J. Philip Drummond

Calculation of supersonic turbulent reacting coaxial jets

The mixing and subsequent combustion within turbulent reacting shear layers is examined. To conduct this study, a computer program has been written to solve the axisymmetric Reynolds-averaged, Navier-Stokes equations. Turbulence is modeled using three algebraic turbulence models, and the chemical kinetics is modeled using a seven-species, seven-reaction, finite-rate chemistry model. Three separate flowfields are investigated. The effect of turbulent mixing upon the extent of combustion is demonstrated. No single turbulence model considered accurately predicted the degree of mixing for all three cases.

Numerical Study of Staged Fuel Injection for Supersonic Combustion

A parametric study of staged (multiple) perpendicular fuel injector configurations has been conducted using a computer code that solves the two-dimensional elliptic Navier-Stokes equations. The program computes the turbulent mixing and reaction of hydrogen fuel and air and allows the study of separated regions of the flow immediately preceding and following the injectors. The validity of the code is demonstrated in a cold flow helium injection study with a single injector. Results are presented that describe the flowfield near opposing staged injectors over a range of parameters. Parameters that are varied include injector size, fuel split, and distance between injectors. Comparisons of the configurations are made to assess their mixing and potential flameholding qualities.

Efficient Calculation of Chemically Reacting Flow

A SEMI-IMPLICIT finite-volume formulation is used to jrVstudy flow with chemical reactions. In this formulation, the source terms resulting from the chemical reaction are treated implicitly and the resulting system of partial differential equations is sovled using two time-stepping schemes. The first is based on the Runge-Kutta method and the second on an Adams predictor-corrector method. Results show that improvements in computational efficiency depend to a large extent on the manner in which the source term is treated. Furthermore, analysis and computation indicate that the RungeKutta method is more efficient than the Adams methods for these systems of differential equations. Finally, an adaptive time-stepping scheme is developed to study problems involving shock ignition. Calculations for a hydrogen air system agree well with other methods.

A numerical study of mixing enhancement in supersonic reacting flow fields

Work has been underway for a number of years at the NASA Langley Research Center to develop a supersonic combustion ramjet or scramjet that is capable of propelling a vehicle at hypersonic speeds in the atmosphere or beyond. A recent part of that research has been directed toward the optimization of the scramjet combustor, and in particular the efficiency of fuel-air mixing and reaction in the engine. A supersonic, spatially developing and reacting mixing layer serves as an excellent physical model for the mixing and reaction processes that take place in a scramjet combustor, This paper describes a study of fuel-air mixing and reaction in a supersonic mixing layer and discusses several techniques that were applied for enhancing the mixing processes and the overall combustion efficiency in the layer. Based on the results of this study, an alternate fuel injector configuration was computationally designed, and that configuration significantly increased the amount of fuel-air mixing and combustion over a given combustor length that was achieved.

Suppression and Enhancement of Mixing in High-Speed Reacting Flow Fields

Work is underway at the NASA Langley Research Center to develop a hydrogen-fueled supersonic combustion ramjet, or scramjet, that is capable of propelling a vehicle at hypersonic speeds in the atmosphere. Recent research has been directed toward the optimization of the scramjet combustor and, in particular, the efficiency of fuel-air mixing and reaction taking place in the engine. With increasing Mach number, the degree of fuel-air mixing through natural convective and diffusive processes is significantly reduced leading to an overall decrease in combustion efficiency and thrust. Even though the combustor flow field is quite complex, it can be viewed as a collection of spatially developing and reacting supersonic mixing layers or jets from fuel injectors mixing with air, one of which serves as an excellent physical model for the overall flow field. This work is focused on understanding the mechanisms of mixing (or lack thereof) and on the development of techniques for its enhancement in compressible turbulent reacting flows. Results generated by direct numerical simulations (DNS) are first used to demonstrate the mechanisms for reduced mixing in shear layers. To counter the effects of suppressed mixing, several mixing enhancement techniques are then discussed. The most successful approaches involve longitudinal vorticity induced into the flow field. Several means for inducing vorticity are studied and assessed.

Development of a Pulsed Combustion Actuator For High-Speed Flow Control

This paper describes the flow within a prototype actuator, energized by pulsed combustion or detonations, that provides a pulsed jet suitable for flow control in high-speed applications. A high-speed valve, capable of delivering a pulsed stream of reactants a mixture of H2 and air at rates of up to 1500 pulses per second, has been constructed. The reactants burn in a resonant chamber, and the products exit the device as a pulsed jet. High frequency pressure transducers have been used to monitor the pressure fluctuations in the device at various reactant injection frequencies, including both resonant and off-resonant conditions. The combustion chamber has been constructed with windows, and the flow inside it has been visualized using Planar Laser-Induced Fluorescence (PLIF). The pulsed jet at the exit of the device has been observed using schlieren.

