Fuhua Ma

System Performance and Thermodynamic Cycle Analysis of Airbreathing Pulse Detonation Engines

A modular approach to the study of system performance and thermodynamic cycle efficiency of airbreathing pulse detonation engines (PDEs) is described. Each module represents a specific component of the engine, and its dynamic behavior is formulated using conservation laws in either one or two spatial dimensions. A framework is established for assessing quantitatively the influence of all known processes on engine dynamics. Various loss mechanisms limiting the PDE performance are identified. As a specific example, a supersonic PDE for high-altitude applications is studied comprehensively. The effects of chamber configuration and operating sequence on the engine propulsive efficiency are examined. The results demonstrate the existence of an optimum cycle frequency and valve close-up time for achieving maximum performance in terms of thrust and specific impulse. Furthermore, a choked convergent-divergent nozzle is required to render the PDE competitive with other airbreathing propulsion systems, such as gas-turbine and ramjet engines.

Propulsive Performance of Airbreathing Pulse Detonation Engines

The propulsive performance of airbreathing pulse detonation engines at selected flight conditions is evaluated by means of a combined analytical/numerical analysis. The work treats the conservation equations in axisymmetric coordinates and takes into account finite-rate chemistry and variable thermophysical properties for a stoichiometric hydrogen/air mixture. In addition, an analytical model accounting for the state changes of the working fluid in pulse detonation engine operation is established to predict the engine performance in an idealized situation. The system under consideration includes a supersonic inlet, an air manifold, a valve, a detonation tube, and a convergent-divergent nozzle. Both internal and external modes of valve operation are implemented. Detailed flow evolution is explored, and various performance loss mechanisms are identified and quantified. The influences of all known effects (such as valve operation timing, filling fraction of reactants, nozzle configuration, and flight condition) on the engine propulsive performance are investigated systematically. A performance map is established over the flight Mach number of 1.2-3.5. Results indicate that the pulse detonation engine outperforms ramjet engines for all the flight conditions considered herein. The benefits of pulse detonation engines are significant at low-supersonic conditions, but gradually decrease with increasing flight Mach number.

Thrust Chamber Dynamics and Propulsive Performance of Single-Tube Pulse Detonation Engines

This pape r deals with the modeling and simulation of the thrust chamber dynamics in an airbreathing pulse detonation engine (PDE). The system under consideration includes a supersonic inlet, an air manifold, a rotary valve, a single -tube combustor , and a convergent -divergent nozzle. The analysis accommodates the full conservation equations in two - dimensional coordinates, along with a calibrated one - progress -variable chemical reaction scheme for a stoichiometric hydrogen/air mixture. The combustion and flow dynamics involved in typical PDE operations are carefully examined. In addition, a flow -path based performance prediction model is established to estimate the theoretical limit of the engine propulsive performance. Various performance loss mechanisms, including refilling process, mismatch of nozzle exit flow conditions with the ambient state, nozzle flow divergence, and internal flow dynamics, are identified and quantified. The internal flow loss, which mainly arises from the shock waves within the chamber, play s a dominant role in degrading the PDE performance. The effects of engine operating parameters and nozzle configurations on the system dynamics are also studied in depth. Results indicate the existence of an optimum operating frequency for achieving a be st performance margin. For a given cycle period and purge time, the performance increases with decreasing valve close -up time in most cases. On the other hand, a larger purge time decreases the specific thrust but increases the specific impulse for a giv en cycle period and valve close -up time. The nozzle throat area affects both the flow expansion process and chamber dynamics, thereby exerting a much more significant influence than the other nozzle geometrical parameters.

Acoustic Characterization of an Ethylene-Fueled Scramjet Combustor with a Cavity Flameholder

*† ‡ § ** †† ‡‡ The occurrence of combustion oscillations has recently raised serious concerns about the development of scramjet engines. Previous studies on supersonic combustion for high-speed airbreathing propulsion applications indicated that combustion may take place in subsonic regions, such as boundary layers and recirculation zones in flame-holding cavities. During this process, a longitudinal mode of thermoacoustic instability may develop in a spatial domain reaching from the shock train to the flame zone. The present work experimentally and analytically investigates such thermoacoustic instabilities inside an ethylene-fueled scramjet combustor with a recessed cavity flameholder. High-speed pressure transducers are utilized to record acoustic signals. The effects of fuel/air equivalence ratio, fueling scheme, and simulated flight conditions on the stability characteristics of the combustor are examined systematically. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. The measured oscillation frequencies agree well with the characteristic frequencies related to the acoustic feedback loop between the shock and flame and the acoustic-convective feedback loop between the fuel injection and flame.

Interactions Between Shock and Acoustic Waves in a Supersonic Inlet Diffuser

The interactions between shock and acoustic waves in a supersonic inlet diffuser are investigated numerically. The model treats the viscous flowfield in an axisymmetric, mixed-compression inlet operating under supercritical conditions. It is solved by means of a finite-volume approach using a four-stage Runge-Kutta scheme for temporal derivatives and the Harten-Yee upwind total-variation-diminishing scheme for spatial terms. Various distinct flow structures, including shock/boundary-layer and shock/shock interactions, are studied under the effects of externally imposed pressure oscillations at the diffuser exit over a wide range of forcing frequencies and amplitudes. As a result of the terminal shock oscillation induced by the impressed disturbances and the cyclic variation of the oblique/normal shock intersection, large vorticity fluctuations are produced in the radial direction. The characteristics of the shock/boundary-layer interactions (such as the size of the separation bubble, the terminal shock configuration, and the vorticity intensity) are also greatly influenced by the acoustic-driven shock oscillation. The overall response of the inlet aerodynamics to acoustic waves can be characterized by the mass-transfer and acoustic-admittance functions at the diffuser exit. Their magnitudes decrease with increasing frequency. A supersonic inlet acts as an effective acoustic damper, absorbing disturbances arising downstream. Severe flow distortion, however, may arise from shock oscillation and subsequently degrade the combustor performance.

