Vaneless Diffuser Performance in a Super-Critical Carbon Dioxide Centrifugal Compressor With Real Gas Effects

Super-critical Carbon dioxide (s-CO2) flows are neither incompressible nor ideal gas flows. Unlike perfect gases, the enthalpy of s-CO2 near the critical point is a strong function of pressure. Incorporation of these effects is necessary for accurate modeling of flows in centrifugal compressor vaneless diffusers. This study reviews the existing vaneless diffuser modeling technique, and modifications are made to incorporate real gas effects. Like the existing procedure, the proposed formulation does not require multiple iterations for convergence. The results are obtained in a single step using a marching technique. Hence, this model can be incorporated in standard centrifugal compressor design and analysis tools, especially for super-critical carbon dioxide flows, subject to experimental validation.

Fast anisotropic Mg and H diffusion in wet forsterite

Mg diffusion, which is important for properties of forsterite such as conductivity and deformation, is a strong function of water content. The mechanism behind this effect, however, has not been fully elucidated. In this study we use Density Functional Theory to predict the diffusivity of 〖(2H)〗_Mg^X and we find that they are around 1000 times slower than H-free Mg vacancies V_Mg^''. In most wet conditions the concentration of 〖(2H)〗_Mg^X is much higher than that of V_Mg^'' and thus the primary effect of water on increasing the Mg-diffusion rate in forsterite is by producing large numbers of H-bearing Mg vacancies. A water induced increase in diffusion rate is predicted to be accompanied by a large increase in diffusional anisotropy primarily in the [001] direction. Using a previously developed model of H distribution in forsterite we predict that the effect of water on Mg diffusion is strongly dependent upon environmental conditions such as pressure or temperature. An exponent (r) describing the relationship of water concentration to Mg diffusion is found to vary between 0.5-1.6 across common experimental conditions with pressure decreasing this exponent and temperature increasing it. With 100 wt. ppm water Mg diffusion rates are predicted to increase by over 2 orders of magnitude at high temperature and low pressure (2000 K, 0 GPa) and by over 3.5 orders of magnitude at low temperature and high pressure (1000 K, 10 GPa) while the anisotropy of diffusion is predicted to increase by ~2/over 5.5 orders of magnitude respectively. A conversion from “dry” to “wet” rheological laws is predicted to occur at <~1 ppm. These results suggest that Mg diffusion in wet forsterite could vary considerably throughout mantle conditions in ways that cannot be captured with a simple one component equation. Finally we considered the effects of the diffusion of H-bearing Mg vacancies on conductivity in forsterite and olivine. We combined our diffusivity results with experimentally determined results for phonon conductivity but this predicted significaly lower conductivities than have been observed experimentally in olivine, particularly at low temperatures (~1000 K). This suggests that the effect of water on olivine conductivity is not primarily due to bulk 〖(2H)〗_Mg^X diffusion and operates via a different unknown mechanism.

Interplay between Brownian motion and cross-linking kinetics controls bundling dynamics in actin networks

Morphology changes in cross-linked actin networks are important in cell motility, division, and cargo transport. Here we study the transition from a weakly cross-linked network of actin filaments to a heavily cross-linked network of actin bundles through microscopic Brownian dynamics simulations. We show that this transition occurs in two phases: first, a composite bundle network of small and highly aligned bundles evolves from cross linking of individual filaments; second, small bundles coalesce into the clustered bundle state. We demonstrate that Brownian motion speeds up the first phase of this process at a faster rate than the second. We quantify the time to reach the composite bundle phase and show that it is a strong function of mesh size only when the concentration of cross links is small, and that it remains roughly constant if we decrease the relative ratio of cross linkers as we increase the actin concentration. Finally, we examine the dependence of the bundling timescale on filament length, finding that shorter filaments bundle faster because they diffuse faster.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame with Coupled Experiments and Simulations

Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot gases as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. To understand these results, we use large-eddy simulation to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame.

Construction Time Estimation Function for Canadian Utility Scale Power Plants

Construction time and time overruns for infrastructure projects have been frequently studied; however, the construction time of power plants has not been studied. This lack of study is problematic, as more renewable energy power plants, such as wind and solar, are planned for many jurisdictions. Accurately estimating the construction time of a power plant will assist construction planning, budget estimates, and policy development encouraging the use of more renewable sources. The construction times of utility scale power plants in Canada were studied using publicly available data. Multiple linear regression analysis techniques were applied to the data to generate construction time estimation functions for all power plants together, and for individual technologies. The analyses reveal that construction time is sensitive to jurisdiction and the decade of construction, indicating that decisions made by individual Canadian provincial governments at different times had statistically significant impacts on construction time. The analyses also indicated that construction time is a strong function of installed capacity, independent of technology. This finding suggests that large solar or wind energy facilities will encounter longer construction times similar to large hydroelectric facilities.

Failure in Confined Brazilian Tests on Sandstone

Strength of rocks in the confined tension region, where the minimum principal stress is tensile, has only infrequently been measured and is not well understood. Quasi-static confined Brazilian tests under a range of confining stresses (2.76 to 27.58 MPa) where used to determine the strength of sandstone in the confined tension region. The test results indicate that the strength in the confined tension region was a strong function of the intermediate principal stress: increasing the intermediate principal stress significantly increased the strength of the sandstone. The strength data were well fit by the Mogi–Coulomb criterion, which accounts for the intermediate principal stress. Unconfined Brazilian strength data were not well fit to the Mogi–Coulomb criterion derived from the confined Brazilian test data, consistent with a transition from tensile to shear processes dominating failure with increasing confining pressure. Observations of post-failure fracture surfaces reveal more indication of shear processes with increasing confining pressure. Numerical simulations from combined finite-discrete element method are compared to the experimental results and reflect similar conditions for failure compared to the experimental tests in the confined tension region.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

High Energies and Radiation Effects

In this chapter we characterize the high-energy spectra of protons that can penetrate shielding and determine the radiation dose to humans and equipment in space. High-energy spectral breaks or “knees”, seen in all large SEP events, determine the contribution of highly penetrating protons. The streaming limit, discussed earlier, places an upper bound on particle fluences early in events and the radial variation of intensities is important for near-solar and deep-space missions. The streaming limit is a strong function of radial distance from the Sun. We also consider requirements for a radiation storm shelter for deep space, a mission to Mars, suitability of exoplanets for life, and radiation-induced chemistry of the upper atmosphere of Earth.