Microscale combustion: Technology development and fundamental research

This paper describes the design of a nonlinear robust adaptive controller for a flexible air-breathing hypersonic vehicle model. Because of the complexity of a first-principle model of the vehicle dynamics, a control-oriented model is adopted for design and stability analysis. This simplified model retains the dominant features of the higher-fidelity model, including the nonminimum phase behavior of the flight-path angle dynamics, the flexibility effects, and the strong coupling between the engine and flight dynamics. A combination of nonlinear sequential loop closure and adaptive dynamic inversion is adopted for the design of a dynamic state-feedback controller that provides stable tracking of the velocity and altitude reference trajectories and imposes a desired set point for the angle of attack. A complete characterization of the internal dynamics of the model is derived for a Lyapunov-based stability analysis of the closed-loop system, which includes the structural dynamics. The proposed methodology addresses the issue of stability robustness with respect to both parametric model uncertainty, which naturally arises when adopting reduced-complexity models for control design, and dynamic perturbations due to the flexible dynamics. Simulation results from the full nonlinear model show the effectiveness of the controller.

Nonlinear Robust Adaptive Control of Flexible Air-Breathing Hypersonic Vehicles

The thermochemistry of cellulose pyrolysis has been studied by a combination of differential scanning calorimetry and thermogravimetric analysis. Additionally, the vapor pressure and heat of vaporization of levoglucosan have been determined by an effusion method. The cellulose pyrolysis has been carried out under inert gas at heating rates from 0.1 to 60 K/min. The main cellulose thermal degradation pathway is endothermic, in the absence of mass transfer limitations that promote char formation. The endothermicity is estimated to be about 538 J/g of volatiles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decomposition products. Pyrolysis can be driven in the exothermic direction by char-forming processes that compete with tar-forming processes. The formation of char is estimated to be exothermic to the extent of about 2 kJ/g of char formed. Low heating rates, in concert with mass transfer limitations, serve to drive the pyrolysis ...

Thermal Effects in Cellulose Pyrolysis: Relationship to Char Formation Processes

A critical review and comparison of the initiation of combustion processes by conventional electric spark or thermal means with laser sources is presented. A description of the fundamentals of ignition processes is used as the basis for interpretation of experimental and theoretical studies of laser ignition. It is shown that many features of laser ignition can be understood on the basis of simple thermal concepts, particularly when the effects of thermal or radical losses are considered. It is proposed that the main advantages of laser sources are likely to be in the timing and placement of ignition rather than the inherent energy requirements. Potential applications to combustion systems of practical importance, e.g., high-speed propulsion systems, are discussed and instructive laboratory-scale experiments are suggested.

Laser versus conventional ignition of flames

This paper addresses issues related to robust output-feedback control for a model of an airbreathing hypersonic vehicle. The control objective is to provide robust velocity and altitude tracking in the presence of model uncertainties and varying flight conditions, using only limited state information. A baseline control design based on a robust full-order observer is shown to provide, in nonlinear simulations, insufficient robustness with respect to variations of the vehicle dynamics due to fuel consumption. An alternative approach to robust output-feedback design, which does not employ state estimation, is presented and shown to provide an increased level of performance. The proposed methodology reposes upon robust servomechanism theory and makes use of a novel internal model design. Robust compensation of the unstable zero dynamics of the plant is achieved by using measurements of pitch rate. The selection of the plant's output map by sensor placement is an integral part of the control design procedures, accomplished by preserving certain system structures that are favorable for robust control design. The performance of each controller is comparatively evaluated by means of simulations of a full nonlinear model of the vehicle dynamics and is tested on a given range of operating conditions.

Robust Linear Output Feedback Control of an Airbreathing Hypersonic Vehicle

Using two mathematical models (one allowing for pressure variations, and the other assuming the pressure to be uniform), the minimum ignition energies of a stoichiometric methane-air mixture are calculated. The effect of the initial pressure wave generated by the spark on minimum ignition energy is studied by comparing the results of the two models. The pressure wave is found to affect the values of minimum ignition energy for ignition times shorter than 2 (is. The dependence of minimum ignition energy on the kernel radius and ignition time are also investigated. The results show that as the kernel radius is decreased to small values, the minimum ignition energy reaches a constant value. The ignition times that give an optimum value of minimum ignition energy are in the range of 3μs to 100μs. Our results show that minimum ignition energy is strongly dependent on the ignition parameters, and a minimum value of 0.005 mJ is found for an ignition kernel radius of 0.0635 mm and an ignition time of 27....

