Ablation is a means of thermal protection based on physicochemical transformations of solid substances by convective or radiation heat flow. The heat-shield effect is the sum of the heat of phase and chemical transformations of the substance and the reduction of the heat flow when the ablation products are forced into the surrounding medium (see Heat Protection). Ablation can be referred to as a sacrificial method of heat protection, since in order to maintain acceptable heat conditions in a body, its surface layer is partially destroyed. Ablation can, as a rule, be allowed in objects of single application; for instance, the re-entry space vehicles, combustion chambers and the nozzle units of solid-propellant rocket engines. The use of ablative facing has a number of advantages over other methods of heat protection. The main advantage is the self-regulation process, i.e., the change in the ablation rate depending on the level of pressure and temperature of the gas flowing across the surface. Thanks to high values of heat of physicochemical transformations and to the injection heat effect, the use of ablative facing materials exceeds substantially in efficiency that of systems functioning on the heat storage principle or on the principle of convective cooling (see Heat Protection). Together with penetrating cooling, ablative facings form the class of active heat protection, the basis for which is the direct effect on the process of heat transfer from the surrounding medium to the body.
The most commonly used ablative materials are the composites, i.e., materials consisting of a high-melting point matrix and an organic binder. The matrix can be glass, asbestos, carbon or polymer fibers braided in different ways. In some cases, a honeycomb construction can be used, filled with a mixture of organic and nonorganic substances and possessing high heat-insulating characteristics (as used, for instance, on the space vehicle ”Apollo”).
Shown in Figure 1 is a schematic model of the destruction of a composite material from a high-melting point matrix and an organic binder. The characteristic property of such heat-shielding coverings is the presence of two fronts or zones, to be more exact, in which physicochemical transformations take place. In convective heating, a viscous melt film can be formed on the surface of such composite materials. Despite its thinness, the film strongly affects the destruction process. In particular, the coalescence of particles of the surface layer prevents their erosion blow-off by the flow. The melt film also reduces the rate of oxidation of chemically-active components of the material by the incoming flow of gas.
Further into the surface lies a comparatively thick layer of charred organic binder reinforced by high-melting fibers. Still deeper is the thermal decomposition zone, where a mixture of volatile and solid (coke) components is formed. The volatile components filtered through the porous matrix are injected into the boundary layer of the incoming gas flow. An intensive sublimation of glass or other oxides which form high-melting fibers occurs on the surface of the melt film. The fraction of gaseous ablation products in the total ablation mass can, therefore, be high. The particles of coke are practically pure carbon; thus, at the melting temperature of glass they remain solid. The spreading film of glass “breaks out” the porous structure of the charred layer and carries away the particles of coke. The later, in turn, affects the flow of the melt, increasing its effective viscosity (see Melting).
At high temperatures, the coke particles in the melt film are not inert components – they interact actively both with glass and with any oxidant present in the gas flow. Tens of various strongly interacting components can exist in the boundary layer over the surface of the composite heat-shielding covering. The choice of a theoretical model for the destruction process of such materials, presents considerable difficulties. However, on the basis of extensive experimental and theoretical studies of thermophysical, thermodynamic and strength phenomena which attend the process of the incident flow effect, we have succeeded in creating a schematic model or a mechanism for the destruction of a heat-facing layer. Such a mechanism has been designed only for some classical representatives of the range of composites (see Sublimation, Melting). At the same time, advances in chemistry and materials technology extend the possibilities of selecting improved ablation materials. In this context, a demand arose for some unique parameter to compare various types of ablative materials convenient for both theoretical and experimental studies. One such parameter is the effective enthalpy of destruction, symbolized as heff.
The effective enthalpy defines the total thermal energy expenditure necessary to break down a unit mass of ablative material. The problem of comparing numerous ablative materials is most easily demonstrated for a quasi-stationary destruction (see Heat Conduction) when the velocity of all isotherms or destruction fronts inside the material coincides with the velocity of the outer surface displacement. In this case, the temperature profile inside the heat-shielding covering is described by a set of exponents, and the heat flux spent on heating inner layers does not depend on the material thermal conductivity λ∑.
Let us first consider a destruction process under conditions of exposure to convective heating. The thermal balance on a destructing surface (Figure 2) can be written as follows:
Here, (α/cp)0 is the heat transfer coefficient, and he and hw are the enthalpies of the gases in the incoming flow and the wall, respectively. In contrast to a nondestructing ablative facing, the convective heat flux supplied from without is expended not only for heating the material ( ) and by radiant re-emission of the four heated surfaces ( εσT4W ) but also for the surface (with mass loss rate and bulk (with mass loss rate physicochemical transformations, whose thermal effects are evaluated as ΔQw and ΔQ∑. If a melt film is formed on the surface of a heat-shielding covering, then , where is the mass loss rate of a substance in a molten form. The total thermal effect of the bulk failure ΔQ∑ contains not only the heat of matrix melting, but also the thermal effect of the thermal decomposition of an organic binder, the heat of heterogeneous interaction between the glass and coke inside the charred layer, etc. In a similar manner, the thermal effect of surface destruction ΔQw must account for the thermal effect of evaporation of a melted film and the burning of the coke particles in the incoming flow of gas.
