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Abstract: Phase change material (PCM) suspensions have received wide spread attention for increased thermal storage in various thermal systems such as heat sinks for electronics and solar thermal applications. To achieve further heat transfer enhancement, this paper investigates the effect of focusing micron-sized phase-change particles (PCMs) to a layer near the heated wall of a parallel plate channel. A numerical model for fully-developed laminar flow with a constant heat flux applied to one wall is developed. Melting of the focused PCMs is incorporated using a temperature-dependent effective heat capacity. The effect of channel height, height of the focused PCM stream, heat flux, and fluid properties on the peak local Nusselt number (Nu∗ ) and the averaged Nusselt number over the melting length (Numelt ) are investigated. Compared to the thermally-developed Nusselt number for this geometry (Nuo =5.385), Numelt and Nu∗ enhancements of 8% and 19% were determined, respectively. The local heat transfer performance is optimized when the PCMs are confined to within 30% of the channel height. The present work provides an extended understanding of local heat transfer characteristics during melting of flowing PCM suspensions, and offers a new method for enhancing heat transfer performance in various thermal-fluidic systems. [Copyright &y& Elsevier]

Abstract: The development of efficient solar thermal receivers has received significant interest for solar to electrical power conversion and heating applications. Volumetric flow receivers, where the incoming solar radiation is absorbed in the volume of a heat transfer fluid (HTF), promise reduced heat loss at the surface compared to surface absorbers. In order to efficiently store the thermal energy in the volume, nanoparticles can be suspended in the HTF to absorb the incoming radiation. In such systems, compact models are needed to design and optimize the performance. This paper presents an analytical model that investigates the effect of heat loss, particle loading, solar concentration and channel height on receiver efficiency. The analytical model was formulated by modeling the absorption of solar radiation by the suspended nanoparticles as a volumetric heat release inside the flowing HTF. The energy equation was solved with the surface heat losses modeled using a combined radiative and convective heat loss coefficient. The analytical solution provides a convenient tool for predicting the effect of different parameters, in terms of dimensionless numbers (Pe, NuE , , and θamb ), on two-dimensional temperature profiles and system performance. By combining the receiver efficiency with a power generation efficiency, idealized by the Carnot efficiency, an optimum receiver length where the total efficiency is maximized is determined. However, in practice, the maximum efficiency depends on the maximum allowable temperature of the working HTF. As a case study, predictions were made for Therminol® VP-1 with suspended graphite nanoparticles in a 1cm deep channel with a solar concentration of 10. The model predicts an optimum total system efficiency of 0.35 for a dimensionless receiver length of 0.86. Finally, the analytical model was used to estimate the optimum efficiency and the corresponding optimum receiver length for different design configurations with varying NuE and . The results from this paper will help guide experimental design of volumetric flow receivers for solar thermal based power systems. [Copyright &y& Elsevier]