Theodore L. Bergman

Three-dimensional simulation of thermal plasma spraying of partially molten ceramic agglomerates

Thermal plasma spraying of agglomerated nanostructured ceramic particles has been studied using computational fluid dynamics. The plasma jet is modeled as a mixture of Ar-H2 plasmas issuing into a quiescent atmosphere. The particles, modeled as micron-sized spheres, are introduced into the jet outside the plasma gun exit with radial injection. The existence of a simple target in front of the plasma gun is taken into account. The trajectories and state histories of particles of various sizes during their flight through the jet are presented. Moreover, the solid-liquid interface within the particles is tracked in an attempt to predict the amount of unmelted material retained in these particles at various axial distances from the gun exit. The effects of turbulence in the jet on these particle histories are accounted for. It is shown that, for the range of particle sizes and the plasma gun operating conditions studied, both the deposition location and the retained unmolten fraction are strongly affected by the size of the particles. The predictions are significant in terms of showing general trends, which will be useful in identifying processing windows for producing optimally nanostructured coatings.

Melting in Cylindrical Enclosures: Numerical Modeling and Heat Transfer Correlations

A finite volume model for analyzing melting with natural convection in vertical cylindrical enclosures is developed. The model is verified through detailed comparison with previous experimental and numerical results. Based on the modeling results of outward melting around isothermal cylinders, correlations for the transient heat transfer rate and the total thermal energy stored in the melt are proposed for a wide range of Rayleigh numbers and several representative aspect ratios. Furthermore, a theoretical relationship between the latent energy storage and the total thermal energy storage in the melt is derived and shown to agree well with the numerical results.

Design and characterization of a laminar flow-through dissolution apparatus: Comparison of hydrodynamic conditions to those of common dissolution techniques

A flow-through dissolution apparatus was designed and evaluated to screen small quantities of pharmaceutical drug compounds early in development. The apparatus was designed to mount on a microscope slide such that a compacted solid drug was positioned flush along one wall and the fluid flow in the apparatus was laminar flow in a rectangular duct. Stereomicroscopic digital images and Raman spectra of the solid were taken during dissolution and the effluent dissolution medium was collected in fractions to determine the dissolution rate by fluorescence or HPLC/UV. Three compounds, triamterene, ketoprofen, and β-naphthoic acid were investigated in the dissolution flow cell at various hydrodynamic conditions. In conditions where no solvent-mediated conversion was expected, there was a decrease in dissolution rate with time in the flow through cell that was associated with surface smoothing. This phenomenon also occurred in rotating disk experiments. In either case, the magnitude and time course of the decrease in dissolution rate with time is generally different enough to distinguish from the decrease in dissolution rate due to solvent-mediated conversion.

Thermal modeling of plasma spray deposition of nanostructured ceramics

A thermal model for plasma spray deposition of ceramic materials onto metallic substrates has been developed. The enthalpy-based control volume formulation of the heat transfer processes has been used to study the temperature evolution in a two-dimensional substrate and in the coating as it is grown. In this paper, additional melting of ceramic splats after deposition is examined, with a view to predicting the retention of nanostructures in a spray consisting of agglomerated, nanometer-sized particles. Initial results for thin coatings indicate that when the mean temperature of the incoming particles is close to the fusion point of the ceramic material, the nanostructure distribution in the coating is largely determined by the composition of the spray. However, with thicker coatings, additional melting due to prolonged plasma gas heating combined with increased thermal resistance in the underlying coating leads to a loss of nanostructure.

A Comparison Study of Sensible and Latent Thermal Energy Storage Systems for Concentrating Solar Power Applications

Thermal energy storage (TES) provides a key opportunity to reduce the cost of concentrating solar power generation. In this article transient heat transfer performance and operational characteristics of sensible TES systems (made of liquid solar salt) and latent TES systems (made of sodium nitrate undergoing liquid-solid phase change), all enclosed in vertical annuli, are numerically simulated. The results show that the latent TES systems can operate with a much higher energy density than the sensible TES systems, and that compact latent TES systems are capable of offering both high energy density and a satisfactory charging/discharging rate.

Numerical Modeling of Alternate Melting and Solidification

A finite-volume model is used to analyze alternate melting and solidification, which is the fundamental operational mode of latent thermal energy storage (LTES) systems. The simulated cases include: (1) melting of tin with natural convection, (2) alternate melting and solidification of sodium nitrate, and (3) cyclic phase change of gallium. For each case, temporal evolution of the heat transfer rate and liquid fraction is presented. In addition, snapshots of phase interface, temperature, pressure, and liquid velocity distributions are presented. The implications of the modeling results are discussed.

Transient Natural Convection in Vertical Annuli: Numerical Modeling and Heat Transfer Correlation

A finite volume model is developed for analyzing transient natural convection in vertical annuli, which are assumed to be isothermally heated (or cooled) from the inner surface and insulated from horizontal and outer surfaces. Based on the numerical results, the dependence of transient heat transfer on governing dimensionless parameters is investigated. It is found that the dimensionless accumulated heat transfer as a function of dimensionless time can be satisfactorily approximated by a simple correlation, which explicitly includes the effects of the Rayleigh and Prandtl numbers as well as the enclosure aspect ratio and height-inner radius ratio.

Simulation of two-dimensional, low-Pr natural convection in harmonically oscillated, differentially heated enclosures

Abstract This study deals with the interaction between (1) buoyancy-induced convection within a liquid metal that is housed in a square, differentially heated enclosure and (2) externally imposed excitation in the form of harmonic rocking about the enclosure centerpoint. The superposition of buoyancy and Coriolis forces leads to complex fluid flow and heat transfer. The transition to chaotic convection is accelerated, and heat transfer rates are reduced as the enclosure is excited at the fundamental frequency of osculation associated with the pun buoyancy-driven case. Average neat transfer rates are correlated for Pr = 0.02 and 0.03 liquids, with heat transfer being more sensitive to external rocking for the Pr = 0.03 case.

A model for radiative cooling of a semitransparent molten glass jet

Transfer of molten glass from location to location typically involves a pouring process, during which a stream of glass is driven by gravity and cooled by combined convective and radiative heat transfer. This study of the thermal and fluid mechanics aspects of glass pouring is motivated by the glass casting of vitrified, surplus weapons-grade plutonium. Here, a mathematical model for the radiative cooling of a semitransparent molten glass jet with temperature-dependent viscosity has been developed and is implemented numerically. The axial velocity and jet diameter variations along the length of the jet, the axial bulk mean temperature distributions, and the centerline-to-surface glass temperature distributions are determined for different processing conditions. Comparisons are also made between the semitransparent predictions, which are based on a spectral discrete ordinates model, and predictions for an opaque medium.

Scaling analysis and prediction of thermal aspects of the plasma spraying process using a discrete particle approach

On the basis of a discrete particle approach, a scaling analysis was used to predict features of the thermal plasma spraying process. Correlations were obtained using the analysis and they were subsequently used to predict two important features: the state of the particle at the moment of impact on the substrate, and the nature of solidification process. Limitations and restrictions were also identified in the development of the analysis that can be used to infer the resulting structure of coating. The correlations that were developed might be utilized in optimizing the thermal plasma spraying process, as well as in producing new types of coatings.