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Lefkowitz J.,Princeton University | Ju Y.,Princeton University | Stevens C.,Innovative Scientific Solutions, Inc. | Hoke J.,Innovative Scientific Solutions, Inc. | And 2 more authors.
49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference | Year: 2013

An experimental investigation of the effectiveness of a nanosecond duration repetitively-pulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C2H4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies (≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.

Lefkowitz J.K.,Princeton University | Guo P.,Princeton University | Ombrello T.,Aerospace Systems Directorate | Won S.H.,Princeton University | And 4 more authors.
Combustion and Flame | Year: 2015

A nanosecond repetitively pulsed (NRP) discharge in the spark regime has been investigated as to its effectiveness in reducing ignition time, both in a flow tube and a pulsed detonation engine (PDE). The flame-development time for methane-air mixtures in the flow tube is found to be a function of the total ignition energy and the pulse repetition frequency. Schlieren imaging revealed that at low pulse-repetition frequency (0-5kHz), ignition kernels formed by the discharge are each transported away from the discharge gap before the following pulse arrives. At higher pulse-repetition frequencies (≥10kHz), multiple pulses are all coupled into a single ignition kernel, thus the resulting ignition kernel size and the total energy deposition into the kernel are increased, resulting in a faster transition into a self-propagating flame. Imaging of the NRP discharge in air revealed that at high pulse frequencies (>10kHz) and peak pulse amplitude (>9kV), the plasma emission is not quenched in-between pulses, resulting in a building up of heat and radicals in the center of the ignition kernel. Optical emission spectra revealed the presence of electronically excited N2, O, and N, as well as O+ and N+, during and between the discharge pulses. Numerical modeling of the plasma indicated that reactions of excited species mainly lead to the production of O atoms and the increase of gas temperature, which shortens induction chemistry timescales, and thus reduces the flame-development time through both kinetic and thermal mechanisms. Ignition of aviation gasoline-air mixtures by NRP discharge in a PDE also demonstrated a noticeable reduction in ignition time as compared to an automotive aftermarket multiple capacitive-discharge ignition system. © 2015 The Combustion Institute.

News Article | November 13, 2015
Site: phys.org

The researchers are calling for a change similar to the one that transformed the genre of very large-scale integration, or VLSI, leading to unprecedented advances in computing and electronics in recent decades. "We are saying that this progress occurred precisely because the VLSI community embraced the need to transform itself, building from scratch the tools necessary to understand, optimize, and validate extremely complex, dynamic systems, ultimately creating the most transformative industry of the past century," said Timothy Fisher, Purdue University's James G. Dwyer Professor of Mechanical Engineering and director of the Center for Integrated Thermal Management of Aerospace Vehicles (CITMAV). "The thermal community today faces a similar challenge and opportunity. " The center was launched in 2014 by the Aerospace Systems Directorate of the U.S. Air Force Research Laboratory and also is supported by an industry consortium. Researchers involved in the center have authored a research paper they hope will provide the basic elements of a framework for change in areas including thermal storage, heat exchangers, thermal controls, "uncertainty quantification" and system-level modeling. The paper is available online and will be published in an upcoming issue of the Journal on Thermophysics and Heat Transfer. Whereas previous research has focused on individual components within a complex system such as an aircraft, researchers are working to better control heating by learning precisely how it occurs in the whole system. Major challenges stem from the fact that the systems undergo rapid changes in heat loads, introducing extreme cooling demands and a level of uncertainty that must be better understood. New research is needed to probe the processes behind these changing, or "transient," heating events. "Almost all of the studies done on high-flux cooling techniques have waited for them to reach steady state," Fisher said. "However, we are really focused on what happens in this transient process. We need to learn how random it is and how repeatable it is from one test to another. Those sorts of things haven't been really explored in depth before." Future work will aim to quantify uncertainty in a dynamic event, such as the dramatic heating that takes place on the skin of aircraft traveling at hypersonic speeds, or faster than Mach 5. "More generally, if you suddenly have a need even in commercial aircraft to put cooling in a certain place, for example systems for onboard air conditioning, we really need to understand how the systems work together under those circumstances," he said. Another example is when aircraft take off from airports in hot desert climates, he said. Innovations are needed to improve thermal storage technologies designed to maintain a specific temperature range for systems to function properly. "You want to tamp down temperature fluctuations, so you have to put the energy someplace temporarily," he said. Advanced materials are needed to handle spiking fluctuations of heat, said Fisher, who is leading work to develop new thermal-management approaches with nanoscale carbon materials. "Normally thermal technologies are heavy and they are slow. They may take minutes to take up and control heat, and we need something that's lightweight and fast, on the order of 1-to-10 seconds," he said. "We need new thermal storage technologies that are much faster than what we now have." The modern aircraft is a collection of complicated and heterogeneous subsystems including propulsion, flight control and actuation, environmental control and pressurization, electrical power generation, and cooling and thermal management. It is important to understand how these separate subsystems interact and participate in the operation of the overall aircraft system. Many of the subsystems require a certain range of operating temperatures to function effectively. To ensure that this remains the case during the entire mission, a thermal management system must be integrated in a system-level architecture. Recently, a shift has resulted in conceptual design through the use of integrated modeling and simulation. "For example, the differences in subsystem interactions between the fourth-generation aircraft such as an F-16 and fifth-generation aircraft such as the F-22 and F-35 fighter aircraft are substantial," said Fisher, also a professor of aeronautics and astronautics. "The interactions are becoming larger and more frequent in number and, thus, more complex. As a result, it is not possible to examine the aircraft thermal management subsystem in isolation; instead, it must be viewed in the context of all the interactions between the subsystems that occurs onboard the aircraft." One of the major goals of CITMAV is to create an integrated, cohesive research program involving several major tenets including the development of models that capture high-rates, and high-amounts of heat transfer, said John Doty, an associate professor in the Department of Engineering Management and Systems at the University of Dayton. "The high rate of heat transfer necessitates that the models be posed and implemented dynamically," Doty said. "Common practices for model-based validation typically employ a steady-state, or quasi-steady state, approach for model development, which is inadequate to capture the important physics of the dynamic heat transfer." Another major component is how uncertainty is adapted to models in a rigorous statistical manner to capture its magnitudes and sources and determine how it influences various responses. Models also must be validated with experiments, he said. Model-based predictive controls must be developed to manage the high-rate thermal responses. "Additionally, optimized analyses enable processes to be modeled in nearly real time so that we can adjust on the fly and potentially even include near-future predictions and anticipation," Doty said. "Taken in total, all of these major efforts are being developed and integrated to support advanced thermal management systems for aerospace vehicles." More information: J. Doty et al. Dynamic Thermal Management for Aerospace Technology: Review and Outlook, Journal of Thermophysics and Heat Transfer (2015). DOI: 10.2514/1.T4701

