Combustion Process

In describing the combustion processes, it is convenient and helpful to the understanding to divide the combustion chamber into a series of discrete zones, as shown in Fig. 9-1 for a typical configuration. It has a flat injector face with many small injection orifices for introducing both fuel and oxidizer liquids as many discrete individual streams, jets, or thin sprays or sheets. The relative thicknesses of these zones, their behavior, and their transitions are influenced by the specific propellant combination, the operating conditions (pressure, mixture ratio, etc.), the design of the injector, and chamber geometry. The boundaries between the zones shown in Fig. 9-1 are really not flat surfaces and do not display steady flow. They are undulating, dynamically movable, irregular boundaries with localized changes in velocity, temporary bulges, locally intense radiation emissions, or variable temperature. Table 9-1 shows the major interacting physical and chemical processes that occur in the chamber. This table is a modification of tables and data in Refs. 9-2 and 9-3.

The combustion behavior is propellant dependent. If the fuel were hydrogen that has been used to cool the thrust chamber, the hydrogen would be gaseous and fairly warm (60 to 500 K); there would be no liquid hydrogen droplets and no evaporation. With hypergolic propellants there is an initial chemical reaction in the liquid phase when a droplet of fuel impinges on a droplet of oxidizer. Experiments show that the contact can create local explosions and enough energy release to suddenly vaporize a thin layer of the fuel and the oxidizer locally at the droplet's contact face; there immediately follows a vapor chemical reaction and a blow-apart and breakup of the droplets, due to the explosion shock wave pressure (Refs. 9-4 and 9-5).

Injection/ Rapid- Streamtube atomization combustion combustion Transonic-flow zone zone

Injection/ Rapid- Streamtube atomization combustion combustion Transonic-flow zone zone

Supersonic expansion zone

Two-dimensional sonic-flow line

FIGURE 9-1. Division of combustion chamber into zones for analysis. (Reprinted with permission from Ref. 8-1, copyright by AIAA.)

Two-dimensional sonic-flow line

Supersonic expansion zone

FIGURE 9-1. Division of combustion chamber into zones for analysis. (Reprinted with permission from Ref. 8-1, copyright by AIAA.)

TABLE 9-1. Physical and Chemical Processes in the Combustion of Liquid Propellants




Liquid jets enter chamber at relatively low velocities Sometimes gas propellant is injected Partial evaporation of liquids Interaction of jets and high pressure gas

Impingement of jets or sheets Formation of liquid fans Formation of droplets Secondary breakup of drops Liquid mixing and some liquid-liquid chemical reaction Oscillations of jets or fans as they become unstable during breakup Vaporization begins and some vapor reactions occur

Droplet gasification and diffusion Further heat release from local chemical reactions Low gas velocities and some cross flow Heat absorbed by radiation and conduction from blowback of turbulent gases from the hot reaction zone Acceleration to higher velocities Vaporization rate influenced by pressure or temperature oscillations and acoustic

Mixing and Reaction

Expansion in Chamber

Turbulent mixing (three-dimensional) Multiple chemical reactions and major heat releases

Interactions of turbulence with droplets and chemical reactions Temperature rise reduces densities Local mixture ratios, reaction rates, or velocities are not uniform across chamber and vary rapidly with time Some tangential and radial flows

Chemical kinetics causes attainment of final combustion temperature and final equilibrium reaction gas composition Gas dynamics displays turbulence and increasing axial gas velocities Formation of a boundary layer Acceleration to high chamber velocities Streamlined high-velocity axial flow with very little cross flow

Rapid Combustion Zone

In this zone intensive and rapid chemical reactions occur at increasingly higher temperature; any remaining liquid droplets are vaporized by convective heating and gas pockets of fuel-rich and fuel-lean gases are mixed. The mixing is aided by local turbulence and diffusion of the gas species.

The further breakdown of the propellant chemicals into intermediate fractions and smaller, simpler chemicals and the oxidation of fuel fractions occur rapidly in this zone. The rate of heat release increases greatly and this causes the specific volume of the gas mixture to increase and the local axial velocity to increase by a factor of 100 or more. The rapid expansion of the heated gases also forces a series of local transverse gas flows from hot high-burning-rate sites to colder low-burning-rate sites. The liquid droplets that may still persist in the upstream portion of this zone do not follow the gas flow quickly and are difficult to move in a transverse direction. Therefore, zones of fuel-rich or oxidizer-rich gases will persist according to the orifice spray pattern in the upstream injection zone. The gas composition and mixture ratio across the chamber section become more uniform as the gases move through this zone, but the mixture never becomes truly uniform. As the reaction product gases are accelerated, they become hotter (due to further heat releases) and the lateral velocities become relatively small compared to the increasing axial velocities.

