Estelle Iacona

Upscaling Statistical Methodology for Radiative Transfer in Porous Media: New Trends

The morphology of a porous medium is now generally known from X and γ ray tomography techniques. From these data and radiative properties at the pore scale, a homogenized medium associated with a porous medium phase is exhaustively characterized by radiative statistical functions, i.e., by a statistical cumulative extinction distribution function, absorption, and scattering cumulative probabilities and a general scattering phase function. The accuracy is only limited by the tomography resolution or the geometrical optics validity. When this homogenized medium follows the Beer’s laws, extinction, absorption, and scattering coefficients are identified from these statistical functions; a classical radiative transfer equation (RTE) can then be used. In all other cases, a generalized radiative transfer equation (GRTE) is directly expressed from the radiative statistical functions. When the homogenized medium is optically thick at a spatial scale such as it is practically isothermal, the radiative transfer can simply be modeled from a radiative Fourier’s law. The radiative conductivity is directly determined by a perturbation technique of the GRTE or RTE. An accurate validity criterion of the radiative Fourier’s law has recently been defined. Some paths for future research are finally given.

Bubble Formation and Detachment in Variable Gravity Environment Under the Influence of Electric Fields

The formation and detachment of air bubbles injected into a stagnant, isothermal liquid through an orifice in a plane surface were studied at various reduced gravity levels under the influence of electric fields. Reduced gravity experiments were carried out in NASA’s reduced gravity aircraft, the KC-135. The objective of this study is to investigate and explore the possibility of substituting the buoyancy force with the electric field force by applying external electric fields in two phase flow and boiling. Two top electrode geometries were used: flat and off-axis spherical, to generate a uniform and non-uniform electric field, respectively. The bubble life cycle was first experimentally visualized, and then the characteristic dimensions of the bubble and its volume at the moment of detachment were measured using digital image processing. It is shown that both the level of gravity and the magnitude of the electric field significantly affect the bubble formation and detachment. Under microgravity with uniform electric fields, an increasing electric potential from U=0 to 20 kV decreases the detachment volume Vd by 51%. In a nonuniform electric field with U=20 kV, the detachment volume decreases by 80% as the gravity increased from 0.006g to 0.374g. The volume flow rates in variable gravity and terrestrial conditions are 2.57x10-7 m3/s and 3.33x10-7 m3/s, respectively. Acknowledgments: This research was supported by a NASA research grant. The experiments in the KC-135 aircraft were carried out by Cila Herman, Gorkem Suner, Steven Marra, and Ed Scheinerman. The support by the KC-135 crew and NASA Glenn Research Center was invaluable for the successful completion of the experiments. Variable gravity, nonuniform electric field U=20kV Microgravity, uniform electric field Microgravity, U=0V Electric field distribution

Determination of Anisotropic Absorption and Extinction Coefficients of a Tomographed Real Porous Medium

The direct general identification method of the radiative properties of high porosity media, developed and validated for virtual statistically isotropic media in [1], has been applied to a real statistically anisotropic medium. This medium has a transparent fluid phase and an opaque gray diffuse solid phase. It is modelled by a semi-transparent equivalent medium characterized by extinction and absorption coefficients β and κ. These quantities are directly determined from the morphology data obtained by X-ray tomography and from the absorptivity of the solid phase. The application of this approach to a mullite sample has established that β and κ are homogeneous but depend on direction. This last feature has to be accounted for by a radiative transfer method for this type of medium.Copyright

Electric Field Effect on Bubble Detachment in Variable Gravity Environment

The subject of the present study, the process of bubble detachment from an orifice in a plane surface, shows some resemblance to bubble departure in boiling. Because of the high heat transfer coefficients associated with phase change processes, boiling is utilized in many industrial operations and is an attractive solution to cooling problems in aerospace engineering. In terrestrial conditions, buoyancy is responsible for bubble removal from the surface. In space, the gravity level being orders of magnitude smaller than on earth, bubbles formed during boiling remain attached at the surface. As a result, the amount of heat removed from the heated surface can decrease considerably. The use of electric fields is proposed to control bubble behavior and help bubble removal from the surface on which they form. The objective of the study is to investigate the behavior of individual air bubbles injected through an orifice into an electrically insulating liquid under the influence of a static electric field. Bubbl...

Enhancement of Pool Boiling Heat Transfer and Control of Bubble Motion in Microgravity Using Electric Fields - BCOEL

The BCOEL project focuses on improving pool boiling heat transfer and bubble control in microgravity by exposing the fluid to electric fields. The electric fields induce a body force that can replace gravity in the low gravity environment, and enhance bubble removal from thc heated surface. A better understanding of microgravity effects on boiling with and without electric fields is critical to the proper design of the phase-change-heat-removal equipment for use in space-based applications. The microgravity experiments will focus on the visualization of bubble formation and shape during boiling. Heat fluxes on the boiling surface will be measured, and, together with the measured driving temperature differences, used to plot boiling curvcs for different electric field magnitudes. Bubble formation and boiling processes were found to be extremely sensitive to g-jitter. The duration of the experimental run is critical in order to achieve steady state in microgravity experiments. The International Space Station provides conditions suitable for such experiments. The experimental appararus to be used in the study is described in the paper. The apparatus will be tested in the KC-135 first, and microgravity experiments will be conducted on board of the International Space Station using the Microgravity Science Glovebox as the experimental platform.