Guillaume Fau

Permeation of inert and supercritical reactive fluids through metallic and composite media

Large heat load are encountered in hypersonic and space flight applications due to the high vehicle speed (over Mach 5, i.e. 5000 km.h -1 ) and to the combustion heat release. If passive and ablative protections are a way to ensure the thermal management, the active cooling is probably the most efficient way to enable the structures withstanding of such large heat load. In some conditions, transpiration cooling will be used. In this paper, the permeation of fuels and other fluids through porous media is studied up to 1150 K and 60 bars. A dedicated experimental bench has been established to ensure the monitoring of temperature, pressure, mass flow rate and chemical composition (Gas Chromatograph, Mass Spectrometer, Infra-Red spectrometer) in stationary and transient conditions. The tests on metallic and composite samples have been conducted with N2, CH4, H2+CH4 mixtures and synthetic fuels (n-C12H26). The pressure losses comparison with the mass flow rate has enabled the determination depending on the temperature of the Darcian

Flash Pyrolysis of High Density PolyEthylene

The inert and oxidative flash pyrolysis of High Density Poly-Ethylene (HDPE) is studied up to 20 000 K.s-1, under pressure up to 3.0 MPa and at temperature ranging from 1000 K to 1500 K. These conditions are considered to represent those waited onboard a hybrid rocket engine using HDPE as solid fuel. Recycling applications may also find some interest. The pyrolysis products are analysed by Gas Chromatograph, Flame Ionisation Detector and Mass Spectrometer to quantify the effects of each physical parameter on the HDPE decomposition. The classical products distribution diene-alkene-alkane for each carbon atoms number is shown to be modified at such high temperature because of the pyrolysis of primary products. The pressure effect, which is generally neglected in HDPE pyrolysis studies found in open literature, is proved to be a major factor (up to one order of magnitude on the ethylene mass fraction). The heating rate presents noticeable consequences on the pyrolysis products distribution with a larger formation of light species while heavier ones are favoured under oxidative pyrolysis conditions. The experimental data should serve in the future to improve the accuracy of kinetic mechanisms for later use in numerical computing.

Determination of Darcian permeability of porous material by infrared spectrometry

The active cooling of aerospace structures can be performed by the use of porous materials. It requires characterizing its permeability to predict the cooling efficiency by means of Computational Fluid Dynamics codes. The Darcian term is generally deduced experimentally from a relationship between the mass flow rate and the pressure drop through the porous media. Due to thermo-chemical process involved in the cooling, the permeability can change. It is currently not possible with common techniques to determine these variations during the flight. This paper presents a novel approach taking advantage of well known flow behaviour in chemical reactor engineering in order to propose a real-time in situ quantification of the Darcian permeability. The residence time distribution is analyzed thanks to tracer injection and to associated experimental measures of Infra-Red signals. The IR peak characteristics (height, width, surface, rising and falling gradients) and time delays are analyzed and correlated to physical parameters (mass flow rate, injected mass of tracer, properties of fluids). The peak height and surface and the rising gradient vary linearly in the same sense as the injected tracer mass while the falling gradient varies in the opposite sense. Both gradients decrease with a mass flow rate increase. The time delay between injection and detection of the tracer is quite constant except when changing the fluid nature. A design of experiments allowed estimating quantitatively the influence of each physical parameter on the optical one of the IR signal. Thanks to this first understanding, the Darcian permeability is linked to the observed IR signal. A linear regression is proposed with the peak width which is judged to be the most relevant parameter. Finally, an analytical approach is developed to fit ordinary differential equations to IR peak measures and to correlate the parameters of the law to the Darcian permeability. Several laws (linear, logarithmic and power) are proposed depending on the parameter but the linear regression involving the coefficient noted β1 is the most promising. One of the advantages of this method is to be able measuring the permeability during the flight and not only on-ground with specific test facility.

Benchmark of Experimental Determination Methods of Gas Permeabilities.