Gaseous ablation products which penetrate into the boundary layer cause a reduction of a convective heat flow due to the so-called “injection effect.” We can evaluate the blocking action of the injection effect by a linear approximation (see Heat Protection):
Here, γ is the dimensionless coefficient of injection (γ < 1), which in the general case depends on flow conditions in the boundary layer (laminar or turbulent) and the ratio of molecular masses of the gas injected and the incoming flow. Unlike other effects influencing the absorption of the heat energy supplied, the injection effect rises steeply with the increasing velocity or temperature of the incoming flow and finally becomes predominant.
If we denote the share of gaseous ablation products in the total mass loss of the substance by Г (Г = / ), then we can obtain a generalized characteristic of destruction power, namely, the effective enthalpy of destruction, heff:
The effective enthalpy determines the amount of heat which can be “blocked” when breaking down a unit mass of covering material (whose surface temperature is Tw) through physicochemical processes. The higher the effective enthalpy, the better the heat-shielding material. We place emphasis on the independence of the effective enthalpy from the geometrical dimensions or the shape of the body. Actually, as distinct from a heat flux whose value, with the given parameters of the incoming flow (pe, he), is inversely proportional to (where RN is the typical dimension of the body; for instance, the radius of curvature in the vicinity of the critical point), the effective enthalpy is unaffected either by the shape or the dimension of the body. This qualifies it as a parameter for relating laboratory and real heat-loading situations.
We can see from the definition of effective enthalpy that in all cases when Г ≠ 0, it must increase substantially with the rise in the enthalpy of the stagnated flow he. The parameters of the incoming gas flow (pressure Pe and enthalpy he) can effect heff through changes in the temperature of the destructing surface Tw, the fraction of the ablation which is in gaseous form Г and the thermal effect of surface processes ΔQw. The effect of surface temperature Tw on heff can be considered to be rather limited. A typical dependence of Tw, Г and heff on enthalpy he and pressure Pe in breaking down glass reinforced plastics in an air flow is shown in Figures 3, 4 and 5. The flow condition (laminar or turbulent) in the boundary layer determines the injection coefficient γ (see Heat Protection), which affects radically the dependence of heff on he (Figure 6 ). If the ablative material does not contain oxides, then, as a rule, the share of gasification Г is close to unity. For graphite-like heat-shield covering, in particular, Г = 1. In this case, however, the thermal effect of surface processes ΔQw varies from a negative value on carbon burning C + O2 = CO2 to a positive value upon its sublimation. An extra liberation of heat upon burning brings about surface overheating relative to the equilibrium value of the temperature for a heat-insulated wall. In this case, the effective enthalpy becomes negative and the notion of heff loses practical sense. The dimensional rate of destruction is often used as an alternative parameter for generalizing the experimental and the design data:
Its advantage is that the function (he ) is always positive and besides, the temperature of the destructing surface Tw and the emissivity ε are not warranted. Typical dependences of on the stagnation enthalpy he for Teflon, glass-reinforced plastic and graphite breaking down in air flow are shown in Figure 7.
Figure 7. Dimensionless destruction rate ( ) as a function of stagnation enthalpy (he) for various materials breaking down in an air flow.
Combined radiation-convection heating of the surface of an ablative material can considerably change the mechanism of its destruction. The injection of gaseous disintegration products in cases where they do not possess high absorption coefficients, slightly reduces the intensity of the radiation component of the heat flow. As the ratio grows, the mechanism of destruction of the majority of ablative materials more closely resembles sublimation and thermal decomposition. This is due to a rapid decrease in the contribution of convective and diffusion transfer in the boundary layer while injecting gaseous products, to the ceasing of melt film flow and to the absence of burning on the destructing surface.
The heat balance on the surface of an ablative material in case of high levels of radiation of heat flows is simplified as follows:
Here, Kα, w is the absorption coefficient, which depends on the spectrum of incident radiation heat flow (λ) and on the spectral distribution of the destructing surface emissivity ελ (λ):
When no mechanical cracking or melting of a heat-shielding material occurs, the total rate of ablation coincides with and the notion of effective enthalpy of the material under intensive radiation heat influence can be introduced as:
Table 1 shows the results of the evaluation of parameters h, Kα, w (in the 0.2 < λ < 1 μm spectral range) and hR for various substances.
|Material||h, kJ/kg||Kα, w||hR, kJ/kg|
An analysis of the data presented in Table 1 allows us to reach a paradoxical conclusion: under the influence of intensive radiation, the effective enthalpies of destruction of graphite and Teflon become about equal. We should note that the ablation rate of graphite, as compared to magnesium oxide, does not differ so strongly as the other values of the effective enthalpies given in the table. This is associated with the fact that the temperature of graphite destruction is almost half as great, and, therefore, the levels of the reemitted energy differ by an order of magnitude. Nonetheless, the main conclusion that can be drawn in analyzing Table 1 is that by decreasing the absorption coefficient of the destructing surface (Kα, w), we can obtain a greater efficiency of ablation than by increasing the heat of sublimation.