Haoyue Weng

Multidimensional Modeling of Pyrolysis Gas Transport Inside Charring Ablative Materials

The behavior of pyrolysis gas transport in arcjet test samples is numerically studied. The simulation of the pyrolysis gas flow inside a porous material is presented, using two different geometries. The effects of allowing the gas to flow out of the sidewall are especially highlighted. Results show that the flow inside the test article is complex, and that the zero-dimensional or one-dimensional assumption made in most material response codes are not necessarily valid for certain geometries. The importance of three-dimensionality for modeling an ablative test article is addressed.

Numerical Investigation of Thermal Response Using Orthotropic Charring Ablative Material

An orthotropic material model is implemented in a three-dimensional material response code, and numerically studied for charring ablative material. Model comparison is performed using an iso-Q sample geometry. The comparison is presented using pyrolysis gas streamlines and time series of temperature at selected virtual thermocouples. Results show that orthotropic permeability affects both pyrolysis gas flow and thermal response, but orthotropic thermal conductivity essentially changes the thermal performance only. The pyrolysis gas flow is hypothesized to contribute to the thermal response of the material as it convects energy through the porous medium.

Simulation of Flow-tube Oxidation on the Carbon Preform of PICA

The oxidation of carbon fibers at high temperature is a subject of great interest in atmospheric re-entry science. To correctly evaluate the parameters involved, and test oxidation models, it is necessary to use a combination of flow and material solver. This works presents the full three-dimensional, implicit coupling of a computational fluid dynamics and material response solver, with the goal of reproducing the results of a series of flow-tube experiments recently performed. Results are presented for the geometry of interested, using a simplified numerical setup. This effort is the part of an ongoing project to develop a fully coupled system that solves fluid dynamics and material response under atmospheric entry conditions.

Numerical Investigation of Pyrolysis Gas Blowing Pattern and Thermal Response using Orthotropic Charring Ablative Material

An orthotropic material model is implemented in a three-dimensional material response code, and numerically studied for charring ablative material. Model comparison is performed using an iso-Q sample geometry. The comparison is presented using pyrolysis gas streamlines and time series of temperature at selected virtual thermocouples. Results show that orthotropic permeability aects both pyrolysis gas ow and thermal response, but orthotropic thermal conductivity essentially changes the thermal performance of the material. The eect of orthotropic properties may have practical use such that the material performance can be manipulated by altering the angle of orthotropic orientation.

Multi-Dimensional Modeling Pyrolysis Gas Flow inside Charring Ablators

During an atmospheric entry/re-entry, when a spacecraft travels at hypersonic speed, a strong bow shock is formed in front of the entering vehicle. Such a shock results in an enormous amount of aerodynamic heat, part of which is transferred to the thermal protection system (TPS). Of the many TPS options, charring ablators have gained popularity in recent years for their e↵ectiveness and light weight. They are made of a fibrous non-pyrolyzing matrix (usually carbon or silicon carbide) and impregnated with pyrolyzing material (often phenolic resin). Phenolic Impregnated Carbon Ablator (PICA), as used for the MSL and the Stardust missions, is one such kind of material.1 The idea behind this type of material is to dissipate part of the energy through pyrolysis and ablation. Pyrolysis is the process in which the phenolic polymer gradually carbonizes at high temperature, losing mass and generating pyrolysis gases. These gases are then expelled through the porous structure of the material and blown into the chemical reacting boundary layer. The other phenomenon, surface ablation, refers to the mass removal of the char (composed of non-pyrolyzed and residual carbonized material) through oxidation, sublimation and spallation. Much research has been done on this topic. However, most of the simulation tools available in the literature are one or two-dimensional.2–6 Admittedly, a one-dimensional solution is mostly adequate for design purposes; for predictive analysis, however, it might not be su cient to take into account all phenomena taking place inside the charring ablator. For example, materials like PICA possess orthotropic properties. The thermal conductivity in the “in-plane” direction is significantly higher than in the “through-the-thickness” direction. Thus, the one-dimensional response models usually underestimate the centerline temperature rise.7 Similarly, the permeability of PICA material is higher in the “in-plane” than in the “through-the-thickness” direction, which is in accord with the anisotropic microstructure of the carbon fiber matrix.8 If the pyrolysis gases blowing rate along a curved surface is concerned, a one-dimensional model might not be accurate. But more importantly, it has been hypothesize that surface mass fluxes are greatly influenced by the geometry of the material tested.9 This is of great importance when small test-articles are employed to derive and validate models that are used in very di↵erent geometrical configurations. For instance, as it is not feasible to fit an entire heat-shield in ground tests facilities such as arc-jets or ICP torches, samples of a few inches are being used for validation and model calibration.10–17 Most ablation code, if not all, use simple analysis for the gas transport; the pyrolysis gas is either assumed to instantly exit at the surface (0D assumption), or simply travel along a normal line (1D assumption). In this research e↵ort, the multi-dimensionality behavior of the pyrolysis gas inside samples are presented. Using samples comparable to the ones used in ground testing facilities, it is shown that those assumption

Effects of Water Presence on Low Temperature Phenomenon in Porous TPS Materials

During the development of the MSL Entry, Descent, and Landing Instrumentation suite, extensive arc jet ground testing was performed at NASA Ames Research Center on PICA models with embedded thermocouples. During these tests, a low temperature phenomenon was consistently observed through thermocouple measurements deep within the material. This anomaly, referred to here as the “hump”, consists of a change in concavity of the temperature profile well below the maximum temperature and is seen in various TPS materials and atmospheric conditions, and typically occurs around 40 ◦C. The “hump” temperatures in the MEDLI test series correlate well with the known saturation curve of water when plotted against the stagnation pressure. It is proposed that the observed “hump” is a result of the heat of vaporization during the endothermic phase transition of water within the TPS material. This is supported by the known absorption of water by PICA from the atmosphere prior to testing or flight. The presented material response model captures energy effects of phase transition from a pre-existing water presence. This work shows that water presence currently appears to be the most probable cause for the phenomenon, which is observed in multiple different porous TPS materials.

Numerical Investigation on Charring Ablator Geometric Effects: Study of Stardust Sample Return Capsule Heat Shield

Sample geometry is very influential in small charring ablative articles where 1D assumption might not be accurate. In heat shield design, 1D is often assumed since the nose radius is much larger than the thickness of charring. Whether the 1D assumption is valid for the heat shield is unknown. Therefore, the geometric effects of Stardust sample return capsule heat shield are numerically studied using a material response program. The developed computer program models material charring, conductive heat transfer, surface energy balance, pyrolysis gas transport and orthotropic material properties in 3D Cartesian coordinates. Simulation results show that the centerline temperatures predicted by 3D model are quite close to 1D model at the surface, but not the case inside the material. The pyrolysis surface gas blowing behaviors are quite similar but differences are observed at later time. Orthotropic model predicted a very different heat shield response to both the isotropic model and the 1D model.