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Highlights • Oxidation of FiberForm under O 2 , CO 2 and CO flows accurately quantified. • Gaseous products from the oxidation of FiberForm measured using calibrated mass spectrometry. • Global porous media permeability of FiberForm measured during oxidation. • Micro-structure of virgin and oxidized material characterized with scanning electron microscopy. Abstract We carried out laboratory experiments to study the decomposition of carbon fibers at high temperature and studied the material used in charring carbon phenolic ablators for planetary probe heatshields. Porous plug samples were exposed to controlled rates of molecular oxygen, carbon dioxide, and carbon monoxide at furnace-heated flow-tube reactor temperatures between 700 and 1500 K and pressures between 2000 and 6200 Pa. We used calibrated mass spectroscopy to make time-resolved quantitative measurements of decomposition products of the gases downstream of the test sample. Absolute and differential pressure measurements were used to determine the high-temperature permeability of the material and to monitor changes during material decomposition, and we characterized the microstructure of the porous sample using scanning electron microscopy. We found substantial carbon fiber oxidation for O 2 flows, resulting in increasing CO/CO 2 production ratios with increasing temperature. The CO 2 was shown to react with carbon fibers following a Boudouard reaction at temperatures above 1200 K. [ABSTRACT FROM AUTHOR]

Highlights • New method for estimation of thermal properties of charring ablators is proposed. • Sensitivity analysis shows thermal conductivity has stronger effect on temperature. • Estimation of thermal properties in order can shorten the inversion time. • Thermal properties can be estimated by both simulations and test data. Abstract The accurate assessment of thermal properties is crucial for charring materials simulation and design. In this work, a new inversion method for estimating temperature-dependent thermal conductivity and specific heat capacity from temperature data is presented for charring ablators. Firstly, a one-dimensional thermal response model with surface recession is developed to simulate the thermal behavior of charring ablator. Then based on the developed model, sensitivity analysis is conducted to investigate the correlation between thermal parameters and temperature for determining inversion sequence and find that virgin thermal conductivity has the biggest influence on temperature which should be estimated at first. Finally, the temperature-dependent thermal conductivities and specific heat capacities are obtained by the inversion method from temperature data. The inversion values from simulation temperature data coincide with the reference values, which the average inversion error of thermal conductivities is 4.3% and the average inversion error of specific heat capacities is 3.1%. And the calculated thermal response temperature employing the inversion thermal properties shows a good agreement with the arc jet test data, which the average error is 8.5%. This inversion method can accurately determine unknown temperature-dependent thermal properties such as thermal conductivity and specific heat capacity, which provides an insight into the analysis and design of thermal protection materials for spacecraft. [ABSTRACT FROM AUTHOR]

As a typical fiber reinforced resin-based ablative material, silica/phenolic composite has important applications in the thermal protection of high-speed aircrafts. At high temperature, it can form a carbonized layer, whose surface emittance is directly related to its capacity of radiative heat dissipation. In this work, carbonization experiments of the silica/phenolic composite and pure phenolic resin were designed to obtain their corresponding carbonized samples. The spectral emittances in the wavelength range from 2.5 to 25  μ m of the carbonized samples were measured by a Fourier transform infrared spectroscopy with a blackbody furnace and a sample heater at temperatures from 200 to 500 °C. The results showed that the spectral emittance of the carbonized ablative material is close to that of the carbonized phenolic resin. The spectral emittances of the two carbonized samples increase with temperature and almost do not change with wavelength, indicating that the carbonized samples have the nature of a grey body. A prediction model of the total emittance was constructed based on the grey body assumption and a geometric model extracted from structure characterization of the carbonized ablative material. The predicted total emittances and those obtained from the integration of the measured spectral results are all higher than 0.8, and the deviations are below 10%. Theoretical predictions of the model show that the total emittance of the carbonized ablative material increases with the increasing porosity ( ϕ ) and total emittance ( ε s ) of the fiber bundle covered with carbonized phenolic resin. Both the experimental and theoretical results reveal that the high total emittance of the carbonized silica/phenolic ablative material can be mainly attributed to the high emittance of the carbonized phenolic resin. [ABSTRACT FROM AUTHOR]

