Damage Assessment of Mullite Ceramic Foams in Radiant Gas Burners

Abstract:

The damage imposed on open-cell mullite ceramic foams was evaluated in pre-mixed radiant gas burners. After exposure to the prevailing combustion environment, foams suffered moderate strength degradation as a result of thermal stresses being imposed on the material during service. There was evidence of chemical attack during combustion although thermal shock measurements suggest that damage sustained by the foams results mainly from thermal shock rather than chemical degradation. Indeed, samples from burners subjected to ageing tests did not show additional damage compared to those subjected to short ageing tests indicating that most of damage occurred during start-up. For comparison purposes, a set of ceramic foam samples were subjected to a water quench test so that the extent to which the foams were damage by exposure to the combustion environment, under well controlled conditions, could be established. The strength retained after thermal shock by open-cell mullite foams decreased gradually with increasing quench temperatures. This suggests a cumulative damage mechanism reflecting an increase in damage throughout the material rather than sudden failure owing to propagation of pre-existing cracks along a plane. Damage in mullite foams was mainly localised at the top layer of the burners where higher temperatures and steeper thermal gradients were imposed on the material. Surprisingly, needle-like mullite crystals with a large aspect ratio were also found to have grown at the surface of the burners via a vapour feed gas-liquid catalyst-solid needle-like growth (VLS) mechanism.