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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Comparative study of computational efficiency of two LU schemes for non-equilibrium reacting flows

The systems of equations governing chemically nonequilibrium flows are solved numerically by two lower-upper(LU) algorithms, namely, LU-SSOR (Yoon and Jameson, 1987) and LU-SW (Steger and Warming, 1981). The methods of flux-Jacobian splitting differentiate the two LU algorithms. The LU-SSOR gives a simpler implicit operator with less temporal damping, while the LU-SW converges faster but requires more computation for each iteration. Relative efficiencies and mutual merits of these two LU algorithms are compared. The results show that LU-SW with upwind-differenced right-hand-side gives the best convergence.

Tsai, Y.-L. Peter↗
NASA NTRS1989

Mixing and combustion enhancement in supersonic reacting flows

Research has been conducted for a number of years at the NASA Langley Research Center to develop a supersonic combustion ramjet (scramjet) capable of propelling a vehicle at hypersonic speeds in the atmosphere or beyond. Recently, that research has been directed toward the optimization of the scramjet combustor, and in particular the efficiency of fuel-air mixing and reaction in the engine. This paper describes a study of fuel-air mixing and reaction in a supersonic flow field, and discusses several techniques that were applied for enhancing the mixing processes and overal combustion efficiency in the flow. Based on the results of the study, an alternate fuel injector configuration was computationally designed, and that configuration significantly increased the amount of fuel-air mixing and combustion over a given combustor length that was achieved.

Drummond, J. Philip↗
NASA NTRS1991

Computational aspects of chemically reacting flows

An implicit, time-dependent approach is taken to solve the inviscid conservative equations in a vertex-based finite-volume formulation. The convective fluxes are estimated by a modified Riemann solver, and higher-order accuracy is obtained by means of the MUSCL. Implicit techniques are adopted to deal with the equation stiffness as a result of the clustered grid and chemical source term. The real-gas Roe matrix and the ADI and LU schemes have been tested on the sample problems: hypersonic flow over blunt configurations, such as the forebody of a double ellipse, and the near wake of a conic aerobrake. Temperature distribution and standoff distance are validated against the shock-fitting, central-differencing results. Equilibrium temperature may be recovered by artificially increasing the chemical rates at the wall. The present method satisfactorily resolves the flow structure, including the strong expansion behind the flat base. The relative merits and performance of the real-gas TVD and implicit schemes are discussed briefly.

Li, C. P.↗
NASA NTRS1991

Finite-rate chemistry effects in a Mach 2 reacting flow

UV spontaneous vibrational Raman scattering and laser-induced predissociative fluorescence (LIPF) are combined and applied to a supersonic flame. For the first time, simultaneous measurements of temperature, major species (H2, O2, N2, H2O), and minor species (OH) concentrations are obtained with a 'single' excimer laser in a supersonic-lifted hydrogen-air diffusion flame. In the supersonic flame, a small amount of reaction occurs upstream of the lifted flame base, due to shock wave interactions and mixing with hot vitiated air. The strong turbulent mixing and high total enthalpy fluctuations lead to nonequilibrium values of temperature, and major and minor species concentrations. Combustion occurs farther downstream of the lifted region where slow three-body recombination reactions result in superequilibrium OH concentrations that depress the temperatures below their equilibrium values. Farther downstream, ambient air entrainment contaminates flame properties.

Cheng, T. S.↗
NASA NTRS1991

A flowfield coupled excitation and radiation model for nonequilibrium reacting flows

A second-order method is developed to correct a radiative transfer analysis for possible local thermodynamic nonequilibrium (LTNE) effects. It uses a two-species excitation model for nitrogen with chemical raction rates obtained from the Kunc and Soon (1989) atomic transition method. A vibrational energy equation is added to calculate a third temperature, which describes the average vibrational energy state of all the diatomic species. A new diffusional model is developed to improve the calculation of the diffusional fluxes of mass and energy, and second-order radiative correction factors are developed for a two-step excitation model for atomic nitrogen. Radiation reaching the wall with the second-order LTNE model shows a greater IR-line contribution and less UV-line center absorption than the first-order one.

Gally, Thomas A.↗
NASA NTRS1992

Shock-turbulence interactions in a reacting flow

A specific reactive flow configuration, the interaction of a detonation wave with convected homogeneous isotropic weak turbulence (which can be constructed by a Fourier synthesis of small amplitude shear waves) is addressed. The effect of chemical heat release on the rms fluctuations downstream of the detonation is presented as a function of Mach number. In addition, for the particular case of the von Karman spectrum, the one dimensional power spectra of these flow quantities is given.

