Investigation of the Gas-surface Interaction of Innovative Carbon Composite Ablators in the VKI Plasmatron
Start Date
2-3-2011 8:25 AM
Description
For a new class of low density carbon/resin composite ablators, which has been introduced and successfully applied to flight by the Stardust mission, the process of ablation is not only restricted to the surface but can also occur in-depth of the material if oxygen is able to diffuse into the porous material. The occurrence of such new porous carbon/resin composites requires an important effort in theoretical and experimental investigation for an adequate understanding of the ablation process to enable development and validation of material response models. At the von Karman Institute for Fluid Dynamics (VKI) research activities were developed to establish a methodology for experimental characterization of innovative low-density ablators using the inductively coupled 1.2MW Plasmatron facility. A comprehensive setup of measurement techniques was applied to the facility in order to determine and characterize the in-situ material response of ablative samples in different test conditions. Optical emission spectroscopy was utilized to address the thermo-chemistry of the plasma free-stream and its interaction with the ablating sample. In addition microscopic analysis tools for sample examination, at the carbon fibre length scale (~10μm), are used to investigate the material physics. The degradation behaviour of the material is then being analyzed by scanning electron microscopy to be able to evaluate the depth of degradation and the thinning of the carbon-fibres. In particular, to provide information about the diffusion/reaction competition of oxygen, which controls the oxidation of carbonized resin and exposed fibres in-depth. Material surface properties, as emissivity, are also determined in-situ using an IR-radiometer combined with two-colour pyrometer measurements. Preliminary results showed that nitridation, leading to CN (CN violet & CN red), is highly apparent in pure nitrogen plasma flows but significantly drops when oxygen is involved, speaking for dominant oxidation reactions (CO, CO2, NO). Additionally, different chemical mechanisms were found to occur rather in nitrogen than in air plasma. In such a way, diatomic carbon (C2 Swan) transitions were highly radiating after injection of the sample into N2 plasma but truncated after a few seconds. This was not observed with air as test gas. As expected, oxygen is the driving force to provoke reactions as the system undergoes the ablation process, but its uncertain state of diffusion into the porous material and on the contrary, reactions undergone in the absence of oxygen, necessitate the usage of appropriate micro- and spectroscopic tools.
Investigation of the Gas-surface Interaction of Innovative Carbon Composite Ablators in the VKI Plasmatron
For a new class of low density carbon/resin composite ablators, which has been introduced and successfully applied to flight by the Stardust mission, the process of ablation is not only restricted to the surface but can also occur in-depth of the material if oxygen is able to diffuse into the porous material. The occurrence of such new porous carbon/resin composites requires an important effort in theoretical and experimental investigation for an adequate understanding of the ablation process to enable development and validation of material response models. At the von Karman Institute for Fluid Dynamics (VKI) research activities were developed to establish a methodology for experimental characterization of innovative low-density ablators using the inductively coupled 1.2MW Plasmatron facility. A comprehensive setup of measurement techniques was applied to the facility in order to determine and characterize the in-situ material response of ablative samples in different test conditions. Optical emission spectroscopy was utilized to address the thermo-chemistry of the plasma free-stream and its interaction with the ablating sample. In addition microscopic analysis tools for sample examination, at the carbon fibre length scale (~10μm), are used to investigate the material physics. The degradation behaviour of the material is then being analyzed by scanning electron microscopy to be able to evaluate the depth of degradation and the thinning of the carbon-fibres. In particular, to provide information about the diffusion/reaction competition of oxygen, which controls the oxidation of carbonized resin and exposed fibres in-depth. Material surface properties, as emissivity, are also determined in-situ using an IR-radiometer combined with two-colour pyrometer measurements. Preliminary results showed that nitridation, leading to CN (CN violet & CN red), is highly apparent in pure nitrogen plasma flows but significantly drops when oxygen is involved, speaking for dominant oxidation reactions (CO, CO2, NO). Additionally, different chemical mechanisms were found to occur rather in nitrogen than in air plasma. In such a way, diatomic carbon (C2 Swan) transitions were highly radiating after injection of the sample into N2 plasma but truncated after a few seconds. This was not observed with air as test gas. As expected, oxygen is the driving force to provoke reactions as the system undergoes the ablation process, but its uncertain state of diffusion into the porous material and on the contrary, reactions undergone in the absence of oxygen, necessitate the usage of appropriate micro- and spectroscopic tools.