16 Uncertainty Treatment Applications: High-Enthalpy Flow Ground Testing
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16.3.4 Gas-Surface Interactions
As with gas-phase chemistry mechanisms describe previously through the term ˙
w j ,
heterogeneous chemistry plays an important role in the description of the thermochemical environment surrounding hypersonic vehicles. In particular, an accurate
prediction of the heat flux to the surface of a vehicle may depend strongly on the
correct solution of catalysis and ablation processes occurring at the gas-surface
interface, known in general as gas-surface interactions (GSI). These processes may
strongly affect the composition of the boundary layer, which in turn may alter the
convective or radiative heating at or near the vehicle surface. For Martian entries,
these effects have also been shown to be important downstream as ablated species
radiate and increase the radiative flux to the back-shell of the vehicle. Typically, GSI
models are implemented as boundary conditions along the vehicle surface.
For non-ablating thermal protection systems, catalysis may play an important
role in the heating to the surface of the TPS. In particular, for Earth entries, catalytic
recombination of the N 2 , O 2 , and NO at the surface are well known. Catalytic
reactions do not participate in the surface mass balance but can promote substantial
heat transfer. As an example, the reactions described above release approximately
950, 500, and 630 kJ per mole of product, respectively.
Surface participating reactions involve both heat and mass transfer between the
surface and surrounding gas. As such, these reactions always include reactants
originating from the TPS material. Examples include the nitridation or oxidation
of solid carbon C(s), the passive and active oxidation of silicon carbide, and the
sublimation of solid carbon to form C, C 2 , or C 3 . In general, there are two types of
models which are used to describe surface chemistry:
Specified Reaction Efficiency Models Perhaps the most widely used GSI model in
hypersonic for its easy implementation. These models describe the mass production
rate of a species at the surface as the product of the mass flux of the reactant
impinging on the surface Γ i and a reaction efficiency γ r which takes a value between
0 and 1. This model is especially relevant for reusable material protection systems
[30].
Finite-Rate Models The model presented above assumes that gas-surface reactions
occur in a single step, however, in reality these reactions are the result of multiple
processes which occur at finite rates. In general, surface reactions are only allowed
at a finite number of active sites on the surface. These active sites are highly
dependent on the topology and chemical structure of the surface and are reaction
dependent (see Fig. 16.5). Finite-rate surface chemistry models describe the net
surface reactions as a series of the elementary processes [40, 41].
Each of these processes may occur at different temperature dependent rates.
Furthermore, the reverse processes are related to the forward reaction through an
equilibrium constant. The heat flux to the surface of a material is then related to
the rate of change in the density of active sites. Thus, a finite-rate surface chemistry
model ensures the conservation of mass and energy at the surface.
531
16.3.4 Gas-Surface Interactions
As with gas-phase chemistry mechanisms describe previously through the term ˙
w j ,
heterogeneous chemistry plays an important role in the description of the thermochemical environment surrounding hypersonic vehicles. In particular, an accurate
prediction of the heat flux to the surface of a vehicle may depend strongly on the
correct solution of catalysis and ablation processes occurring at the gas-surface
interface, known in general as gas-surface interactions (GSI). These processes may
strongly affect the composition of the boundary layer, which in turn may alter the
convective or radiative heating at or near the vehicle surface. For Martian entries,
these effects have also been shown to be important downstream as ablated species
radiate and increase the radiative flux to the back-shell of the vehicle. Typically, GSI
models are implemented as boundary conditions along the vehicle surface.
For non-ablating thermal protection systems, catalysis may play an important
role in the heating to the surface of the TPS. In particular, for Earth entries, catalytic
recombination of the N 2 , O 2 , and NO at the surface are well known. Catalytic
reactions do not participate in the surface mass balance but can promote substantial
heat transfer. As an example, the reactions described above release approximately
950, 500, and 630 kJ per mole of product, respectively.
Surface participating reactions involve both heat and mass transfer between the
surface and surrounding gas. As such, these reactions always include reactants
originating from the TPS material. Examples include the nitridation or oxidation
of solid carbon C(s), the passive and active oxidation of silicon carbide, and the
sublimation of solid carbon to form C, C 2 , or C 3 . In general, there are two types of
models which are used to describe surface chemistry:
Specified Reaction Efficiency Models Perhaps the most widely used GSI model in
hypersonic for its easy implementation. These models describe the mass production
rate of a species at the surface as the product of the mass flux of the reactant
impinging on the surface Γ i and a reaction efficiency γ r which takes a value between
0 and 1. This model is especially relevant for reusable material protection systems
[30].
Finite-Rate Models The model presented above assumes that gas-surface reactions
occur in a single step, however, in reality these reactions are the result of multiple
processes which occur at finite rates. In general, surface reactions are only allowed
at a finite number of active sites on the surface. These active sites are highly
dependent on the topology and chemical structure of the surface and are reaction
dependent (see Fig. 16.5). Finite-rate surface chemistry models describe the net
surface reactions as a series of the elementary processes [40, 41].
Each of these processes may occur at different temperature dependent rates.
Furthermore, the reverse processes are related to the forward reaction through an
equilibrium constant. The heat flux to the surface of a material is then related to
the rate of change in the density of active sites. Thus, a finite-rate surface chemistry
model ensures the conservation of mass and energy at the surface.