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Results for “mechanical engineering”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Rotating detonation combustor operability and aero-thermal performance with an integrated diverging nozzle
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Reduced order model for flow distribution in tapered headers of U-type heat exchangers
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Energy Storage in Lightweight Aggregate and Pervious Concrete Infused with Phase Change Materials
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On the role of low frequency modes in the thermal and momentum mixing in parallel plane jets
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Uncertainty analysis of steady-state measurements with a hot-filament type calorimetric emissometer
Calorimetric emissometers measure total hemispherical emissivity by measuring the heat transferred from a heated sample to its surroundings under a vacuum. The accuracy of emissometers standardized by the ASTM C835-06 are well understood. This work uses the Guide to the Evaluation of Uncertainty in Measurement (GUM) for the propagation uncertainties for an ASTM compliant emissometer. The GUM method was able to develop a measurement model and expressions to determine the uncertainty for other emissometers of this type. Data on ‘as-received’ Hastelloy X was used to develop a detailed uncertainty analysis of the emissivity measurement. Data on ‘as-received’ SS 347 and sandblasted A387 Gr. 91 and previous data by the group on A508/A533B were used to determine uncertainty over the ranges 0.16 to 0.81. For all samples, relative uncertainties in emissivities varied from 0.77% to 2.5% when using a fluxgate magnetometer sensor (FMS) to measure the DC heating current. Data on Hastelloy X using a Hall-effect sensor for DC current and low alloy steel showed the DC current and voltage across the test section to be dominate sources of uncertainty. When these sources were reduced, the specimen temperature and the surface area of the test sections were main sources of uncertainty in the emissivity, especially at higher temperatures. As thermal expansion of the surface was considered in the calculations, correlation between specimen temperature and surface area was examined. It was found to be a small contribution to emissivity's uncertainty despite the differences in linear CTE and its uncertainty for the materials analyzed in this study. For low temperatures, the chamber temperature can be a significant source of uncertainty if not sufficiently cooled. The GUM was also briefly compared to uncertainty from the 2nd and 3rd expansions of the Taylor series. We found that the results were the same when rounding to two significant figures.
Quantitative assessment of eddy viscosity rans models for turbulent mixed convection in a differentially heated plane channel
Turbulent mixed convection between two vertical, infinite parallel plates at different temperatures is studied using various two-equation turbulence models. The numerical simulations are performed at a turbulent Reynolds number of Re τ = 150 and a Grashof number of Gr = 9.6 × 10 5 . Comparisons are made against the highly trusted DNS results. Consistent with the DNS approach, the current simulations are performed using constant properties and the Boussinesq approximation to predict the influence of buoyancy. Previous studies have provided assessments of two-equation turbulence models for various scenarios, but often rely on a qualitative “eye” test in order to determine the most appropriate model to predict a given flow. This study aims to provide a new form of quantitative assessment that accounts for both the physics captured by the turbulence model as well as the magnitude of the system response quantities (SRQ) using a modified symmetric mean absolute percent error (SMAPE) method. This method is designed to be approachable to researchers at any level and can be applied to system response quantities from multiple research fields. Uncertainty quantification is also performed to determine the discretization error for each turbulence model. Recommendations are made as to which turbulence models best capture the physics – hydrodynamically and thermally – using both local and global validation metrics. Lastly, a sensitivity analysis is performed on the damping functions used in the most accurate models. This underpins the potential of model developments and adjustments most worth pursuing for buoyant flows. Finally, this framework provides a more physics-based comparative analysis of the selected turbulence models.
Analysis of Laminar Convective Heat Transfer Over Structured Non-Wetting Surfaces
Structured non-wetting surfaces provide alternating no-slip and partial slip boundary conditions to the fluid flow which, in turn, affects the convective heat transfer performance over the surfaces. In this work, an analytical model is developed for the interfacial Nusselt number, the overall Nusselt number and a thermal hydraulic performance factor for fluid flow in a cylinder patterned with structured non-wetting surfaces, for the two cases of uniform wall heat flux and uniform wall temperature. In addition, by considering the stability of the Cassie state of wettability and its transition to the Wenzel state for flow over superhydrophobic surfaces, the present model overcomes certain limitations of the previously reported studies in the literature. Based on the analytical formulations and the stability constraints, the present paper provides optimum design maps for tailoring structured non-wetting surfaces for maximizing convective heat transfer and the combined thermal-hydraulic performance in applications. Use of the design maps on example cases is also discussed. It is shown that the use of structured non-wetting surfaces is most effective for low Reynolds numbers and/or small cylinder radius.
Numerical simulation of falling film flow hydrodynamics over round horizontal tubes
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A note on thermal history kernel for unsteady heat transfer of a spherical particle
When a particle is subjected to an unsteady ambient flow, in terms of either time-dependent relative velocity or time-dependent temperature difference, the net heat transfer from the particle cannot be calculated based on the quasi-steady heat transfer correlation alone. Due to unsteady evolution of the thermal boundary layer, there is also a history contribution to heat transfer. The history contribution to heat transfer is expressed as a convolution integral of past evolution of temperature difference between the particle and the surrounding. While Basset history force and its finite Reynolds number extension have been well studied, similar understanding of unsteady heat transfer and thermal history kernel is lacking. Here, we use existing particle-resolved simulation results to develop a finite Peclet number thermal history kernel, which when used with the convolution integral is demonstrated to accurately predict unsteady heat transfer over a range of Peclet numbers and particle-to-fluid heat capacity ratio.