Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E6: Reacting Flows: High Speed |
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Chair: Seong-kyun Im, Worcester Polytechnic Institute Room: 105 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E6.00001: Model Scramjet Inlet Unstart Induced by Mass Addition and Heat Release Seong-kyun Im, Damiano Baccarella, Brendan McGann, Qili Liu, Lydiy Wermer, Hyungrok Do The inlet unstart phenomena in a model scramjet are investigated at an arc-heated hypersonic wind tunnel. The unstart induced by nitrogen or ethylene jets at low or high enthalpy Mach 4.5 freestream flow conditions are compared. The jet injection pressurizes the downstream flow by mass addition and flow blockage. In case of the ethylene jet injection, heat release from combustion increases the backpressure further. Time-resolved schlieren imaging is performed at the jet and the lip of the model inlet to visualize the flow features during unstart. High frequency pressure measurements are used to provide information on pressure fluctuation at the scramjet wall. In both of the mass and heat release driven unstart cases, it is observed that there are similar flow transient and quasi-steady behaviors of unstart shockwave system during the unstart processes. Combustion driven unstart induces severe oscillatory flow motions of the jet and the unstart shock at the lip of the scramjet inlet after the completion of the unstart process, while the unstarted flow induced by solely mass addition remains relatively steady. The discrepancies between the processes of mass and heat release driven unstart are explained by flow choking mechanism. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E6.00002: Linear stability analysis of scramjet unstart Ik Jang, Joseph Nichols, Parviz Moin We investigate the bifurcation structure of unstart and restart events in a dual-mode scramjet using the Reynolds-averaged Navier-Stokes equations. The scramjet of interest (HyShot II, Laurence et al., AIAA2011-2310) operates at a free-stream Mach number of approximately 8, and the length of the combustor chamber is 300mm. A heat-release model is applied to mimic the combustion process. Pseudo-arclength continuation with Newton-Raphson iteration is used to calculate multiple solution branches. Stability analysis based on linearized dynamics about the solution curves reveals a metric that optimally forewarns unstart. By combining direct and adjoint eigenmodes, structural sensitivity analysis suggests strategies for unstart mitigation, including changing the isolator length. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E6.00003: Application of Filtered Spherical Harmonics Radiation Transport to High-Speed Reactive Flow Ryan Houim, Elaine Oran Radiative heat transfer is an important, but often neglected, process in high-speed reacting and multiphase flow applications. Some scenarios, such as dust explosions in coal mines, can have regions that are nearly transparent and other regions with high dust concentration that are optically thick. Most approximations to the radiative transfer equation (RTE) are not valid in both limits simultaneously. Issues also arise when solving approximations to the RTE that can often require the solution of elliptic equations. Many compressible hydrodynamic codes use explicit time-marching and block-structured adaptive-mesh-refinement algorithms. Adapting these codes to solve elliptic equations is not always straightforward. Recently, filtered spherical harmonics (FP$_N$) approximations to the RTE have been developed. The FP$_N$ equations are hyperbolic and, as a result, can be solved using algorithms that are similar Godunovâ€™s method for compressible fluid flow. The FP$_N$ model is also valid in optically thick and thin situations provided that the order, $N$, is high enough. We show that the FP$_N$ equations are a promising alternative to traditional RTE approximations. Challenging test cases that involve both free-streaming and optically thick regions will be presented. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E6.00004: Vibrational nonequilibrium in chain branching reactions of hydrogen combustion using quasi-classical trajectory analysis Stephen Voelkel, Venkat Raman, Philip Varghese In high-speed reactive flows in scramjets, thermal nonequilibrium is introduced in the flow via shock waves. Though rotational and translational energy modes relax back to equilibrium quickly, vibrational relaxation is comparable to the bulk mixing and reaction timescales. The discrepancy between vibration and rotation/translation energy distributions can dramatically alter on the initiation of the fuel oxidation process. For continuum-scale applications, thermal nonequilibrium effects are derived from the rovibrational state-specific reaction and scattering rates associated with the chemical mechanism. In this work, the state-specific reaction rates are calculated for the chain branching reactions in the hydrogen combustion mechanism using a quasi-classical trajectory (QCT) framework. The state-specific rates are incorporated into a multiple temperature continuum-scale model whereby each species is characterized by a Boltzmann distribution parametrized by its own vibrational temperature. The flame ignition rates are implemented in a CFD code to simulate a reactive coflow. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E6.00005: In Search of Reaction Rate Scaling Law for Supersonic Combustion Foluso Ladeinde, Zhipeng Lou, Wenhai Li As a way of employing the flamelet approach, which was developed essentially for incompressible flows, to model supersonic combustion, the role ascribed to pressure has not been very convincing. That is, the reaction rate is often scaled on the square of the pressure in the finite Mach number flow field relative to the usually atmospheric static pressure field used in the generation of the flamelet library. This scaling assumption is quite simple and will therefore be very attractive if it has a sound theoretical basis and it works for a large selection of high-speed combustion flows. We try to find some justifications for different scaling laws, with the hope of coming up with a more universally-acceptable flamelet procedure for supersonic combustion. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E6.00006: Optimizing Simplified One-Step Chemical Models for High Speed Reacting Flows Alp Ozgen, Ryan W. Houim, Elaine S. Oran One of the most important and difficult parts of constructing a multidimensional numerical simulation of a hydrocarbon reacting flow is finding a reliable and affordable model of the chemical and diffusive properties. Full detailed chemical models of these systems contain too many reactions and chemical species to be practical for realistic scenarios. The objective of our work is to create the simplest model possible that can reproduce the time-dependence of the energy input and the conversion from fuel to products. To that end, we are developing a procedure optimizing parameters in the most simplified ``one-step'' model. An important requirement of this model is that it reproduces known flame and detonation properties. Multidimensional numerical simulations using the new model are compared to deflagration-to-detonation experiments in channels containing ethylene and oxygen. [Preview Abstract] |
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