Ofodike A. Ezekoye

A Review of Numerical and Experimental Characterization of Thermal Protection Materials - Part III. Experimental Testing

Thermal protection materials are required to preserve structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. A comprehensive literature survey was conducted to review the experimental test methods as well as erosion and heat transfer data relevant to the performance of thermal protection materials for different applications. Laboratory scale apparatus, small-scale and subscale solid rocket motors were used to test these thermal protection materials. A wide range of thermal protection materials such as cork, fiber-reinforced phenolics, fiber-reinforced silicone, filled-elastomers, filled-silicone nanocomposites, and phenolic fiber-reinforced nanocomposites were included. This paper is the third in a four-part comprehensive literature survey that is grouped into numerical modeling, properties characterization, experimental testing, and advanced nozzle throat material. I. Introduction hermal protection materials are required to protect structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. A thorough literature survey was conducted to review the numerical and experimental characterization of these thermal protection materials for different military and aerospace applications. The literature survey is grouped into: (a) numerical modeling, 1 (b) materials thermophysical properties characterization, 2 (c) experimental testing, and (d) advanced nozzle throat material. In this paper, only the experimental testing literature will be discussed. The numerical modeling 1 and thermophysical properties 2 reviews were published previously, and advanced nozzle throat material will be presented in a subsequent review.

Soot oxidation and agglomeration modeling in a microgravity diffusion flame

The global evolution of a microgravity diffusion flame is detailed. Gas species evolution is computed using a reduced finite rate chemical mechanism. Soot evolution is computed using various combinations of existing soot mechanisms. Radiative transfer is coupled to the soot and gas phase chemistry processes using a P1 spherical harmonics radiation model. The soot agglomeration model was examined to note the dependence of soot growth and oxidation processes on soot surface area predictions. For limiting cases where agglomeration was excluded from the soot evolution model, soot primary particle sizes and number concentrations were calculated, and the number of primary particles per aggregate was inferred. These computations are compared with experimental results for microgravity and nonbuoyant flame conditions.

Thermoplastic polyurethane elastomer nanocomposites: morphology, thermophysical, and flammability properties

Novel materials based on nanotechnology creating nontraditional ablators are rapidly changing the technology base for thermal protection systems. Formulations with the addition of nanoclays and carbon nanofibers in a neat thermoplastic polyurethane elastomer (TPU) were melt-compounded using twin-screw extrusion. The TPU nanocomposites (TPUNs) are proposed to replace Kevlar-filled ethylene-propylene-diene-monomer rubber, the current state-of-the-art solid rocket motor internal insulation. Scanning electron microscopy analysis was conducted to study the char characteristics of the TPUNs at elevated temperatures. Specimens were examined to analyze the morphological microstructure during the pyrolysis reaction and in fully charred states. Thermophysical properties of density, specific heat capacity, thermal diffusivity, and thermal conductivity of the different TPUN compositions were determined. To identify dual usage of these novel materials, cone calorimetry was employed to study the flammability properties of these TPUNs.

Kinetics and Thermophysical Properties of Polymer Nanocomposites for Solid Rocket Motor Insulation

Thermal protection materials are required to protect structural components of space vehicles during the reentry stage, missile launching systems, and solid rocket motors. Novel materials based on nanotechnology creating nontraditional ablators are rapidly changing the technology base for thermal protection systems. In this study, different polymer nanocomposite compositions were created by melt-compounded montmorillonite nanoclays or carbon nanofibers in a neat thermoplastic polyurethane elastomer polymer using twin-screw extrusion. The kinetic and thermophysical properties that are required to analyze the ablation characteristics were measured for selective thermoplastic polyurethane materials. Properties of the nanomodified systems were then compared against those of the current state-of-the-art insulation material, Kevlar®-filled ethylene-propylene-diene monomer and the neat thermoplastic polyurethane elastomer, for the investigation of kinetic parameters using the isoconversion technique. Based on temperatures at peak weight-loss rate, Kevlar-filled ethylene-propylene-diene monomer outranked the proposed formulations at all heating rates. Recognizing that ablation performance is a complex function of kinetic and thermophysical/transport properties, a surrogate test for ablation performance was investigated. Samples of neat and nanomodified thermoplastic elastomer were run in an oxygen-consumption (cone) calorimeter. The peak heat release rates of the nanomodified samples were substantially less than that of the neat thermoplastic elastomer.

