Optimization of Hypersonic Airbreathing Propulsion Systems through Mixed Analysis Methods
DellaFera, Andrew Brian
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Accurate flow path modeling of scramjet engines is a key step in the development of an airframe integrated engine for hypersonic vehicles. A scramjet system model architecture is proposed and implemented using three different engine components: the isolator, combustor, and nozzle. For each component a set of intensive properties are iterated to match prescribed conditions, namely the mass flow. These low-fidelity one-dimensional models of hypersonic propulsion systems are used in tandem with Sandia Labs' Dakota optimization toolbox with the goal of accelerating the design and prototyping process. Simulations were created for the various components of the propulsion system and tied together to provide information for the entire flow-path of the engine given an inlet state. The isolator model incorporated methods to compute the intensive properties such as temperature and pressure of the flow path whether a shock-train exists internally as a dual-mode ramjet or if the engine is operating as a pure scramjet with a shock free isolator. A Fanno flow-like model was implemented to determine the friction losses in the isolator and a relation is iterated upon to determine the strength and length of the shock train. Two combustor models were created, the first of which uses equilibrium chemistry to estimate the state of the flow throughout the combustor and nozzle. Going one step further, the second model uses a set of canonical reactors to capture the non-equilibrium effects that may exist in the combustor/nozzle. The equilibrium combustor model was created to provide faster calculations in early iterations, and the reactor model was created to provide more realistic data despite its longer computational time. The full engine model was then compared and validated with experimental data from a scramjet combustor rig. The model is then paired with an optimization toolbox to yield a preliminary engine design for a provided design space, using a finite element analysis to ensure a feasible design. The implemented finite element analysis uses a coarse mesh with simple geometry to reduce computational time while still yielding sufficiently accurate results. The results of the optimization are then available as the starting point for higher fidelity analyses such as 2-D or 3-D computational fluid dynamics.
General Audience Abstract
Ramjets and scramjets are the key to sustained flight at speeds above five times the speed of sound. These propulsion systems pose a challenging simulation environment due to the wide range of flow seen by the system structure. A scramjet simulation model is formulated using a series of combustion models with the goal of accurately modelling the combustion processes throughout the engine. The combustor model is paired with an isolator model and the engine model is compared against previous studies. A structural analysis model is then paired with the engine simulation, and the combined model is used within an optimizer to find an optimum design.
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