Tahir Gokcen

N2-CH4-Ar Chemical Kinetic Model for Simulations of Atmospheric Entry to Titan

Ad etailed chemical kinetic model for N2-CH4-Ar mixtures is developed fo rn onequilibrium simulation of shock layers formed in front of probes entering Titan’s atmosphere. The detailed kinetic model uses up-to-date chemical reaction mechanisms and reaction rates, and it is validated against existing shock tube experiments. A reduced kinetic model is also developed through sensitivity analysis of chemical reactions in the detailed model and reproduces the chemical kinetics of major species within the parameter space that may be encountered during Titan atmospheric entry. The reduced model, having fewer species and reactions than the detailed model, is better suited to coupled reacting CFD flowfield calculations. I. Introduction Shock layers formed in front of planetary probes entering the atmosphere of the Saturn moon Titan ar ee xpected to be in thermochemical nonequilibrium. Two examples of these probes are the CassiniHuygens probe scheduled to enter the Titan atmosphere in December 2004, and a Titan probe under consideration by the In-Space Propulsion program at NASA for future aerocapture missions. Titan’s atmosphere is known to be composed primarily of molecular nitrogen, methane, and argon. The relative mole fractions of species are uncertain at this time but believed to be N2 (80-98%), CH4 (2-10%), Ar (0-10%). Methane dissociates behind a strong shock wave at typical hypersonic entry conditions (e.g., speeds of 6-6.5 km/s, nonequilibrium temperatures of 5000-15,000 K, and pressures of 0.01-0.05 atm), an dc yano radical (CN) is formed as a result of the nonequilibrium chemistry. Since CN is known to be a stron gr adiator,the probes are expected to experience significant radiative heating as a result of the nonequilibrium radiation emission from the shock layer. Implications of shock layer nonequilibrium in Titan atmospheric entry were apparently first pointed out and analyzed by Park, 1 and later by others, e.g., Nelson et al., 2 Park and Bershader, 3 and Park. 4 Several aerothermal analyses carried out relatively recently in Refs. 5-11 also indicate that the radiative heating due to CN radiation will be a significant or even the dominant portion of the total heating. The radiative heat flux at the stagnation point was predicted to be as much as 0.5-7 times the convective heat flux. There are many reasons for such large variation in the predictions: freestream conditions, the vehicle nose radius (or shock stand-off distance), the uncertainty of the CH4 mole fraction, radiation-flowfield coupling, and the models used for chemistry and radiation. Clearly, a chemical kinetic model plays a pivotal role in the flowfield simulation of an onequilibrium shock layer and prediction of its radiation. The chemical kinetic model most commonly used i na erothermal analyses so far was originally proposed by Nelson et al. 2 (Nelson-91 model). However, it is found that this model has several inconsistencies with respect to the current literature: it does not include important species and reactions for methane decomposition and CN formation; and the reaction rates used are significantly different from the current literature values. Therefore, there is a need to update th eN elson-91model or to develop a new chemical kinetic model for N2-CH4-Ar mixtures. The present paper gives a short evaluation of the Nelson-91 model, and proposes a new chemical kinetic model consistent with the current literature for simulations of Titan atmospheric entry. For the development of a new chemical kinetic model, the following approach is taken. First, a detailed chemical kinetic model for N2-CH4-Ar mixtures is developed using up-to-date chemical reaction mechanisms and reaction rates. Second, the detailed model is validated against four sets of existing shock tube experiments.

Computational Analysis of Arc-Jet Stagnation Tests Including Ablation and Shape Change

Coupled fluid-material response analyses of arc-jet stagnation tests conducted in a NASA Ames Research Center arc-jet facility are considered. The fluid analysis includes computational Navier-Stokes simulations of the nonequilibrium flowfield in the facility nozzle and test box as well as the flowfield over the models. The material response analysis includes simulation of two-dimensional surface ablation and internal heat conduction, thermal decomposition, and pyrolysis gas flow. For ablating test articles including shape change, the material response and fluid analyses are coupled to take into account changes in surface heat flux and pressure distributions with shape. The ablating material used in these arc-jet tests was a phenolic impregnated carbon ablator. Computational predictions of surface recession, shape change, and material response are compared with the experimental measurements.