As a phenolic resin-based ablative material, high silica/phenolic composite is widely used in aerospace field. However, the effective thermal conductivity of carbonized ablative material formed during ablation process is rarely reported. In this work, carbonized sample of the high silica/phenolic ablative material was obtained by a carbonization process, and its effective thermal conductivity was measured from 100 to 970 °C. In addition, an analysis model of the effective thermal conductivity of the carbonized ablator consisting of fiber yarns and carbonized phenolic was established based on the result of structure analysis. The measured value of the effective thermal conductivity of the carbonized phenolic was used to inverse the transverse effective thermal conductivity of the fiber yarns. When the inversed values were adopted in different empirical models, it was found that the Clayton model is suitable for predicting the effective thermal conductivity of the carbonized high silica/phenolic. [ABSTRACT FROM AUTHOR]

Charring ablation materials are widely used for thermal protection systems in a vehicle during hypersonic reentry. The pyrolysis gases from the charring materials can react with oxygen in the boundary layer, which makes the surface ablation rate decrease. The problem of protection of pyrolysis gases combustion against charring materials’ surface ablation is solved by the detached normal shock wave relations and the counterflow diffusion flame model. The central difference format for the diffusion term and the upwind scheme for the convection term are used to discretize the mathematical model of the counterflow diffusion flame. Numerical results indicate that the combustion of pyrolysis gases in the boundary layer can completely protect the materials surface from recession when the velocity of pyrolysis gases injecting to the boundary layer is higher than the critical velocity. There is an allometric relationship between the critical velocity and Mach number, and the combustion heat has little influence on the temperature distribution originating from the aerodynamic heating. This study will be helpful for the design of the thermal protection system in hypersonic reentry vehicles. [ABSTRACT FROM AUTHOR]

Ablation experiments of C/C composite materials were conducted by using arc plasma jet. The ablated surfaces of the C/C composite material samples were monitored by using the online spectrum diagnoses during the ablation experiments. Based on energy balance, mass conservation and chemical equilibrium equations, surface ablative model was established for the C/C composite materials to study the ablation process and the thermal environment parameters. The simulation results are in good agreement with the theoretical solution and experimental results. The ablative experiment and analytical model can be recognized as the basic research method for the thermal protection materials in the aerospace applications. [ABSTRACT FROM AUTHOR]

Abstract: Ablative materials can sustain very high temperatures in which surface thermochemical processes are significant enough to cause surface recession. Existence of moving boundary over a wide range of temperatures, temperature-dependent thermophysical properties of ablators, and no prior knowledge about the location of the moving surface augment the difficulty for predicting the exposed heat flux at the receding surface of ablators. In this paper, the conjugate gradient method is proposed to estimate the unknown surface recession and time-varying net surface heat flux for these kinds of problems. The first order Tikhonov regularization is employed to stabilize the inverse solution. Considering the complicated phenomena that are taking place, it is shown via simulated experiment that unknown quantities can be obtained with reasonable accuracy using this method despite existing noises in the measurement data. [Copyright &y& Elsevier]

Abstract: We develop an analytical Knudsen layer model at the ablative surface in gas flow; this model takes into account the temperature and velocity gradients in the bulk gas and the rebounding of gas molecules by the ablative wall back into the gas region. In addition, this model uses a bimodal velocity distribution function which preserves the laws of conservation of mass, momentum and energy within the Knudsen layer and converges to the Chapman–Enskog velocity distribution function at the outer boundary of the Knudsen layer. This model enables us to obtain the boundary conditions at the ablative surface in gas flow for arbitrary condensation and accommodation coefficients, which can be used for computational fluid dynamics simulation of ablation avoiding “micro” modelling of the evaporation process at the mean free path scale. [Copyright &y& Elsevier]

