lecture_20

lecture_20 - 16.512, Rocket Propulsion Prof. Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 20: Combustion of Liquid Propellants Simplified Drop-Wise Liquid Combustion Theory 1. Introduction In this lecture we examine in more depth the fluid phenomena which dominate the operation, and hence the design, of a liquid rocket. We start by considering the two phase aspects of combustion, which usually determine the combustor size. Next we examine the stability of combustor operation and the design modification required to achieve it. Heat transfer to walls, wall compatibility and wall cooling form the next topic. We conclude with some notions in nozzle aerodynamic design and nozzle optimization. 2. Drop Vaporization and Spray Combustion Returning to Fig. 12.1 (lecture 12), we note again the existence of a zone AC where the propellants go from a series of liquid jets issuing through a multiplicity of small injector holes, through breakup of these jets into droplets, impingement (in some designs) of jets or droplet streams on each other, dispersion of the droplets into a recirculating mass of combustion products, evaporation of the droplets, interdiffusion of the vapors and kinetically controlled combustion. These are obviously complicated processes, and a comprehensive analysis good enough for first principles design requires large-scale computation [18, 19, 20]. In fact, the largest number of existing liquid rocket combustors, those dating from before 1970, were developed mainly through empirical methods, supplemented by very extensive testing. Improved modeling and computational capabilities have more recently permitted a more direct approach, with fewer hardware iterations, but theory is still far from completely developed in this area, and serves at this point mainly to ascertain trends and verify mechanisms. For an in-depth discussion of liquid propellant combustion, see Refs. 21 and 22. Here we will only review the fundamental concepts which underlie current spray combustion models. 2.1 Single-Drop Combustion Even when liquids are originally injected into the chamber, actual combustion takes place in the gas phase, following vaporization of both, oxidizer and fuel. In fact, vaporization is usually the rate–controlling step in the whole process, although its rate is itself affected by the reactions occurring near each droplet. In a few instances, both propellants enter the chamber as gases or easily vaporized liquids, and then gas phase mixing is the limiting step. (An example is the Shuttle SSME, where the hydrogen is vaporized in the cooling circuit, and the oxygen is partially vaporized by the hydrogen in a co-flowing heat exchanger arrangement ahead of the injectors, the rest being atomized and evaporated shortly after injection). 16.512, Rocket Propulsion Lecture 20 Prof. Manuel Martinez-Sanchez Page 1 of 16
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In a fuel- rich environment, vapor from a fuel droplet diffuses without reacting into the surrounding gas, while heat diffuses inwards to supply the latent heat of evaporation. In the same environment, vapor from an oxidizer drop
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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_20 - 16.512, Rocket Propulsion Prof. Manuel...

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