Transient Ablation Investigation Of C/SiC Composite Material In Oxygen-rich Environment

The thermal protection is very important for the development of ramjet technology. The ablation expriments can evaluate and measure the thermal protection performance and service life of the engine structural elements. C/SiC composite material is a new type of strategic materials, with low density, high temperature resistance, high specific modulus, high specific strength, low brittle, anti-ablation, anti-oxidation and high reliability. It's ideal material of the thermal protection and thermal structure for ramjet. However,Engineering calculations lack ablation models and methods, so this thesis conducts study on the subject.Broadly speaking, the ablation of material can be divided into two parts:thermo-chemical ablation and mechanical erosion, the former refers to the material surface oxidation and decomposition reaction in high temperature and high pressure environment, the latter refers to rupture and mechanical failure in fiber materials due to pressure and shear force in high speed flow or thermal stress damage. According to the specific environment of ramjet combustion chamber, the ablative materials explose in high temperature, high pressure,high heat flux, high oxidation gas concentration and high Mach number environment, in such a harsh environment, just considering the thermo-chemical ablation is unreasonable,mechanical erosion must also be incorporated into the system and both should be taken into consideration in ablation.In this paper, the ablation of C/SiC material in the unsteady state is the research object.Because oxidation mechanism of SiC is different at different temperature, this paper establishes different oxidation models. Comprehensive considering the influence of temperature, pressure, oxidizing component (O2 andH2O) concentrations,the thermo-chemical ablation model of C/SiC material under unsteady state conditions is builded. At the same time,through absorting the relevant experience and achievements of predecessors, we establish the mechanical erosion model which is semi-expirical, the model takes the effect of erosion rate,shear stress and temperature on the mechanical denudation into account. So a general ablation model including thermo-chemical ablation and mechanical erosion has been builded. The ablation model is programmed into the UDY program and fit to FLUENT solver, then realize the transient coupled for flow field and the ablation procedure, the transient ablation calculation can be achieved.Based on Plasma wind tunnel to heat the air(with water) for C/SiC flat ablation test. the experimental was validated by numerical simulation and ablation calculation. From the trend of ablation thickness,the calculation results are in good agreement with the experiment in a certain extent, serious ablation area occurred in the upstream, but because of the influence factors of material deformation and flow field is not stable, the test has some errors. Specimen 1 ablation examination last for 60s, the maximum actual ablation thickness is 7.28mm, and the results of the calculation thickness is 7.11mm,the absolute error and relative error ware-0.17mm, -2.3%; specimen 2 ablation last for 50s and cumulative maximum ablation thickness was 2.01mm, calculated value was 0.31mm higher than the experimental value, the relative error was 15.4%. To sum up, errors keep in a small range, it indicated that the ablation model and calculation method is effective,feasible,can predict the ablation of C/SiC composite material in a certain extent,laid the foundation for the ablation of C/SiC composites in ramjet calculation and thermal protection design.It shows that the ablation thickness increases with the increase of Wall temperature,pressure, oxidizing component (O2 and H2O) concentrations or adjunction of other oxidizing components by quantitative analysis of the impact for ablation of these four factors. Both thermo-chemical ablation rate and mechanical denudation rate and the ablation surface temperature are in great relationship. calculation shows that, the main thermo-chemical ablation area and mechanical erosion area stay in the position of upstream, and the high temperature zone are at the same location. The thermo-chemical ablation rate and mechanical denudation rate are in the same order of magnitude, in this condition, thermo-chemical ablation accounted for the major share.Finally, this paper predicts the ablation of ramjet nozzle, calculation results show that the main ablation zone stay in the nozzle throat, the thermo-chemical ablation in the convergent section is more obvious, and mechanical denudation in the throat is particularly intense.