Lorenzo Valdevit
University of California, Irvine
Journal paper
Publication Types:
Influence of Configuration on Materials Selection for Actively-Cooled Combustors
Abstract
The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.
Materials Property Profiles for Actively Cooled Panels: An Illustration for Scramjet Applications
Abstract
A scheme for identifying and visualizing the material properties that limit the performance of candidate materials for actively cooled aerospace propulsion components is presented and illustrated for combustor panels for Mach 7 hypersonic vehicles. The method provides a framework for exploring the nonlinear interactions between design and materials optimization. By probing the active constraints along the border of feasible design space, the limiting properties have been elucidated for a representative group of candidate materials. Property vectors that enhance design options have also been determined. For one of the promising candidate alloys (the Ni-based superalloy, INCONEL X-750), the possibilities of reclaiming design space and lowering optimal combustor panel weight by tailoring its strength properties are assessed.
Feasibility of metallic structural heat pipes as sharp leading edges for hypersonic vehicles
Abstract
The influence of combustor size and shape on material feasibility is explored using (structural and fuel) weight, as well as fuel economy as metrics. A materials selection methodology developed for actively cooled rectangular panels has been embellished to include cylindrical/annular configurations. The procedure incorporates an analytical model for temperature and stress distributions subject to thermomechanical loads representative of hypersonic flight conditions. The model has been numerically verified using finite element simulations. By combining the model with optimization routines, materials robustness maps have been produced, depicting the range of thermal loads and fuel flow rates that satisfy all design constraints. A wide selection of high-temperature materials has been investigated. Comparisons of cylindrical and rectangular combustors are made for the leading candidates. It is established that the cylindrical designs allow both lighter optimal structures as well as greater robustness and fuel economy.
Organic substrates for flip chip design: a thermo-mechanical model that accounts for heterogeneity and anisotropy
Abstract
We present a thermo-mechanical characterization of organic substrates that accounts for heterogeneity both in the in-plane and out-of-plane directions. Systematic observation of the board files of a number of substrates of commercial interest reveals primarily three recurrent topological arrangements of copper and polymer; for each arrangement, the in-plane effective thermo-elastic properties are calculated via appropriate composite materials models. The averaging process in the out-of-plane direction (i.e. the stacking effect) is performed using standard laminated plate theory. The model is successfully applied to various regions of three organic substrates of interest (mainly differing in core thickness): the analytically calculated effective Young’s moduli (E) and coefficients of thermal expansion (CTE) are shown to be typically within 10% of the experimental measurements. An important attribute of this model is its ability to provide substrate description at various levels of complexity: a few effective properties are outputted that can be useful for further purely analytical investigations; at the same time, the model provides the full stiffness matrix for each region of the substrate, to be used for more detailed finite elements simulations of higher-level structures (e.g. silicon die/underfill/substrate/cooling solution assemblies). Preliminary application of this model to the warp analysis of a flip-chip is presented in the end.
A materials selection protocol for lightweight actively cooled panels
Abstract
This article provides a materials selection methodology applicable to lightweight actively cooled panels, particularly suitable for the most demanding aerospace applications. The key ingredient is the development of a code that can be used to establish the capabilities and deficiencies of existing panel designs and direct the development of advanced materials. The code is illustrated for a fuel-cooled combustor liner of a hypersonic vehicle, optimized for minimum weight subject to four primary design constraints (on stress, temperatures, and pressure drop). Failure maps are presented for a number of candidate high-temperature metallic alloys and ceramic composites, allowing direct comparison of their thermostructural performance. Results for a Mach 7 vehicle under steady-state flight conditions and stoichiometric fuel combustion reveal that, while C–SiC satisfies the design requirements at minimum weight, the Nb alloy Cb752 and the Ni alloy Inconel X-750 are also viable candidates, albeit at about twice the weight. Under the most severe heat loads (arising from heat spikes in the combustor), only Cb752 remains viable. This result, combined with robustness benefits and fabrication facility, emphasizes the potential of this alloy for scramjets.
Structural performance of near-optimal sandwich panels with corrugated cores
Abstract
An experimental and computational study of the bending response of steel sandwich panels with corrugated cores in both transverse and longitudinal loading orientations has been performed. Panel designs were chosen on the basis of failure mechanism maps, constructed using analytic models for failure initiation. The assessment affirms that the analytic models provide accurate predictions when failure initiation is controlled by yielding. However, discrepancies arise when failure initiation is governed by other mechanisms. One difficulty is related to the sensitivity of the buckling loads to the rotational constraints of the nodes, as well as to fabrication imperfections. The second relates to the compressive stresses beneath the loading platen. To address these deficiencies, existing models for core failure have been expanded. The new results have been validated by experimental measurements and finite element simulations. Limit loads have also been examined and found to be sensitive to the failure mechanism. When face yielding predominates, appreciable hardening follows the initial non-linearity, rendering robustness. Conversely, for designs controlled by buckling (either elastic or plastic) failure initiation is immediately followed by softening. The implication is that, when robustness is a key requirement, designs within the face failure domain are preferred.
Optimal active cooling performance of metallic sandwich panels with prismatic cores
Abstract
All-metallic sandwich panels with prismatic cores are being currently investigated for combined structural and active cooling performance. We present a new approach to active cooling performance, and use it to optimize the panel geometry for four different systems: aluminum-air, aluminum-water, aluminum-gasoline and titanium-gasoline. The results show that some geometric parameters can be fixed without much detriment in thermal performance. Moreover, while optimal core densities are typically 25–50%, near-optimal results can be obtained with densities as low as 10%. These findings provide considerable geometric flexibility when attempting combined thermal and structural optimization.
Structurally optimized sandwich panels with prismatic cores
Abstract
Multifunctional sandwich panels with corrugated and prismatic diamond cores have been analyzed and their behavior compared with panels designed using truss and honeycomb cores. Failure mechanism maps have been devised that account for interactions between core and face members during buckling. The optimal dimensions and the minimum weight have been evaluated. The load capacities predicted for near-optimal designs have been validated by conducting selected finite element calculations. Designs that use diamond prismatic cores (with corrugation order 4) are slightly more weight efficient than trusses, when optimized for a specific loading direction. Honeycomb cores, while somewhat more weight efficient, especially at lower load capacities, are not amenable to the fluid flows needed for cooling. We conclude that the diamond prismatic topology is the most weight efficient among designs amenable to simultaneous load bearing and active cooling.
Active cooling by metallic sandwich structures with periodic cores
Abstract
We review the thermal characteristics of all-metallic sandwich structures with two dimensional prismatic and truss cores. Results are presented based on measurements in conjunction with analytical modeling and numerical simulation. The periodic nature of these core structures allows derivation of the macroscopic quantities of interest—namely, the overall Nusselt number and friction factor—by means of correlations derived at the unit cell level. A fin analogy model is used to bridge length scales. Various measurements and simulations are used to examine the robustness of this approach and the limitations discussed. Topological preferences are addressed in terms scaling relations obtained with three dimensionless parameters—friction factor, Nusselt number and Reynolds number—expressed both at the panel and the cell levels. Countervailing influences of topology on the Nusselt number and friction factor are found. Case studies are presented to illustrate that the topology preference is highly application dependent.