The thermal load protection of hypersonic and space vehicle structures can be achieved by either passive or active methods, such as ablative materials or active cooling. For the latter, porous Ceramic Matrix Composite media offer a possibility to exploit thermal protection by means of transpiration cooling due to their higher permeability. Ceramic materials used for regenerative cooling have a much smaller permeability (several orders of magnitude of difference). The cooling techniques based on fluid transpiration are particularly interesting for reusable systems. However, one of the related key issues is the determination of permeability parameters such as the Darcys and Forchheimers terms which are highly dependent on the fabrication process, cracks, delamination and heterogeneities notably. After a review of available permeation laws, the present paper aims at proposing an analytical and applied comparison of two of them (one based on the international norm ISO4022 and one derived for compressible flows). To apply these mathematically equivalent laws, a cross verification and validation has been realized on two different test rigs with different porous media (metallic and composite) with a range of Darcian permeability varying from 10 -17 m² to 10 -11 m². The PRISME test bench has a lower accuracy for thick samples (over 3 mm) due to lateral permeation while the DLR rig is free from such a phenomenon thanks to an innovative sealing (which is however not adapted to samples thinner than 3 mm). The two test rigs are found to be complementary since the PRISME one is more accurate for structures related to regenerative cooling (10 -13 m² and lower) while the DLR one is better for active cooling structures (10 -14 m² and higher). The range of permeability is very thin for cross verification but the results are nevertheless judged to be satisfactory (discrepancy around 14 % for reference samples). Finally, both methods for permeability determination are suitable and present discrepancies lower than a factor 2, which is still not negligible. However, this can be reduced to 50% by applying a DLR in-house code. The one based explicitly on compressibility is chosen more specifically for its easiest application to Computational Fluid Dynamics codes as it is based on the inlet conditions for the porous media instead of internal mean conditions with the ISO4022 method.

Modeling the spatio-temporal evolution of permeability during coking of porous material

Transpiration cooling is one of the most efficient cooling techniques, but one which generates complex phenomena that are difficult to model, and this all the more in that a reactive fluid such as an endothermic fuel is used. Above a certain temperature, such fuel is pyrolysed and, thanks to its endothermic behaviour, this ensures the active cooling of the hot walls of the combustion chamber. However, one of the consequences of this thermal decomposition is the unwanted formation of coke which blocks the porous material (both on the surface and in the interior). This gradual blocking reduces the materials permeability and thus the efficiency of the cooling system. Modelling the permeability distribution of porous materials is thus a key parameter in better understanding transpiration cooling. The present article shows several models intended to estimate the variation in time and space of the permeability of a material (stainless steel) during its coking. The fluid circulating in this porous material is n-dodecane that is maintained at a high temperature. Following a presentation of the measurement device and the measured experimental data of the mean permeability, two categories of model are studied, notably discontinuous mesh models (with 2 and 3 meshes) and continuous analytical models (linear and exponential). The results obtained show that discontinuous models with 2 and 3 meshes are very close in measuring the temporal evolution of the thickness of the coked zone of the porous material. They also revealed that the exponential model is more appropriate than the linear model in estimating the spatiotemporal evolution of the permeability. Additionally, the evolution of the coking rate in the porous material was determined as a function of time and the results show behaviour similar to that indicated in the literature. Lastly, the average Darcy permeability was linked to the mass of coke deposit in the porous material, the result of which reveals a quasi-linear decrease. Nomenclature Latin letters e = Sample thickness e j [m] = Thickness of each layer j K = Hydraulic conductivity tensor K D = Darcians permeability K Davg = Average Darcian permeability K D0 = Initial Darcian permeability K F = Forchheimers permeability K j = Darcian permeability of each layer j P = Pressure ΔP = Pressure drop t = time T = temperature V = Mean fluid velocity Greek Letters e = Overall open porosity μ = Dynamic viscosity ρ = Fluid density o = Diameter

Effect of Flow Configuration on Darcian and Forchheimer Permeabilities Determination in a Porous Composite Tube