Jackson, T. L.↗
NASA NTRS1992

A computational investigation of mixing and reacting flows in supersonic combustors

Results of the numerical simulation of 15 degree downstream helium injection into a unconfined Mach 6 airstream are presented. Both details of downstream mixing and mean flow are in good agreement with experimental data. Results of the numerical simulation of similar hydrogen injection into a high enthalpy (Mach 17) confined Mach 6 airstream are presented with favorable comparison to experimental plume structure and wall data. The low enthalpy inflow from the unconfined case was then provided to the high enthalpy geometry in order to study the feasibility of using low enthalpy simulations of mixing for scramjet flight performance estimation. Results indicate that the mixing is substantially lower for the high enthalpy case but can be shown to appropriately scale using the inflow velocity. Production and decay of axial vorticity, cross-flow velocities, and the mean-flow velocities of these confined flows are then related and discussed to illustrate the effect of residence time on jet mixing.

Riggins, D. W.↗
NASA NTRS1992

An iodine hypersonic wind tunnel for the study of nonequilibrium reacting flows

A pilot scale hypersonic wind tunnel operating on pure iodine vapor has been designed and tested. The wind tunnel operates intermittently with a run phase lasting approximately 20 minutes. Successful recirculation of the iodine used during the run phase has been achieved but can be improved. Relevant issues regarding the full scale facility's design and operation, and the use of iodine as a working gas are discussed. Continuous wave laser induced fluorescence was used to monitor number densities within the plume flowfield, while pulsed laser induced fluorescence was used in an initial attempt to measure vibrational energy state population distributions. Preliminary nozzle flow calculations based on finite rate chemistry are presented.

Pham-Van-diep, G. C.↗
NASA NTRS1992

A new two-temperature dissociation model for reacting flows

A new two-temperature dissociation model for flows undergoing compression is derived from kinetic theory. The model minimizes uncertainties associated with the two-temperature model of Park. The effects of the model on AOTV type flowfields are examined and compared with the Park model. Calculations are carried out for flows with and without ionization. When considering flows with ionization, a four temperature model is employed. For Fire II conditions, the assumption of equilibrium between the vibrational and electron-electronic temperatures is somewhat poor. A similar statement holds for the translational and rotational temperatures. These trends are consistent with results obtained using the direct simulation Monte Carlo method.

Olynick, David R.↗
NASA NTRS1992

A code validation strategy and facility for nonequilibrium, reacting flows

The rotational and vibrational population distributions in a hypersonic flow of hot iodine vapor were investigated using a pilot wind tunnel. The tunnel provides run times of about 20 min, with a scale-up to a larger facility appearing to be possible. It will provide a 20-cm-diameter, Mach 9 flow of iodine vapor with only small amounts of free stream nonequilibrium. An analysis of the flow response to a normal shock wave indicates that significant chemistry will occur in model flow fields in the larger facility.

Muntz, E. P.↗
NASA NTRS1992

Investigation of the aerothermodynamics of hypervelocity reacting flows in the ram accelerator

New diagnostic techniques for measuring the high pressure flow fields associated with high velocity ram accelerator propulsive modes was experimentally investigated. Individual propulsive modes are distinguished by their operating Mach number range and the manner in which the combustion process is initiated and stabilized. Operation of the thermally choked ram accelerator mode begins by injecting the projectile into the accelerator tube at a prescribed entrance velocity by means of a conventional light gas gun. A specially designed obturator, which is used to seal the bore of the gun, plays a key role in the ignition of the propellant gases in the subsonic combustion mode of the ram accelerator. Once ignited, the combustion process travels with the projectile and releases enough heat to thermally choke the flow within several tube diameters behind it, thereby stabilizing a high pressure zone on the rear of the projectile. When the accelerating projectile approaches the Chapman-Jouguet detonation speed of the propellant mixture, the combustion region is observed to move up onto the afterbody of the projectile as the pressure field evolves to a distinctively different form that implies the presence of supersonic combustion processes. Eventually, a high enough Mach number is reached that the ram effect is sufficient to cause the combustion process to occur entirely on the body. Propulsive cycles utilizing on-body heat release can be established either by continuously accelerating the projectile in a single propellant mixture from low initial in-tube Mach numbers (M less than 4) or by injecting the projectile at a speed above the propellant's Chapman-Jouguet detonation speed. The results of experimental and theoretical explorations of ram accelerator gas dynamic phenomena and the effectiveness of the new diagnostic techniques are presented in this report.

Hertzberg, A.↗
NASA NTRS1993

The validation and application of numerical modelling to supersonic mixing and reacting flows

This study demonstrates that the computational fluid dynamics (CFD) code GASP can be used to predict the features for flowfields representative of scramjet combustors. The validation program undertaken was composed of two parts: a low enthalpy mixing study and a simplified autoignition study. The low enthalpy mixing study was done using low angle helium injection into a Mach 6 airstream over a flat plate. Both matched and overpressurized injection cases were numerically modelled. The agreement with experimental data for the farfield bulk mixing in both cases was reasonable. The combustion studies modelled premixed hydrogen-air reaction with the results being evaluated qualitatively. GASP proved to be highly sensitive to the chemical species present for flow near the ignition temperature, but provided reasonable qualitative results for a subsonic flameholder model.

Vitt, Paul H.↗