Commentary, J Nucl Ene Sci Power Generat Technol Vol: 12 Issue: 4

Radiative Heat Transfer in Molten Salts

Sheng Zhang^*

Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA

*Corresponding Author: Sheng Zhang
Department of Nuclear Engineering and Radiological Sciences
University of Michigan, Ann Arbor
MI 48109, USA
E-mail: shengzh@umich.edu

Received date: 20 June, 2023, Manuscript No. JNPGT-23-103391;
Editor assigned date: 22 June, 2023, PreQC No. JNPGT-23-1033911 (PQ);
Reviewed date: 07 July, 2023, QC No. JNPGT-23-103391;
Revised date: 14 July, 2023, Manuscript No. JNPGT-23-103391(R);
Published date: 21 July, 2023, DOI: 10.4172/2325-9809.1000344.

Citation: Zhang S (2023) Radiative Heat Transfer in Molten Salts. J Nucl Ene Sci Power Generat Technol 12:4.

Description

Molten salts exhibit promising attributes for applications involving high-temperature heat transfer and thermal energy storage, considering their thermophysical properties, nuclear properties, cost-effectiveness, material compatibility, stability, and flammability. They have been proposed for utilization in molten salt reactors, fusion reactors, and concentrating solar power plants.

While the heat transfer characteristics of molten salts have been extensively studied, most research has primarily focused on conductive and convective heat transfer, with limited consideration given to radiative heat transfer in molten salts. However, it is important to recognize that radiative heat transfer may not be negligible, especially in laminar flows of molten salts. Therefore, it becomes crucial to analyze and understand radiative heat transfer in molten salts, as well as identify the specific conditions under which radiative heat transfer should be considered.

Molten salts

At standard temperature and pressure, molten salt exists in a solid state. However, when subjected to increased temperatures, it transitions into a liquid state. The term "molten salt" encompasses a wide range of salt compounds, such as fluoride, nitrate, and chloride salts. It is commonly used in various applications, particularly in hightemperature heat transfer and thermal energy storage systems. For example, fluoride salts, such as FLiBe (LiF-BeF₂, 66-34 mol%) are promising coolants for molten salt reactors and blanket fluids for fusion reactors. Nitrate salts, such as NaNO₃-KNO₃ (60-40 wt%), and chloride salts, such as MgCl₂-KCl-NaCl (60-20-20 mol%), are promising candidates for thermal energy storage systems, offering efficient and sustainable solutions for energy applications.

Conductive, convective, and radiative heat transfer

Heat transfer in molten salts is a complex phenomenon involving the combined effects of conductive, convective, and radiative heat transfer. While correlations have been proposed for conductive and convective heat transfer in molten salts, there are relatively few correlations available for the coupled conductive, convective, and radiative heat transfer.

To analyze the coupled heat transfer in molten salts, a general energy conservation equation can be employed for a controlled volume. However, the calculation of the divergence of the radiative heat flux presents a challenge, as it relies on the radiation intensity, which should be solved via the Radiative Transfer Equation (RTE). The RTE is derived from the radiant energy conservation of a cell with a unit volume in an optically semi-transparent fluid like molten salts, which absorb, emit, and scatter radiant energy. It is an integrodifferential equation that involves integrals and derivatives of the radiation intensity.

Finding a theoretical solution for the radiation intensity in an absorbing, emitting, and scattering medium, such as molten salts, is difficult due to the multidimensional and nonhomogeneous effects involved. Assumptions need to be made in order to solve the RTE, resulting in various radiation models implemented in Computational Fluid Dynamics (CFD) software like ANSYS Fluent or simplified radiation models in in-house system-level codes such as NACCO.

Modeling and simulation

ANSYS Fluent and STAR-CCM⁺ are two widely, used CFD tools in the field of thermal fluid sciences.

ANSYS fluent offers five radiation models: The P-1 model, Rosseland model, Discrete Transfer Radiation Model (DTRM), Discrete Ordinates (DO) model, and Surface-to-Surface (S2S) model. On the other hand, STAR-CCM+ provides two radiation models: The Discrete Ordinates Method (DOM) and S2S models

The selection of a specific radiation model depends on several factors, including the nature of the surface (diffuse/non-diffuse), wavelength dependence of radiation, presence of scattering, and optical thickness. Generally, the P-1 and Rosseland models have moderate computational demands but offer lower accuracy compared to the DTRM, DO, and DOM models, and are suitable for optically thick media. The DTRM model is applicable over a wide range of optical thickness but assumes gray radiation. The S2S model is not suitable for optically semi-transparent fluids.

The DO model in ANSYS Fluent or DOM model in STAR-CCM⁺ provides distinct advantages over other radiation models, including:

• Applicability across the full range of optical thickness,
• Flexibility in choosing between gray or non-gray models, and
• Improved accuracy by increasing the number of solid angles or ordinates. Therefore, the DO model in ANSYS Fluent or DOM model in STAR-CCM⁺ is recommended for analyzing radiative heat transfer in molten salts.

In certain cases, such as molten salts in passive safety systems reliant on natural circulation, evaluating radiative heat transfer using the aforementioned CFD tools may not be cost-effective. In such scenarios, utilizing in-house system-level codes like NACCO, which encompass modules for heat transfer (conductive, forced/mixed/natural convective, radiative), fluid dynamics, closure models, and numerical schemes, is more appropriate for analyzing these systems.

Scenarios for considering radiative heat transfer

Experimental and numerical evidence has shown that radiative heat transfer can be considered negligible for molten salts in transitional and turbulent regimes. Conventional heat transfer correlations, such as the Dittus-Boelter correlation, Gnielinski correlation, Sieder-Tate correlation, and Hausen correlation, which do not account for radiative heat transfer, can be reasonably applied to molten salts with an uncertainty of around ± 20%.

However, it is important to note that under certain conditions, such as molten salts in passive safety systems or during reactor shutdown in pebble-bed/prismatic cores, the flow of molten salt becomes laminar. In such cases, it is necessary to evaluate radiative heat transfer as it has the potential to significantly enhance heat transfer. Ignoring radiative heat transfer under these conditions would not be appropriate.

Conclusion

In general, radiative heat transfer is negligible in molten salts under transitional and turbulent flow conditions. However, it becomes significant and should be evaluated for laminar flows of molten salts. To assess radiative heat transfer in molten salts within small geometries, various radiation models can be employed, such as the DO model in ANSYS Fluent or DOM model in STAR-CCM+. On the other hand, for larger-scale molten salt systems, it is more appropriate to utilize system-level codes, such as NACCO, which incorporate simplified radiation models for evaluating radiative heat transfer.