Mesler entrainment is the formation of large numbers of small bubbles which occurs when a drop strikes a liquid reservoir at a relatively low velocity. Existing studies of Mesler entrainment have focused almost exclusively on water as the working fluid in a nominally clean state, where even very small levels of contamination can cause significant changes in surface tension that affect the repeatability of the results. Herein water combined with the soluble surfactant Triton X-100 is used as the working fluid in an attempt to stabilize the state of the water surface. Despite this approach, nominally identical drops did not always result in the same bubble formation event. Accordingly, Mesler entrainment was quantified by its frequency of occurrence for drops having the same nominal diameter and impact velocity. This frequency of occurrence was found to be well correlated to both the Weber number and the shape of the drop on impact. V

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An important factor identified for the efficiency of falling particle concentrating solar applications is the falling particle curtain opacity. Low curtain opacity results in increased radiative losses. Candidate multi-stage configurations that can increase particle-curtain opacity were simulated for the existing 1 MWth falling particle on-sun receiver at Sandia’s NSTTF. In the candidate configurations, falling particles were collected periodically in sloped troughs spanning the width of the receiver. A small lip at the front of each trough causes particles to accumulate, allowing subsequent particles to spill over. Particle surface boundary conditions were represented with an empirically based model created to approximate particle behavior observed in testing. Curtain opacity increased using a multi-stage approach and decreases in radiative losses were outweighed by decreases in advective losses which were the dominant loss mechanism. The ability to alter the flow of air within the receiver using multi-stage release resulted in the greatest efficiency gains by reducing advective losses. Additionally, multi-stage release substantially decreased back wall temperatures within receiver.

Falling particle receivers are an emerging technology for use in concentrating solar power systems. In this work, quartz tubes cut in half to form tube shells (referred to as quartz half-shells) are investigated for use as a full or partial aperture cover to reduce radiative and advective losses from the receiver. A receiver subdomain and surrounding air volume are modeled using ANSYS® Fluent®. The model is used to simulate fluid dynamics and heat transfer for the following cases: (1) open aperture, (2) aperture fully covered by quartz half-shells, and (3) aperture partially covered by quartz half-shells. We compare the percentage of total incident solar power lost due to conduction through the receiver walls, advective losses through the aperture, and radiation exiting out of the aperture. Contrary to expected outcomes, simulation results using the simplified receiver subdomain show that quartz aperture covers can increase radiative losses and, in the partially covered case, also increase advective losses. These increased heat losses are driven by elevated quartz half-shell temperatures and have the potential to be mitigated by active cooling and/or material selection.

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The thermal performance of a candidate next-generation falling particle receiver (FPR) is analyzed subject to various expected operating conditions. This receiver design was created from the result of an extensive optimization study and developed to support the Generation 3 Particle Pilot Plant (G3P3) project. Previous analysis demonstrated high thermal efficiencies for the receiver at nominal quiescent conditions, but further analysis was required to demonstrate that the receiver could maintain that thermal performance in a wide range of anticipated environments. In this study, the thermal efficiency was numerically evaluated using a CFD model for different wind conditions and shown to maintain a thermal efficiency above 83% for considered wind conditions. Moreover, the effect of radiative spillage from the incoming concentrated solar beam on the receiver exterior was investigated using ray tracing and CFD models. The exterior wall material temperature limits were not exceeded for the anticipated design power from the heliostats. Additional features were numerically explored including the addition of a chimney to capture particle fines and waste heat and a multi-stage concept to maximize curtain opacity. Particle fines of 10 μm were shown to preferentially flow into this chimney rather than out of the aperture, and the multi-stage design decreased radiative losses and minimized wall temperatures behind the particle curtain.