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Results for “nonequilibrium steady states”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Carbon vaporization into a nonequilibrium, stagnation-point boundary layer

The heat transfer to the stagnation point of an ablating carbonaceous heat shield, where both the gas-phase boundary layer and the heterogeneous surface reactions are not in chemical equilibrium, is examined. Specifically, the nonequilibrium changes in the mass fraction profiles of carbon species calculated for frozen flow are studied. A set of equations describing the steady-state, nonequilibrium laminar boundary layer in the axisymmetric stagnation region, over an ablating graphite surface, is solved, with allowance for the effects of finite rate of carbon vaporization.

Suzuki, T.↗
NASA NTRS1988

Accelerating the computation of steady state solutions for non-equilibrium, chemically reacting flow fields

The objective of the study is to develop a technique for accelerating the computation of steady state solutions for nonequilibrium chemically reacting flow fields using the time marching method. To achieve this, a better initial condition is introduced for the time marching scheme to achieve a faster convergence to the steady state solution. This is done by replacing the source terms of the original system of equations, which are responsible for the sitffness, by approximate source terms. It is shown that the saving in computation time increases in direct proportion to the stiffness of the flow model, with a factor of 10 increase in convergence rate observed for the models tested in this study.

Krispin, Jacob↗
NASA NTRS2012

Modeling of Laser Vaporization and Plume Chemistry in a Boron Nitride Nanotube Production Rig

Flow in a pressurized, vapor condensation (PVC) boron nitride nanotube (BNNT) production rig is modeled. A laser provides a thermal energy source to the tip of a boron ber bundle in a high pressure nitrogen chamber causing a plume of boron-rich gas to rise. The buoyancy driven flow is modeled as a mixture of thermally perfect gases (B, B2, N, N2, BN) in either thermochemical equilibrium or chemical nonequilibrium assuming steady-state melt and vaporization from a 1 mm radius spot at the axis of an axisymmetric chamber. The simulation is intended to define the macroscopic thermochemical environment from which boron-rich species, including nanotubes, condense out of the plume. Simulations indicate a high temperature environment (T > 4400K) for elevated pressures within 1 mm of the surface sufficient to dissociate molecular nitrogen and form BN at the base of the plume. Modifications to Program LAURA, a finite-volume based solver for hypersonic flows including coupled radiation and ablation, are described to enable this simulation. Simulations indicate that high pressure synthesis conditions enable formation of BN vapor in the plume that may serve to enhance formation of exceptionally long nanotubes in the PVC process.

Gnoffo, Peter A.↗
NASA NTRS1992

A hybrid multigrid technique for computing steady-state solutions to supersonic flows

Recently, Li and Sanders have introduced a class of finite difference schemes to approximate generally discontinuous solutions to hyperbolic systems of conservation laws. These equations have the form together with relevant boundary conditions. When modelling hypersonic spacecraft reentry, the differential equations above are frequently given by the compressible Euler equations coupled with a nonequilibrium chemistry model. For these applications, steady state solutions are often sought. Many tens (to hundreds) of super computer hours can be devoted to a single three space dimensional simulation. The primary difficulty is the inability to rapidly and reliably capture the steady state. In these notes, we demonstrate that a particular variant from the schemes presented can be combined with a particular multigrid approach to capture steady state solutions to the compressible Euler equations in one space dimension. We show that the rate of convergence to steady state coming from this multigrid implementation is vastly superior to the traditional approach of artificial time relaxation. Moreover, we demonstrate virtual grid independence. That is, the rate of convergence does not depend on the degree of spatial grid refinement.

Sanders, Richard↗
NASA NTRS1990

Radiative shocks and nonequilibrium chemistry in the early universe - Galaxy and primordial star formation

The nonequilibrium radiative cooling, recombination, and molecule formation behind steady-state shock waves in primordial composition gas is studied. The hydrodynamical conservation equations are solved as well as equations for nonequilibrium ionization, recombination, and molecule formation and the equation of radiative transfer. The shocked gas is found to cool faster than it can recombine; as a result it is able to form an H2 concentration as high as 0.001 or higher via the formation of H(-) and H2(+) intermediaries due to the enhanced nonequilibrium ionization at 10,000 K. With such an H2 concentration, the gas cools by rotational-vibrational line excitation of H2 molecules to well below the canonical final temperature of 10,000 K for a molecule-free gas without metals. It is shown that, as the level of external ionizing and dissociating radiation flux is increased, the formation of and cooling by H2 molecules can be inhibited and delayed.

Shapiro, Paul R.↗
NASA NTRS1979

Disequilibrium condensation environments in space - A frontier in thermodynamics

The thermal-disequilibrium aspect of the problem of dust-particle formation from a gas phase in an open space environment is discussed in an effort to draw attention to the space condensation environment as an interesting arena for application and extension of the ideas and formalisms of nonequilibrium thermodynamics. It is shown that quasi-steady states with a disequilibrium between the gas-phase kinetic temperature and the condensed-phase internal temperature appear to be the norm of condensation environments in space. Consideration of the case of condensation onto a bulk condensed phase indicates that these quasi-steady states may constitute Prigogine dissipative structures. It is suggested that a proper study of the process of condensation in a space environment should include any effects arising from thermal disequilibrium.