Nonequilibrium Ablation of Phenolic Impregnated Carbon Ablator

Phenolic Impregnated Carbon Ablator is the forebody heatshield material for the Mars Science Laboratory, the Stardust sample-return capsule, and the SpaceX Dragon vehicle. In previous work an equilibrium ablation and thermal response model was developed. Model predictions were compared with data from many stagnation arcjet tests conducted over a range of stagnation heat flux and pressure from 107 W/cm 2 at 2.3 kPa to 1100 W/cm 2 at 84 kPa. In general, model predictions compared well with the data for surface recession, surface temperature, in-depth temperature at multiple thermocouples, and char depth. The uncertainty of the recession predictions was greatest for test conditions with low heat flux or low pressure. Additional testing has been performed at conditions down to 40 W/cm 2 and 1.6 kPa. The new test data suggest that nonequilibrium effects become important for prediction of PICA ablation at heat flux or pressure below about 80 W/cm 2 and 10 kPa, respectively. In this work we investigate two modifications to the ablation model to account for these nonequilibrium effects. Model predictions are compared with the arcjet test data.

Computation of axisymmetric and ionized hypersonic flows using particle and continuum methods

Comparisons between particle and continuum simulations of hypersonic near-continuum flows are presented. The particle approach employs the direct simulation Monte Carlo (DSMC) method, and the continuum approach solves the appropriate equations of fluid flow. Both simulations have thermochemistry models for air implemented including ionization. A new axisymmetric DSMC code that is efficiently vectorized is developed for this study. In this DSMC code, particular attention is paid to matching the relaxation rates employed in the continuum approach. This investigation represents a continuum of a previous study that considered thermochemical relaxation in one-dimensional shock waves of nitrogen. Comparison of the particle and continuum methods is first made for an axisymmetric blunt-body flow of air at 7 km/s. Very good agreement is obtained for the two solutions. The two techniques also compare well for a one-dimensional shock wave in air at 10 km/s. In both applications, the results are found to be sensitive to various aspects of the chemistry model employed.

Effects of freestream nonequilibrium on convective heat transfer to a blunt body

The axisymmetric Navier- Stokes equations are solved numerically for nonequilibrium airflows over a hemisphere. A formulation with a three-temperature thermochemical model has been employed to simulate vibrationally excited and partially dissociated airflow. A flow condition that has a total enthalpy of 25 MJ/kg and a surface pressure of 0.076 atm is studied. Computed stagnation point heat transfer using finite catalytic boundary conditions at the surface is compared with classical results. Departures from the classical heat transfer predictions caused by nonequilibrium effects are assessed for arcjet testing applications. A Damkohler number analysis is used to characterize the extent of flowfield nonequilibrium. It is shown that characterization of the thermodynamic state of the gas at the boundary-layer edge and within the boundary layer is needed to interpret the heat transfer measurements and to determine the surface catalytic efficiency.

Effect of Non-equilibrium Surface Thermochemistry in Simulation of Carbon Based Ablators

This study demonstrates that coupling of a material thermal-response code and a flow solver using a nonequilibrium gas/surface-interaction model provides time-accurate solutions for the multidimensional ablation of carbon-based charring ablators. The material thermal-response code used in this study is the two-dimensional implicit thermal response and ablation program, which predicts the charring-material thermal response and shape change on hypersonic space vehicles. Its governing equations include total energy balance, pyrolysis-gas mass conservation, and a three-component decomposition model. The flow code solves the reacting Navier–Stokes equations using the data-parallel-line-relaxation method. Loose coupling between the material-response and flow codes is performed by solving the surface mass balance in the flow code and the surface energy balance in the material-response code. Thus, the material surface recession is predicted by finite rate gas/surface-interaction boundary conditions implemented in...

Computational Analysis of Semi-Elliptical Nozzle Arc-Jet Experiments: Calibration Plate, Wing Leading Edge

This paper reports computational analysis in support of experiments in a high enthalpy arcjet wind tunnel at NASA Ames Research Center. These experiments were conducted in the NASA Ames 60-MW Interaction Heating Facility and include surface temperature measurements of swept-wing leading edge shaped pylon models. Surface temperatures of the arcjet pylon models were measured with thermocouples, an infrared camera, and a pyrometer. During the facility characterization runs, surface pressure and heat flux measurements on a water-cooled calibration plate were obtained. The present analysis comprises computational simulations of the nonequilibrium flowfield in the facility (the nozzle and the test box) and comparisons with the experimental measurements. The value of computational fluid dynamics simulations in planning and analysis of a complex arcjet test configuration is demonstrated.