Dependence of Minimum Ignition Energy on Ignition Parameters

Flow past a backward-facing step is computed using three different turbulence modeling approaches; Detached Eddy Simulation (DES), Unsteady Reynolds Averaged Navier-Stokes (URANS), and the newly developed Partially Averaged Navier-Stokes (PANS) [1, 2] method. PANS is a family of closure models parameterized by different ratios of resolved-to-modeled kinetic energy and dissipation. The corresponding resolution control parameters are f k (the ratio of unresolved to total kinetic energy) and f ϵ (the ratio of unresolved-to-total dissipation). The main objectives of this study are: (i) to investigate the merits of PANS relative to the other two methods by comparing against existing experimental data; and (ii) to examine the effect of the PANS parameter f ϵ on the computed flow field. In addition, we study the sensitivity of PANS results to grid-size and the level of unsteadiness in the inflow. Overall, the results clearly demonstrate the suitability of PANS for computing unsteady turbulent flows at a reasonab...

Flow Past a Backward-Facing Step: Comparison of PANS, DES and URANS Results with Experiments

Coupling between plate vibration and acoustic radiation

Computational e uid dynamics tools have been used extensively in the analysis and development of the X-43A Hyper-X Research Vehicle. A signiecant element of this analysis is the prediction of integrated vehicle aeropropulsive performance,which includes an integration of aerodynamic and propulsion eoweelds. Thedevelopment of the Mach7X-43Arequiredapree ightassessmentoflongitudinalandlateral-directionalaeropropulsivecharacteristics nearthetargeteight-testcondition.Thedevelopmentofthispree ightdatabasewasaccomplishedthroughextensive aerodynamic wind-tunnel testing and a combination of three-dimensional inviscid airframe calculations and cowlto-tail scramjet cycle analyses togenerate longitudinal performance increments between mission sequences. These increments were measured directly and validated through tests of the Hyper-X e ight engine and vehicle e owpath simulator in the NASA Langley Research Center 8-Foot High Temperature Tunnel. Predictions were reened with tip-to-tailNavier‐Stokescalculations,which alsoprovidedinformation onscramjetexhaustplumeexpansion inthe aftbody region. A qualitative assessment of lateral-directional stability characteristics was made through a series of tip-to-tail inviscid calculations, including a simulation of the powered scramjet eight-test condition. Additional comparisons with wind-tunnel forceand momentdataas well as surfacepressuremeasurements from theHyper-X e ight engine and vehicle eowpath simulator model and wind-tunnel testing were made to assess solution accuracy.

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Integrated Aeropropulsive Computational Fluid Dynamics Methodology for the Hyper-X Flight Experiment

This paper is concerned with the propagation of uncertainties in the values of turbulence model coefficients and parameters in turbulent flows. These coefficients and parameters are determined from experiments performed on elementary flows and they are subject to uncertainty. The widely used k–e turbulence model is considered. It consists of model transport equations for the turbulence kinetic energy and rate of turbulent dissipation. Both equations involve various model coefficients about which adequate knowledge is assumed known in the form of probability density functions. The study is carried out for the flow over a 2D backward-facing step configuration. The Latin Hypercube Sampling method is employed for the uncertainty quantification purposes as it requires a smaller number of samples compared to the conventional Monte-Carlo method. The mean values are reported for the flow output parameters of interest along with their associated uncertainties. The results show that model coefficient variability has significant effects on the streamwise velocity component in the recirculation region near the reattachment point and turbulence intensity along the free shear layer. The reattachment point location, pressure, and wall shear are also significantly affected.Copyright © 2010 by ASME

Abdelkader Frendi

Papers

Dependence of Minimum Ignition Energy on Ignition Parameters

Flow Past a Backward-Facing Step: Comparison of PANS, DES and URANS Results with Experiments

Coupling between plate vibration and acoustic radiation

Integrated Aeropropulsive Computational Fluid Dynamics Methodology for the Hyper-X Flight Experiment

Uncertainty Quantification of Turbulence Model Coefficients via Latin Hypercube Sampling Method