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In an actual combustion, 100% of the fuel energy is not available to increase the energy of the air stream. The first non-availability results because the atmospheric air is not at absolute zero. That loss of available energy is called a Carnot loss. Typically the Carnot loss is about 21% of the input energy, that is 79% is available. The second non-availability in the combustor results from the temperature gradient in the combustor from the center of the combustor to the cooler wall. Typically for metal walls in gas turbine engines and other airbreathing engines that loss is about 10%, so now 69% of the available combustor energy is available to produce thrust. The third non-availability results from the energy required to mix the fuel and air at high combustor flow speeds [Swithenbank, 1969]. This latter energy loss is a function of the kinetic energy of the fuel entering the combustor compared to the kinetic energy of the air stream. These three non-availabilities are due to basic thermodynamics and gas dynamics. Nothing at this point has been included in terms of friction and shock wave losses in the engine module. The ratio of the kinetic air stream energy to the hydrogen-air combustion heat addition is presented in Figure 4.3 for the three energy non-availabilities.

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