*Hypersonic Vehicles - Applications, Recent Advances, and Perspectives*

**Figure 6.**

*Density variations from stagnation, via transonic, to supersonic and hypersonic flows. (a) Mach zero. (b) Mach 1 flow. (c) Mach 2 flow. (d) Mach 3 flow. (e) Mach 4 flow. (f) Mach 4.5 flow. (g) Mach 5 flow. (h) Mach 6 flow.*

*Plasma Preheating Technology for Ablation Studies of Hypersonic Reentry Vehicles DOI: http://dx.doi.org/10.5772/intechopen.100129*

#### **Figure 7.**

*Flow and species parameters at Mach 4.5 flow from simulations. (a) Bow shock standoff distance. (b) CO mass fraction [29].*

#### **Figure 8.**

*Boundary layer flow parameters from simulations at Mach 4.5 and Mach 6.*

maximum value of 0.23 kgm<sup>3</sup> at about 4 mm from the wall; and finally dropping to a minimum value of 0.033 kgm<sup>3</sup> at the wall. The static temperature rises from 60 K to 272 K across the shock, maintaining this relatively constant value to about 4 mm from the hot surface, then rises to a maximum value of about 2500 K at the wall, which explains the density change [34]. The turbulence kinetic energy is greatest at the shock and falls to almost zero at the stagnation point, suggesting a laminar

boundary layer. The gas velocity also decreased across the shock from about 700 m/ s to about 51 m/s, and then experiencing a small rise immediately after the shock and gradually falling to zero at the stagnation point.

No changes in the gas properties were evident until approximately 4 mm from the wall as shown in **Figure 9**. The mass fraction of N2 behaves in a similar way to that of O2 with a sharp drop near the surface as a result of the increase in mass fraction of reaction products entering the flow from the surface. The mass fraction of molecular oxygen dropped from 21% to about 8% at the wall while that of molecular nitrogen dropped from 79% to about 67% at the wall. Neither the molecular oxygen nor nitrogen concentration dropped to zero at the wall as the temperature reached by the gas was not sufficient for complete dissociation. This indicates that a mixture of N2 and O2 was still present in the reacting boundary layer. A contribution to the reduced concentration of molecular oxygen results from its consumption in combustion. The molecular oxygen was only dissociated in very close proximity to the surface as the gas temperature only reached the dissociation temperature within 0.0025 mm from the wall. Species transports are driven by flow properties. Thermally initiated chemical reactions are the formation process of all carbonaceous species in **Figure 9** except for the carbon sublimation species, C, C2, C3. The sublimation species C, C2, and C3 were almost zero. The CO2 species are formed from further oxidation of CO species, thus making CO2 a secondary reaction. The result also shows that CO2 has a sharp drop from the peak value to almost zero at the surface within the experimental temperature limit of 2530 K. The CN species distribution also show a similar pattern to that of CO2. This also suggests that CN is not a product of direct surface reaction within the experimental temperature limit. This further supports the absence of dissociated nitrogen atoms which would otherwise aid direct formation of CN at the surface [29]. The simulations did

**Figure 9.** *Boundary layer species concentrations from simulations at Mach 4.5 and Mach 6.*

### *Plasma Preheating Technology for Ablation Studies of Hypersonic Reentry Vehicles DOI: http://dx.doi.org/10.5772/intechopen.100129*

not predict atomic nitrogen in the present work. This is because the temperature needed to cause N2 dissociation was not achieved in the present work. The CO formation was the major contributor to graphite mass loss and contributions from all other carbonaceous species were insignificant [29]. All carbonaceous species have their peak values around the stagnation region at the wall. The mass fraction of CO species rises to a maximum value at the wall. Mass fraction of CO2 increases to a maximum near the wall and then drops closer to the wall. Simulation results from CFD show that the peak mass fraction of CO at the surface was about 7.9%. The CO2 mass fraction was about six orders of magnitude lower than that of CO.

The information contained in this material is a new experimental method for heating material samples using a high temperature plasma arc fixture. This new fixture can be integrated within cold hypersonic flow wind tunnels in order to measure material ablation and mass loss due to aerodynamic flow effects. The mass loss measurements and microscopic images can be obtained to quantitatively and qualitatively observe material mass loss and surface modifications. Finite rate ablation chemistry has been used in the present work along with volumetric and wall surface reactions [29]. The technique makes it possible to document mass loss from their specimens using dimensional values. This invention focuses on ground-based characterisation techniques for thermal protection material analysis, thus making it a valuable tool for aerothermodynamics of reentry studies.

Well prepared experiments with clear scope and structure will be of importance to the hypersonic research community, especially in the field of new ground-testing facilities for ablation analysis. The presented technology is useful for detailed material characterisation, for example, in the context of material response model validation. It is a new test facility, which can adopt a new measurement technique to generate data for model calibration and validation. The technique is able to accurately replicate the hypersonic flow characteristics and heatshield conditions at reentries. This technology is indispensable when it comes to presenting a new experimental technique for ablation analysis, highlighting new techniques for ablation measurements and providing experimental data for code validation. The key contributions to reentries include: (1) the development of the new test technique, (2) the quantifiable measurements of mass loss, and (3) connecting the measurements made to ablation theory or models.
