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Active thermal protection system

Active thermal protection of the Ajax hypersonic vehicle is based on the chemical endothermic conversion of the initial hydrocarbon fuel at the expense of utilization of thermal losses associated with aerodynamic heating of the airframe and power plant operation. This conversion is carried out in catalytic reactors installed in heat-stressed parts of the vehicles and it allows:

  • increase fuel cooling resources through physical-and-chemical transformations of the initial components;
  • provide cooling of the structure not only at the expense of heat removal by convection and radiation but also absorbing it in the process of chemical reaction directly on the protected surface;
  • obtain hydrogen-containing fuel mixture being supplied to the combustion chamber for improving energy and ecological characteristics of the combustion process.

The basic idea of using chemical heat recovery for cooling and fuel conversion is based on the fact that the gaseous energy carriers with their heating accompanied by endothermic chemical reactions, taking place in them, optimally solve both tasks in hand. Attractiveness of chemically reacting gas as a coolant is determined by two main factors. First, due to the thermal effect of the endothermic reaction the total heat capacity of gas increases significantly, therefore at the same temperature of gas heating it is possible to considerably increase the heat removal from the cooling surface. Second, the process of heat transfer in a chemically reacting gas, along with the usual convective heat transfer, involves the diffusion transport of latent heat of reaction, which in contrast to the convective heat transfer does not create a significant temperature ratio. Thus, at the parallel operation of both types of heat transfer in the boundary layer on the cooled wall, a significant increase in the combined heat release is observed

Among many existing endothermic reactions, we consider the reaction of hydrocarbon steam conversion. The specific form of steam conversion reaction depends on the process conditions (temperature, pressure, water/hydrocarbon ratio, etc.) Thus, at high temperatures (t > 1000 º C) the reactions practically proceed up to the formation of H2 and CO only (high-temperature conversion): 

At low temperatures (t < 400 º C) the product yield is strongly shifted toward the formation of CH4 and CO2 and the resulting reaction of gasification can be represented as (low-temperature conversion):

In the general case, the reactions are accompanied by two other independent reversible reactions which determine the equilibrium composition of the converted gas:

Unfortunately, the process of hydrocarbon decomposition is complicated by adverse reactions of free carbon (coke) formation. One way to reduce coke formation is application of two-stage scheme of decomposition.  The thing is that the rate of deposition of coke on the surface of nickel-containing catalysts decreases in the series: ethylene > benzene > heptane > hexane > butane > methane. Besides, the ratio of C/H in the molecule of methane is minimal. Therefore, if methane serves as the feedstock, the formation of carbon deposits is not a difficult problem in general.

Example of a combustion chamber with an active thermal cooling

Coming from the fuel preparation system hydrocarbon duel is directed through two channels. The main part of it goes (is fed) to the combustion chamber. A smaller part goes to the thermo-chemical reactor (TCR), one of the walls of which is a cylindrical inner wall of the TCR. Water vapor goes to the TCR as well. Resulting from steam conversion hydrogen is mixed with the initial fuel. Thus, hydrogen-enriched methane gets in the combustion chamber significantly improving fuel mixture quality (in particular, fuel calorific value rises). Some of the released energy creates heat flux to the wall of the reactor which is used for steam conversion reaction. Reduction of temperature of combustor wall takes place at that.