Two-Phase Vorticoacoustic Flow Interactions in Solid-Propellant Rocket Motors

Two-phase e ow interactions with vorticoacoustic oscillations in simulated solid-propellant rocket motors have been studied numerically using a combined Eulerian ‐Lagrangian approach. The model accommodates the complete conservation equations in axisymmetric coordinates and, consequently, allows for a detailed treatment of particle dynamics and unsteady motor internal e ow evolution. Emphasis is placed on the interphase coupling between the gas and particle e elds under the ine uence of acoustic excitation and turbulence dispersion and the intraphase interactions among particles such as collision and coalescence. The study demonstrates that acoustic oscillations provide additional mechanisms to transfer energy from periodic motions to turbulence, leading to an enhanced level of turbulence intensity and an early transition from laminar to turbulence. On the other hand, turbulence-induced eddy viscosity tends to suppress vortical e ow motions caused by acoustic waves. The thermal and momentum relaxation times of particles, along with acoustic characteristic time, play an important role in dictating thetwo-phase e ow interactionswith oscillatory motorinternal e ows. Amaximum attenuation of acoustic waves occurs when those timescales become comparable. Small particles, however, usually exert greater ine uence on thedispersion ofacousticwave through its effectivemodie cation of mixturecompressibility. Particleintraphase interactions are signie cant mainly in situations with a wide range of particle size distribution.

Thermoacoustic Flow Instability in a Scramjet Combustor

*† ‡ § ¶ This paper deals with the thermoacoustic instability and ensuing flow oscillation in a scramjet engine, a phenomenon commonly known as combustion instability. The analysis is based on a quasi-one-dimensional treatment of unsteady flow motion, which simulates the main features of the oscillatory flowfields in both the isolator and combustor. The model also accommodates the response of local heat release to acoustic excitation. The calculated oscillation frequency agrees well with the measured values of around 350 Hz. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. Their influence on the acoustic oscillation characteristics is identified.

Internal Flow Dynamics in a Valveless Airbreathing Pulse Detonation Engine

The internal flow dynamics in a valveless airbreathing pulse detonation engine operating on ethylene fuel is studied numerically. The system involves no mechanical valves in the air flowpath, and the isolation between the inlet and combustor is achieved through gas-dynamic means. The valve operation timing for the fuel injection and initiator is determined based on the local flow conditions. The analysis accommodates the full conservation equations in axisymmetric coordinates and takes into account simplified finite-rate chemistry and variable properties for an ethylene/air/oxygen system. The detailed flow evolution and detonation dynamics during the limit-cycle operation is explored systematically. The calculated pressure history and propulsive performance agree well with experimental data. A sensitivity study of operation timing is also conducted to further elucidate the system dynamics and to provide guidelines for engine design optimization.

A Comprehensive Study of Combustion Oscillations in a Hydrocarbon-Fueled Scramjet Engine

*† ‡ § ¶ The occurrence of combustion oscillations has recently raised a serious concern in the development of scramjet engines. This phenomenon results from the mutual coupling between the unsteady heat release and local flow fluctuations in the flame zone, and has been commonly observed in other types of airbreathing systems such as ramjet and gas-turbine engines. In a scramjet engine, acoustic waves may arise in an unsteady combustion process and propagate upstream through various subsonic flow regions (such as boundary layers, recirculation zones in flame-holding areas, and regions behind precombustion shock waves). These waves then modify the local flowfield and create vortical and entropy disturbances convected downstream into the flame zone. The ensuing pressure and velocity fluctuations perturb the heat-release process and generate acoustic waves traveling upstream. A feedback loop is thus established causing large-amplitude flow oscillations in the engine. Recent experiments have demonstrated the existence of flow oscillations in a hydrocarbonfueled scramjet engine facility with frequencies of 100-160 Hz for liquid JP-7 fuel and 300360 Hz for gaseous ethylene fuel. The present work attempts to establish an integrated theoretical/numerical framework to investigate the combustion oscillations in a scramjet combustor equipped with aerodynamic ramp fuel injectors and a cavity flameholder. Various underlying mechanisms responsible for driving instabilities in a combustor are explored systematically.

Dynamics Combustion Characteristics in Scramjet Combustors with Transverse Fuel Injection

A comprehensive DES quality numerical analysis has been carried out for reacting flows in constant-area and divergent scramjet combustor configurations with and without a cavity. Transverse injection of hydrogen is considered over a broad range of injection pressure. The corresponding equivalence ratio of the overall fuel/air mixture ranges from 0.167 to 0.50. The work features detailed resolution of the flow and flame dynamics in the combustor, which was not typically available in most of the previous studies. In particular, the oscillatory flow characteristics are captured at a scale sufficient to identify the underlying physical mechanisms. Much of the flow unsteadiness is related not only to the cavity, but also to the intrinsic unsteadiness in the flowfield. The interactions between the unsteady flow and flame evolution may cause a large excursion of flow oscillation. The roles of the cavity, injection pressure, and heat release in determining the flow dynamics are examined systematically.