Daniel Bivolaru

Plasma-Assisted Ignition in Scramjets

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. To compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.

Plasma effect on shock waves in a supersonic flow

An experimental study of the plasma effect on the structure of an attached conical shock front appearing at the front end of a cone-shaped model has been carried out in a Mach 2.5 stream. The tip and the body of the model are designed as the cathode and anode, which are separated by a conical-shaped ceramic insulator providing a 5 mm gap for gaseous discharge. The electric field intensity near the cathode is enhanced by the sharpness of the tip. The experimental results show that the diffused discharge can produce plasma distributed symmetrically around the tip in the region in front of the shock wave. It is observed that such plasma can cause shock wave moving upstream with its shock front detached from the model. The shock front is also becoming more and more diffusive and having an increasing shock angle as seen in the shadow video graphs of the flow. A physical mechanism of the observed plasma effect on this type of shock wave is also presented.

Observation of shock wave elimination by a plasma in a Mach-2.5 flow

An experimental study on the influence of a plasma on the structure of an attached conical shock front appearing at the front end of a missile-shaped model has been carried out in a Mach-2.5 flow. The tip and the body of the model are designed as the cathode and anode for gaseous discharge, which produces a spraylike plasma moving around the tip. It is observed that the plasma has caused the shock front to separate from the model. The shock wave moves upstream in the form of a detached bow shock a sensible distance away from the model tip. The detached shock front appears to be highly dispersed in its new location as seen in the shadow video graphs of the flow. As the discharge current increases, experimental evidence shown in the video further reveals a distinct state of the flow without the presence of any shock wave.

Observation of supersonic shock wave mitigation by a plasma aero-spike

A wind-tunnel model in the shape of a 30° half-angle truncated-cone is designed to generate a strong bow shock behind a weak conical (oblique) shock wave attached to the tip of a protruding central-electrode, in a non-ionized supersonic flow. Plasma is generated between two shocks by an on-board discharge. Its effect on shock waves is explored. The results show that the plasma spike has drastically modified the original complicated shock structure to a simple structure having only a single conical shock attached to the tip of the model, similar to the one generated by a perfect cone.

Characteristics of an arc-seeded microwave plasma torch

The design and operation of a portable microwave plasma torch is presented. An arc plasma torch running at 60 Hz and generated by a torch module is installed on the bottom wall in the narrow section of a tapered S-band rectangular cavity, and is used to seed the microwave discharge at the location where the microwave electric field is at a maximum. This tapered cavity is designed to support the TE/sub 103/ mode. With seeding, only low Q cavity and moderate microwave power (time average power of 700 W) are needed. The microwave-enhanced discharge increases the size, cycle energy, and duty cycle of the torch plasma considerably. This torch can be run without introducing gas flow to stabilize the arc and microwave discharges. Adding gas flow can increase not only the size of the torch plasma, but also its cycle energy which reaches a plateau of about 12 J/per cycle for a gas flow rate exceeding 0.393 l/s. The electron density and excitation temperature, and the composition of torch species are determined by emission spectroscopy. It is shown that, at the bottom of the torch close to the cavity wall, electrons distribute quite uniformly across the core of the torch with density and excitation temperature determined to be about 7/spl times/10/sup 13/ cm/sup -3/ and 8000 K, respectively. It is also found that this torch produces an abundance of reactive atomic oxygen.

Aerodynamic modification of supersonic flow around truncated cone using pulsed electrical discharges

An experimental study on the use of plasma to improve blunt body aerodynamics in a supersonic flow is presented. Shadow, schlieren, and plasma glow imaging techniques were used simultaneously for flow and plasma visualization. The discharge current and voltage, as well as the flow pressure and temperature at different locations on the surface of the model, were measured. With a proper aspect ratio 0.81 of the physical spike length to the frontal diameter of a 60-deg truncated cone-cylinder designed for the wind-tunnel model, the on-board pulsed electrical discharge produced a conically distributed plasma around the cathode, which modified the main shock wave structure from a detached bow shock to a tip attached conical shock wave

Focal-plane imaging of crossed beams in nonlinear optics experiments

An application of focal-plane imaging that can be used as a real time diagnostic of beam crossing in various optical techniques is reported. We discuss two specific versions and demonstrate the capability of maximizing system performance with an example in a combined dual-pump coherent anti-Stokes Raman scattering-interferometric Rayleigh scattering experiment (CARS-IRS). We find that this imaging diagnostic significantly reduces beam alignment time and loss of CARS-IRS signals due to inadvertent misalignments.

Characterization of a Combined CARS and Interferometric Rayleigh Scattering System

This paper describes the characterization of a combined Coherent anti-Stokes Raman Spectroscopy and Interferometric Rayleigh Scattering (CARS-IRS) system by reporting the accuracy and precision of the measurements of temperature, species mole fraction of N2, O2, and H2, and two-components of velocity. A near-adiabatic H2-air Hencken burner flame was used to provide known properties for measurements made with the system. The measurement system is also demonstrated in a small-scale Mach 1.6 H2-air combustion- heated supersonic jet with a co-flow of H2. The system is found to have a precision that is sufficient to resolve fluctuations of flow properties in the mixing layer of the jet.

Single-pulse Multi-point Multi-component Interferometric Rayleigh Scattering Velocimeter

A simultaneous multi-point, multi-component velocimeter using interferometric detection of the Doppler shift of Rayleigh, Mie, and Rayleigh-Brillouin scattered light in supersonic flow is described. The system uses up to three sets of collection optics and one beam combiner for the reference laser light to form a single collimated beam. The planar Fabry-Perot interferometer used in the imaging mode for frequency detection preserves the spatial distribution of the signal reasonably well. Single-pulse multi-points measurements of up to two orthogonal and one non-orthogonal components of velocity in a Mach 2 free jet were performed to demonstrate the technique. The average velocity measurements show a close agreement with the CFD calculations using the VULCAN code.

Direct-View Multi-Point Two-Component Interferometric Rayleigh Scattering Velocimeter

This paper describes an instantaneous velocity measurement system based on the Doppler shift of elastically scattered laser light from gas molecules (Rayleigh scattering) relative to an incident laser. The system uses a pulsed laser as the light source, direct-viewing optics to collect the scattered light, an interferometer to analyze spectrally the scattered light mixed with the incident laser light, and a CCD camera to capture the resulting interferogram. The system is capable of simultaneous, spatially (~0.2 mm 3 ) and temporally (~40 ns) resolved, multiple point measurements of two orthogonal components of flow velocity in the presence of background scattered light, acoustic noise and vibrations, and flow particulates. Measurements in a large-scale axi-symmetric Mach 1.6 H2-air combustionheated jet running at a flow sensible enthalpy specific to Mach 5.5 hypersonic flight are performed to demonstrate the technique. The measurements are compared with CFD calculations using a finite-volume discretization of the Favre-averaged Navier-Stokes equations (VULCAN code). I. Introduction oth experimental and computational fluid dynamics methods are widely used in the design and analysis of hypersonic air-breathing engine flow paths. Computational fluid dynamics (CFD) methods employ models that are based on statistical properties of flow turbulence. These statistical properties can be known only when multiple flow properties are measured simultaneously and instantaneously, and when the spatial and temporal scales of the turbulent fluctuations are resolved. Correlations between those properties lead to a more detailed understanding of complex flow behavior and aid in the development of multi-parameter turbulence models for computational fluid dynamics codes 1