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NIA Seminar by Joe Iannelli |
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Date: October 25, 2006
Time: 11:00am (Note new time)
Location: NIA, Rm 137
Additional Information: Bio | Presentation (.pdf)
Calculation of Through Flows in Turbojet Engines
and Supersonic Inlets with Flow Control
Joe Iannelli, The City University of London, UK
This presentation describes the computational prediction of steady and unsteady flows throughout an entire turbojet engine and in a supersonic inlet with flow control. For these predictions, the developed computational procedure couples the Euler and Navier-Stokes equations with a characteristics-based parabolic perturbation that induces a continuum multi-dimensional upwind. The procedure also employs a non-linearly stable implicit Runge-Kutta scheme and a preconditioned GMRES iterative numerical linear-algebra method, with a ten-fold convergence-rate improvement over the baseline method.
One of the most challenging issues in turbojet flow prediction consists in modeling the spinning compressor and turbine. The developed computational procedure models the effects of these two rotating components, within the linear-momentum and energy Euler / Navier-Stokes equations, by suitable blade-force and shaft-power source terms that depend on the compressor/turbine angular speed and satisfy the 2nd law of thermodynamics. The resulting system is then tightly coupled with the rotational dynamics equation of the compressor, turbine and connecting shaft. As a consequence, the computational procedure predicts the time variation of the shaft angular speed as it relates to the steady as well as unsteady spatial distributions of turbojet axial flow speed, pressure, temperature and total enthalpy.
In supersonic-flight turbojets, the inlet critically matches the supersonic free stream with the subsonic flow needed at the axial-compressor entrance. The inlet must first start, hence swallow an initial external bow shock; it then decelerates the established convergent-duct supersonic flow with minimal stagnation-pressure loss, which requires the presence of a weak essentially normal shock downstream of the inlet throat. Although geometrically simple, an internal-compression inlet cannot be theoretically started for flight Mach numbers equal to or greater than 2.0, without any form of flow control. Operation of an inlet at off-design conditions, additionally, can then lead to rapid disgorgement of the steady divergent-duct shock, with abrupt loss of thrust and rise in drag, a situation known as unstart. The Air Force and NASA continuously seek methods to improve start conditions and mitigate unstart. This presentation will also detail the performance of a distributed flow control system, from inlet entrance to throat, which activates transfer of flow energy or mass, linear momentum and total enthalpy. This system succeeds in mitigating unstart and starting an internal-compression inlet without any theoretical limitation on the supersonic flight Mach number. It achieves these objectives by rapidly attracting the external bow shock into the inlet and preventing the eventual steady shock from racing upstream.
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