(351d) Modeling Radiative Heat Transfer in High-Temperature Liquid-Salts

Recent technological advancements and needs for high-temperature heat have increased interest in both nuclear and solar high-temperature liquid-salt systems. Growing nuclear concepts include the Fluoride-salt-cooled High-temperature Reactor (FHR) that uses solid fuel and a liquid salt coolant, the Molten Salt Reactor (MSR) with fuel dissolved into the salt coolant, and high-magnetic-field fusion machines with immersion salt coolant blankets [1]. In parallel high-temperature concentrated solar power systems propose using liquid-salts as volumetric absorbers and for energy storage [2]. What these design have in common is that they utilize high-temperature chloride and fluoride salts that may experience the effects of participating media radiative heat transfer. While some work has been conducted on the thermophysical properties of these fluids [3], very little is known about their radiative properties. Because these salts operate between 600-1000C and radiative heat transfer increases as T⁴, thermal radiation has the potential to become a significant heat transfer mechanism by decreasing surface structure temperatures while increasing temperature uniformity, especially in off-normal higher-temperature conditions. Therefore, new, innovative research in this area is required.

DESCRIPTION OF WORK

The purpose of this work is to create a methodology for understanding and modeling the effects of radiative heat transfer in high-temperature liquid-salt systems. First, spectral absorption coefficients of candidate salts will be measured using Fourier Transform Infrared Spectroscopy. Then, using that data, computational fluid dynamics (CFD) simulations, validated by scaled experiments, will be run quantify the impact of thermal radiation to the overall heat transfer. By demonstrating that CFD can capture the correct physics, complex geometries with new salts can be analyzed for use in various heat transfer applications.

BACKGROUND

At the temperatures of the above energy applications, liquid-salts may act as participating media where thermal radiation can be absorbed and re-emitted within the fluid. This heat transfer mechanism is controlled mainly by the absorption coefficient, k. Therefore, to model radiative heat transfer, k must be well characterized in regions of significant emissive power. For high-temperature applications, radiation is emitted by the salt itself between 600-1000C, so our focus will be on the near to mid IR wavelengths (2-12µm). Theoretical calculations have shown that some chloride and fluoride liquid-salt absorption edges may occur in that region [4]. Additionally, nuclear salts can contain impurities that can greatly increase the absorptivity of the salt in the near IR.

EXPERIMENTAL METHODS

1. Measurement of Absorption Coefficient

The spectral absorption coefficients, k(λ), of candidate chloride and fluoride salts will be measured using Fourier Transform Infrared (FTIR) spectroscopy. The FTIR apparatus is composed of two, concentric nickel cuvettes with transparent, diamond windows, a light source, and light detector. Liquid-salt is placed in the outer cuvette while the inner cuvette is free to move up and down, allowing for variable attenuation thickness. A broad-spectrum light source is then passed through the diamond windows and attenuated by the liquid-salt. Using Beer’s law and taking the ratio of attenuated intensities (I) at multiple thicknesses (Δx), all background attenuation can be cancelled out and the absorption coefficient of the salt can be isolated.

2. CFD Modeling with Experiments

CFD simulations are being conducted and will be validated by experimental testing to prove understanding and predictability of the heat transfer behavior. Historically, CFD radiation work has focused on surface-to-surface radiation with a small number of participating media scenarios (i.e. combustion gases) where the index of refraction is close to 1. This work will expand upon those studies and increase the applicability of CFD modeling to advanced liquid-salt concepts with participating media. This leads to enhancements in the accuracy of wall temperature predictions as well as calculated energy distributions within the fluid.

In this work, high-fidelity 3D CFD simulations are conducted using STAR-CCM+, a vetted commercial code. These simulations are run at high temperatures with participating media multiband thermal radiation models that have been validated against participating media coupled with conduction and convection test cases with known numerical solutions. Using STAR-CCM+, radiation models can be turned on and off to isolate the effects of radiation. These results are then confirmed by running experiments with hot, liquid-salts. However, because radiative heat transfer cannot be removed in real, high-temperature experiments, Dowtherm A, a well documented simulant fluid [5], is used to match important dimensionless parameters at lower temperatures. High-temperature liquid-salt and Dowtherm A experiments can then be directly compared to CFD simulations with and without thermal radiation.

Preliminary CFD work is currently being used to inform experimental design choices. By using this iterative CFD/experimental process, we are able to minimize potential sources of error as well as ensure experiments capture heat transfer and flow regimes of interest. Initial liquid-salt tank simulations with an immersion heater confirm that, by accounting for participating media, reduced surface temperatures and increased fluid temperature uniformity are observed.

PATH FORWARD

The apparatus designed to measure absorption coefficients of chloride and fluoride salts is currently under construction. Once results are obtained, they can then be used in CFD simulations to obtain preliminary results. Currently, theoretically calculated absorption coefficients are being used.

REFERENCES

1. FORSBERG, D. WHYTE, “Converging Fission and Fusion Systems Toward High-Temperature Liquid-Salt Coolants: Implications for Research and Development and Strategies,” Trans, ICCAP, San Francisco, CA (2016).
2. SLOCUM ET AL., “Concentrated Solar Power on Demand,” Solar Energy 85:1519-1529 (2011).
3. FORSBERG ET AL., “Liquid Salt Applications and Molten Salt Reactors,” Revue Generale Nucleaire, 4 (2007).
4. CHALEFF ET AL., “Radiation Heat Transfer in Molten Salt FLiNaK,” Nuclear Technology, 196 (2016).
5. THERMAL HYDRAULICS WORKING GROUP, “Fluoride-Salt-Cooled High-Temperature Reactor Benchmarking White Paper,” University of California, Berkeley, (2016).