Sheldon R. Tieszen

Guidance on risk analysis and safety implications of a large liquefied natural gas (LNG) spill over water.

While recognized standards exist for the systematic safety analysis of potential spills or releases from LNG (Liquefied Natural Gas) storage terminals and facilities on land, no equivalent set of standards or guidance exists for the evaluation of the safety or consequences from LNG spills over water. Heightened security awareness and energy surety issues have increased industrys and the publics attention to these activities. The report reviews several existing studies of LNG spills with respect to their assumptions, inputs, models, and experimental data. Based on this review and further analysis, the report provides guidance on the appropriateness of models, assumptions, and risk management to address public safety and property relative to a potential LNG spill over water.

Gaseous hydrocarbon-air detonations

Abstract Detonation cell width measurements were made on mixtures of air and methane, ethane, dimethylether, nitroethane, ethylene, acetylene, propane, 1,2-epoxypropane, n-hexane, 1-nitrohexane, mixed primary hexylnitrate, n-octane, 2,2,4-trimethylpentane, cyclooctane, 1-octene, cis-cyclooctene, 1,7-octadiene, 1-octyne, n-decane, 1,2-epoxydecane, pentylether, and JP4. Cell width measurements were carried out at 25 and 100°C for some of these fuelair mixtures. For the stoichiometric alkanes, alkenes, and alkynes, there is a very slight decrease in detonation cell width with increasing initial temperature from 25°C to 100°C, although the differences are within the experimental uncertainties in cell width measurements. Also within the uncertainty limits of the measurements, there is no variation in detonation cell width with increase fuel molecular weight for n-alkanes from ethane to n-decane. Molecular structure is found to affect detonability for C8 hydrocarbons, where the saturated ring structure is more sensitive than the straight-chain alkane, which is more sensitive than the branched-chain alkane. Unsaturated alkenes and alkynes are more sensitive to detonation than saturated alkanes. However, the degree of sensitization decreases with increasing molecular weight. Addition of functional groups such as nitro, nitrate, epoxy, and ethers is found to significantly reduce the detonation cell width from the parent n-alkane. Nitrated n-alkanes can be more sensitive than hydrogenair mixtures. The increase in sensitivity of epoxy groups appears to be related to the oxygen to carbon ratio of the molecule. A numerical model, using a simplified ZND analysis and a detailed chemical kinetic reaction mechanism, was used to interpret the experimental results. The model indicates the effect that each factor—fuel molecule size, fuel structure, initial temperature, bond saturation, and inclussion of different functional groups—has on the computed induction length under detonation conditions.

Experimental study of the flow field in and around a one meter diameter methane fire

Abstract Simultaneous temporally and spatially resolved, 2-D velocity fields are obtained using Particle Image Velocimetry (PIV) in a one-meter diameter methane fire. The flow rate of methane is 0.066 kg/m2-s, comparable to fuel burning rates in a large JP8 pool fire. Raw PIV images are recorded with 35 mm cinematography at 200 images/s. They are digitized and post-processed to obtain velocity data for a region ∼0.8 m high by 1 m wide centered on the centerline of the flame and extending from just above the surface of the burner to include the fuel core, near-field combusting zones, and surrounding air. The data cover 11 puff cycles of the fire. Instantaneous, phase-, and time-averaged 2-D velocity plots (103 × 82 vectors) are obtained for each of 1331 time-planes (121 time-planes per puff cycle) spaced 5 ms apart. Each vector represents a statistical estimate of the velocity in 2.1 cm by 2.1 cm by 0.8 cm volumes, which are overlapped by 50% in the vector plots. Time-averaged turbulent statistics ( u′ 2 , v′ 2 , & u′v′ ) are also presented. Boundary conditions have been carefully measured and the results are intended for validation of numerical simulations of the fire behavior. The results clearly show the dominant effect of puffing, measured at 1.65 cycles/s for this fire, on the temporal and spatial development of the velocity field.

A heuristic model of turbulent mixing applied to blowout of turbulent jet diffusion flames

Abstract A phenomenological study has been conducted on jet flames near blowout for the purpose of determining the blowout mechanism. The authors show the successful blowout correlation of Broadwell et al. [ Twentieth Symposium (International) on Combustion , 1984, p. 303] can be derived from the assumptions of Vanquickenborne and van Tigglen [ Combust. Flame 10:59 (1966)], namely, that blowout is a competition between the local premixed turbulent flame speed and the local flow velocity. The authors argue that the role of coherent, large-scale, rotational structures found in jet turbulence is to enhance the turbulent flame speed near blowout. Experiments were conducted which show that nearly the entire cross-section of the jet is combusting in a premixed flame near blowout. This is distinct from a lifted flame that combusts only near the outer edge of the jet. The length and time scales used in the derivation of the blowout mechanism are compared with those observed in the experiments and found to be consistent with the data.