Flammability Studies of a Novel Class of Thermoplastic Elastomer Nanocomposites

The thermal insulation properties of thermoplastic polyurethane elastomer nanocomposites were characterized at different heat fluxes. Thermoplastic polyurethane elastomer was modified with different loadings of montmorillonite nanoclays and carbon nanofibers (CNFs) via twin screw extrusion processing. The addition of nanoparticle into thermoplastic polyurethane elastomer resulted in the formation of a char layer and modified the thermal insulative properties of the material. It was found that thermoplastic polyurethane elastomer with 10 wt% CNFs and with 5 wt% nanoclays gave the best thermal performance with respect to protecting a substrate. The surface temperature of the thermoplastic polyurethane elastomer-clay nanocomposites did not vary much with addition of clay particles while the surface temperature of the thermoplastic polyurethane elastomer-CNF nanocomposites varied more substantially. Some of the trends in surface and substrate temperature measurements with nanomodification can be described using a simple energy balance model that takes into account the basic heat transfer mechanisms.

Volumetric PIV and 2D OH PLIF imaging in the far-field of a low Reynolds number nonpremixed jet flame

Cinematographic stereoscopic PIV with temporal and spatial resolution ranging from 2.6 to 5.5 Kolmogorov scales, which is sufficient to accurately represent most of the dissipation structures, is used in conjunction with Taylor’s frozen flow hypothesis to generate quasi-instantaneous pseudo-volumes of the three-component velocity field in the far-field of a nonpremixed jet flame at the jet exit Reynolds number (Red) of 8000. The 3D data enable the computation of the nine components of the velocity gradient tensor and other important kinematic quantities. The volumetric PIV is combined with single-shot simultaneous OH PLIF imaging to mark the instantaneous reaction zone at one plane in the reconstructed volume. The combined datasets enable the investigation of the relationship between the reaction zone and the fully-3D representations of strain, vorticity, kinetic energy dissipation and dilatation, and of the impact of heat release on the structure of turbulence. In this Red = 8000 flame, it is observed that sheet-like layers of vorticity and dissipation tend to coincide and are aligned with the OH layers, an effect that is believed to be due to the stabilizing effect of heat release on this relatively low Reynolds number jet flame. Furthermore, the spatial organization of the strain field is predominantly driven by the presence of the flame rather than turbulence. Finally, intense dissipation is mostly due to the laminar shear caused by the presence of the flame rather than to the strain generated by vortical structures as typically observed in nonreacting jets.

Inverse design of thermal systems with dominant radiative transfer

Abstract The design of thermal systems often involves the specification of two conditions, typically both temperature and heat flux distributions on some surfaces or within media to perform particular tasks. Examples are in annealing ovens, dryers, chambers for rapid thermal processing of semiconductor wafers, utility and chemical furnaces, infrared ovens, and many others. Conventional design techniques require specification of one and only one boundary condition on each surface of a system, requiring trial-and-error solutions to achieve a design that satisfies the second specified condition. Here, the methods of inverse analysis are applied to the ill-conditioned equations that result when two conditions are specified for a particular surface. It is demonstrated that the use of inverse methods can result in multiple designs that each provide the required conditions; that designs are generated that might not be found through the conventional approach; and that the use of inverse design can lead the designer to efficient and novel designs for thermal systems.

