Radiative Heat Transfer In The High Temperature Flowing Gas Radiation Receiver

Author : Mohamed Abou Dbai
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (125 download)

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Download or read book Radiative Heat Transfer and Stability in High-temperature Fluoride-salt Flow Systems written by Mohamed Abou Dbai and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Fluoride Salt-Cooled High-Temperature Reactors (FHRs) use high Prandtl high-temperature FLiBe (2LiF-BeF2) salt to transport energy. The salt coolant has a freezing point of 459 °C and a boiling point of 1400 °C at atmospheric pressure. Flow perturbations due to overcooling transients may affect the performance and stability behavior of natural circulation systems in FHRs. Furthermore, due to the high-temperature operation (500-700 °C) and the near-to-mid infrared optical transparency nature of the salt coolant, radiative heat transfer in participating media can affect the temperature distribution. In this work, the stability behavior of natural circulation systems using FLiBe coolant is investigated. A new 1-D linear stability analysis that accounts for salt freezing is developed, and the Nyquist Stability Criterion is employed to estimate the stability boundary on a Grashof versus Reynolds plot. The loop response to the disturbances caused by the freezing of salt in the coldest heat exchanger is expressed in the form of pressure drop due to pipe contraction. The simulations have shown that the system is more likely to become unstable as the Reynolds number increases. In the event of freezing of salt, the model predicted that the stability boundary is dependent on the Biot number of the frozen layer; however, the increase in friction losses has a stabilizing effect on the natural circulation flow. Additionally, the contribution of radiative heat transfer (RHT) to conductive-convective heat transfer, for FLiBe molten salt, is investigated computationally and the Nusselt ratio, Nu_total/Nu_(no-rad), was implemented to quantify the magnitude of these RHT effects. Under forced convection, Nu_total/Nu_(no-rad) reaches a peak value of 1.76 at Ï4_D=4, and is less than 1.1 for Ï4_D0́−0.1 and Ï4_D60. Under aiding-flow mixed convection, Nu_total/Nu_(no-rad) reaches a peak of 1.34 at Ï4_D=4.2 and is less than 1.1 for Ï4_D0́−0.4 and Ï4_D36. Under opposing-flow mixed convection, Nu_total/Nu_(no-rad) reaches a peak of 1.81 at Ï4_D=4.2 and is less than 1.1 for Ï4_D0́−0.2 and Ï4_D50. The influence of wall emissivity and pipe diameter is also discussed in this work. Overall, for a vertical heated tube, radiative heat transfer effects lead to enhancement of heat transfer by as high as a factor of two, and they depend on the optical thickness of the flow, mixed convection environment (Gr\/Re^2 and direction of flow relative to gravity), surface emissivity, and entrance effects. Lastly, a multi-band thermal radiation method was developed that is capable of capturing the spectral dependence of the absorption coefficient for FLiBe at a reasonable computational cost. The method was deployed to investigate the thermal performance of the UW natural circulation FLiBe loop under various power levels and operating conditions. The numerical results were interpreted and analyzed against experimental data to assess the ability of the CFD model to predict the flow and heat transfer behavior of the salt and to evaluate the impact of radiative heat transfer. The CFD simulations predicted that thermal radiation enhances the overall heat transfer by more than 18% and that RHT acts to reduce wall temperature while improving temperature uniformity within the salt.