Experimental study of a turbulent buoyant helium plume

An experimental study has been performed on the dynamics of a large turbulent buoyanthelium plume. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) while helium mass fraction was determined by planar laser-induced fluorescence (PLIF). PIV and PLIF were performed simultaneously in order to obtain velocity and mass fraction data over a plane that encompassed the plume core, the near-field mixing zones and the surrounding air. The Rayleigh–Taylor instability at the base of the plume leads to the vortex that grows to dominate the flow. This process repeats in a cyclical manner. The temporally and spatially resolved data show a strong negative correlation between density and vertical velocity, as well as a strong 90° phase lag between peaks in the vertical and horizontal velocities throughout the flow field owing to large coherent structures associated with puffing of the turbulent plume. The joint velocity an mass fraction data are used to calculate Favre-averaged statistics in addition to Reynolds-(time) averaged statistics. Unexpectedly, the difference between both the Favre-averaged and Reynolds-averaged velocities and second-order turbulent statistics is less than the uncertainty in the data throughout the flow field. A simple analysis was performed to determine the expected differences between Favre and Reynolds statistics for flows with periodic fluctuations in which the density and velocity fields are perfectly correlated, but have the phase relations as suggested by the data. The analytical results agreewith the data, showing that the Favre and Reynolds statistics will be the same to lead order. The combination of observation and simple analysis suggests that for buoyancy-dominated flows in which it can be expected that density and velocity are strongly correlated,phase relations will result in only second-order differences between Favre- and Reynolds-averaged data in spite of strong fluctuations in both density and velocity.

The influence of initial pressure and temperature on hydrogen-air-diluent detonations☆

Abstract We have studied the influence of pressure and temperature on the detonability of hydrogen-air-diluent mixtures diluted with steam or carbon dioxide. Detonation cell width measurements have been obtained from experiments conducted in a 0.43-m-diameter heated detonation tube. Calculations from a Zeldovich-von Neumann-Doring (ZND) model of a detonation with a detailed chemical-kinetic reaction mechanism for hydrogen oxidation are used to correlate the data. The data show a significant reduction in the ability of a diluent (excess air or hydrogen, carbon dioxide, or steam) to inhibit a detonation as the temperature is increased from 293 to 373 K. Only a small decrease in the cell width is observed with increasing pressure between approximately 1 and 3 atm for hydrogen-air mixtures diluted with steam or excess air. For conditions beyond which data exist, calculations based on the model yield results that indicate similar detonabilities of all mixtures considered at low initial pressures or high initial temperatures. Additionally, these results indicate minima in the cell width for variations in the initial pressure and temperature. The cell minima represent approximately the location of a change in a rate-limiting mechanism corresponding to the extended classical second-limit criterion.

Large eddy simulation and experimental measurements of the near-field of a large turbulent helium plume

Large eddy simulations (LES) are conducted of a large, 1 m in diameter, turbulent helium plume. The plume instability modes and flow dynamics are explored as a function of grid resolution with and without the use of subgrid scale (SGS) models. LES results reproduce well-established varicose puffing mode instabilities as well as secondary “finger-like” azimuthal instabilities leading to the breakdown of periodically shed toroidal vortices. Simulation results of time-averaged velocity and concentration fields show excellent agreement with experimental data collected from Sandia’s FLAME facility using particle image velocimetry and planar laser induced fluorescence measurement techniques. For locations very near the base of the plume, i.e., X/Dp<0.5, the LES overpredicts the measured root-mean squared streamwise velocity and concentration and, in addition, is found to be highly sensitive to grid resolution. The cause of these discrepancies is attributed to unresolved buoyancy-induced vorticity generation on ...

A Spatially Developing One-Dimensional Turbulence (ODT) Study of Soot and Enthalpy Evolution in Meter-Scale Buoyant Turbulent Flames

The interaction between soot and enthalpy evolution in a buoyant turbulent flame, exhibiting key attributes of a fire, is studied using a novel approach. This approach is based on a spatially evolving form of the one-dimensional turbulence (ODT) model that resolves the full range of scales, in a single spatial dimension, from the scale of the plume evolution to that of the soot layers. The model is compared with limited experimental data available. The evolved flow field of a meter-scale flame is then analyzed both in terms of conventional spatial averages and conditional averages. The conditional moments of the terms in the soot and enthalpy evolution equations are analyzed to elucidate the balance between the large-scale evolution (advective terms), the source terms, and the turbulent mixing (dissipative terms). The results show the significant influence of the mixture evolution on the transport of soot relative to the flame (in mixture-fraction space).

A Turbulence Model for Buoyant Flows Based on Vorticity Generation

A turbulence model for buoyant flows has been developed in the context of a k-{var_epsilon} turbulence modeling approach. A production term is added to the turbulent kinetic energy equation based on dimensional reasoning using an appropriate time scale for buoyancy-induced turbulence taken from the vorticity conservation equation. The resulting turbulence model is calibrated against far field helium-air spread rate data, and validated with near source, strongly buoyant helium plume data sets. This model is more numerically stable and gives better predictions over a much broader range of mesh densities than the standard k-{var_epsilon} model for these strongly buoyant flows.

Internal Pressure Loads Due to Gaseous Detonations

The internal loading of structures and confinements by gaseous detonation is studied using a multidimensional strong-shock physics code. Hydrogen-air-steam mixtures are used in calculations to show phenomena that apply qualitatively to any detonable gaseous fuel-oxidant-diluent mixture. Several variables are considered with respect to loading: (a) inert layers of various thicknesses; (b) deflagration-to-detonation transition(DDT) location as compared with direct initiation; and (c) some variations in geometry or confinement. Relatively thin inert layers are shown to increase the peak reflected shock pressure over that which would occur if the inert layer were not there. Inert layers may also increase impulse under some circumstances. DDT increases peak reflected pressures over those seen for direct initiation because of precompression of unburned gases. DDT may also increase impulses. Peak reflected pressures and impulses are greater in edges and corners than on flat surfaces. Internal obstruction tends to randomize the energy in a detonation wave, decreasing the impulse on structures, and allowing the pressure to equilibrate more rapidly than if there were no obstruction.