Shape deposition manufacturing with microcasting

Abstract: "This paper provides a brief overview of an emerging application for solid freeform fabrication known as Shape Deposition Manufacturing (SDM) with Microcasting. The SDM microcasting process has been used to manufacture complex geometric shapes from CAD models. This novel manufacturing process is briefly described, and a sample artifact is shown. Our current research is described, involving the thermal behavior of the process, the bonding of deposited layers, and droplet fluid dynamics. We have gained significant understanding of the relationship between process parameters and the final quality of artifacts created by microcasting, and continue to investigate the effect of process parameters to develop a systematic representation of the parameter design space; current efforts are directed towards improving the numerical simulations to more accurately predict and control the microcasting process.

In-plane phonon transport in thin films

The in-plane phononthermal conductivities of argon and siliconthin films are predicted from the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phononproperties obtained from harmonic and anharmonic lattice dynamics calculations. The input required for the lattice dynamics calculations is obtained from interatomic potentials: Lennard-Jones for argon and Stillinger–Weber for silicon. The effect of the boundaries is included by considering only phonons with wavelengths that fit within the film and adjusting the relaxation times to account for mode-dependent, diffuse boundary scattering. Our model does not rely on the isotropic approximation or any fitting parameters. For argon films thicker than 4.3 nm and silicon films thicker than 17.4 nm, the use of bulk phononproperties is found to be appropriate and the predicted reduction in the in-plane thermal conductivity is in good agreement with results obtained from molecular dynamics simulation and experiment. We include the effects of boundary scattering without employing the Matthiessen rule. We find that the Matthiessen rule yields thermal conductivity predictions that are at most 12% lower than our more accurate results. Our results show that the average of the bulk phonon mean free path is an inadequate metric to use when modeling the thermal conductivity reduction in thin films.</p

Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions

A literature review of critical heat flux (CHF) experimental visualizations under subcooled flow boiling conditions was performed and systematically analyzed. Three major types of CHF flow regimes were identified (bubbly, vapor clot and slug flow regime) and a CHF flow regime map was developed, based on a dimensional analysis of the phenomena and available experimental information. It was found that for similar geometric characteristics and pressure, a Weber number (We)/thermodynamic quality (x) map can be used to predict the CHF flow regime Based on the experimental observations and the review of the available CHF mechanistic models under subcooled flow boiling conditions, hypothetical CHF mechanisms were selected for each CHF flow regime, all based on a concept of wall dry spot overheating, rewetting prevention and subsequent dry spot spreading. Even though the selected concept has not received much attention (in term or theoretical developments and applications) as compared to other more popular DNB models, its basis have often been cited by experimental investigators and is considered by the authors as the “most-likely” mechanism based on the literature review and analysis performed in this work. The selected modeling concept has the potential to span the CHF conditions from highly subcooled bubbly flow to early stage of annular flow and has been numerically implemented and validated in bubbly flow and coupled with one- and three-dimensional (CFD) two-phase flow codes, in a companion paper. [Le Corre, J.M., Yao, S.C., Amon, C.H., in this issue. A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one and three-dimensional computer codes. Nucl. Eng. Des.].</p

A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one- and three-dimensional computer codes

Based on a review of visual observations at or near critical heat flux (CHF) under subcooled flow boiling conditions and consideration of CHF triggering mechanisms, presented in a companion paper [Le Corre, J.M., Yao, S.C., Amon, C.H., 2010. Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions. Nucl. Eng. Des.], a model using a two-dimensional transient thermal analysis of the heater undergoing nucleation was developed to mechanistically predict CHF in the case of a bubbly flow regime. The model simulates the spatial and temporal heater temperature variations during nucleation at the wall, accounting for the stochastic nature of the boiling phenomena. It is postulated that a high local wall superheat occurring underneath a nucleating bubble at the time of bubble departure can prevent wall rewetting at CHF (Leidenfrost effect). The model has also the potential to evaluate the post-DNB heater temperature up to the point of heater melting. Validation of the proposed model was performed using detailed measured wall boiling parameters near CHF, thereby bypassing most needed constitutive relations. It was found that under limiting nucleation conditions; a peak wall temperature at the time of bubble departure can be reached at CHF preventing wall cooling by quenching. The simulations show that the resulting dry patch can survive the surrounding quenching events, preventing further nucleation and leading to a fast heater temperature increase. The model was applied at CHF conditions in simple geometry coupled with one-dimensional and three-dimensional (CFD) codes. It was found that, within the range where CHF occurs under bubbly flow conditions (as defined in Le Corre et al., 2010), the local wall superheat underneath nucleating bubbles is predicted to reach the Leidenfrost temperature. However, a better knowledge of statistical variations in wall boiling parameters would be necessary to correctly capture the CHF trends with mass flux (or Weber number).</p

