Transient Marangoni convection in hanging evaporating drops

Abstract. A combined experimental and numerical analysis has been carried out to study Marangoni effects during the evaporation of droplets. The experiments are performed with pendant drops of silicone oils (with different viscosities) and hydrocarbons. The temperature of the disk sustaining the drop is rapidly increased or decreased in order to study transient heating or cooling processes. The velocity field in the droplet is evaluated monitoring the motion of tracers in the meridian plane, using a laser sheet illumination system and a video camera. Surface temperature distributions of the drops are detected by infrared thermocamera. The numerical model is based on axisymmetric Navier-Stokes equations, taking into account the presence of Marangoni shear stresses and evaporative cooling at the liquid-air interface. Marangoni flows cause a larger, more uniform surface temperature, increasing heat transfer from disk to droplet, as well as evaporation rate. When Marangoni effects are negligible, larger surface temperature differences occur along the drop surface and heat transfer is relatively small. The role of Marangoni and buoyancy flows in silicone oils with different viscosities and hydrocarbons is discussed and correlations are presented between experimental and numerical results. Keywords: Viscosity of liquids; diffusive momentum transport, Evaporation and condensation, Surface tension and related phenomena, Flows in ducts, channels, nozzles, and conduit

Testing ultra-high temperature ceramics for thermal protection and rocket applications

The work is focused on experimental aerothermodynamic characterization of Ultra-High-Temperature Ceramic materials for aerospace applications. These materials are assuming an increasing importance in aerospace research because their high temperature resistance makes them interesting to develop components for extreme applications, such as thermal protection systems for hypersonic or atmospheric reusable re-entry vehicles, specific components for propulsion, combustion chambers, engines intakes or rocket nozzles. In order to test the materials behavior in extreme relevant environments, different facilities are available, including supersonic arc-jet wind tunnels and lab-scaled rocket motors. Typical activities include the design of prototypes for the experimental campaigns, tests on material samples in both re-entry and combustion environments, numerical modelling and simulations of reacting flows around test articles. To investigate the typical materials behavior in atmospheric re-entry conditions, relevant tests are carried out with arc-jet facilities, with a maximum flow total enthalpy higher than 20 MJ/kg, supersonic Mach number and temperatures up to 2000°C in a gas atmosphere with high concentration of atomic oxygen. The facility, fed by an electric arc-heater and working in a continuous fashion, is able to reproduce, for long exposition times, heat fluxes and dynamic pressures of interest to simulate hypersonic flight conditions, in order to test the ablation resistance of TPSs, as well as to investigate aero-thermochemical issues. Samples with different shapes have been investigated to simulate the conditions reached in the stagnation point of a re-entry vehicle, with flat or hemispherical specimens, or on leading edges. Typical sizes of the samples are in the order of 1 cm and the maximum heat flux is in the order of 10 MW/m2. Larger facilities allow to test models of larger size. Particular attention has been focused on thermo-chemical surface instabilities of SiC-ZrB2 ceramics in high enthalpy dissociated supersonic air flows resulting from the catalytic activity of a surface oxide scale. Pyrometry and thermography techniques allow to perform temperature and emissivity measurements on the samples. The microstructures of the UHTCs sample are analyzed thanks to a collaborative partnership with CNR-ISTEC Institute in Faenza (Italy); the most promising materials are characterized by layered multiphase configurations of oxide scales protecting the unoxidized core material. The experimental activities are supported by numerical simulations, that become a viable and largely recommended tool not only to predict the thermo-chemical evolution of the gas but also to characterize the flow-field surrounding the proof article inside the (ground) testing chamber. In addition, because the flow conditions generated in the high-enthalpy plasma wind tunnel are very complex, the verification of the free-stream flow conditions must be a combined effort of experimental diagnostics and computational fluid dynamics (CFD) simulations. Furthermore, the Aerospace Propulsion Laboratory allows, besides the characterization of hybrid rocket propellants, for which it is mainly conceived, also to investigate rocket components or subsystems manufactured in innovative materials, such as nozzles and nozzle inserts, in highly relevant operating conditions. Computational models for numerical simulations of the rocket internal ballistics and of the nozzle exhaust jet are developed to support the experimental activities. This presentation will be an overview of the main research activities performed so far, as well as of the current programs, regarding the characterization of Ceramic Matrix Composites with an Ultra-High-Temperature Ceramic matrix (UHTCMC), in both propulsion and atmospheric re-entry environments, in the framework of the European Project C3HARME – Next Generation Ceramic Composites for Combustion Harsh Environment and Space

Buoyancy and surface tension-driven convection around a bubble

A combined experimental and numerical analysis has been carried out to study the behavior of a bubble under a horizontal heated surface. In this configuration, the interaction between buoyancy and surface tension driven convection produces complex fluid dynamic structures. An instability occurs in the form of an oscillatory three-dimensional fluctuation of the thermal and velocity field when a critical temperature difference is exceeded. The structure of this flow regime has been investigated with transient three-dimensional simulations carried out for normal gravity and zero gravity conditions. The velocity field around the bubble has been experimentally analyzed with a ``laser sheet'' technique for the flow visualization and a Wollaston prism interferometer has been utilized to capture the oscillatory temperature field. Good correlations are shown between experimental and numerical results. Keywords: Thermal convection, Buoyancy-driven instabilities, Surface-tension-driven instability, Interaction

