2d model for Unsteady Burning of Heterogeneous ap/Binder Solid Propellants




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Название2d model for Unsteady Burning of Heterogeneous ap/Binder Solid Propellants
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2D Model for Unsteady Burning of Heterogeneous AP/Binder Solid Propellants


Sergey T. Surzhikov*, J.J. Murphy, Herman Krier**

Center for Simulation of Advanced Rockets

University of Illinois at Urbana-Champaign

1206 West Green Street

Urbana, IL 61801


Abstract


The burning of a periodic sandwich of the solid oxidizer, ammonium perchlorate (A) and solid fuel, HTPB is modeled by considering two-dimensional energy balances in both the solid and gas phases, two-dimensional gas species concentration, considering a reduced chemistry model for three global reactions and eight chemical species. Full heat coupling between the solid and gas phase allows the prediction of the AP, binder, and average regression rates.

Flame structure including the AP decomposition flame and the diffusion flames with the binder are predicted to occur within regions ranging from 10 m to 200 m.

Solutions are presented for various AP/binder ratios, at solid rocket pressures, ranging from 40-100 atm. Parametric studies identify the sensitivity of the burning rates to the chemical kinetics constants and the pyrolysis relations, as well as the solid-phase heat exchange coefficient, .

1. Introduction1



A numerical simulation of unsteady heterogeneous propellant combustion has been developed. The simulation uses a two-dimensional, periodic sandwich configuration, and includes a realistic, albeit simplified, chemical reaction scheme valid for propellants consisting of ammonium perchlorate (AP) oxidizer and hydroxyl-terminated polybutadiene (HTPB) binder. We are using this model to examine the following of physical phenomena:

  • radiation augmented burning

  • pressure-coupled burning-rate response

  • effects of heterogeneity along the burning direction

  • effects of acceleration on flame structure.

Heterogeneous propellant combustion is a very complicated process. The complexity starts with the material itself. Composite propellants are a mix of crystalline oxidizer and polymer binder, and have heterogeneous length scales on the order of 100 m or more. When these materials are burned, flames with characteristics of both premixed flames and diffusion flames can form in the gas phase. The surface of the burning propellant is multi-phase and three-dimensional, resembling a boulder field on which molten binder can flow. It is important to realize that significant chemistry is occurring in the gas phase, in the solid phase, and at the surface. When materials such as aluminum are added to the mix, the complexity increases by another order of magnitude. It is safe to say that all models previously developed or currently in development account for only a fraction of these phenomena.

The aspects we choose for study here include several transient phenomena, as well as the effects of radiation and acceleration. First, we developed a computational model of heterogeneous propellant combustion. Chemistry is approximated using either a two- or three-step global chemical kinetics scheme. Pyrolysis of the binder and oxidizer is modeled using simple Arrhenius surface reactions. The surface is assumed to be a planar, gas-solid interface – all multi-phase phenomena are ignored. Heat transfer within each solid phase is assumed to be either quasi-one-dimensional or fully two-dimensional. Rudimentary heat transfer between the binder and oxidizer is allowed. The phases are fully coupled, so the burning rate can be determined from the model.

Because the model approximates both the geometry and the chemistry of a burning propellant sandwich, burning-rate predictions from the model are compared to burning-rate data for propellant sandwiches. We also study the effects of thermal radiation and compare predictions of how the burning rate is affected by radiation with some experimental results.

The response of the burning rate to pressure oscillations can also be studied. This problem is important to the study of combustion-driven instabilities in solid rocket motors. Specifically, the differences in the responses of the oxidizer and the binder, which can be studied with our model, are of interest.

Composite propellants are heterogeneous in all three directions, rather than just one. The transients that occur when an AP particle is uncovered and then ignited are likely an important part of AP-HTPB propellant combustion. The combustion “noise” which occurs as a result of this process may have a stabilizing or destabilizing effect on the rocket motor. The dynamics of this inherently unsteady process may also couple with “forcing” such as the pressure oscillations which occur when a rocket is unstable. Such questions are investigated here with a “staggered sandwich”, or ”checker-board” model. As the propellant burns down, the binder and oxidizer are periodically switched. The resulting dynamics are isolated and studied, to gain insight into how this heterogeneous phenomenon affects both steady and unsteady burning processes in the propellant.

The model can also used to study acceleration effects on the flame structure of a propellant. Acceleration of the burning surface of a propellant in a solid rocket motor can occur in several situations. One example is when the motor is stabilized by spinning it around its longitudinal axis. When combustion-driven instability is a problem, the resulting pressure oscillations can flex the casing of the motor and the propellant inside it. In come cases, accelerations or as much as 10000 g have been measured, although accelerations in the range of 100 to 1000 g are much more common. The Froude number,

,

where u is the gas flow velocity, is the acceleration, L is the space scale of the process, which characterizes the effect of acceleration in this model, becomes of order one at around of acceleration. Thus, buoyancy becomes important to the flowfield, and can significantly alter the flame structure.

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