BYPASS FLOW PATTERN CHANGES AT TURBO-RAM TRANSIENT OPERATION OF A COMBINED CYCLE ENGINE

Shinichi Takata

Researcher, Kakuda Space Center, NASDA

Toshio Nagashima

Professor, Department of Aeronautics and Astronautics, Univ. Tokyo

Susumu Teramoto

Lecturer, Department ofAeronautics and Astronautics, Univ. Tokyo

Hidekazu Kodama

Assistant General Manager, Advanced Technology Department, IHI Co, Ltd

Abstract Turbo-Ramjet Combined Cycle Engine is composed of a turbofan engine and a ramjet engine that is expected to be a candidate of future hypersonic propulsion systems. It can operate as turbofan mode for low speed range (Mach number 0~3), then changes into ramjet mode for the higher speed range(Mach number 2.5~5). In order to achieve smooth mode transition, it inevitably employs vari­able geometry components, so that the latter control sequence has to be carefully scheduled to maintain air fbw rate and thrust at cruise condition during the mode transition. The present study therefore examines the flaw pattern changes due to ram air mixing with fan bypass air, that are not available from the transient en­gine thermo-cycle simulation. CFD calculation showed that such ft>w patterns be feasible, yielding rather good agreement between the thermo-cycle and flow simulations with respect to total engine air flow rate and thrust.

1. Introduction

Turbo-Ramjet Combined Cycle Engines (CCE) are expected to be a promis­ing choice for the next generation hypersonic fight propulsion system, which was successfully demonstrated a couple of years ago by Japanese HYPR project with US and EU international collaboration. (Miyagi et al., 1995a, Miyagi

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K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 345-356. © 2006 Springer. Printed in the Netherlands.

et al., 1995b, Takeo et al., 1999) A key issue in the project was concerning the transition in the engine operation from turbofan mode (Mach number 0^3) to ramjet mode (Mach number 3^5) that takes place at Mach 2.5 through 3.0 climb around the altitude of 20 km. The engine thrust and air fbw rate need to be maintained for smooth and stable acceleration to the hypersonic cruise, so that a sequence of controls for variable geometry mechanism was adopted. HYPR engine employed a tandem arrangement of the turbo core engine and the ram combustor, attaching 2D variable area nozzle at the exhaust end. The ram air is introduced through Mode Selector Valve (MSV) at the entrance of the ram inlet duct which wraps around the core fan engine and mixes in the bypass duct with the bypass fan air through forward-Variable Area Bypass In­jector (FVABI). Upon opening MSV to start the ramjet mode and depending upon the rear pressure level, a flow reversal or recirculation of the bypass fan air jet through mixing in the ram inlet duct is alarmed to cause unstable flows, which may lead to the ram combustion instability.

The primary objective of this paper is therefore to investigate whether the conjectured control sequences suit to yield safe and smooth transition or not, by means of flow pattern visualization in accordance with CFD simulation.

In the following, a series of flow patterns in the ram inlet/bypass duct and the rear mixing zone, representing important stages as the engine shifts from turbofan to ramjet, will be shown, which may not be adequately consistent in the sense that the domain interface conditions should be also available as a part of the solution in a fully coupled dynamic analysis of the fan and core engine fbw fields. The latter treatment requires quite a heavy CPU burden, so that a transient thermo-cycle model was presently adopted to yield the proper temporal values of the thermo dynamic parameters at the interfaces. Since the turbo-ram mode transition is scheduled during a time interval of several tens of seconds, long enough for each control sequence to be well settled down, the present predictions are expected to be satisfactory for such engine operation.