Hybrid propellant rockets have been proposed as a valuable choice for future-generation space propulsion systems, and the Computational Fluid Dynamics (CFD) modeling of this class of devices has raised a considerable interest in the scientific community. The development of suitable simplified chemical kinetics mechanisms, representing a trade off between CFD accuracy and computational costs, is of great interest.
In this context, a set of simplified chemical kinetics mechanisms for hybrid rocket applications using gaseous oxygen (GOX) and hydroxyl-terminated polybutadiene (HTPB) will be obtained starting from a 561-species, 2538-reactions, detailed chemical kinetics mechanism for hydrocarbon combustion.
This set mechanisms will be used for predictions of the oxidation of butadiene, the primary HTPB pyrolysis product.
The simplification will be carried out systematically by means of a Computational Singular Pertur-bation (CSP) based algorithm. The simplification algorithm will be fed with the steady-solutions of classical flamelet equations, these being representative of the non-premixed nature of the combus-tion processes characterizing a hybrid rocket combustion chamber. The flamelet steady-state solu-tions will be obtained employing pure butadiene and gaseous oxygen as fuel and oxidizer bounda-ry conditions for a range of imposed values of strain rate and background pressure. Finally, a com-prehensive strategy will be employed to obtain simplified mechanisms capable of reproducing the main flame features in the whole pressure range considered.
The simplification strategy, even though based on a well established method, is applied for the first time starting from a dataset generated by means of a reactive systems including transport processes, such as a diffusive flame.
The CSP analysis has already been employed for the analysis of reactive flows to inquire the mu-tual role of reactive and transport processes [1¿3]. So far the CSP-based automatic chemical mechanism simplification procedure has been successfully employed in the context of purely reactive systems [4, 5]. The capabilities of aforementioned automatic procedure was repeatedly as-sessed in describing the ignition transient of batch reactors, as well as in numerical predictions laminar and turbulent premixed flames [6]. In a diffusive, stably burning diffusive flame, the chemical source term is balanced by diffusion, which impedes the chemical equilibrium to be reached [2] and, because of its non-premixed nature, the mixture equivalence ratio varies from pure fuel to pure oxidizer.
In the present project the CSP-based automatic procedure for the simplification of chemical kinetic mechanisms will be adapted for the first time to consistently use a flamelet-generated dataset. As a consequence, the improvement in the algorithm capabilities is two fold. On one hand, it allows to populate the chemical composition state with states that could not be visited by a purely reactive system evolution, such as the combustion of mixtures richer that the auto-ignition limit. On the other hand, it does not take into account those processes which are not relevant to a stably burning non-premixed flame, such as the auto-ignition dynamics.
The result of this work will be a set of skeletal mechanisms for butadiene/oxygen combustion, useful for reactive flow modeling of hybrid rocket engine.
The algorithm improvements will be assessed in terms of capability of pointing out those reactive processes which are the most relevant to the competition between chemical kinetics and transport processes.
- Dissemination
The main findings resulting from the project will be published on International Archival Journals and presented at National and International Conferences. The Journals where we plan to publish our findings will most likely be Combustion and Flame, J. Comput. Phys., Combustion Theory and Modeling, J. Fluid Mechanics. Moreover, we plan to activate a dedicated web site.