Experimental Investigation of Radiative Properties and Heat Transfer in Particulate Media

Abstract

Particulate media consisting of solid particles and a gas phase have important applications in thermal and chemical energy systems. Specifically, next-generation concentrated solar power (CSP) plants aim to use sand-like ceramic particles as heat transfer and thermal energy storage (TES) media to increase operating temperatures from ~600 °C to 1000 °C and facilitate round-the-clock solar electricity production at a competitive cost. Therefore, there is a need to understand the flow and heat transfer behavior of particulate media, especially at high temperatures where radiation is an important mode of energy transfer. The radiative properties of particulate media govern quantities such as solar absorptance and thermal emittance that are critical to the performance of a solar receiver, which converts concentrated sunlight to thermal energy. However, there is limited experimental data available for temperature-dependent radiative properties. Therefore, a primary objective of this work is to experimentally measure temperature-dependent radiative properties of ceramic particles that have relevance to solar energy systems. Specifically, the reflectance and thermal emittance of composite metal oxide particles were measured up to 1000 °C, and these measurements highlight the importance of accounting for temperature-dependent radiative properties in the context of solar receivers. Surfaces of several high-temperature materials relevant to CSP, including alumina (Al2O3) and Inconel alloy, were also characterized under various environmental conditions to assess the impacts of thermal cycling and surface oxidation on their radiative properties. All measurements were performed with a Fourier transform infrared spectrometer (FTIR), which is well-suited to investigate the infrared spectrum relevant to thermal radiation (1–20 μm). Custom modifications were required to perform high-temperature measurements and mitigate the effects of detector saturation. Results from this study point to the merit of measuring temperature-dependent radiative properties; however, in the absence of these capabilities, room-temperature measurements after thermal cycling were shown to be a reasonable substitute.

In addition to radiative property measurements, a series of experiments were conducted to enhance understanding of flow and heat transfer behavior in dilute and dense particle flows. For dilute flows of free-falling particles, a thermopile-based transmittance technique was developed to quantify particle concentration. This has implications for the performance of solar receivers, where particle curtains are used to volumetrically absorb concentrated sunlight and convert it to thermal energy. Heat transfer behavior of dense particle flows in a channel was assessed by leveraging non-contact techniques—thermal imaging and pyrometry—to obtain particle temperatures up to 550 °C in a 20 kW moving-bed particle heat exchanger for CSP. These measurements were used to experimentally evaluate local particle-to-wall heat transfer coefficients on the order of 100–200 W/m2K. Enhancement in the heat transfer coefficient was observed when increasing particle mass flow rate, which correlated with a transition in heat transfer behavior from advection-limited to conduction-limited. Finally, ongoing and future work focuses on addressing knowledge gaps on heat transfer characterization of dense particle flows up to 700 °C for a wider range of flow rates with a customized high-temperature particle flow channel.

Overall, this work highlights progress in addressing knowledge gaps regarding heat transfer in particulate media, with an emphasis on ceramic particles used in CSP applications. In particular, this work presents the development of methods to help characterize temperature-dependent radiative properties up to 1000 °C and techniques for improved characterization of heat transfer coefficients in dense granular flows up to 700 °C.