Using Fuel Cell on board of aircraft imposes to extract light species (such as Hydrogen and light hydrocarbons) from the liquid fuel which is stored and used, possibly at temperatures where a fuel pyrolysis occurs. Natural porosity of composite material could be used to filtrate the selected species. Hence the permeability of the porous media becomes one of the key parameter to be accurately measured. It is often determined experimentally in laboratory with disc samples (outlet of the flow is achieved through the porous material) and normal flow. However, this configuration is far from the realistic one consisting of tubes (a main flow is found additionally to the one through the material, tangential permeability). Therefore, the effect of a second outlet on the Darcys and Forchheimers permeabilities characterization should be studied (despite the permeability is an intrinsic property of the material itself and it should not be dependent on the test apparatus). This paper focuses on a new way of using an existing test bench for the determination of Darcys and Forchheimers permeabilities of C/SiC porous composite tube by taking two outlets into account. Operating parameters (temperature, pressure and mass flow rate) are measured for three different configurations: i) secondary outlet (S.O) is 0% open ii) S.O is 50% open and iii) S.O is 100% open. Then Darcys and Forchheimers permeabilities are computed by ISO and P 2 methods using a direct search algorithm. Obtained results from different methods are compared and discussed. They are in agreement with the literature data which guarantees the reliability of the test bench and of related measures. Nomenclature Acronym CMC = Ceramic Matrix Composite PO/SO ratio = ratio of the flowrate from the Primary Outlet over the one from the Secondary Outlet P.O = Primary outlet 1 PhD student, CE Group, PRISME Laboratory, hussain.najmi@insa-cvl.fr. 2 Associate Professor, DMS Group, PRISME Laboratory, eddy.el-tabach@univ-orleans.fr. 3 Tenure Professor, CE Group PRISME Laboratory, khaled.chetehouna@insa-cvl.fr. 4 Full Professor, CE Group, PRISME Laboratory, nicolas.gascoin@insa-cvl.fr, Senior AIAA Member. 5 Chief Engineer, Hypersonic Programs MBDA, francois.falempin@mbda-systems.com, Senior AIAA Member. American Institute of Aeronautics and Astronautics 2 S.O = Secondary outlet LatinLetters a g = Grain area d g = Grain diameter d p = Pore diameter K D = Darcians permeability K F = Forccheimers permeability L = Length P inlet = Pressure inlet P outlet = Pressure outlet R e = Reynolds pore number T = Temperature V = Velocity Greek Letters e = Overall open porosity µ = Dynamic viscosity ρ = Density

High-End Experiments on Regenerative Cooling: Test Bench Design

Regenerative cooling is widely used in high-pressure and high-thrust fuel-cooled rocket engines, also suitable for hypersonic structures. The propellant duality in terms of functions (fuel and coolant) makes the thermal and combustion management quite challenging. Dynamics of the system must be studied to develop regulation and control strategies which should be performed with a response time lower than the lowest characteristic time found in supersonic combustion ramjets, i.e. about 1 ms. The present work aims at setting experiments at lab scale by simplifying the additional difficulty of supersonic flow, to determine appropriate regulation dynamics for latter model and control developments. A combustion chamber was dimensioned with similitude rules in terms of heat flux density, conversion rate, chemical compositions, dynamics. Computational Fluid Dynamics was developed to dimension the experimental bench. It was found that a pyrolysis rate up to 100% can be obtained using ethylene as fuel at 50 bar and 1200 K and with a residence time of about 100 s. Combustion with air (adiabatic flame temperature up to 2400 K) will provide the required heat flux density. The operating range in terms of fuel pressure (10-50 bar), of fuel mass flow rates (50-100 mg s) and of equivalence ratio (0.8 to 1.0) have been certified.

Indirect Infra-Red Determination of Darcian permeability for cooling applications.

The active cooling of aerospace structures is expected by use of porous material such as Ceramic Matrix Composite. This technology involves a coolant flow through the porosity to enhance the heat transfers. It requires characterizing the permeability to predict its efficiency. The Darcys and Forchheimers terms are used in Computational Fluid Dynamics codes to simulate tests of cooling. These parameters are generally deduced experimentally from relationship between the mass flow rate and the pressure drop through the porous media. This paper presents a novel approach taking advantage of well known flow behaviour in chemical reactor engineering. The residence time distribution is analyzed thanks to tracer injection and to associated experimental measures of Infra-Red signal. The IR peak characteristics (height, width, surface, rising and falling gradients) and time delays are analyzed and correlated to physical parameters (mass flow rate, injected mass of tracer, nature of fluids). The peak height and surface and the rising gradient vary linearly in the same sense as the injected tracer mass while the falling gradient varies in the opposite sense. Both gradients decrease with a mass flow rate increase. The time delay between injection and detection of the tracer is quite constant except when changing the fluid nature. A design of experiments allowed estimating quantitatively the influence of each physical parameter on the optical one of the IR signal. Thanks to this first understanding, the Darcys permeability is linked to the observed IR signal to try providing first relationship to be used later in the opposite to estimate the Darcian term on the basis of IR signal. A linear regression is proposed between this permeation term and the peak width which is judged to be the most relevant parameter. Finally, an analytical approach is developed to fit ordinary differential equation to IR peak measure and to correlate the parameters of the law to the Darcys permeability. Several laws (linear, logarithmic and power) are proposed depending on the parameter but the linear regression involving the coefficient noted 1 β is the most promising.