Heating Rate and Nanoparticle Loading Effects on Thermoplastic Polyurethane Elastomer Nanocomposite Kinetics

Studies of the thermal degradation of a thermoplastic polyurethane elastomer are performed to understand its kinetics. Kinetic parameters are an important part of characterizing the behavior of a material used in high temperature environments. Many engineering applications require that the materials used have high resistance to thermal degradation. Experiments using polymers loaded with nanoparticles have shown that the kinetic parameters can be vastly different than the kinetic parameters of the neat polymer. The kinetic parameters change in such a way that the thermal degradation of materials loaded with nanoparticles is delayed compared to that of the neat material. The effect of loading the rmoplastic polyurethane elastomer with montmorillonite organoclay and multiwall carbon nanotubes is presented along with the kinetic parameters calculated for each material. The thermogravimetric analyses of these materials are conducted at low heating rates (from 20 to 100 o C/min (CPM )). A study will be presented to analyze if the kinetics calculated from these data sets are valid parameters to describe the kinetics of experiments conducted at much higher heating rates (up to 500 CPM ).

Modeling fan-driven flows for firefighting tactics using simple analytical models and CFD

Airflow control has become a large part of the tactical toolbox that firefighters use to combat fires. Control of airflow requires managing the impact of environmental conditions (i.e., wind) and optimally using mechanically generated flows from fans to drive air and combustion products through predetermined vents. This article discusses the ability of analytical and computational models to predict flow variables associated with the use of positive pressure ventilation. To make these predictions, it is shown that various levels of approximation and a knowledge of (the often neglected) structure leakage rates are required. This study details experiments and simulations of airflow rates associated with fan-induced pressure differences between the environment and a structure.

Thermophysical properties characterization of thermoplastic polyurethane elastomer nanocomposites

Thermal protection materials are required to protect structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. Novel materials based on nanotechnology creating nontraditional ablators are rapidly changing the technology base for thermal protection systems. The introduction of inorganic nanomaterials as additives into polymer systems has resulted in polymer nanostructured materials exhibiting multifunctional, high- performance polymer characteristics beyond traditional polymer composites possess. In this study, different polymer nanocomposite compositions were created by melt-compounded montmorillonite nanoclays or carbon nanofibers in a neat thermoplastic polyurethane elastomer (TPU) polymer using twin-screw extrusion. These materials were characterized for their thermal and kinetic properties. Scanning electron microscopy analysis was performed for microstructural studies. Selective results were then compared against the current state-of-the-art insulation material, Kevlar®-filled ethylene-propylene-diene rubber (EPDM) as well as the neat TPU for the investigation of properties enhancement. I. Introduction Thermoplastic polyurethane elastomer nanocomposites (TPUNs) are a novel class of insulation materials developed by Air Force Research Laboratory (AFRL) that are lighter, exhibits better erosion and insulation characteristics, and possesses a more cost-effective manufacturing process than the current baseline material, Kevlar®-filled ethylene-propylene-diene rubber (EPDM). The proposed research combines numerical modeling and experimental characterization of TPUNs for solid rocket motor (SRM) insulation. The TPUN thermophysical properties are characterized using thermogravimetric analysis (TGA) for kinetic parameters, differential scanning calorimetry (DSC) for specific heats, laser flash for thermal diffusivity and thermal conductivity, and dilatometry for coefficient of thermal expansion, as well as other analysis techniques are used in this study. This paper will summarize our research progress on the characterization of thermal behavior and thermophysical properties of these TPUNs, which will later be used in our numerical modeling research. A. Behavior of Thermal Protection Materials Thermal protection materials are required to protect structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. Polymeric composites have been used as ablative thermal protection systems (TPS) for a variety of military and aerospace applications. Thermal protection materials such as carbon phenolics and carbon-carbon composites are used as spacecraft heat shields, and insulation and nozzle assembly materials for SRMs. These materials are exposed to a thermochemical as well as particle impinging flow and are subjected to high temperatures in excess of