George J.,MetroLaser, Inc. | Jenkins T.,MetroLaser, Inc. | Grady N.,Ohio State University | Sutton J.,Ohio State University | Rice B.,Aerospace Systems Directorate
32nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference | Year: 2016

An unseeded measurement technique was developed for simultaneous, spatially resolved measurements of density, temperature, and velocity for potential use in high speed wind tunnel flows. A variant of filtered Rayleigh scattering (FRS) was developed that takes advantage of unique monotonic relationships between molecular vapor cell filter transmissions and flow parameters of interest. The method relies on laser Rayleigh scattering from naturally present nitrogen or oxygen molecules, and, therefore, requires no seeding of the flow. Thermodynamic properties of the test gas are obtained by measuring the transmission of Rayleigh scattered light through multiple iodine vapor cell filters, which have sharp spectral cut-offs that can be tailored to provide good sensitivity to the properties of interest. A unique set of transmission coefficients, measured from signals from photomultiplier detectors, define a given thermodynamic state, providing a non-intrusive instantaneous measurement. During this study, we demonstrated the technique with single-point measurements of temperature and velocity in a supersonic flow, laying the groundwork for later variants that will enable two-dimensional measurements of these quantities in various supersonic flow fields, e.g., boundary layers, shock-wave/boundary layer interactions, etc. Additional experiments were carried out using spectrally pure laser output to demonstrate feasibility of obtaining near-wall measurements with sufficient signal to noise ratios. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Beck J.A.,Aerospace Systems Directorate | Justice J.A.,Aerospace Systems Directorate | Scott-Emuakpor O.E.,Aerospace Systems Directorate | George T.J.,Aerospace Systems Directorate | Brown J.M.,Aerospace Systems Directorate
Journal of Aerospace Engineering | Year: 2015

An n-harmonic traveling-wave excitation (nTWE) system that was developed for the U.S. air force research laboratory turbine engine fatigue facility (TEFF) is described. This traveling-wave system is designed to simulate engine order excitations on stationary integrally bladed rotors (IBRs) and dual flow-path IBRs (DFIBRs) for the purpose of measuring forced response amplification due to mistuning. This new system extends beyond traditional traveling-wave systems by its capability to generate a combination of n engine order (EO) harmonics for IBRs as well as different EO harmonics simultaneously to both inner- and outer-blade sets of DFIBRs. This system can excite both IBRs and DFIBRs of differing sizes and number of blades with electromagnetic excitation. Scanning vibrometry mounted above the test specimen is used to measure any localization due to mistuning. Sources of error in the nTWE system are discussed and experimental forced response levels are shown for a 22-blade IBR. © 2015 American Society of Civil Engineers.

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