Methods for Prediction of High-Speed Reacting Flows in Aerospace Propulsion

ESEARCH to develop high-speed airbreathing aerospacepropulsion systems was underway in the late 1950s. A majorpart of the effort involved the supersonic combustion ramjet, orscramjet, engine. Work had also begun to develop computationaltechniques for solving the equations governing the flow through ascramjet engine. However, scramjet technology and the computa-tional methods to assist in its evolution would remain apart foranother decade. The principal barrier was that the computationalmethods needed for engine evolution lacked the computertechnology required for solving the discrete equations resultingfromthenumericalmethods.Eventoday,computerresourcesremainamajorpacingitem inovercomingthisbarrier.Significantadvanceshave been made over the past 35 years, however, in modeling thesupersonic chemicallyreacting flowin ascramjet combustor. Toseehow scramjet development and the required computational toolsfinally merged, we briefly trace the evolution of the technology inboth areas.We begin with a review of the history of efforts to model thescramjet environment and thenconcentrate onmore recent activitiesthat lead to today’s computational capabilities. The NationalAeroSpace Plane (NASP) technology program provided strongmotivation for advancing the computational capabilities of thecountryinboththegovernmentandprivatesectors.RequiredgroundtestfacilitieswithsufficienttesttimeswerelimitedtoaroundMach8,and higher Mach numbers, achievable in pulse facilities, could onlybe maintained for the order of milliseconds. In addition, the numberof facility cycles available to parameterize a given engine flow-path were limited, and the facilities were expensive to operate.Computationalcapabilitieswereneededtofilleachoftheseareasthatexistedingroundtestfacilities.AlthoughtheNASPprogramwasnotsuccessful in developing a vehicle, it did spawn the development ofnewcomputational algorithms.TheHyper-XProgram,beginningin1995, revived high-speed computational research and development.A flight program is the catalyst that drives technology developmentandsynthesizesalloftheeffortsintoaunifiedtoolfordevelopmentofthe ultimate experiment, the flight of a hypersonic vehicle. Thegenesisofmostofthecurrentdaystate-of-the-artcomputationaltoolsfor scramjet research and development began with the Hyper-Xprogram. This paper attempts to cover this story from NASP andHyper-Xtothepresentday.Webeginwithabriefhistoryofscramjetdevelopment leading up to the NASP Program. Although this paperwill use the history of scramjet development as a roadmap for theevolution of computational tools, the reader interested in a moregenerallookatthehistoryshouldconsultthepapersbyBillig[1]andCurran[2]ontechnologyanditsissuesandHallion[3]onhypersonicsystems.FollowingpioneeringeffortsofFerri[4],Dugger[5],andWebberand MacKay [6] in the 1950s, a significant increase in research todevelopscramjetengineconceptsoccurredinthe1960s.In1965,theNASA Langley Research Center initiated the Hypersonic ResearchEngine (HRE) project to develop a high-speed air breathingtechnology for hypersonic cruise vehicles [7]. The goal of the HREproject was to flight test a regeneratively cooled, hydrogen-fueled pylon-mounted scramjet on the X-15 research airplane anddemonstrate design performance levels. The HRE did not reach theflight demonstration stage due to cancellation of the X-15 program,but the ground-based program did continue and resulted in thedevelopment and construction of two variable geometry enginemodels.Workwiththesemodelssignificantlyincreasedthescramjettechnology database to be applied in more advanced configurations.Following completion of the HRE project, attention moved topropulsion concepts that would provide high performance wheninstalled on a vehicle. The original concept, a pylon-mounted HRE,would have resulted in excessive levels of external drag, and so thepylon was removed, and work began to highly integrate the engine

Toward a High-Frequency Pulsed-Detonation Actuator

This paper describes the continued development of an actuator, energized by pulsed detonations, that provides a pulsed jet suitable for flow control in high-speed applications. A high-speed valve, capable of delivering a pulsed stream of reactants a mixture of H2 and air at rates of up to 1500 pulses per second, has been constructed. The reactants burn in a resonant tube and the products exit the tube as a pulsed jet. High frequency pressure transducers have been used to monitor the pressure fluctuations in the device at various reactant injection frequencies, including both resonant and off-resonant conditions. Pulsed detonations have been demonstrated in the lambda/4 mode of an 8 inch long tube at approximately 600 Hz. The pulsed jet at the exit of the device has been observed using shadowgraph and an infrared camera.

Numerical modeling of turbulent supersonic reacting coaxial jets

The paper considers the mixing and subsequent combustion within turbulent reacting shear layers. A computer program was developed to solve the axisymmetric Reynolds averaged Navier-Stokes equations. The numerical method integrates the Reynolds averaged Navier-Stokes equations using a finite volume approach while advancing the solution forward in time using a Runge-Kutta scheme. Three separate flowfields are investigated and it is found that no single turbulence model considered could accurately predict the degree of mixing for all three cases.

Numerical Solution for Perpendicular Sonic Hydrogen Injection into a Ducted Supersonic Airstream

This note discusses a computer program being developed to study the flow field near opposing perpendicular fuel injectors in scramjets. The MacCormack time-split, finite difference relaxation technique was used to solve the full two-dimensional compressible Navier-Stokes equations along with energy and species equations. By using this technique, a program was developed to consider the turbulent nonreacting flow of hydrogen and air in a rectangular duct. A damping term, proportional to the second derivative of pressure and temperature, was used to produce a stable solution behind the hydrogen jet in the neighborhood of the recompression shock. A case using actual conditions encountered in current scramjet design was analyzed, with results agreeing qualitatively with experimental observations.