Implicit Coupling Approach for Simulation of Charring Carbon Ablators

This study demonstrates that coupling of a material thermal response code and a flow solver with nonequilibrium gas–surface interaction for simulation of charring carbon ablators can be performed using an implicit approach. The material thermal response code used in this study is the three-dimensional version of fully implicit ablation and thermal response program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting Navier–Stokes equations using data-parallel line relaxation method. Coupling between the material response and flow codes is performed by solving the surface mass balance in the flow solver and the surface energy balance in the material response code. Thus, the material surface recession is predicted in the flow code, and the surface temperature and pyrolysis gas injection rate are computed in the material response code. It is demonstrated that the time-lagged explicit approach is sufficient for simulations at low s...

Validation of a Three-Dimensional Ablation and Thermal Response Simulation Code

The 3dFIAT code simulates pyrolysis, ablation, and shape change of thermal protection materials and systems in three dimensions. The governing equations, which include energy conservation, a three-component decomposition model, and a surface energy balance, are solved with a moving grid system to simulate the shape change due to surface recession. This work is the first part of a code validation study for new capabilities that were added to 3dFIAT. These expanded capabilities include a multi-block moving grid system and an orthotropic thermal conductivity model. This paper focuses on conditions with minimal shape change in which the fluid/solid coupling is not necessary. Two groups of test cases of 3dFIAT analyses of Phenolic Impregnated Carbon Ablator in an arc-jet are presented. In the first group, axisymmetric iso-q shaped models are studied to check the accuracy of three-dimensional multi-block grid system. In the second group, similar models with various through-the-thickness conductivity directions are examined. In this group, the material thermal response is three-dimensional, because of the carbon fiber orientation. Predictions from 3dFIAT are presented and compared with arcjet test data. The 3dFIAT predictions agree very well with thermocouple data for both groups of test cases.

Evaluation of thermochemical models for particle and continuum simulations of hypersonic flow

Computations are presented for one-dimensional, strong shock waves that are typical of those that form in front of a re-entering spacecraft. The fluid mechanics and thermochemistry are modeled using two different approaches. The first employs traditional continuum techniques in solving the Navier-Stokes equations. The second approach employs a particle simulation technique, the direct simulation Monte Carlo method (DSMC). The thermochemical models employed in these two techniques are quite different. The present investigation presents an evaluation of thermochemical models for nitrogen under hypersonic flow conditions. Four separate cases are considered that are dominated in turn by vibrational relaxation, weak dissociation, strong dissociation, and weak ionization. In near-continuum, hypersonic flow, the nonequilibrium thermochemical models employed in continuum and particle simulations produce nearly identical solutions. Furthermore, the two approaches are evaluated successfully against available experimental data for weakly and strongly dissociating flows. SPACE-VEHICLE passing through the Earths atmo- sphere will traverse a number of different flow regimes. At lower altitudes, the fluid density is sufficiently large for the flow to be considered in thermochemica l equilibrium. However, as the vehicle ascends higher into the atmosphere, the molecular collision rate falls, and low-density effects be- come increasingly important. Continuum methods are successfully applied to flows in which the collision rate of the gas is sufficient to maintain Boltzmann energy distributions for the various thermal modes of the gas. It is not necessary that the temperatures associated with each of the different modes be equal, or that chemical equilibrium prevails. Particle methods, such as the direct sim- ulation Monte Carlo method (DSMC), are successfully ap- plied to flows in which a reduced collision rate no longer supports equilibrium energy distributions. As the numerical cost of this technique is proportional to the fluid density, application has mainly been limited to rarefied flows. The computation of flow properties for the flight trajec- tories of many space vehicles require the use of both contin- uum and particle methods mentioned above. The interface between the different flow regimes is therefore of great im- portance. Clearly, it is desirable to obtain consistent results with these numerical methods in an overlapping near-contin- uum flow regime. Although the thermochemical models em- ployed in continuum and particle methods are quite different, under conditions of thermochemical equilibrium they are ex- pected to provide identical solutions. The relationship be- tween the continuum and particle simulations under condi- tions of thermochemica l nonequilibrium , however, has not been thoroughly investigated. Therefore, it is the purpose of this article to study this relationship by computing typical hypersonic flows with both the continuum and particle sim- ulation methods. Evaluation of the thermochemical models is made through the computation of four different cases. The flow conditions in the studies are given in Table 1 and are chosen to examine the effects of vibrational relaxation, dissociation, and ion- ization. These processes are considered in an accumulative sense through a gradual increase in the initial enthalpy of the flow. The continuum and particle approaches employed in this work are briefly described below.