Evaluation of Convective Heat Transfer Coefficients With CFD for Heat Flux Calculation in Combustion Chamber

Abstract Packages for the transport of high-level radioactive waste are designed to withstand severe accidents. To obtain approval for transport, these packages must adhere to the specification-based criteria of regulations established by the International Atomic Energy Agency (IAEA). To ensure compliance with the regulations, mechanical and thermal tests need to be conducted regarding the package type. The requirements define mechanical tests followed by a thermal test, including criteria ensuring the package design’s ability to withstand severe accidents. Heavyweight packages for the safe transport of radioactive materials are equipped with impact limiters, which are often built with porous materials such as densely packed wood reinforced by steel sheet structures. These components absorb the kinetic energy during the impact of the package in drop tests and thus dampen the acceleration of other package components which supports the package to meet the requirements of the IAEA regulations. Following the mechanical tests, the package must, with its predamaged impact limiters, endure a thermal test defined precisely in the IAEA regulations. The thermal test is defined as a 30-min, fully engulfing 800 °C fire and a following time under ambient conditions for a sufficient period to ensure that temperatures in the specimen decrease in all parts of the specimen. During and following the thermal test, the specimen shall not be artificially cooled, and any combustion of materials of the specimen shall be permitted to proceed naturally. A wood-filled impact limiter can continue to release thermal energy during an ongoing combustion process, thus defining relevant package temperatures. Heat flux from a potentially burning impact limiter to the package is important for the safety evaluation of transport packages. A test setup was developed to approach the energy flow investigation and examine the combustion behaviour of porous materials encapsulated in predamaged cylindrical metal enclosures under various conditions. The setup consists of a combustion chamber for thermal tests under adjustable and defined boundary conditions. The temperature development of the test specimens can be observed from outside using a thermographic imager with high-definition cameras, and the mass loss of the test specimen can be measured in the combustion chamber. Convective heat transfer coefficients for various boundary conditions must be defined for use with experimentally gathered test specimen surface temperature data for heat flux evaluations. The airflow conditions in the combustion chamber were analysed using computational fluid dynamics (CFD) calculations in OpenFOAM with respect to the convective heat transfer coefficients at the surface of a hot test specimen. A convergence study was performed, and sensitivity analyses for different test specimen surface temperatures and exhaust gas volume flows were conducted.