Spectral emissivity measurement of free-falling particle groups

Abstract

A solar energy receiver is a device that absorbs solar radiation and transfers the generated heat energy to a heat transfer medium. As a heat transfer medium in solar energy receivers, solid particles can safely absorb strong solar radiation flux and achieve a higher operating temperature. Hence, they have attracted considerable research interest. The emissivity of particles is an important parameter in numerical simulation and efficiency analysis of solid particle receivers. However, it is rarely reported, especially for dispersed particles flowing in receivers. In this study, a measurement system for the normal spectral emissivity of free-falling particles is designed and developed. The system is based on the energy method and uses an FTIR spectrometer as a radiation measurement device, which can achieve measurement in the wavelength range of 2.5–16 µm below 800 °C. The emissivity of the sheet SiC material is measured to verify the reliability of the system. The results are in good agreement with the data published in NIST. Furthermore, the effects of the particle material, temperature, and particle group thickness on the particle group emissivity are studied. It is found that the emissivity of gray ceramic sand particles is not sensitive to the temperature, whereas that of alumina ceramic particles is positively correlated with the temperature. The emissivity of the two particles increases rapidly with the release width of the particle group, which is related to the increase in the mass flow rate of the particle group. Finally, uncertainty analysis of the measurement system is performed. The results measured in this paper can provide the original emissivity data for heat transfer simulation and efficiency evaluation in the receiver.