Victor A. Miller

Supersonic Combustion of Pylon-Injected Hydrogen in High-Enthalpy Flow with Imposed Vortex Dynamics

The ignition and combustion characteristics of the hydrogen plume issued from two pylon-type injectors in a Mach 2.4, high-enthalpy airflow are presented. Specifically, the focus of the study is on the effects of the imposed interaction and subsequent dynamics of a system of selected supersonic streamwise vortices on the reacting plume morphology and its evolution. The design phase of the experimental campaign was carried out with a reduced-order model, with the goal of identifying peculiar interactions among streamwise vortical structures introduced in the flow of interest. Two vortex interaction modes have been selected and later implemented using ramp-type vortex generators positioned symmetrically and asymmetrically on the pylon injectors, as prescribed by the results of the simulations reported here. Hydrogen/air combustion experiments, aimed at investigating the selected cases, were conducted in the expansion tube facility of the High-Temperature Gas Dynamics Laboratory at Stanford University. Stagn...

SpectraPlot.com: Integrated Spectroscopic Modeling of Atomic and Molecular Gases

Abstract SpectraPlot is a web-based application for simulating spectra of atomic and molecular gases. At the time this manuscript was written, SpectraPlot consisted of four primary tools for calculating: (1) atomic and molecular absorption spectra, (2) atomic and molecular emission spectra, (3) transition linestrengths, and (4) blackbody emission spectra. These tools currently employ the NIST ASD, HITRAN2012, and HITEMP2010 databases to perform line-by-line simulations of spectra. SpectraPlot employs a modular, integrated architecture, enabling multiple simulations across multiple databases and/or thermodynamic conditions to be visualized in an interactive plot window. The primary objective of this paper is to describe the architecture and spectroscopic models employed by SpectraPlot in order to provide its users with the knowledge required to understand the capabilities and limitations of simulations performed using SpectraPlot. Further, this manuscript discusses the accuracy of several underlying approximations used to decrease computational time, in particular, the use of far-wing cutoff criteria.

Secondary Diaphragm Thickness Effects and Improved Pressure Measurements in an Expansion Tube

T HE aim of this Technical Note is to present practical considerations for measuring pressure in an expansion-tube flow facility; some aspects of this work can be applied to pressure measurement strategies in other impulse (e.g., reflected shock tunnel) or continuous flow facilities that may suffer from similar issues. We assume that the reader is familiar with the terminology and general operating principles regarding expansion tubes; formore information regarding expansion tubes, please refer to the seminal work of Trimpi [1]. In many studies relevant to aerospace engineering, propulsion, and fluid mechanics, pressure measurement is often a diagnostic of primary interest [2–7], but obtaining high-quality (i.e., high signalto-noise, or SNR) pressure measurements can be a challenge in expansion-tube facilities (or other impulse facilities). Because of the short test times in impulse facilities, high-bandwidth pressure transducers are necessary to capture transients of interest, but as we show in this work, certain transducer architectures are more susceptible to noise than others. Analog or digital signal processing can be used to filter some noise from the measurements, but at the expense of measurement bandwidth or accuracy; thus, the engineer must compromise between bandwidth and SNR to acquire reliable pressure measurements. Practical information regarding the acquisition of optimal (i.e., high SNR) pressure measurements in impulse facilities is sparse; however, some articles do provide useful details of implementation, for example Dufrene et al. [8] present specifics of pressure instrumentation in an expansion-tube facility, and Beresh et al. [9] report on the response characteristics of a variety of pressure transducers. Expansion-tube facilities also produce a contaminated freestream flow due to the presence of the secondary diaphragm; the primary shock wave may reflect off the secondary diaphragm [10], and secondary diaphragm fragments of a broad range of sizes are entrained in the test gas [11]. We have found that diaphragm fragments impacting the structure in which pressure transducers are mounted can have a significant, adverse effect on pressure measurement quality. An example of this effect is presented in Fig. 1, which shows a pressure time history alongwith a schlieren image taken at the instant in time when a diaphragm fragment impacts the model that houses the pressure transducers. The shadowed shape in the schlieren image is a flat platewith a pitot-pressure transducer (PCB113A26)mounted above the plate in a conical housing; directly below the pitot opening, a wall-mounted transducer (PCB 112A22) simultaneously measures the wall static pressure. (We will interpret these pressure traces in Sec. III.A, but for now, we only demonstrate the level of noise in the two traces.) The displayed still schlieren image is taken from a high-speed schlieren video that captures the full event during a test. A diaphragm fragment can be seen impacting the top of the model, at which time both pressure traces become extremely noisy. We have also observed the spontaneous eruption of noise in pressure time histories in many other tests, and generally, the time at which the pressure traces become noisy varies from shot to shot, and the onset of noise never occurs before the arrival of the test gas. These introductory observations suggest that diaphragm fragments impacting the model are a major source of noise in these pressure measurements. To test the hypothesis that diaphragm fragment impacts are a source of noise in pressure measurements, we make pressure measurements as a function of secondary diaphragm thickness, expecting thinner diaphragms (and less massive fragments) to cause less noise. Two types of pressure transducers are also tested, two different piezoelectric sensors and two different piezoresistive sensors, and the noise characteristics of each sensor are compared. As a secondary objective, we also test whether the secondary diaphragm thickness affects test-gas conditions as inferred from shock speed measurements. Last, a practical solution for minimizing noise in piezoelectric transducers is provided.