De, B. R.↗
NASA NTRS1976

Monte Carlo calculations of diatomic molecule gas flows including rotational mode excitation

The direct simulation Monte Carlo method was used to solve the Boltzmann equation for flows of an internally excited nonequilibrium gas, namely, of rotationally excited homonuclear diatomic nitrogen. The semi-classical transition probability model of Itikawa was investigated for its ability to simulate flow fields far from equilibrium. The behavior of diatomic nitrogen was examined for several different nonequilibrium initial states that are subjected to uniform mean flow without boundary interactions. A sample of 1000 model molecules was observed as the gas relaxed to a steady state starting from three specified initial states. The initial states considered are: (1) complete equilibrium, (2) nonequilibrium, equipartition (all rotational energy states are assigned the mean energy level obtained at equilibrium with a Boltzmann distribution at the translational temperature), and (3) nonequipartition (the mean rotational energy is different from the equilibrium mean value with respect to the translational energy states). In all cases investigated the present model satisfactorily simulated the principal features of the relaxation effects in nonequilibrium flow of diatomic molecules.

Yoshikawa, K. K.↗
NASA NTRS1972

Nonequilibrium ionization measurements in hydrogen-helium mixtures

Time-resolved emission measurements of several atomic line and continuum radiation channels were made behind the incident shock wave of an electric arc shock tube. Test times and nonequilibrium ionization times were obtained for shock velocities up to 2.5 X 10,000 m/sec in a 0.2 H2 - 0.8 He gas mixture. The shock-heated test gas was found to be free from driver gas contamination, and the test times were adequate to achieve steady state conditions. An activation energy of 4 eV was obtained from the nonequilibrium ionization time measurements. Modifications of experimental technique to determine the effect of test gas impurity level on ionization time measurements are discussed.

Leibowitz, L. P.↗
NASA NTRS1990

The effects of nonequilibrium ionization on the radiative losses of the solar corona

The emissivity of the ions of carbon and oxygen has been recalculated for a set of solar coronal loop models with a steady state siphon flow. The ion densities were calculated from the plasma velocities, temperatures, and densities of the models, and large departures from equilibrium were found. For purposes of comparison, the emissivity was calculated with and without the approximation of ionization equilibrium. Considerable differences in the radiative loss function Lambda(T) curve between equilibrium and nonequilibrium conditions were found. The nonequilibrium Lambda(T) function was then used to solve again the steady state flow equations of the loop models. The differences in the structure of these models with respect to the models calculated adopting the Lambda(T) curve in equilibrium are discussed.

Spadaro, D.↗
NASA NTRS1984

Macrosegregation in aluminum alloy ingot cast by the semicontinuous direct chill method

A theoretical model of the semicontinuous DC casting method is developed to predict the positive segregation observed at the subsurface and the negative segregation commonly found at the center of large commercial-size aluminum alloy ingot. Qualitative analysis of commercial-size aluminum alloy semicontinuous cast direct chill (DC) ingot is carried out. In the analysis, both positive segregation in the ingot subsurface and negative segregation at the center of the ingot are examined. Ingot subsurface macrosegregation is investigated by considering steady state casting of a circular cross-section binary alloy ingot. Nonequilibrium solidification is assumed with no solid diffusion, constant equilibrium partition ratio, and constant solid density.

Yu, H.↗
NASA NTRS1992

Radiative shocks and hydrogen molecules in pregalactic gas - The effects of postshock radiation

Previous calculations of radiative shocks in pregalactic gas are generalized and substantially improved. The hydrodynamical conservations are solved, along with the rate equations for nonequilibrium ionization, recombination, and molecule formation, and the equation of radiative transfer, for steady-state shocks in a gas of primordial compositions. This calculation self-consistently includes the effects of the diffuse postshock emission as well as a possible external radiation flux on the postshock flow and the preshock ionization levels. A previous result is confirmed that the shocked gas cools faster than it can recombine and, as a result, is able to form an H2 concentration as high as 0.003 via the formation of H(-) and H2(+) intermediaries due to the enhanced nonequilibrium ionization fraction at 10,000 K. It is shown that, as the level of the external radiation flux is increased, the formation of and cooling by H2 molecules can be inhibited and delayed. The detailed radiation spectra and ionizing photon number fluxes emergent from these shocks are presented.

Kang, Hyesung↗
NASA NTRS1989

A fully-coupled implicit method for thermo-chemical nonequilibrium air at sub-orbital flight speeds

A CFD technique is described in which the finite-rate chemistry in thermal and chemical nonequilibrium air is fully and implicitly coupled with the fluid motion. Developed for use in the suborbital hypersonic flight speed range, the method accounts for nonequilibrium vibrational and electronic excitation and dissociation, but not ionization. The steady-state solution to the resulting system of equations is obtained by using a lower-upper factorization and symmetric Gauss-Seidel sweeping technique through Newton iteration. Inversion of the left-hand-side matrices is replaced by scalar multiplications through the use of the diagonal dominance algorithm. The code, named CENS2H (Compressible-Euler-Navier-Stokes Two-Dimensional Hypersonic), is fully vectorized and requires about 8.8 x 10 to the -5th sec per node point per iteration using a Cray X-MP computer. Converged solutions are obtained after about 2400 iterations. Sample calculations are made for a circular cylinder and a 10 percent airfoil at 5 deg angle of attack. The calculated cylinder flow field agrees with that obtained experimentally. The code predicts a 10 percent change in lift, drag, and pitching moment for the airfoil due to the thermochemical phenomena.

Park, Chul↗
NASA NTRS1991

A survey of the role of thermodynamic stability in viscous flow

The stability of near-equilibrium states has been studied as a branch of the general field of nonequilibrium thermodynamics. By treating steady viscous flow as an open thermodynamic system, nonequilibrium principles such as the condition of minimum entropy-production rate for steady, near-equilibrium processes can be used to generate flow distributions from variational analyses. Examples considered in this paper are steady heat conduction, channel flow, and unconstrained three-dimensional flow. The entropy-production-rate condition has also been used for hydrodynamic stability criteria, and calculations of the stability of a laminar wall jet support this interpretation.

Horne, W. C.↗