparing with that of Bernardi and Newman (1987) and validated using limited thermal battery discharge-performance data from the open literature (Dunning 1981) and from Sandia (Guidotti 1996). Moreover, a new Liquid Chemkin Software Package was developed, which allows the user to handle manly aspects of liquid-phase kinetics, thermodynamics, and transport (particularly in terms of computing properties). Lastly, a Lattice-Boltzmann-based capability was developed for modeling pore- or micro-scale phenomena involving convection, diffusion, and simplified chemistry; this capability was demonstrated by modeling phenomena in the cathode region of a thermal battery cell.

Validation predictions of a 13 m/s cross-wind fire for Fuego and the University of Waterloo dataset.

Detailed herein are the results of a validation comparison. The experiment involved a 2 meter diameter liquid pool of Jet-A fuel in a 13 m/s crosswind. The scenario included a large cylindrical blocking object just down-stream of the fire. It also included seven smaller calorimeters and extensive instrumentation. The experiments were simulated with Fuego. The model included several conduction regions to model the response of the calorimeters, the floor, and the large cylindrical blocking object. A blind comparison was used to compare the simulation predictions with the experimental data. The more upstream data compared very well with the simulation predictions. The more downstream data did not compare very well with the simulation predictions. Further investigation suggests that features omitted from the original model contributed to the discrepancies. Observations are made with respect to the scenario that are aimed at helping an analyst approach a comparable problem in a way that may help improve the potential for quantitative accuracy.

Experimental investigation of a cylinder in turbulent thermal convection with an imposed shear flow.

An experimental investigation is made into the fluid mechanics and heat transfer of a circular cylinder immersed in a wall-bounded turbulent mixed-convection flow of water. The cylinder is oriented spanwise to the forced channel flow and within the thermal boundary layer of the heated lower wall. The flow channel is capped with a cold, near-adiabatic upper wall producing a fully turbulent gap Rayleigh number of 10 8 . A low-speed crossflow is applied to advect the turbulent thermal plumes over the cylinder surface. We present spatially resolved cylinder-surface heat-flux data alongside 2-D PIV imaging of the streamwise and wall- normal velocity components for two flow conditions in the mixed-convection heat-transfer regime. The measured cylinder-wake flowfield reflects the complex coupling between the separated wake flow, the highly turbulent freestream and the buoyant wall and cylinder boundary layers. A method for measurement of spatially resolved surface heat fluxes based on the measured cylinder-surface temperature distribution and a well-posed two-dimensional solution to the conduction problem in the cylinder wall is presented. The resulting spatially resolved flux measurements show enhanced surface heat transfer, which results from the intense buoyancy generated free-stream turbulence and mixing in the cylinder wake. This work extends the literature on thermal convection with crossflow well into the turbulent regime and is, to our knowledge, the first investigation of surface heat-transfer to an object of engineering importance placed in this type of turbulent mixed-convection flowfield. The data are currently being utilized for validation of mixed- convection turbulence models at Sandia and comparisons between the computational and experimental results are presented.

Transient PVT measurements and model predictions for vessel heat transfer. Part II.

A series of experiments consisting of vessel-to-vessel transfers of pressurized gas using Transient PVT methodology have been conducted to provide a data set for optimizing heat transfer correlations in high pressure flow systems. In rapid expansions such as these, the heat transfer conditions are neither adiabatic nor isothermal. Compressible flow tools exist, such as NETFLOW that can accurately calculate the pressure and other dynamical mechanical properties of such a system as a function of time. However to properly evaluate the mass that has transferred as a function of time these computational tools rely on heat transfer correlations that must be confirmed experimentally. In this work new data sets using helium gas are used to evaluate the accuracy of these correlations for receiver vessel sizes ranging from 0.090 L to 13 L and initial supply pressures ranging from 2 MPa to 40 MPa. The comparisons show that the correlations developed in the1980s from sparse data sets perform well for the supply vessels but are not accurate for the receivers, particularly at early time during the transfers. This report focuses on the experiments used to obtain high quality data sets that can be used to validate computational models. Part II of this report discusses how these data were used to gain insight into the physics of gas transfer and to improve vessel heat transfer correlations. Network flow modeling and CFD modeling is also discussed.

Comparison of high pressure transient PVT measurements and model predictions. Part I.

A series of experiments consisting of vessel-to-vessel transfers of pressurized gas using Transient PVT methodology have been conducted to provide a data set for optimizing heat transfer correlations in high pressure flow systems. In rapid expansions such as these, the heat transfer conditions are neither adiabatic nor isothermal. Compressible flow tools exist, such as NETFLOW that can accurately calculate the pressure and other dynamical mechanical properties of such a system as a function of time. However to properly evaluate the mass that has transferred as a function of time these computational tools rely on heat transfer correlations that must be confirmed experimentally. In this work new data sets using helium gas are used to evaluate the accuracy of these correlations for receiver vessel sizes ranging from 0.090 L to 13 L and initial supply pressures ranging from 2 MPa to 40 MPa. The comparisons show that the correlations developed in the 1980s from sparse data sets perform well for the supply vessels but are not accurate for the receivers, particularly at early time during the transfers. This report focuses on the experiments used to obtain high quality data sets that can be used to validate computational models. Part II of this report discusses how these data were used to gain insight into the physics of gas transfer and to improve vessel heat transfer correlations. Network flow modeling and CFD modeling is also discussed.