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lecture_12 - 16.512, Rocket Propulsion Prof. Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 12: Review of Equilibrium Thermochemistry Thermochemistry of Combustion for Propulsion Devices 12.1.1 Introduction The general principles that govern chemical reactions (species conservation, enthalpy balance and chemical equilibrium) have been introduced in previous subjects. In the next three Sections we will briefly examine the application of these principles to Propulsion. The sketch in Fig 12.1 indicates the main regions. Fig. 12.1. Main regions in a liquid-fuel combustor In a combustion chamber which could belong to a ramjet or rocket, or, with minor variations, to many other technical devices which utilize hot gas flows. It is frequent to supply fuel (and sometimes oxidizer as well) in liquid form, and so the first region involves a series of colliding liquid jets or sprays, designed to break up these liquids into fine droplets. These now find themselves immersed in the hot combustion gases, and evaporate rapidly, a step which must necessarily precede actual chemical reaction. Mixing of the oxidizer and fuel vapors typically proceeds via the strong turbulence existing in the combustor, and once this mixing has actually 16.512, Rocket Propulsion Lecture 12 Prof. Manuel Martinez-Sanchez Page 1 of 10
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achieved intimate molecular level interdiffusion within the turbulent eddies, reaction occurs usually on a time scale which is short compared to those for vaporization and mixing. The combustion chamber design must take into account the kinetics of these steps to arrive at a dimension which guarantees completion of the combustion reactions without waste of volume. If this is so, the last portion of the combustor contains at any time a mixture of combustion products which have reached a state of thermodynamic equilibrium, and which are about to flow into their working devices, be it a simple nozzle, a turbine (also preceded by its nozzles) or other configurations. Equilibrium does not necessarily mean complete combustion since, at high temperatures, various decomposition reactions are thermodynamically favored. In a high speed Ramjet with a subsonic combustion chamber the incompleteness of the combustion is so extreme –due to the high temperature prevailing- that, beyond a certain flight speed, no net heat release can be obtained. It is important to distinguish between this kind of combustion incompleteness, which is unavoidable at high temperatures, and is due to the activation of reverse reactions, and a more common but avoidable incompleteness, which occurs when the combustion chamber is inadequately sized and the vaporization-mixing-reaction processes are not completed in the residence time allowed to each fluid particle. A first objective of our analysis is to determine the actual gas conditions (temperature, composition) at the exit of an equilibrated combustor, for a set of prescribed reactant flow rates. These flow rates, together with the discharge nozzle geometry, also determine a combustor pressure, which we will here assume as an independently prescribed quantity.
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This note was uploaded on 11/07/2011 for the course AERO 16.512 taught by Professor Manuelmartinez-sanchez during the Fall '05 term at MIT.

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lecture_12 - 16.512, Rocket Propulsion Prof. Manuel...

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