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Experimental and numerical study of the formation of thermophysical characteristics of carbon composite materials. Part 2. Numerical analysis of the performance of a refractory carbon composite material
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Carbon/carbon composites have been typically used to protect a rocket nozzle
from high temperature oxidizing gas. Based on the Fourier’s law of heat
conduction and the oxidizing ablation mechanism, the ablation model with
non-linear thermal conductivity for a rocket nozzle is established in order
to simulate the one-dimensional thermochemical ablation rate on the surface
and the temperature distributions by using a written computer code. As the
presented results indicate, the thermochemical ablation rate of a solid
rocket nozzle calculated by using actual thermal conductivity, which is a
function of temperature, is higher than that by a constant thermal
conductivity, so the effect of thermal conductivity on the ablation rate of a
solid rocket nozzle made of carbon/carbon composites cannot be neglected.
Experimental data for carbon–carbon constituent materials are combined with a three-dimensional stationary heat-transfer finite element analysis to compute the average transverse and longitudinal thermal conductivities in carbon–carbon composites. Particular attention is given in elucidating the roles of various micro-structural defects such as de-bonded fiber/matrix interfaces, cracks and voids on thermal conductivity in these materials. In addition, the effect of the fiber precursor material is explored by analyzing PAN-based and pitch-based carbon fibers, both in the same type pitch-based carbon matrix. The finite element analysis is carried out at two distinct length scales: (a) a micro scale comparable with the diameter of carbon fibers and (b) a macro scale comparable with the thickness of carbon–carbon composite structures used in the thermal protection systems for space vehicles. The results obtain at room temperature are quite consistent with their experimental counterparts. At high temperatures, the model predicts that the contributions of gas-phase conduction and radiation within the micro-structural defects can significantly increase the transverse thermal conductivity of the carbon–carbon composites.
Ablation modeling is addressed from the material point of view. Multi-scale surface roughness of two ablated carbon-based materials is first analyzed by SEM and micro-tomography. Then, a 3D reaction-diffusion model is set up and solved to explain the observed morphologies at micrometer scale. To take into account the complex crystalline structure of graphite, the model has been enriched by incorporation of an anisotropic heterogeneous reactivity. The models are solved either analytically or using a homemade simulation code. Results are in correct agreement with observations. This approach contributes to the understanding of the physico-chemical coupled
phenomena involved in ablation and of the material intrinsic behavior.
Carbon material thermal conductivity is an important factor for providing thermal shock resistance. The effect of production factors on the reproducibility of thermal conductivity values during manufacture of three-dimensional reinforced high-density carbon-carbon composite is considered. An average level of thermal conductivity applies to more than 82% of workpieces. From 4.6 to 12.5% of workpieces have thermal conductivity not less than 44 and not more than 70 W/(m·K). It is established that a change in carbon fiber treatment temperature from 1600 to 2400°C may lead to an increase in materials thermal conductivity by 0.21 W/(m·K) for each 100°C. Adifference in framework structure may change average thermal conductivity within the limits of 3 W/(m·K). Variation of material apparent density from 1.89 to 1.98 g/cm³ gives a change in thermal conductivity by 1.1 W/(m·K) for each +0.1 g/cm³. A connection is established for a carbon-carbon composite of electrical conductivity ρ and thermal conductivity λ, corresponding in form to the Weidemann-Franz rule λρ = const.
A steady-state, one-dimensional ablative surface model has been developed for coupling with hypersonic CFD applications in two- and three-dimensions. Details and assumptions of the steady-state model are presented along with the CFD boundary condition implementation and associated loose-coupling methodology. A preliminary sensitivity analysis for coefficients present within the ablation model kinetics and flow transport properties is also presented for one of the primary quantities of interest in ablation modeling, the recession rate. Finally, the coupled approach is applied to a sphere-cone graphite model, corresponding to an arcjet stream experimental configuration performed at NASA's Ames Research Center. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc.
Billet deformation as a result of high-temperature treatment is observed and measured. A connection is established for material deformation with structure. Deformation of carbon fiber with constant loading in the temperature range from 20 – 2000°C is measured. Temperature ranges are established when there is possibly adjustment of deformation under action of an external load. The load capable of deforming carbon material or restoring initial billet shape is calculated. Groups of carbon materials are proposed for manufacturing shaping mandrels for high-temperature treatment.
Carbon-carbon composites rank first among ceramic composite materials with a spectrum of properties and applications in various
sectors. These composites are made of fibres in various directions and carbonaceous polymers and hydrocarbons as matrix precursors.
Their density and properties depend on the type and volume fraction of reinforcement, matrix precursor used and end heat treatment
temperature. Composites made with thermosetting resins as matrix precursors possess low densities (1.55–1.75g/cm3) and well-distributed microporosity whereas those made with pitch as the matrix precursor, after densification exhibit densities
of 1.8–2.0g/cm3 with some mesopores, and those made by the CVD technique with hydrocarbon gases, possess intermediate densities and matrices
with close porosities. The former (resin-based) composites exhibit high flexural strength, low toughness and low thermal conductivity,
whereas the latter (pitch- and CVD-based) can be made with very high thermal conductivity (400–700 W/MK) in the fibre direction.
Carbon-carbon composites are used in a variety of sectors requiring high mechanical properties at elevated temperatures, good
frictional properties for brake pads in high speed vehicles or high thermal conductivity for thermal management applications.
However, for extended life applications, these composites need to be protected against oxidation either through matrix modification
with Si, Zr, Hf etc. or by multilayer oxidation protection coatings consisting of SiC, silica, zircon etc.
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