DSMC Aero-Thermo-Dynamic Analysis of a Deployable Capsule for Mars Entry

A deployable capsule is made of flexible, high temperature resistant fabric, folded at launch and deployed in space at the beginning of the re-entry. This kind of capsule thanks to lightness and to low costs can be an alternative to the current “conventional“ capsules. The present authors already analyzed the trajectory and the aerodynamic behavior of such a kind of capsule during the Earth re-entry. In that study an aerodynamic longitudinal stability analysis and an evaluation of the thermal and mechanical loads for a possible, suborbital re-entry demonstrator, was carried out in both continuum and rarefied regimes. The results verified that a stable equilibrium condition is verified around the zero angle of attack and an unstable equilibrium condition is verified around the 180 angle of attack; therefore the capsule turned out to be self-stabilizing. In the present paper the trajectory, the longitudinal stability, the thermal and mechanical loads of the same capsule has been evaluated for a possible use in Mars entry. The present study is aimed at providing preliminary information considering both the diversity of the two atmospheres and the diversity of the two types of entry: ballistic, sub-orbital for Earth, direct for Mars and therefore of the initial entry velocity. The parameters were compared with those along the Earth re-entry. As the computer tests have been carried out at high altitudes, therefore in rarefied flow fields, the use of Direct Simulation Monte Carlo codes has been mandatory. The computations involved both global aerodynamic quantities (drag and longitudinal moment coefficients) and local aerodynamic quantities (heat flux, pressure and skin friction distributions along the capsule surface). The results verified that the capsule at high altitude (100 km) in Mars entry is not self-stabilizing; it is stable both around the nominal attitude or at zero angle of attack and around the reverse attitude or at 180 deg angle of attack. Furthermore, due to the much higher entry velocity, the local quantities are of several orders of magnitude higher than the ones in Earth re-entry

Aero-Thermo-Dynamic Analysis of a Low Ballistic Coefficient Deployable Capsule in Earth Re-Entry

The paper deals with a microsatellite and the related deployable recovery capsule. The aero-brake is folded at launch and deployed in space and is able to perform a de-orbiting controlled re-entry. This kind of capsule, with a flexible, high temperature resistant fabric, thanks to its lightness and modulating capability, can be an alternative to the current “conventional” recovery capsules. The present authors already analyzed the trajectory and the aerodynamic behavior of low ballistic coefficient capsules during Earth re-entry and Mars entry. In previous studies, aerodynamic longitudinal stability analysis and evaluation of thermal and aerodynamic loads for a possible suborbital re-entry demonstrator were carried out in both continuum and rarefied regimes. The present study is aimed at providing preliminary information about thermal and aerodynamic loads and longitudinal stability for a similar deployable capsule, as well as information about the electronic composition of the plasma sheet and its possible influence on radio communications at the altitudes where GPS black-out could occur. Since the computer tests were carried out at high altitudes, therefore in rarefied flow fields, use of Direct Simulation Monte Carlo codes was mandatory. The computations involved both global aerodynamic quantities (drag and longitudinal moment coefficients) and local aerodynamic quantities (heat flux and pressure distributions along the capsule surface). The results verified that the capsule at high altitude (150 km) is self-stabilizing; it is stable around the nominal attitude or at zero angle of attack and unstable around the reverse attitude or at 180 deg angle of attack. The analysis also pointed out the presence of extra statically stable equilibrium trim points

Arc-jet testing of UHTC demonstrators with a sharp profile

The ultra-high-temperature ceramics (UHTCs) are currently the most studied key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth\u27s atmosphere. They are characterized by sharp profiles to increase performance and maneuvrability. Wedges with a very sharp profile (0.2 mm radius of curvature) and blunt hemispheric articles (5 mm radius of curvature) were produced in the system ZrB2-SiC. The dynamic response to oxidation of such UHTC demonstrators was studied under aero-thermal heating using a high enthalpy supersonic flow of a N2/O2 gas mixture in a plasma wind tunnel. Microstructural features of the reaction scale developed upon oxidation were analyzed and correlated to test conditions through Computational Fluid Dynamics (CFD) simulations. The outputs of CFD simulations matched the in-situ determinations and the materials evolution during arc-jet testin

Recent achievements on fabrication, properties and arc-jet testing of sharp UHTC leading edges

The present contribution is addressed to offer an overview of recent achievements on some borides-based composites that are conventionally classified as ultra-high temperature ceramics (UHTCs) for their extremely high melting points.UHTCs are actively studied as key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth\u27s atmosphere: increased performances and better maneuvrability are gained only through the design of very sharp profile. Sharp wedges and test articles with blunter profiles were fabricated in the ZrB2 base system, using SiC particulate or SiC short fibers as second phase: microstruture and fundamental thermo-mechanical characteristics were determined. The dynamic response to oxidation of such UHTCs was studied under aero-thermal heating using high enthalpy supersonic flows in arc-jet plasma wind tunnel. Microstructural changes were analyzed and correlated to specimen\u27s size and shape and test conditions through Computational Fluid Dynamics (CFD) simulation