The combustion process is not a steady flow process. Some people believe that the combustion is locally so intense that it approches localized explosions that create a series of shock waves. When observing any one specific location in the chamber, one finds that there are rapid fluctuations in pressure, temperature, density, mixture ratio, and radiation emissions with time.

Injection/Atomization Zone

Two different liquids are injected with storable propellants and with liquid oxygen/hydrocarbon combinations. They are injected through orifices at velocities typically between 7 and 60 m/sec or about 20 to 200 ft/sec. The injector design has a profound influence on the combustion behavior and some seemingly minor design changes can have a major effect on instability. The pattern, sizes, number, distribution, and types of orifices influence the combustion behavior, as do the pressure drop, manifold geometry, or surface roughness in the injection orifice walls. The individual jets, streams, or sheets break up into droplets by impingement of one jet with another (or with a surface), by the inherent instabilities of liquid sprays, or by the interaction with gases at a different velocity and temperature. In this first zone the liquids are atomized into a large number of small droplets (see Refs. 9-3 and 9-6). Heat is transferred to the droplets by radiation from the very hot rapid combustion zone and by convection from moderately hot gases in the first zone. The droplets evaporate and create local regions rich either in fuel vapor or oxidizer vapor.

This first zone is heterogeneous; it contains liquids and vaporized propellant as well as some burning hot gases. With the liquid being located at discrete sites, there are large gradients in all directions with respect to fuel and oxidizer mass fluxes, mixture ratio, size and dispersion of droplets, or properties of the gaseous medium. Chemical reactions occur in this zone, but the rate of heat generation is relatively low, in part because the liquids and the gases are still relatively cold and in part because vaporization near the droplets causes fuel-rich and fuel-lean regions which do not burn as quickly. Some hot gases from the combustion zone are recirculated back from the rapid combustion zone, and they can create local gas velocities that flow across the injector face. The hot gases, which can flow in unsteady vortexes or turbulence patterns, are essential to the initial evaporation of the liquids.

The injection, atomization and vaporization processes are different if one of the propellants is a gas. For example, this occurs in liquid oxygen with gaseous hydrogen propellant in thrust chambers or precombustion chambers, where liquid hydrogen has absorbed heat from cooling jackets and has been gasified. Hydrogen gas has no droplets and does not evaporate. The gas usually has a much higher injection velocity (above 120 m/sec) than the liquid propellant. This causes shear forces to be imposed on the liquid jets, with more rapid droplet formation and gasification. The preferred injector design for gaseous hydrogen and liquid oxygen is different from the individual jet streams used with storable propellants, as shown in Chapter 8.

Stream Tube Combustion Zone

In this zone oxidation reactions continue, but at a lower rate, and some additional heat is released. However, chemical reactions continue because the mixture tends to be driven toward an equilibrium composition. Since axial velocities are high (200 to 600 m/sec) the transverse convective flow velocities become relatively small. Streamlines are formed and there is relatively little turbulent mixing across streamline boundaries. Locally the flow velocity and the pressure fluctuate somewhat. The residence time in this zone is very short compared to the residence time in the other two zones. The streamline type, inviscid flow, and the chemical reactions toward achieving chemical equilibrium presist not only throughout the remainder of the combustion chamber, but are also extended into the nozzle.

Actually, the major processes do not take place strictly sequentially, but several seem to occur simultaneously in several parts of the chamber. The flame front is not a simple plane surface across the combustion chamber. There is turbulence in the gas flow in all parts of the combustion chamber.

The residence time of the propellant material in the combustion chamber is very short, usually less than 10 milliseconds. Combustion in a liquid rocket engine is very dynamic, with the volumetric heat release being approximately 370 MJ/m3-sec, which is much higher than in turbojets. Further, the higher temperature in a rocket causes chemical reaction rates to be several times faster (increasing exponentially with temperature) than in turbojet.

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