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Factors Affecting Pyrotechnic Performance
Published in Ajoy K. Bose, Military Pyrotechnics, 2021
(a) Thermal Effusivity (e): Thermal effusivity is related to the speed at which thermal equilibrium can be reached. In other words, a material's thermal effusivity is the measure of its ability to exchange thermal energy with its surroundings. Effusivity (sometimes called the heat penetration coefficient) is the rate at which a material can absorb heat. It is the property that determines the contact temperature of two bodies that touch each other. It is defined as “the square root of the product of the material’s thermal conductivity and its volumetric heat capacity (ρ Cp)” and is mathematically expressed as e=(λρCp)1/2 where λ = thermal conductivity of the material (W.m.−1K−1), ρ = density of the material (Kg.m−3). Cp = specific heat capacity of the material (J. Kg−1K−1).
Sensitivity Analysis of Transient Critical Heat Flux by RIA Under High-Pressure Flow Boiling Conditions in TRTL
Published in Nuclear Technology, 2023
Nicholas A. Meehan, Seok Bin Seo, Trevor K. Howard, Nicholas R. Brown
The thermal properties of heater materials are key parameters for estimating CHF under both steady-state and transient conditions. Early studies on pool boiling experiments using various heater materials reported strong evidence that material properties effect transient CHF (Refs. 30, 31, and 32). Thermal properties, including density and specific heat capacity, as well as thermal conductivity, are often investigated in the combination of other parameters, such as thermal effusivity, thermal diffusivity, thermal activity, and the heat transfer time constant (HTTC). One previous study33 reported a strong correlation between the thermal effusivity of the heater and CHF, while another34 described the occurrence of CHF using the thermal diffusivity of heater materials. The thermal activity–based correlations could account for the pool boiling CHF for some heater materials,35–37 although it has yet to be applied for aluminum-based heat surfaces.38 A recent study at UTK used HTTC to account for the effect of thermal properties under transient conditions and yielded the exact characterization of the contribution of HTTC on the peak fuel and outer cladding temperatures, as well as the time occurrence of CHF using sensitivity and uncertainty analysis.39
Effect of Heater Material and Thickness on the Steady-State Flow Boiling Critical Heat Flux
Published in Nuclear Technology, 2020
Soon K. Lee, Maolong Liu, Nicholas R. Brown, Kurt A. Terrani, Youho Lee
Thermal effusivity is a measure of a material’s ability to exchange thermal energy to its contacting environment. Increasing thermal effusivity increases the interface temperature between solid and liquid, which increases the number of active nucleation sites by sharpening the thermal gradient of the liquid microlayer. Increasing the number of active nucleation sites promotes effective heat dissipation, thereby delaying the irreversible dry spot formation.7 Thermal activity considers finite cladding thickness with effusivity to account for the size effect.