Supersonic Combustion and Flame-Holding Characteristics of Pylon Injected Hydrogen in a Mach 2.4 High Enthalpy Flow

This work investigates the structure of the reaction zones downstream of a pylon injecting hydrogen fuel into a Mach 2.4 high enthalpy flow, with the aim of characterizing the behavior and evolution of the expected vortical structures. The pylon injector incorporates expansion ramps that serve as vortex generators. Fuel is injected through a thin slit along the entire length of the base of the ramps. Tests were conducted in the Stanford University Expansion Tube Facility at the High Temperature Gasdynamics Laboratory. All the experiments were conducted at a stagnation enthalpy of 2.8MJ/kg, a static temperature of 1400K, a static pressure of 40kPa, and Mach number of 2.4. The total temperature of the injected hydrogen was 300K. The ignition and flame holding characteristics were investigated using schlieren, time-integrated OH* chemiluminescence, and instantaneous OH planar laser-induced fluorescence (OH PLIF) imaging. OH PLIF was applied to capture the evolution of the reactive vortical system in planes normal to the freestream flow direction at a distance of 1.8cm, 4.3cm, 7.6cm and 10.7cm from the fuel exit plane. Resulting images show the distribution of OH radicals in the plume; these images also show a peculiar geometric pattern in the plume shape, suggesting that the vortices generated by the pylon configuration, and the consequent vortex dynamics, played a dominant role in the mixing and combustion processes.

Rapid Chemiluminescent Imaging Behind Reflected Shock Waves

Current shock tube combustion experiments generally assume that the test environment behind a reflected shock wave is quiescent and that ignition processes progress uniformly over the entire test volume. However, various past investigations, including those based on schlieren data and sidewall imaging [1, 2], have observed nonuniform ignition in certain test regimes. Here, we use both conventional diagnostics (pressure, emission, and laser absorption) and a high-speed chemiluminescent imaging system to investigate the ignition behavior of n-heptane/oxygen/argon in shock tubes at long test times (greater than 2 ms), in an attempt to map the boundary of uniform and nonuniform ignition behavior in one of our shock tubes.

Development of a Model Scramjet Combustor

A model scramjet combustor has been developed to reproduce the main flow features that are present in a scramjet engine on a typical hypersonic vehicle; these flow features include shock/boundary layer interactions, shock/jet interactions, and supersonic mixing and combustion. The model combustor has been designed with optical access on three sides via fused-silica windows, and has a single fuel injector on the streamwise centerline capable of injecting fuel perpendicular to the bottom wall. The top wall of the inlet of the model combustor is ramped, with a turning angle of 10 ◦ , which generates a shock train in the model. We subject the model combustor to flows representative of the aerothermodynamic conditions expected in a scramjet combustor (Mach number, M ∼ 2.8, static pressure, P ∼ 40 kPa, temperature, T ∼ 1250 K) using the Stanford 6-inch Expansion Tube Facility. Fuel (hydrogen) is injected transversely into the freestream (air) with a momentum flux ratio J of 2.5, yielding an overall equivalence ratio φ of 0.25. Static pressure measurements on the top wall of the model have been made, and high-speed schlieren images have been acquired to characterize the flow. Additionally, OH* chemilluminescence images have been acquired to identify the burning region. We present the development, design, and features of the combustor, followed by an overview of measurements made thus far, and we conclude with a summary of our future plans.

The reacting transverse jet in supersonic crossflow: physics and properties (invited)

Combustion in the supersonic regime presents several challenges when compared to its low-speed counterpart. Here we review some of these challenges, and we describe some of the key features of one of the canonical flow fields in supersonic combustion: the reacting transverse jet in a supersonic crossflow (JISCF). From a practical standpoint, the key challenges that limit the control of this combustion regime are fast mixing, robust flame holding and stability. In turn, these aspects are controlled by the complex effects introduced by chemistry, compressibility, shocks and shock/flow interactions, turbulence and the underlying coupling among them. Some of their properties are discussed here. In particular, for a JISCF in a Mach 2.4 high enthalpy crossflow, the reaction zone structure, its dependence on near-wall events, boundary layer, and shock/boundary layer interaction are described. We demonstrate the paramount importance of the coupling between boundary layers and compressibility to provide mechanisms for flame stabilization at the wall. Mixing characteristics, overall structure, and the link to global parameters (momentum flux, velocity and density ratios) that characterize the JISCF, and possibly free shear supersonic flows in general, is also highlighted from non-reacting experiments.

Toluene PLIF thermometry in supersonic flows

Toluene PLIF has been applied to image temperature in supersonic flows generated in an expansion tube. Toluene has high fluorescence quantum yield and high temperature sensitivity compared to similar aromatic tracers, and these qualities of toluene make it ideal for imaging supersonic flows where large temperature gradients can exist. Toluene fluorescence exhibits a red shift with increasing temperature, enabling temperature imaging through a single-excitation, two-camera imaging strategy. A single pulse of 266 nm laser light excites the tracer, and two cameras with different optical filters simultaneously image different spectral regions of toluene fluorescence; the ratio of these images is converted to temperature. In this work, the design of the diagnostic is described, the diagnostic is validated through imaging of flow behind incident shockwaves, and the technique is demonstrated on two canonical flowfields, M =2 .3 flow of toluene-seeded nitrogen over a wedge and cylinder.