• Low-temperature ablators can be used for the design of re-entry capsule thermal protection systems. • Camphor is preferred over naphthalene due to its thermophysical properties which prevent melting and maximize shape change. • The validated numerical approach couples CFD analysis with model ablation and mesh evolution. • The Phoebus capsule geometry is chosen as reference for the experimental test campaign through a numerical trade-off. • Numerical rebuilding of experimental results with H-3 blow-down hypersonic wind tunnel conditions is satisfactory. Ground testing of ablative materials aims at providing critical data on the material behavior under hypersonic reentry conditions. This is normally done in plasma wind tunnel facilities. However, non-negligible technical challenges are faced in order to duplicate the real flight conditions, such as inducing the recession of space-relevant ablative materials, which requires sufficiently high inflow total enthalpies, and/or reproducing the actual hypersonic flow velocity, which requires sufficiently high inflow Mach numbers. Often, ground facilities which are providing one requirement are lacking the other one and vice-versa. A possible solution is to use low-temperature ablators in continuous hypersonic blow-down tunnels, where aerodynamic and ablative tests with considerable shape change effects may be performed under reasonably low total temperature conditions and with affordable test durations. These substances are readily available, and they sublimate or ablate in a fashion that can be described fairly accurately by theory. This work has the objective to numerically characterize the shape change of such materials in hypersonic conditions, concurrently providing a validation against literature data and from a dedicated experimental ground test campaign. The numerical procedure relies on ad-hoc mesh generation/evolution strategies taking into account the material shape change, and is based on subsequent steady-state Computational Fluid Dynamics (CFD) computations coupled with a customizable gas-surface interaction wall boundary condition. Preliminary numerical simulations helped the design of the experiments to be carried out in the von Karman Institute (VKI) H-3 hypersonic wind tunnel, in particular for the identification of capsule geometry and size in order to maximize the shape change caused by ablation. Subsequently, camphor is identified as the most suitable low-temperature ablator to be used in the experimental campaign after a thorough analysis of its surface reaction thermodynamics and kinetics. Results from the CFD approach are first compared with a literature experimental test case and then with those of the previously designed experiments, featuring a camphor sub-scale capsule, underlying advantages and limits of the numerical procedure adopted. The obtained numerical and experimental results underline how it is possible to obtain a relevant shape change for relatively small exposure times by using low-temperature ablators in continuous hypersonic blow-down wind tunnels. Hence, results from this work can be used to support the design and sizing of the actual heat shield and the analysis of the capsule's aerodynamics and stability, accounting for shape change effects, by establishing an appropriate similitude between in-flight and on-ground conditions. [ABSTRACT FROM AUTHOR]

• Adesign and fabrication method for flexible flat heat pipe which can be folded repeatedly. • A novel capillary mesh with gradient wettability was fabricated and successfully applied to flexible flat heat pipe. • The preparation method of the nanowires is an innovation. We have improved the growth of nanowire structures with sufficient pore size. • We successfully achieved the characteristics of simultaneous bending of the casing material and the capillary mesh. In this work, we report the fabrication and thermal performance evaluation of flexible flat heat pipes by using laser ablated casing material with aluminum compound packing film and using gradient wetting functional copper meshes covered with nanowires as the wick structure. Deionized water was chosen as working fluid and three different filling ratios (20%, 30%, 40%) of working fluid were loaded into the heat pipe to investigate its impact on thermal performance. The fabricated flexible flat heat pipes can be bended from 0° to 180° in the horizontal operation mode and demonstrated consistently low thermal resistance of 0.525 °C/W or effective thermal conductivity of 1499 W / m · K after repeated bending. Simultaneously, theoretical analysis reveals that bending disturbs the vapor flow from evaporator to condenser in the flexible flat heat pipe, which leads to increased liquid–vapor interfacial thermal resistance in the evaporation section. The purpose of this study is to apply the flexible flat heat pipe to thermal control of deployable structures. [ABSTRACT FROM AUTHOR]