Falling particle receivers are being evaluated as an alternative to conventional fluid-based solar receivers to enable higher temperatures and higher efficiency power cycles with direct storage for concentrating solar power (CSP) applications. This paper presents studies of the particle mass flow rate, velocity, particle-curtain opacity and density, and other characteristics of free-falling ceramic particles as a function of different discharge slot apertures. The methods to characterize the particle flow are described, and results are compared to theoretical and numerical models for unheated conditions. Results showed that the particle velocities within the first 2 m of release closely match predictions of free-falling particles without drag due to the significant amount of air entrained within the particle curtain, which reduced drag. The measured particle-curtain thickness (∼2 cm) was greater than numerical simulations, likely due to additional convective air currents or particle–particle interactions neglected in the model. The measured and predicted particle volume fraction in the curtain decreased rapidly from a theoretical value of 60% at the release point to less than 10% within 0.5 m of drop distance. Measured particle-curtain opacities (0.5–1) using a new photographic method that can capture the entire particle curtain were shown to match well with discrete measurements from a conventional lux meter.

Falling particle receivers are being evaluated as an alternative to conventional fluid-based solar receivers to enable higher temperatures and higher efficiency power cycles with direct storage for concentrating solar power applications. This paper presents studies of the particle mass flow rate, velocity, particle-curtain opacity and density, and other characteristics of free-falling ceramic particles as a function of different discharge slot apertures. The methods to characterize the particle flow are described, and results are compared to theoretical and numerical models for unheated conditions.

In an effort to further improve the rut resistance of asphalt pavements in Florida, the Florida Department of Transportation implemented specifications requiring that the production of asphalt mixes be stopped when the air-void content falls below a critical level. To address the problem of low air voids and rutting in north Florida, a proposal was made to reduce the maximum amount of material allowed to pass the 75-μm sieve (P-75μm) at design for asphalt mixtures containing north Florida limestone aggregates. A field study was then undertaken to determine whether this proposal would adequately resolve the problem of low air voids during production due to high P-75μm. The purpose of the study was to determine the amount of degradation to a typical north Florida limestone material and the subsequent effects that degradation has on air voids. The results indicate that although the north Florida limestone aggregates used in this study did degrade significantly, the asphalt contractor was, in general, able to control the amount of P-75μm material in the mix by wasting the baghouse fines. During production, the air voids were low on a number of samples. The source of these low air voids appears to be related to a combination of a high asphalt content in the mix as well as a high P-75μm content. The findings do not support the proposal to reduce the P-75μm content at design at this time. An unexpected finding of this study was that the bulk specific gravities of the commercial aggregate products were less than expected. The impact of this finding is that the voids in the mineral aggregate (VMA) of the mix at design would not meet minimum specification requirements. Although this could make it difficult for an asphalt mixture to have adequate air voids during production, the primary impact of a low VMA is that the pavement would have poor durability and would potentially become brittle and crack prematurely.

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