Granular media such as Carbo ceramic particles and silica sands have been used as heat transfer media in heat exchangers for solar-thermal energy storage systems. However, thermal transport processes in the particulate heat exchangers are still not fully understood. In particular, it is still not clear whether and how the thermal conductivity of the particles in the near-wall region could be impacted by the particle -wall interaction. While numerous models exist, there is a lack of suitable experimental technique to probe the thermal transport processes of granular media when they are flowing in a heat exchanger. &nbsp;In this work, we report the instrumentation development of high-temperature thermal conductivity measurement of flowing ceramic particles within a confined channel. We use an existing technique called modulated photothermal radiometry (MPR) and first develop it for high temperature measurements (up to 800 deg. C). MPR is a non-contact technique where the surface of the sample is heated by an intensity-modulated laser and it utilizes the intrinsic thermal emission from the specimens for thermometry, which is favorable for the measurement at high temperature in harsh environment. The high-temperature MPR setup is validated by measuring bulk and thin coating materials with known thermal conductivity. We show that the measurement error by MPR is less than 10%. We then develop the technique for measuring stationary ceramic particles up to 700℃. The measured effective thermal conductivity of stationary particles agrees with that measured by the transient hot-wire (THW) method as benchmark. The MPR technique is then used to measure the effective thermal conductivity of moving ceramic particles in a channel with variation in the channel depth (2 – 7 mm), particle size (200 – 400 μm), moving velocity up to 60 mm s‑1 and temperature up to 700℃. We find that the thermal conductivity of particle bed decreases by 10% - 50% as velocity increases from 0 mm s-1 (stationary) to 60 mm s-1 depending on the properties and morphologies of particles. The MPR also shows the potential to separate the bulk thermal conductivity and near-wall thermal resistance of particle bed at the flowing state in a single measurement. Our study is the first in-situ thermal conductivity measurement of moving particle bed. The ability to quantify the effect of moving velocity on the effective thermal conductivity of particle bed and to separate the bulk thermal conductivity and near-wall thermal resistance not only helps optimize the design of particle heat exchanger for thermal storage and reaction systems, but also provides insights into the heat transfer mechanisms in granular materials.

Thermal Conductivity Measurement of Flowing Granular Media Using Modulated Photothermal Radiometry

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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