Thermal design of wearable computers : application to the Navigator 2, thermal management devices, and embedded electronics

technical repor

Material composition and localized heat generation effects on conjugate heat removal from electronic components

Abstract: "A time-dependent conjugate conduction/convection numerical study of four electronic component configurations that differ in material composition and distribution of internal heat generation is conducted in parallel PCBs (Printed Circuit Boards) for laminar and transitional Reynolds numbers. Both material composition and concentration of heat generation are found to affect the spatial distribution of temperature, heat flux, and Nusselt number along the solid-fluid interface. Furthermore, it is found that the distribution of internal heat generation strongly affects the convective resistance at the solid-fluid interface of the component. This leads, for the configurations with local heat generation, to a non-monotonic relationship between the convective heat transport and the Reynolds number for the range of parameters investigated. It is found that conjugate heat transfer can significantly affect the temperature distribution.

Towards a new approach to concurrent thermal design of PCBs

Abstract: "A thermal design methodology suitable for concurrent design of cost-driven electronic systems is proposed and exemplified for a sample printed circuit board (PCB). The design methodology utilizes an evolutionary concept, in which the analysis tools are capable of adjusting their level of complexity as the design evolves, initiating with rough approximate analyses and culminating with a conjugate conduction/convection simulation for a portion of the sample of the PCB. The level of approximation included at each stage is selected with consideration of both time and accuracy constraints. The importance of considering the conjugate problem in generating heat transfer coefficientsfor electronic packages is discussed.The proposed thermal design methodology is then applied to the Vu-Man artifact and the results are described to illustrate the effect that upstream thermal information can have on the evolution of a design.

Size-dependent model for thin film and nanowire thermal conductivity

We present an analytical model for the size-dependence of thin film and nanowirethermal conductivity and compare the predictions to experimental measurements on siliconnanostructures. The model contains no fitting parameters and only requires the bulk lattice constant, bulk thermal conductivity, and an acoustic phonon speed as inputs. By including the mode-dependence of the phonon lifetimes resulting from phonon-phonon and phonon-boundary scattering, the model captures the approach to the bulk thermal conductivity of the experimental data better than gray models based on a single lifetime.</p

Cross-plane phonon transport in thin films

We predict the cross-plane phononthermal conductivity of Stillinger-Weber siliconthin films as thin as 17.4 nm using the lattice Boltzmann method. The thin films are modeled using bulk phononproperties obtained from harmonic and anharmonic lattice dynamics calculations. We use this approach, which considers all of the phonons in the first Brillouin-zone, to assess the suitability of common assumptions. Specifically, we assess the validity of: (i) neglecting the contributions of optical modes, (ii) the isotropic approximation, (iii) assuming an averaged bulk mean-free path, and (iv) the Matthiessen rule. Because the frequency-dependent contributions to thermal conductivity change as the film thickness is reduced, assumptions that are valid for bulk are not necessarily valid for thin films.</p

Disruption of superlattice phonons by interfacial mixing

<p>Molecular dynamics simulations and lattice dynamics calculations are used to study the vibrational modes and thermal transport in Lennard-Jones superlattices with perfect and mixed interfaces. The secondary periodicity of the superlattices leads to a vibrational spectrum (i.e., dispersion relation) that is distinct from the bulk spectra of the constituent materials. The mode eigenvectors of the perfect superlattices are found to be good representations of the majority of the modes in the mixed superlattices for up to 20% interfacial mixing, allowing for extraction of phonon frequencies and lifetimes. Using the frequencies and lifetimes, the in-plane and cross-plane thermal conductivities are predicted using a solution of the Boltzmann transport equation (BTE), with agreement found with predictions from the Green-Kubo method for the perfect superlattices. For the mixed superlattices, the Green-Kubo and BTE predictions agree for the cross-plane direction, where thermal conductivity is dominated by low-frequency modes whose eigenvectors are not affected by the mixing. For the in-plane direction, mid-frequency modes that contribute to thermal transport are disrupted by the mixing, leading to an underprediction of thermal conductivity by the BTE. The results highlight the importance of using a dispersion relation that includes the secondary periodicity when predicting phonon properties in perfect superlattices and emphasize the challenges of estimating the effects of disorder on phonon properties.</p