Influence of Initial Temperature and Convective Heat Loss on the Self-Propagating Reaction in Al/Ni Multilayer Foils

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.

Numerical study of Flow patterns and performance of a coupled cavity-dish system under different focal lengths

The effectiveness of the parabolic dish system (PDS) is greatly affected by the heat losses associated with high temperatures. The complexity of flow and temperature patterns in and around the cavity receiver makes it a challenging task to determine the convective heat loss from the cavity. Various studies have been carried out to determine the convection heat losses from isolated cavities of different shapes. In the presence of dish structure, the free stream wind may affect the stability of structure and the heat losses from the PDS. In this study, effect of focal length on the performance of the coupled cavity-dish system was analyzed using numerical simulations. The loading and the convective heat loss from the cavity were examined with three different cavity positions and different operating conditions in the presence of the dish. The results showed that the shallow dish experienced higher local air velocities near the cavity receiver than in the case of the deep dish. It was concluded that the heat loss is a stronger function of tilt angle rather than focal length, and in essence, the heat losses due to variation of this are negligible.