A computational fluid dynamics (CFD) code has been developed to enable simulation of spray combustion near the fuel injectors in a liquid-fueled rocket engine. This code reflects the three-dimensional (3D), multiphase nature of the flow field in a rocket engine and is capable of modeling even a flow field as complex as one that results from the use of impingement injectors. Unlike prior spray-combustion codes that emphasize physical constraints at the expense of geometric ones, this code implements a compromise between physical and geometric constraints in order to enable analysis and comparison of the performances of alternative engine designs that involve different injector geometries. In particular, this code was constructed to enable prediction of the interactive effects of injector-element impingement angles and impingement points, momenta of individual orifice flows, and the resulting combusting flow.

This computer program includes finite-difference Navier-Stokes CFD subprograms for mathematical modeling of spray combustion from several different perspectives. Two of the subprograms implement models of heterogeneous spray combustion: One of these models, denoted a volume-of-fluid (VOF) model, represents a liquid core of coaxial and impinging jets and the atomization and vaporization thereof. The other, denoted a blob model, represents injected streams as a cloud of droplets that are initially of the size of the injector orifice and that subsequently exhibit particle interaction, vaporization, and combustion. These models are computationally intensive, as they must be to account accurately for the complex combustion and other physics that one seeks to predict.

One of the subprograms implements a model of homogeneous spray combustion, representing the flow as a continuum of multiphase, multicomponent fluids that move without thermal or velocity lags among the phases. This model enables relatively fast computation. To enable the representation of subcritical and supercritical liquid and vapor flows, this subprogram uses a real-fluid model that comprises thermal and caloric equations of state. The great advantage of this real-fluid model is that it is valid over a wide range of pressures and temperatures, making it unnecessary to provide (as in prior approaches) a submodel to represent the effects of surface tension in the subcritical regime; this is important because liquid-fuel rocket engines usually operate at supercritical pressures (for which surface tensions are zero), making it counterproductive to continue to adapt low-pressure spray-combustion models.

Because some rocket engines utilize RP-1 (essentially, kerosine) and liquid oxygen as propellants, the homogeneous-spray subprogram includes a simplified hydrocarbon-combustion model for use in simulations of 3D, multiphase flow. This model does not identify drops or their distribution, but it does enable accurate prediction of film coolant flow and of recirculating flow along the injector face and into an acoustic cavity. Soot is represented as a third phase that behaves as a dense gas. The resolution of the flow field of the reacting, vaporizing propellants predicted by this spray model exceeds that of any other published model developed for the same purpose.

The heterogeneous VOF and blob spray-combustion subprograms include provisions for Euler-Lagrange particle tracking to account for thermal and velocity lag of droplets. These subprograms also use the real-fluid equations of state to calculate thermodynamic properties of fluids.

This program was written by R. C. Farmer and G. C. Cheng of SECA, Inc., and Y. S. Chen of Engineering Sciences, Inc., for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Software category.

MFS-31599


NASA Tech Briefs Magazine

This article first appeared in the October, 2001 issue of NASA Tech Briefs Magazine.

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